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ZOOLOGICAL ARTICLES 
 
ZOOLOGICAL ARTICLES 
 
 CONTRIBUTED TO THE "ENCYCLOPAEDIA BRITANNICA" 
 
 BY 
 
 E. RAY LANKESTER, M.A., LL.D., F.R.S. 
 
 DEPUTY LINACRE PROFESSOR IN THE UNIVERSITY OF OXFORD, AND HOX. FELLOW OF EXETER COLLEGE 
 PRESIDENT OF THE MARINE BIOLOGICAL ASSOCIATION OF THE UNITED KINGDOM ; 
 
 HON. MEMBER OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY; 
 CORRESPONDING MEMBER OJ TH.E ^ADEMY OF SCIENCES OF PHILADELPHIA. 
 
 TO WHICH ARE ADDED KINDRED ARTICLES BY 
 
 W. JOHNSON SOLLAS, LL.D., F.R.S. 
 
 PROFESSOR OP GEOLOGY IX TRINITY COLLEGE, DUBLIN. 
 
 LUDWIG VON GRAFF, PH.D. 
 
 I'ROFESSOE OF ZOOLOGY IX HIE UNIVERSITY OF GRAZ, AUSTKU. 
 
 A. A. W. HUBRECHT, PH.D., LL.D. 
 
 PKOFESSOK OF ZOOLOGY IN THE UNIVERSITY OF CTKKCBT. 
 
 A. G. BOURNE, D.Sc. 
 
 FKOFESSOB OF BIOLOGY IX THE PRESIDENCY COLLEGE, MADRAS. 
 
 W. A. HERDMAN, D.Sc. 
 
 PBOFESSOE OF SATCRAL HISTORY IS THE US1VEKSITY COLLEGE, LIVERPOOL. 
 
 EDINBURGH: ADAM & CHARLES BLACK 
 NEW YORK: CHARLES SCRIBNER'S SONS 
 
 MDCCCXCI 
 
PREFACE. 
 
 T HAVE been anxious to render the articles on various groups of Animals written by 
 -*- me for the Encyclopedia Britannica more readily accessible to the University 
 student than they are when bound up in the large volumes of that great work. The 
 Publishers have very kindly met my wishes in this respect by consenting to issue the 
 present reprint. With my articles on Protozoa, Hydrozoa, Mollusca, Polyzoa, and 
 Vertebrata, are here included, by the kind consent of the authors, the article on Sponges 
 by Professor Sollas, that on Planarians by Professor von Graff, that on Nemertines by 
 Professor Hubrecht, that on Eotifera by Professor Bourne, and that on Tunicata by 
 Professor Herdman. The volume thus forms a treatise on a considerable section of the 
 animal kingdom. Obviously it does not profess to be a complete handbook. Since the 
 articles are reprinted from the original plates, and issued at a low price, it has not been 
 possible to introduce any large additions into the text. Here and there an error, due to 
 oversight, has been corrected, and one or two new figures have been added, rendering the 
 work more complete. The chief additions are the woodcut illustrating recent discoveries 
 concerning the Dinoflagellata (p. 37) ; the note by Professor Sollas on the classification of 
 Monaxonida (p. 39); the woodcut of Scyphomedusse from the Deep-Sea (p. 57); the 
 woodcut fig. 19 on p. 107, which replaces a similar but incorrect figure in the original 
 article, and the woodcut, fig. IA on p. 159, showing forms connecting the Eupolyzoa and 
 other Gephyraea. 
 
 There are one or two matters, by way of addition to or correction of my own articles, 
 which this preface gives me the opportunity of mentioning. 
 
 In regard to the Protozoa, the reader should note that Professor Biitschli's treatise in 
 Bronn's Thierreich is now completed. He has rejected the classification of the Ciliata, 
 which we owe to Stein, and adopts the following Branch A. Gymnostoma ( = Holotricha 
 with chitinised pharynx, Prorodon, Trachelius, &c.) ; Branch B. Trichostoma ( = the 
 remaining Ciliata, all of which have the pharynx ciliated, if present). The Trichostoma 
 are divided into two classes the Aspirotricha, and the Spirotricha. The Aspirotricha 
 are the rest of the Holotricha of Stein, not comprised in the Gymnostoma of this classifi- 
 
 M90603 
 
vi PREFACE 
 
 cation. The Spirotricha are characterised by all possessing the adoral " heterotrichal " band 
 of large cilia ; they are divided into the sub-classes Heterotricha, Hypotricha, Peritricha, 
 and Oligotricha. The two first of these groups correspond with Stein's groups of the same 
 names, whilst the Peritricha of Stein are now divided into Peritricha and Oligotricha, the 
 latter sub-class being formed for such genera as Halteria, Strombidium, and Tintinnus. I 
 consider Blitschli's classification an improvement upon Stein's, with the doubtful exception 
 of the distinct position' assigned to the Oligotricha. 
 
 Tn regard to the Hydrozoa, the most important additions to knowledge since the date 
 of the article are to be found in the large and richly-illustrated monographs by Haeckel 
 (System der Medusen, Jena, 1879-1880 ; " Report on the Deep-Sea Medusae," Challenger 
 Reports, vol. iv., 1882; "Reports on the Deep-Sea Siphonophora," Challenger Reports, 
 vol. xxviii., 1888), and in the remarkable researches of Weissman on the origin of the 
 sexual products (Enstehung der Sexualzellen bei der Hydromedusen, Jena, 1883). The 
 student who takes in hand the actual examination of a specimen of Aurelia aurita by 
 aid of the description given of it in the article Hydrozoa, should also refer to the plates of 
 Ehrenberg's account of this animal (Physikalische Abhandlungen der Konigl. Akad. d. 
 Wissensch., Berlin, 1835), and Mr Minchin's brief but valuable paper on the enclosure of 
 the embryos in minute brood pouches formed by sacculation of the grooves of the oral 
 lobes (Proc. Zool. Soc., 1889, No. xxxix.). 
 
 If I were rewriting the article Mollusca, I should adopt the conclusion of my friend 
 and former pupil, Dr Paul Pelseneer, of Ghent, and remove the Pteropoda from association 
 with the Cephalopoda, not to maintain them as a distinct class, but to place them, as he 
 has done, among the Palliate or Tectibranchiate Opisthobranchiate Gastropoda, to which, it 
 seems, they bear the same relation as do the Natantia to the Azygobranchiate Streptoneura. 
 It appears that the Thecosomate Pteropods are nearly related to the Bullidse and Torna- 
 tellidaB, whilst the Gymnosomate forms are derivable from the Aplysiidse. A careful study 
 of the nervous system convinced Dr Pelseneer that the sucker-bearing lobes of such 
 Gymnosomate Pteropods as Pneumodermon are really cephalic in nature, and innervated 
 from the cerebral ganglion, whilst the sucker-bearing lobes of the Cephalopoda are produc- 
 tions of the foot, and are convincingly demonstrated by Pelseneer (as maintained by me in 
 the article " Mollusca ") to be innervated by the pedal ganglia. The remarkable coincidence 
 in the Pteropoda and Cephalopoda of adoral appendages provided with suckers which had 
 been, to my mind, the chief ground for supposing a genetic relationship between these 
 two sets of forms, proves to be a case of homoplasy. 1 It is, indeed, a very striking case of 
 the parallelism of genetically distinct organs. The whole of this question is ably treated 
 by Pelseneer in Part III. of his " Report on the Pteropoda," published in vol. xxiii. of the 
 Challenger Reports, 1888. The student of molluscan anatomy should not fail to read this 
 
 1 The reader is referred for an explanation of this term, and a discussion of the phenomena concerned, to my article 
 " On the use of the term Homology in Modern Zoology, and the distinction between Homogenetic and Homoplastic 
 Agreements," Ann. and Mag. Nat. Hist., 1870. 
 
PREFACE. vii 
 
 clear and well-illustrated discussion of the structure of the Pteropoda, and of the inferences 
 which may be drawn therefrom as to their affinities. 
 
 In regard to the article Polyzoa, I may mention that I think it preferable to make use 
 of the established term "Gephyraea" in place of that introduced in this article, viz., 
 " Podaxonia." The Gephyraea, then, include the Sternaspidomorpha, Echiuromorpha, 
 Sipunculomorpha, Phoronidomorpha, Polyzoa (Eupolyzoa of the article), Brachiopoda, and 
 Pterobranchia. Concerning the affinities of the first four of these classes with one another, 
 there is little doubt : as to the affinities of the last three with one another, and with the 
 first four we are still in a very uncertain state, and are likely to remain so for some time, 
 owing to the absence of satisfactory embryological data and the difficulty of obtaining such. 
 
 The subject matter of the article Vertebra ta is much more extensive than that of the 
 other chapters, and, owing to limited space, is treated in a much more general way than is 
 the case with the latter. In regard to the Craniata, the intention was to give only a sketch 
 of leading features which should be supplemented by the study of such works as Gegenbaur's 
 Comparative Anatomy, Wiedersheim's Anatomy of Vertebrates, and the special articles on 
 Fishes, Reptiles, Birds, and Mammals, written for the Encyclopedia by eminent autho- 
 rities on those groups. The treatment of the Cephalochorda (Amphioxus) and its relations 
 to the Urochorda is a little more complete, and I therefore take occasion to refer the reader 
 to recent publications, in which our knowledge of this most interesting member of the 
 Vertebrate group has been largely extended. They are Contributions to the Know- 
 ledge of Rhabdopleura and Amphioxus (ubique citata), by E. Ray Lankester (London : 
 J. & A. Churchill, 1889); "The Development of the Atrial Chamber of Amphioxus," by 
 E. Ray Lankester and Arthur Willey, in the Quart. Jour, of Mic. ScL, vol. xxxi., 1890 ; 
 " The Later Larval Development of Amphioxus," by Arthur Willey, B.Sc., in the same 
 Journal, vol. xxxii. ; and "The Excretory Organs of Amphioxus," by F. E. Weiss, B.Sc., 
 also in the Quart. Jour, of Mic. Sci., vol. xxxi. 
 
 The article " Sponges," by Professor Sollas, contains the only summary account of the 
 Porifera written since the recent extraordinary advances in our knowledge of this group. 
 Its incorporation in the present volume cannot fail to be welcome to students. In 
 Professor Bourne's article on Rotifera are given the only extant woodcuts of the important 
 genus Pedalion. This most important form is not figured or discussed in any other general 
 treatise accessible to students. The articles on Planarians and Nemertines, by Professor 
 von Graff and Professor Hubrecht respectively, are brief summaries of what is known, 
 written by the chief living authority on each group. 
 
 E. RAY LANKESTER. 
 
 OXFORD, December 1890. 
 
CONTENTS. 
 
 PAGE 
 
 PROTOZOA, . . l 
 
 SPONGES, 39 
 
 HYDROZOA, . 57 
 
 PLANARIANS, 77 
 
 NEMERTINES, . 83 
 
 ROTIFERA, . 89 
 
 MOLLUSCA, . . 95 
 
 POLYZOA, . 169 
 
 VERTEBRATA, . l73 
 
 TUNICATA, ..... 185 
 
ZOOLOGICAL ARTICLES. 
 
 PROTOZOA 
 
 ~T)ROTOZOA is the name applied to the lowest grade of 
 JtT the animal kingdom, and originated as a translation 
 of the German term "Urthiere." Whilst at first used 
 some forty years ago in a vague sense, without any strict 
 definition, so as to include on the one hand some simple 
 organisms which are now regarded as plants and on the 
 other some animals which are now assigned a higher place 
 in the animal series, the term has within the last twenty 
 years acquired a very clear signification. 
 
 The Protozoa are sharply and definitely distinguished 
 from all the rest of the animal kingdom, which are known 
 by the names " Metazoa " or " Enterozoa." They are 
 those animals which are structurally single "cells" or 
 single corpuscles of protoplasm, whereas the Enterozoa 
 consist of many such units arranged definitely (in the first 
 instance) in two layers an endoderm or enteric cell-layer 
 and an ectoderm or deric cell-layer around a central 
 cavity, the enteron or common digestive cavity, which is 
 in open communication with the exterior by a mouth. 
 
 The Protozoa are then essentially unicellular animals. 
 The individual or person in this grade of the animal king- 
 dom is a single cell ; and, although we find Protozoa which 
 consist of aggregates of such cells, and are entitled to be 
 called " multicellular," yet an examination of the details 
 of structure of these cell-aggregates and of their life- 
 history establishes the fact that the cohesion of the cells 
 in these instances is not an essential feature of the life of 
 such multicellular Protozoa but a secondary and non-essen- 
 tial arrangement. Like the budded "persons" forming, 
 when coherent to one another, undifferentiated " colonies " 
 among the Polyps and Corals, the coherent cells of a com- 
 pound Protozoon can be separated from one another and 
 live independently ; their cohesion has no economic signifi- 
 cance. Each cell is precisely the counterpart of its neigh- 
 bour ; there is no common life, no distribution of function 
 among special groups of the associated cells, and no cor- 
 responding differentiation of structure. As a contrast to 
 this we find even in the simplest Enterozoa that the cells 
 are functionally and structurally distinguishable into two 
 groups those which line the enteron or digestive cavity 
 and those which form the outer body wall The cells of 
 these two layers are not interchangeable ; they are funda- 
 mentally different in properties and structure from one 
 another. The individual Enterozoon is not a single cell ; 
 it is an aggregate of a higher order consisting essentially 
 of a digestive cavity around which two layers of cells are 
 
 disposed. The individual Protozoon is a single cell; a 
 number of these individuals may, as the result of the pro- 
 cess of fission (cell-division), remain in contact with one 
 another, but the compound individual which they thus 
 originate has not a strong character. The constituent 
 cells are still the more important individualities; they 
 never become differentiated and grouped in distinct layers 
 differing from one another in properties and structure; 
 they never become subordinated to the individuality of 
 the aggregate produced by their cohesion ; hence we are 
 justified in calling even these exceptional aggregated 
 Protozoa unicellular. 
 
 By far the larger number of Protozoa are absolutely 
 single isolated cells, which, whenever they duplicate them- 
 selves by that process of division common to these units 
 of structure (whether existing as isolated organisms or as 
 constituents of the tissues of plants or of animals), separ- 
 ate at once into two distinct individuals which move away 
 from one another and are thenceforward strangers. 
 
 Whilst it is easy to draw the line between the Protozoa 
 and the Enterozoa or Metazoa which lie above them, on 
 account of the perfectly definite differentiation of the cells 
 of the latter into two primary tissues, it is more difficult to 
 separate the Protozoa from the parallel group of unicellular 
 plants. 
 
 Theoretically there is no difficulty about this distinction. 
 There is no doubt that organisms present themselves to us 
 in two great series starting in both cases from simple 
 unicellular forms. The one series, the plants, can take up 
 the carbon, hydrogen, oxygen, and nitrogen necessary to 
 build up their growing protoplasm from mineral com- 
 pounds soluble in water, compounds which constitute the 
 resting stage of those elements in the present physical 
 conditions of our planet. Plants can take their nitrogen 
 in the form of ammonia or in the form of nitrates and 
 their carbon in the form of carbonic acid. Accordingly 
 they require no mouths, no digestive apparatus ; their 
 food being soluble in water and diffusible, they absorb at 
 all or many points of their surface. The spreading diffuse 
 form of plants is definitely related to this fact. On the 
 other hand the series of organisms which we distinguish 
 as animals cannot take the nitrogen, necessary to build up 
 their protoplasm, in a lower state of combination than it 
 presents in the class of compounds known as albumens ; 
 nor can they take carbon in a lower state of combination 
 than it presents when united with hydrogen or with 
 
 A 
 
PROTOZOA 
 
 hydrogen and oxygen to form fat, sugar, and starch. 
 Albumens and fats are not soluble in water and diffusible ; 
 they have to be seized by the animal in the condition 
 of more or less solid particles, and by chemical processes 
 superinduced in the living protoplasm of the animal by 
 the contact of these particles they are acted upon, chemic- 
 ally modified, and rendered diffusible. Hence the animal 
 is provided with a mouth and a digestive cavity, and with 
 organs of locomotion and prehension by which it may search 
 out and appropriate its scattered nutriment. Further the 
 albumens, fats, sugars, and starch which are the necessary 
 food of an animal are not found in nature excepting as 
 the products of the life of plants or of animals ; accord- 
 ingly all animals are in a certain sense parasitic upon 
 either plants or other animals. It would therefore seem 
 to be easy to draw the line between even the most minute 
 t unicellular .pjajits .and the similarly minute unicellular 
 'afcimEfcls-XaostgniKg those which feed on the albumens, &c., 
 of other organisms 'by means of a mouth and digestive 
 agpajpatu^ to h;e aninxal -Series, and those which can appro- 
 pfi$ife:tiie-' eifenJenfeMjfxam'monia, nitrates, and carbonates 
 to 'the 'plants.' 
 
 Such absolute distinctions lending themselves to sharp 
 definitions have, however, no place in the organic world ; 
 and this is found to be equally true whether we attempt 
 to categorically define smaller groups in the classification 
 of plants and animals or to indicate the boundaries of the 
 great primary division which those familiar names imply. 
 Closely allied to plants which are highly and specially 
 developed as plants, and feed exclusively upon ammonia, 
 nitrates, and carbonates, we find exceptionally modified 
 kinds which are known as " insectivorous plants " and are 
 provided with digestive cavities (the pitchers of pitcher- 
 plants, &c.), and actually feed by acting chemically upon 
 the albumens of insects which they catch in these diges- 
 tive receptacles. No one would entertain for a moment 
 the notion that these insectivorous plants should be con- 
 sidered as animals. The physiological definition separat- 
 ing plant from animal breaks down in their case ; but the 
 consideration of the probable history of their evolution as 
 indicated by their various details of structure suffices at 
 once to convince the most sceptical observer that they 
 actually belong to the vegetable line of descent or family 
 tree, though they have lost the leading physiological char- 
 acteristic which has dominated the structure of other 
 plants. In this extreme case it is made very obvious that 
 in grouping organisms as plants or as animals we are not 
 called upon to apply a definition but to consider the 
 multifarious evidences of historical evolution. And we 
 find in the case of the Protozoa and the Protophyta that 
 the same principle holds good, although, when dealing 
 with extremely simple forms, it becomes much more diffi- 
 cult to judge of the genetic relationship of an organism in 
 proportion as the number of detailed points of possible 
 agreement with and divergence from other forms to which 
 it may be supposed to be related are few. 
 
 The feeding of plants upon carbonic acid is invariably 
 accompanied by the presence of a peculiar green-colouring 
 matter chlorophyll. In virtue of some direct or indirect 
 action of this chlorophyll the protoplasm of the plant is 
 enabled to seize the carbon of the mineral world the car- 
 bon which has sunk to the lowest resting stage of combina- 
 tionand to raise it into combination with hydrogen and 
 oxygen and ultimately with nitrogen. There are plants 
 which have no chlorophyll and are thus unable to feed 
 upon carbonic acid. They are none the less plants since 
 they agree closely with particular chlorophyll-bearing 
 plants in details of form and structure, mode of growth 
 and reproduction. A large series of these are termed 
 Fungi. Though unable to feed on carbonic acid, they do 
 
 not feed as do animals. They can take their carbon from 
 acetates and tartrates, which animals cannot do, and their 
 nitrogen from ammonia. Even when it is admitted that 
 some of these colourless plants, such as the Bacteria 
 (Schizomycetes), can act upon albumens so as to digest 
 them and thus nourish themselves, it is not reasonable to 
 place the Bacteria among animals, any more than it would 
 be reasonable so to place Nepenthes, Sarracenia, and 
 Drosera (insectivorous Phanerogams). For the structure 
 and mode of growth of the Bacteria is like that of well- 
 known chlorophylligerous minute Algae from which they 
 undoubtedly differ only in having secondarily acquired 
 this peculiar mode of nutrition, distinct from that which 
 has dominated and determined the typical structure of 
 plants. 
 
 So we find in a less striking series of instances amongst 
 animals that here and there the nutritional arrangements 
 which we have no hesitation in affirming to be the leading 
 characteristic of animals, and to have directly and perhaps 
 solely determined the great structural features of the 
 animal line of descent, are largely modified or even alto- 
 gether revolutionized. .The green Hydra, the freshwater 
 Sponge, and some Planarian worms produce chlorophyll 
 corpuscles in the protoplasm of their tissues just as green 
 plants do, and are able in consequence to do what animals 
 usually cannot do namely, feed upon carbonic acid. The 
 possibilities of the protoplasm of the plant and of the 
 animal are, we are thus reminded, the same. The fact 
 that characteristically and typically plant protoplasm ex- 
 hibits one mode of activity and animal protoplasm another 
 does not prevent the protoplasm of even a highly developed 
 plant from asserting itself in the animal direction, or of a 
 thoroughly characterized animal, such as the green Hydra, 
 from putting forth its chlorophylligenous powers as though 
 it belonged to a plant. 
 
 Hence it is not surprising that we find among the 
 Protozoa, notwithstanding that they are characterized by 
 the animal method of nutrition and their forms determined 
 by the exigencies of that method, occasional instances of 
 partial vegetable nutrition such as is implied by the deve- 
 lopment of chlorophyll in the protoplasm of a few members 
 of the group. It would not be inconsistent with what is 
 observed in other groups should we find that there are 
 some unicellular organisms which must, on account of 
 their structural resemblances to other organisms, be con- 
 sidered as Protozoa and yet have absolutely given up alto- 
 gether the animal mode of nutrition (by the ingestion of 
 solid albumens) and have acquired the vegetable mode of 
 absorbing ammonia, nitrates, and carbonic acid. Experi- 
 ment in this matter is extremely difficult, but such " veget- 
 able" or "holophytic nutrition " appears to obtain in the 
 case of many of the green Flagellata, of the Dinoflagellata, 
 and possibly of other Protozoa. 
 
 On the other hand there is no doubt that we may fall 
 into an error in including in the animal line of descent all 
 unicellular organisms which nourish themselves by the 
 inception of solid nutriment. It is conceivable that some 
 of these are exceptional creophagous Protophytes parallel 
 at a lower level of structure to the insectivorous Phanero- 
 gams. In all cases we have to balance the whole of the 
 evidence and to consider probabilities as indicated by a 
 widely-reaching consideration of numerous facts. 
 
 The mere automatic motility of unicellular organisms 
 was at one time considered sufficient indication that such 
 organisms were animals rather than plants. We now know 
 that not only are the male reproductive cells of ferns and 
 similar plants propelled by vibratile protoplasm, but such 
 locomotive particles are recognized as common products 
 (" swarm-spores " and " zoospores ") of the lowest plants. 
 
 The danger of dogmatizing erroneously in distinguish- 
 
PROTOZOA 
 
 ing Protozoa from Protophyta, and the insuperable diffi- 
 culty in really accomplishing the feat satisfactorily, has led 
 at various times to the suggestion that the effort should be 
 abandoned and a group constituted confessedly containing 
 both unicellular plants and unicellular animals and those 
 organisms which may be one or the other. Haeckel has 
 proposed to call this group the Protista (I). 1 On the 
 whole, it is more satisfactory to make the attempt to dis- 
 criminate those unicellular forms which belong to the 
 animal line of descent from those belonging to the veget- 
 able line. It is, after all, not a matter of much conse- 
 quence if the botanist should mistakenly claim a few 
 Protozoa as plants and the zoologist a few Protophyta 
 as animals. The evil which we have to avoid is that some 
 small group of unattractive character should be rejected 
 both by botanist and zoologist and thus our knowledge of 
 it should unduly lag. Bearing this in mind the zoologist 
 should accord recognition as Protozoa to as wide a range 
 of unicellular organisms as he can without doing violence 
 to his conception of probability. 
 
 A very interesting and very difficult subject of speculation forces 
 itself on our attention when we attempt to draw the line between 
 the lowest plants and the lowest animals, and even comes again 
 before us when we pass in review the different forms of Protozoa. 
 
 That subject is the nature of the first protoplasm which was 
 evolved from not-living matter on the earth's surface. \Vas that 
 first protoplasm more like animal or more like vegetable proto- 
 plasm as we know it to-day ? By what steps was it brought into 
 existence ? 
 
 Briefly stated the present writer's view is that the earliest proto- 
 plasm did not possess chlorophyll and therefore did not possess the 
 power of feeding on carbonic acid. A conceivable state of things 
 is that a vast amount of albuminoids and other such compounds 
 had been brought into existence by those processes which cul- 
 minated in the development of the first protoplasm, and it seems 
 therefore likely enough that the first protoplasm fed upon these 
 antecedent steps in its own evolution just as animals feed on 
 organic compounds at the present day, more especially as the 
 large creeping plasmodia of some Mycetozoa feed on vegetable 
 refuse. It indeed seems not at all improbable that, apart from their 
 elaborate fructification, the Mycetozoa represent more closely than 
 any other living forms the original ancestors of the whole organic 
 world. At subsequent stages in the history of this archaic living 
 matter chlorophyll was evolved and the power of taking carbon 
 from carbonic acid. The "green" plants were rendered possible 
 by the evolution of chlorophyll, but through what ancestral forms 
 they took origin or whether more than once, i.e., by more than 
 one branch, it is difficult even to guess. The green Flagellate Pro- 
 tozoa (Volvocinese) certainly furnish a connecting point by which 
 it is possible to link on the pedigree of green plants to the primi- 
 tive protoplasm ; it is noteworthy that they cannot be considered 
 as very primitive and are indeed highly specialized forms as com- 
 pared with the naked protoplasm of the llycetozoon's plasmodium. 
 
 Thus then we are led to entertain the paradox that though the 
 animal is dependent on the plant for its food yet the animal 
 preceded the plant in evolution, and we look among the lower \ 
 Protozoa and not among the lower Protophyta for the nearest j 
 representatives of that firet protoplasm which" was the result of a 
 long and gradual evolution of chemical structure and the starting j 
 point of the development of organic form. 
 
 The Protozoan Cell-Individual compared with the Typical 
 Cell of Animal and Vegetable Tissues. 
 
 MORPHOLOGY. 
 
 The Protozoon individual is a single corpuscle of proto- 
 plasm, varying in size when adult from less than the 
 ToVoth of an inch in diameter (some Sporozoa and Flagel- 
 lata) up to a diameter of an inch (Xummulites), and even ! 
 much larger size in the plasmodia of Mycetozoa. The sub- | 
 stance of the Protozoa exhibits the same general properties 
 irritability, movement, assimilation, growth, and division 
 and the same irremediablechemical alteration as the result 
 of exposure to a moderate heat, which are observed in 
 the protoplasm constituting the corpuscles known as cells 
 which build up the tissues of the larger animals and 
 
 1 These cumbers refer to the bibliography at p. 866. 
 
 plants. There is therefore no longer any occasion to make 
 use of the word " sarcode " which before this identity was 
 established was very usefully applied by Dujardin (2) to 
 the substance which mainly forms the bodies of the 
 Protozoa. Like the protoplasm which constitutes the 
 " cells " of the Enterozoa and of the higher plants, that 
 of the Protozoon body is capable of producing, by chemical 
 processes which take place in its substance (over and above 
 those related merely to its nutrition), a variety of distinct 
 chemical compounds, which may form a deposit in or 
 beyond the superficial protoplasm of the corpuscle or may 
 accumulate centrally. These products are therefore either 
 ectoplastic or entoplastic. The chemical capacities of 
 protoplasm thus exhibited are very diverse, ranging from 
 the production of a denser variety of protoplasm, probably 
 as the result of dehydration, such as we see in the nucleus 
 and in the cortical substance of many cells, to the chemical 
 separation and deposition of membranes of pure chitin or 
 of cellulose or of shells of pure calcium carbonate or quasi- 
 crystalline needles of silica. 
 
 NUCLEUS. The nucleus is probably universally present in 
 the Protozoon cell, although it may have a very simple struc- 
 ture and be of very small size in some cases. The presence 
 of a nucleus has recently been demonstrated by means of 
 appropriate staining reagents in some Protozoa (shell- 
 bearing Reticularia or Foraminifera and many Mycetozoa) 
 where it had been supposed to be wanting, but we are not 
 yet justified in concluding absolutely that there are not 
 some few Protozoa in which this central differentiation of 
 the protoplasm does not exist ; it is also a fact that in the 
 young forms of some Protozoa which result from the 
 breaking up of the body of the parent into many small 
 " spores " there is often no nucleus present. 
 
 In contrast to this it is the fact that the cells which 
 build up the tissues of the Enterozoa are all derived from 
 the division of a nucleated egg-cell and the repeated 
 division of its nucleated products, and are invariably 
 nucleated. The same is true of tissue-forming plants, 
 though there are a few of the lowest plants, such as the 
 Bacteria, the protoplasm of which presents no nucleus. In 
 spite of recent statements (3) it cannot be asserted that 
 the cells or protoplasmic corpuscles of the yeast^plant 
 (Saccharomyces) and of the hyphae of many simple moulds 
 contain a true nucleus. We are here brought to the 
 question " What is a true nucleus ? " The nucleus which 
 is handed on from the egg-cell of higher plants and 
 Enterozoa to the cells derived from it by fission has lately 
 been shown to possess in a wide variety of instances such 
 very striking characteristics that we may well question 
 whether every more or less distinctly outlined mass or 
 spherule of protoplasm which can be brought into view by 
 colouring or other reagents, within the protoplasmic body 
 of a Protozoon or a Protophyte, is necessarily to be con- 
 sidered as quite the same thing as the nucleus of tissue- 
 forming egg-cell-derived cells. 
 
 Researches, chiefly due to Flemming (4), have shown 
 that the nucleus in very many tissues of higher plants 
 and animals consists of a capsule containing a plasma of 
 " achromatin " not deeply stained by reagents, ramifying 
 in which is a reticulum of " chromatin " consisting of fibres 
 which readily take a deep stain (Fig. I., A). Further it is 
 demonstrated that, when the cell is about to divide into 
 two, definite and very remarkable movements take place 
 in the nucleus, resulting in the disappearance of the 
 capsule and in an arrangement of its fibres first in the 
 form of a wreath (Fig. I., D) and subsequently (by the 
 breaking of the loops formed by the fibres) in the form of a 
 star (E). A further movement within the nucleus leads to 
 an arrangement of the broken loops in two groups (F), the 
 position of the open ends of the broken loops being reversed 
 
PROTOZOA 
 
 as compared with what previously obtained. Now the 
 two groups diverge, and in many cases a striated appear- 
 ance of the achromatin substance between the two groups 
 of loops of chromatin is observable (H). In some cases 
 (especially egg-cells) this striated arrangement of the 
 achromatin substance precedes the separation of the loops 
 (G). The striated achromatin is then termed a " nucleus- 
 spindle," and the group of chromatin loops (Fig. I., G, ) 
 
 FIG. I. Karyokinesis of a typical tissue-cell (epithelium of Salamander) after 
 Flemming and Klein. The series from A to 1 represent the successive stages 
 in the movement of the chromatin fibres during division, excepting G, which 
 represents the " nucleus-spindle " of an egg-cell. A , resting nucleus; D, wreath- 
 form; E, single star, the loops of the wreath being broken; F, separation of the 
 star into two groups of U-shaped fibres; H, diaster or double star; I, comple- 
 tion of the cell-division and formation of two resting nuclei. In G the 
 chromatin fibres are marked a, and correspond to the phase shown in F ; they 
 are in this case called the ' ' equatorial plate " ; 6, achromatin fibres forming the 
 nucleus-spindle; c, granules of the cell-protoplasm forming a "polar star. 1 ' 
 Such a polar star is seen at each end of the nucleus-spindle, and is not to be 
 confused with the diaster H. 
 
 is known as "the equatorial plate." At each end of 
 the nucleus-spindle in these cases there is often seen a 
 star consisting of granules belonging to the general proto- 
 plasm of the cell (G, c). These are known as " polar stars." 
 After the separation of the two sets of loops (H) the 
 protoplasm of the general substance of the cell becomes 
 constricted, and division occurs, so as to include a group of 
 chromatin loops in each of the two fission products. Each 
 of these then rearranges itself together with the associated 
 achromatin into a nucleus such as was present in the 
 mother-cell to commence with. This phenomenon is termed 
 " karyokinesis," and has been observed, as stated above, 
 in a large variety of cells constituting tissues in the higher 
 animals and plants. 
 
 There is a tendency among histologists to assume that 
 this process is carried out in all its details in the division 
 of all cells in the higher plants and animals, and accordingly 
 to assume that the structural differentiation of achromatin 
 plasma and chromatin nucleus-fibres exists in the normal 
 nucleus of every such cell. If this be true, it is necessary 
 to note very distinctly that the nucleus of the Protozoon 
 cell-individual by no means conforms universally to this 
 model. As will be seen in the sequel, we find cases in 
 which a close approach is made by the nucleus of Protozoa 
 to this structure and to this definite series of movements 
 during division (Fig. VIII. 3 to 12, and Fig. XXV.); and 
 a knowledge of these phenomena has thrown light upon 
 some appearances (conjugation of the Ciliata) which were 
 previously misinterpreted. But there are Protozoa with a 
 deeply-placed nucleus-like structure which does not pre- 
 sent the typical structure above described nor the typical 
 changes during division, but in which on the contrary the 
 nucleus is a very simple homogeneous corpuscle or vesicle 
 of more readily stainable protoplasm. 
 
 The difficulties of observation in this matter are great, 
 and it is proportionately rash to generalize ; but it appears 
 that we are justified at the present moment in asserting 
 that not all the cells even of higher plants and animals 
 
 exhibit in full detail the structure and movement of the 
 typical cell-nucleus above figured and described; and accord- 
 ingly the fact that such structure and movement cannot 
 always be detected in the Protozoon cell-nucleus must not 
 be regarded as either an isolated phenomenon peculiar to 
 such Protozoon cells, nor must it be concluded that we have 
 only to improve our means of analysis and observation in 
 order to detect this particular structure in all nuclei. It 
 seems quite possible and even probable that nuclei may 
 vary in these details and yet be true nuclei. Some nuclei 
 which are observed in Protozoon cell-bodies may be regarded 
 as being at a lower stage of differentiation and specializa- 
 tion than are those of the epithelial and embryonic cells 
 of higher animals which exhibit typical karyokinesis. 
 Others on the contrary, such as the nuclei of some 
 Eadiolaria (vide infra), are probably to be regarded as 
 more highly developed than any tissue cell-nuclei, and will 
 be found by further study to present special phenomena 
 peculiar to themselves. In some of the highest Protozoa 
 (the Ciliata) it has lately been shown that the nucleus 
 may have no existence as such, but is actually dispersed 
 throughout the protoplasm in the form of fine particles of 
 chromatin-substance which stain on treatment with car- 
 mine but are in life invisible (84). This diffuse condition 
 of the nuclear matter has no parallel, at present known, in 
 tissue-cells, and curiously enough occurs in certain genera 
 of Ciliata whilst in others closely allied to them a solid 
 single nucleus is found. The new results of histological 
 research have necessitated a careful study of the nucleus 
 in its various stages of growth and division in the cell- 
 bodies of Protozoa and a comparison of the features there 
 observed with those established as " typical " in tissue-cells. 
 Accordingly we have placed the figure and explanation of 
 the typical cell-nucleus in the first place in this article for 
 subsequent reference and comparison. 
 
 CORTICAL SUBSTANCE. The superficial protoplasm of 
 an embryonic cell of an Enterozoon in the course of its 
 development into a muscular cell undergoes a change 
 which is paralleled in many Protozoa. The cortical layer 
 becomes dense and highly refringent as compared with the 
 more liquid and granular medullary substance. Probably 
 this is essentially a change in the degree of hydration of 
 the protoplasm itself, although it may be accompanied by 
 the deposition of metamorphic products of the protoplasm 
 which are not chemically to be regarded as protoplasm. 
 The differentiation of this cortical substance (which is not 
 a frequent or striking phenomenon in tissue-cells) may be 
 regarded as an ectoplastic (i.e., peripheral) modification 
 of the protoplasm, comparable to the entoplastic (central) 
 modification which produces a nucleus. 
 
 The formation of " cortical substance " in the Protozoa 
 furnishes the basis for the most important division into 
 lower and higher forms, in this assemblage of simplest 
 animals. A large number (the Gymnomyxa) form no 
 cortical substance ; their protoplasm is practically (except- 
 ing the nucleus) of the same character throughout. A 
 nearly equally large number (the Corticata) develop a 
 complete cortical layer of denser protoplasm, which is 
 distinct from the deeper medullary protoplasm. This 
 layer is permanent, and gives to the body a definite shape 
 and entails physiological consequences of great moment. 
 The cortical protoplasm may exhibit further specialization of 
 structure in connexion with contractile functions (muscular). 
 
 ECTOPLASTIC PRODUCTS CHEMICALLY DISTINCT FROM 
 PROTOPLASM. The protoplasm of all cells may throw down 
 as a molecular precipitate distinct from itself chemical 
 compounds, such as chitin and horny matter and other 
 nitrogenized bodies, or again non-nitrogenous compounds, 
 such as cellulose. Very usually these substances are 
 deposited not external to but in the superficial proto- 
 
PROTOZOA 
 
 plasm. They are then spoken of as cell-cuticle if the cell 
 bounds the free surface of a tissue, or as matrix or cell- wall 
 in other cases. The Protozoon cell-body frequently forms 
 such "cuticles," sometimes of the most delicate and 
 evanescent character (as in some Amoebae), at other times 
 thicker and more permanent. They may give indications 
 (though proper chemical examination is difficult) of being 
 allied in composition to chitin or gelatin, in other instances 
 to cellulose, which is rare in animals and usual in plants. 
 These cuticular deposits may be absent, or may form thin 
 envelopes or in other cases jelly-like substance intimately 
 mixed with the protoplasm (Radiolaria). They may take 
 the form of hooks, tubercles, or long spines, in their 
 older and more peripheral parts free from permeation by 
 protoplasm, though deeply formed in and interpenetrated 
 by it. Such pellicles and cuticles, the deeper layers (if not 
 the whole) of which are permeated by protoplasm, lead 
 insensibly to another category of ectoplastic products in 
 which the material produced by the protoplasm is separated 
 from it and can be detached from or deserted by the proto- 
 plasm without any rupture of the latter. These are 
 
 Shells and Cysts. Such separable investments are 
 formed by the cell-bodies of many Protozoa, a phenomenon 
 not exhibited by tissue-cells. Even the cell-walls of the 
 protoplasmic corpuscles of plant tissues are permeated by 
 that protoplasm, and could not be stripped off without 
 rupture of the protoplasm. The shell and the cyst of the 
 Protozoon are, on the contrary, quite free from the cell- 
 protoplasm. The shell may be of soft chitin-like sub- 
 stance (Gromia, <frc.), of cellulose (Labyrinthula, Dino- 
 flagellata), of calcium carbonate (Globigerina, <fcc.) ( or of 
 silica (Clathrulina, Codonella). The term "cyst" is ap- 
 plied to completely closed investments ("shells" having 
 one or more apertures), which are temporarily produced 
 either as a protection against adverse external conditions 
 or during the breaking up of the parent-cell into spores. 
 Such cysts are usually horny. 
 
 Stalk*. By a localization of the products of ectoplastic 
 activity the Protozoon cell can produce a fibre or stalk of 
 ever-increasing length, comparable to the seta of a 
 Chaetopod worm produced on the surface of a single cell. 
 
 ENTOPLASTIC PRODUCTS DISTINCT FROM PROTOPLASM. 
 Without pausing here to discuss the nature of the finest 
 granules which are embedded as a dust-cloud in the hyaline 
 matrix of the purest protoplasm alike of Protozoa and of 
 the cells of higher animals and plants, and leaving aside 
 the discussion of the generalization that all protoplasm 
 presents a reticular structure, denser trabeculae of extreme 
 minuteness traversing more liquid material, it is intended 
 here merely to point to some of the coarser features of 
 structure and chemical differentiation, characteristic of the 
 cell-body of Protozoa. 
 
 With regard to the ultimate reticular structure of 
 protoplasm it will suffice to state that such structure has 
 been shown to obtain in not a few instances (e.g., Lith- 
 amoeba, Fig. V.), whilst in most Protozoa the methods of 
 microscopy at present applied have not yielded evidence 
 of it, although it is not improbable that a recticular 
 differentiation of the general protoplasm similar to that of 
 the nucleus may be found to exist in all cells. 
 
 Most vegetable cells and many cells of animal tissues 
 exhibit vacuolation of the protoplasm ; i.e., large spaces are 
 present in the protoplasm occupied by a liquid which is not 
 protoplasm and is little more than water with diffusible 
 salts in solution. Such vacuoles are common in Protozoa. 
 They are either permanent, gastric, or contractile. 
 
 Permanent vacuoles containing a watery fluid are some- 
 times so abundant as to give the protoplasm a "bubbly" 
 structure (Thalamophora, Radiolaria, &c.), or may merely 
 give to it a trabecular character (Trachelius, Fig. XXIV. 
 
 14, and Noctiluca, Fig. XXVI. 18). Such vacuoles may 
 contain other matters than water, namely, special chemical 
 secretions of the protoplasm. Of this nature are oil-drops, 
 and from these we are led to those deposits within the 
 cell-protoplasm which are of solid consistence (see below). 
 
 Gastric vacuoles occur in the protoplasm of most Proto- 
 zoa in consequence of the taking in of a certain quantity 
 of water with each solid particle of food, such ingestion of 
 solid food-particles being a characteristic process bound up 
 with their animal nature. 
 
 Contractile vacuoles are frequently but not universally 
 observed in the protoplasm of Protozoa. They are not 
 observed in the protoplasm of tissue-cells. The contrac- 
 tile vacuole whilst under observation may be seen to 
 burst, breaking the surface of the Protozoon and discharg- 
 ing its liquid contents to the exterior ; its walls, formed of 
 undifferentiated protoplasm, then collapse and fuse. After 
 a short interval it re-forms by slow accumulation of liquid 
 at the same or a neighbouring spot in the protoplasm. 
 The liquid is separated at this point by an active process 
 taking place in the protoplasm which probably is of an 
 excretory nature, the separated water carrying with it 
 nitrogenous waste-products. A similar active formation 
 of vacuoles containing fluid is observed in a few instances 
 (Arcella, some Amoebae) where the protoplasm separates a 
 gas instead of liquid, and the gas vacuole so produced ap- 
 pears to serve a hydrostatic function. 
 
 Corpuscular and Amorphous Entof>lastic Solids. Con- 
 cretions of undetermined nature are occasionally formed 
 within the protoplasm of Protozoon cells, as are starch and 
 nitrogenized concretions in tissue-cells (Lithamoeba, Fig. 
 V. cone.). But the most important corpuscular products 
 j after the nucleus, which we have already discussed, are 
 chlorophyll corpuscles. These are (as in plants) concavo- 
 convex or spherical corpuscles of dense protoplasm resem- 
 bling that of the nucleus, which are impregnated superfi- 
 cially with the green-coloured substance known as chloro- 
 phyll. They multiply by fission, usually tetraschistic, 
 independently of the general protoplasm. They occur in 
 representatives of many different groups of Protozoa (Pro- 
 teomyxa, Heliozoa, Labyrinthulidea, Flagellata, Ciliata), 
 but are confined to a few species. Similar corpuscles or 
 | band-like structures coloured by other pigments are occa- 
 sionally met with (Dinoflagellata). 
 
 Recently it has been maintained (Brandt, 5) that the 
 chlorophyll corpuscles of Protozoa and other animals are 
 parasitic Algae. But, though it is true that parasitic Algae 
 occur in animal tissues, and that probably this is the nature 
 of the yellow cells of Radiolaria, yet there seems to be no 
 more justification for regarding the chlorophyll corpuscles 
 of animal tissue-cells and of Protozoa as parasites than 
 there is for so regarding the chlorophyll corpuscles of the 
 leaves of an ordinary green plant. 
 
 Corpuscles of starch, paramylum, and other amyloid 
 substances are commonly formed in the Flagellata, whose 
 nutrition is to a large extent plant-like. 
 
 Entoplastic Fibres. A fibrillation of the protoplasm of 
 the Protozoon cell-body may be produced by differentia- 
 tion of less and more dense tracts of the protoplasm itself. 
 But as distinct from this we find horny fibres occasionally 
 produced within the protoplasm (Heliozoa) having definite 
 skeletal functions. The threads produced in little cavities 
 in the superficial protoplasm of many Ciliate Protozoa, 
 \ known as trichocysts, may be mentioned here. 
 
 Entoplastic Spicules. Needle-like bodies consisting 
 
 either of silica or of a horny substance (acanthin) are 
 
 produced in the protoplasm of many Protozoa (Heliozoa, 
 
 j Radiolaria). These are known as spicules ; they may be 
 
 j free or held together in groups and arranged either radially 
 
 j, or tangentially in reference to the more or less spherical 
 
PROTOZOA 
 
 body of the Protozoon. A similar production of siliceous 
 spicules is observed in the tissue-cells of Sponges. Crys- 
 tals of various chemical nature (silica, calcium carbonate, 
 oxalate, &c.) are also frequently deposited in the protoplasm 
 of the Protozoa, differing essentially from spicules in that 
 their shape is due purely to crystallization. 
 
 GENERAL FORM OF THE PROTOZOON CELL. Those Proto- 
 zoa which have not a differentiated cortical substance, and 
 are known as Gymnomyxa, present very generally an 
 extreme irregularity of contour. Their protoplasm, being 
 liquid rather than viscoas, flows into the most irregular 
 shapes. Their fundamental form when at rest is in many 
 cases that of the sphere ; others are discoidal or may be 
 monaxial, that is to say, show a differentiation of one 
 region or " end " of the body from the other. Frequently 
 the protoplasm is drawn out into long threads or filaments 
 which radiate uniformly from all parts of the spherical or 
 discoidal cell-body or originate from one region to the 
 exclusion of other parts of the surface. 
 
 These non-corticate Protozoa can take solid particles of 
 food into their protoplasm, there to be digested in an 
 extemporized "gastric vacuole," at any part or most parts 
 of their superficies. They have no permanent cell-mouth 
 leading into the soft protoplasm since that soft protoplasm 
 is everywhere freely exposed. 
 
 The corticate Protozoa have (with the exception of some 
 parasites) one, and in the Acinetaria more than one, de- 
 finite aperture in the cortical substance leading into the 
 softer medullary protoplasm. This is the cell-mouth, 
 morphologically as distinct from the mouth of an Entero- 
 zoon as is the hole in a drain pipe from the front door of 
 a house, but physiologically subserving the same distinc- 
 tively animal function as does the mouth of multicellular 
 animals. The general form of the body is in these Proto- 
 zoa oblong, with either monaxial symmetry, when the 
 mouth is terminal, or bilateral symmetry, when the body 
 is oblong and flattened and the mouth is towards one end 
 of what becomes by its presence the " ventral " surface. 
 Though the protoplasm is not nakedly exposed in irregular 
 lobes and long filaments in these corticate Protozoa so as 
 to pick up at all points such food-particles as may fall in 
 its way, yet the protoplasm does in most Corticata project 
 in one or more peculiarly modified fine hair-like processes 
 from the otherwise smooth surface of the cell-body. 
 These processes are vibratile cilia, identical in character 
 with the vibratile cilia of epithelial tissue-cells of Entero- 
 zoa. They are essentially locomotor and current-produc- 
 ing (therefore prehensile) organs, and, whilst unable to 
 ingest solid food-particles themselves, serve to propel the 
 organism in search of food and to bring food into the cell- 
 mouth by the currents which they excite. Either a single 
 vibratile filament is present, when it is called a flagellum, 
 or a row or many rows of cilia are developed. 
 
 Constituent cells of the Enterozoa are well known which 
 closely resemble some of the Gymnomyxa or non-corticate 
 Protozoa in their general form. These are the colourless 
 blood corpuscles or lymph corpuscles or phagocytes (Mecz- 
 nikow, 6) which float freely in the blood and ingest solid 
 particles at any part of their surface as do non-corticated 
 Protozoa ; they exhibit a similar irregularity and muta- 
 bility of outline, and actually digest the particles which 
 they take in. The endodermal digestive cells of some 
 Enterozoa (Coelentera and Planarians) are also naked proto- 
 plasmic corpuscles and can take in solid food-particles. 
 
 No tissue-cells are known which present any close 
 parallel to the mouth-bearing corticate Protozoa. The 
 differentiation of the structure of a single cell has in these 
 forms reached a very high degree, which it is not surpris- 
 ing to find without parallel among the units which build 
 up the individual of a higher order known as an Entero- 
 
 zoon. Cilia are developed on such cell -units (ciliated 
 epithelium), but not used for the introduction of food- 
 particles into the cell. In rare cases (the cilikted " pots " 
 of the vascular fluid of Sipunculus) they act so as to freely 
 propel the ciliated cell through the liquid " blood " of the 
 Enterozoon, as the cilia of a Protozoon propel it through 
 water. An aperture in the cortical substance (or in 
 the cuticular product) of a tissue-cell is sometimes to be 
 observed, but is never (1) used for the ingestion of food- 
 particles. Such an aperture occurs in unicellular glands, 
 where it serves as the outlet of the secretion. 
 
 PHYSIOLOGY. 
 
 Motion. As has just been hinted, the movement of 
 protoplasm, which in the tissue -cells of Enterozoa and 
 higher plants is combined and directed so as to produce 
 effects in relation to the whole organism built up of 
 countless cells, is seen in the Protozoa in a different 
 relation, namely, as subserving the needs of the individual 
 cell of which the moving protoplasm is the main sub- 
 stance. The phenomena known in tissue-cells as " stream- 
 ing" (e.g., in the cells of the hairs of Tradescantia), 
 as local contraction and change of form (e.g., in the 
 corpuscles of the cornea), as muscular contraction, and as 
 ciliary movement are all exhibited by the protoplasm of 
 the cell-body of Protozoa, with more or less constancy, 
 and are intimately related to the processes of hunting, 
 seizing, and ingesting food, and of the intercourse of the 
 individuals of a species with one another and their evasion 
 of hostile agencies. Granule streaming and the implied 
 movement of currents in the protoplasm are seen in the 
 filamentous protoplasm of the Heliozoa, Radiolaria, Reti- 
 cularia, and Noctiluca, and in the cyclosis of the gastric 
 vacuoles of Ciliata. Local contraction and change of form 
 is seen best in the Amcebas and some Flagellata, where it 
 results in locomotion. Definite muscular contraction is 
 exhibited by the protoplasmic band in the stalk of Vorti- 
 cella, by the leg-like processes of the Hypotrichous Ciliata, 
 and by the cortical substance of some large Ciliata. Cili- 
 ary movement ranging from the vibration of filaments of 
 protoplasm temporarily evolved, up to the rhythmic beat 
 of groups of specialized cilia, is observed in all groups of 
 Protozoa in the young condition if not in the adult, and 
 special varieties of ciliary movement and of cilia-like 
 organs will be noted below. For an account of the con- 
 ditions and character of protoplasmic movement generally 
 which cannot be discussed in the present article the reader 
 is referred to Engelmann (7). 
 
 The protoplasm of the cell-body of the Protozoa is drawn 
 out into lobes and threads which are motile and are used 
 as locomotive and prehensile organs. These processes are 
 of two kinds, which are not present on the same cell and 
 are not capable of transmutation, though there are excep- 
 tions to both of these statements. The one kind are 
 termed " pseudopodia," and are either lobose or filamentous 
 or branched and even reticular (Figs. IV. and IX.). The Pro- 
 tozoa which exhibit them are sometimes termed Myxopods. 
 The other kind are cilia and flagella, and are simple threads 
 which are alternately bent and straightened almost inces- 
 santly during the life of the organism. These Protozoa 
 are termed Mastigopods. "Whilst the cilia and flagella are 
 permanent organs, the pseudopodia vary greatly in char- 
 acter ; they are in some cases rapidly expanded and with- 
 drawn in irregular form, and can hardly be said to be more 
 than lobose protuberances of the flowing moving mass of 
 protoplasm. In other cases they are comparatively per- 
 manent stiff threads of protoplasm which can be contracted 
 and can fuse with one another but rarely do so (Heliozoa, 
 Radiolaria). Between these extreme forms of "pseudo- 
 podia " there are numerous intermediate varieties, and the 
 
PROTOZOA 
 
 whole protoplasmic body of the Protozoon may even 
 assume the form of a slowly changing network of threads 
 of greater or less tenuity (Chlamydomyxa, Fig. VI.). 
 
 Nutrition. Typically that is to say, by determinate 
 hereditary tendency the Protozoa take solid food-particles 
 into their protoplasm which form and occupy with the water 
 surrounding them " gastric vacuoles " in the protoplasm. 
 The food-particle is digested in this vacuole, by what 
 chemical processes is not ascertained. It has been shown 
 that the contents of the gastric vacuole give in some cases 
 an acid reaction, and it is not improbable that free acid is 
 secreted by the surrounding protoplasm. It is not known 
 whether any ferment 1 is separated by the protoplasm, 
 but it is probable from observations made on the digestive 
 process of Coelentera (Actiniae) that the ferment is not 
 separated, but that actual contact of the food-particle with 
 the protoplasm is necessary for a " ferment influence " to be 
 exerted. The digestion of a food-particle by a Protozoon 
 is intra-cellular, and has been contrasted with the cavitary 
 digestion of higher animals. In the latter, ferments and 
 acids are poured out by the cells bounding the enteric 
 cavity into that space, and digestion is extra-cellular. In 
 the lowest Enterozoa (many Coelentera and some Planarian 
 worms) it has been shown that food-particles are actually 
 taken up in a solid state by the soft protoplasm of the 
 enteric cells and thus subjected to intra-cellular digestion. 
 There appears to be a gradual transition from this process, 
 in which close contact with living protoplasm is necessary 
 that the solution of an albuminous food-particle may be 
 effected, onwards to the perfectly free cavitary digestion 
 by means of secretions accumulated in the enteron. 
 
 We have not yet any satisfactory observations on the 
 chemistry of intra-cellular digestion either of Protozoa or 
 of Coelentera. 
 
 Certain Protozoa which are parasitic do not take solid 
 food particles ; they (like higher parasites, such as the 
 Tapeworms) live in the nutritious juices of other animals 
 and absorb these by their general surface in a liquid state. 
 The Gregarinae (Sporozoa), many Ciliata, tc., are in this 
 case. Other Protozoa are known which are provided with 
 chlorophyll corpuscles and do not take in solid food, but, 
 apparently as a result of exceptional adaptation in which 
 they differ from closely -allied forms, nourish themselves 
 as do green plants. Such are the Volvocinean Flagellata 
 and some of the Dinoflagelkta. It has also been asserted 
 that other Protozoa (viz., some Ciliata) even some which 
 possess a well-developed mouth can (and experimentally 
 have been made to) nourish themselves on nitrogenous 
 compounds of a lower grade than albumens such, for 
 instance, as ammonium tartrate. Any such assertions 
 must be viewed with the keenest scepticism, since experi- 
 mental demonstration of the absence of minute albuminous 
 particles (e.g., Bacteria) from a solution of ammonium 
 tartrate in which Ciliate Protozoa are flourishing is a 
 matter of extreme difficulty and has not yet been effected. 
 
 Undigested food-remnants are expelled by the protoplasm 
 of the Protozoon cell either at any point of the surface or 
 by the cell-mouth or by a special cell-anus (some Ciliata, 
 see Fig. XXIV. 22). 
 
 Jifspiraiion and Excretion. The protoplasm of the 
 Protozoa respires, that is, takes up oxygen and liberates 
 carbonic acid, and can readily be shown experimentally 
 to require a supply of oxygen for the manifestation of its 
 activity. Xo special respiratory structures are developed 
 in any Protozoa, and as a rule also the products of oxida- 
 tion appear to be washed out and removed from the proto- 
 plasm without the existence of any special apparatus. 
 
 1 The digestive ferment pepsin has been detected by Krukenberg in 
 the plasmodium of the Myeetozoon Fuligo (flowers of tan). See on 
 this subject Zopf (13), p. 88. 
 
 The contractile vacuole which exists in so many Protozoa 
 appears, however, to be an excretory organ. It has been 
 shown to rapidly excrete in a state of solution colouring 
 matters (anilin blue) which have been administered with 
 food particles (8). jfo evidence has been adduced to show 
 whether traces of nitrogenous waste-products are present 
 in the water expelled by the contractile vacuole. 
 
 Chemical Metamorphosis. The form which the various 
 products of the activity of the Protozoon's protoplasm may 
 assume has been noted above. It will be sufficient here 
 to point out that the range of chemical capacities is quite 
 as great as in the cells of the higher Enterozoa. Chitin, 
 cellulose, silicon, calcium carbonate, fats, pigments, and 
 gases can be both deposited and absorbed by it. Owing 
 to the minuteness of the Protozoa, we are at present unable 
 to recognize and do justice to the variety of chemical bodies 
 which undoubtedly must play a part in their economy as 
 the result of the manufacturing activity of their pro- 
 toplasm. See, however, Zopf (13), p. 71. 
 
 Growth and Reproduction. The Protozoon cell follows 
 the same course as tissue-cells, in that by assimilation of 
 nutriment its protoplasm increases in volume and reaches 
 a certain bulk, when its cohesion fails and the viscid 
 droplet divides into two. The coefficient of cohesion 
 varies in different genera and species, but sooner or later 
 the disrupting forces lead to division, and thus to multi- 
 plication of individuals or reproduction. The phenomena 
 connected with the division of the nucleus (already alluded 
 to) will be noticed in particular cases below. 
 
 Whilst simple binary division is almost without excep- 
 tion a chief method of reproduction among the Protozoa, 
 it is also very usual, and probably this would be found if 
 our knowledge were complete to have few exceptions, that 
 under given conditions the Protozoon breaks up rapidly 
 into many (from ten to a hundred or more) little pieces, 
 each of which leads an independent life and grows to the 
 form and size of its parent. It will then multiply by 
 binary division, some of the products of which division 
 will in their turn divide into small fragments. The small 
 fragments are called "spores." Usually the Protozoon 
 before breaking up into spores forms a " cyst " (see above) 
 around itself. Frequently, but not as a necessary rule, 
 two (rarely three or more) Protozoon cell-individuals come 
 together and fuse into one mass before breaking up into 
 spores. This process is known as "conjugation;" and 
 there can be no doubt that the physiological significance 
 of the process is similar to that of sexual fertilization, 
 namely, that the new spores are not merely fragments of 
 an old individual but are something totally new inasmuch 
 as they consist of a combination of the substance of indi- 
 viduals who have had different Life experiences. 
 
 Whilst spore-formation is not necessarily preceded by 
 conjugation, conjugation is not necessarily followed by 
 spore-formation. Among the Mycetozoa the young indi- 
 viduals produced from spores conjugate at a very early 
 period of growth in numbers and form "plasmodia," and 
 after a considerable interval of feeding and growth the 
 formation of spores takes place. Still more remarkable is 
 the fact observed among the Ciliata where two individuals 
 conjugate and after a brief fusion and mixture of their 
 respective protoplasm separate, neither individual (as far 
 as certain genera at least are concerned) breaking up into 
 spores, but simply resuming the process of growth and 
 recurrent binary division with increased vigour. 
 
 There is certainly no marked line to be drawn between 
 reproduction by simple fission and reproduction by spore- 
 formation ; both are a more or less complete dividing of 
 the parent protoplasm into separate masses ; whether the 
 products of the first fission are allowed to nourish them- 
 selves and grow before further fission is carried out or not 
 
8 
 
 PROTOZOA 
 
 does not constitute an essential difference. The fission of 
 the Ciliate Protozoon, Opalina (see below Fig. XXIV. 4-8), 
 is a step from the ordinary process of delayed binary divi- 
 sion towards spore-formation. In some Protozoa spores are 
 produced after encystation by a perfectly regular process 
 of cleavage (comparable to the cleavage of the egg-cell 
 of Enterozoa) first two, then four, then eight, sixteen, 
 and thirty-two fission products being the result (see 
 Fig. XX. 24, 25, <fec.). 
 
 But more usually there is a hastening of the process, 
 and in these cases it is by no means clear what part the 
 parent cell-nucleus takes. An encysted Gregarina (or two 
 conjugated Gregarinse) suddenly breaks up into a number 
 of equal-sized spores, which do not increase in number by 
 binary division and have not been formed by any such 
 process. This multicentral segregation of the parent pro- 
 toplasm is a marked development of the phenomenon of 
 sporulation and remote from ordinary cell-division. How 
 it is related to ordinary cell-division is not known, inas- 
 much as the changes undergone by the nucleus in this 
 rapid multicentral segregation of the parent protoplasm 
 have not been determined. The spores of Protozoa may 
 be naked or encased singly or in groups in little en- 
 velopes, usually of a firm horny substance (see Fig. 
 XX. 23 to 26, and Fig. XXIV. 15 to 18). Whenever 
 the whole or a part of a Protozoon cell divides rapidly 
 into a number of equal-sized pieces which are simultane- 
 ously set free and are destined to reproduce the adult 
 form, the term spore is applied to such pieces, but the 
 details of their formation may vary and also those of their 
 subsequent history. In typical cases each spore produced 
 as the result of the fission of an encysted Protozoon (con- 
 jugated or single) has its own protective envelope, as in 
 the Mycetozoa (Fig. III.) and the Sporozoa (Fig. XVIII.), 
 from which the contained protoplasm escapes by " ger- 
 mination " as a naked corpuscle either flagellate or amcebi- 
 form. In some terminologies the word " spore " is limited 
 to such a " coated " spore, but usually the naked proto- 
 plasmic particles which issue from such " coated " spores, 
 or are formed directly by the rapid fission of the parent 
 Protozoon, are also called " spores." The former condition 
 is distinguished as a " chlamydospore, " whilst the latter are 
 termed " gymnospores." Many Protozoa produce gymno- 
 spores directly by the breaking up of their protoplasm, 
 and these are either " flagellulse " (swarm-spores) or "arnce- 
 bulse " (creeping spores). The production of coated spores 
 is more usual among the lower plants than it is among 
 Protozoa, but is nevertheless a characteristic feature of 
 the Gregarinse (Sporozoa) and of the Mycetozoa. The 
 term " gemma " or " bud-spore " is applied to cases, few 
 in number, where (as in Acinetaria, Fig. XXVI., Spiro- 
 chona, Fig. XXIII. 10, and Reticularia, Fig. X. 8) the 
 spores are gradually nipped off from the parent-cell one 
 or more at a time. This process diifers from ordinary 
 cell-division only in the facts (1) that the products of 
 division are of unequal size the parent-cell being distin- 
 guishable as the larger and more complete in structure, 
 and (2) that usually the division is not binary, but more 
 than one bud-spore is produced at a time. 
 
 Whilst in the binary cell-division of the Protozoa the 
 two products are usually complete in structure at the 
 period of separation, spores and spore-buds are not only of 
 small size and therefore subject to growth before attaining 
 the likeness of the parent, but they are also very often of 
 simple and incomplete structure. The gap in this respect 
 between the young spore and its parent necessarily varies 
 according to the complexity of the parental form. 
 
 In the case of the Eadiolaria, of the Gregarinse, of 
 Noctiluca, and of the Acinetaria, for instance, the spore 
 has before it a considerable process of development in 
 
 structure and not merely of growth, before attaining the 
 adult characters. Hence there is a possible embryology 
 of the Protozoa, to the study of which the same prin- 
 ciples are applicable as are recognized in the study of the 
 embryology of Enterozoa. Embryonic forms of great sim- 
 plicity of structure, often devoid of nucleus, and consist- 
 ing of simple elongate particles of protoplasm, are hatched 
 from the spore-cases of the Gregarinee (Fig. XVII. 13, 14). 
 These gradually acquire a differentiated cortical protoplasm 
 and a nucleus. A very large number of Gymnomyxa pro- 
 duce spores which are termed " monadiform," that is, have 
 a single or sometimes two filaments of vibratile protoplasm 
 extended from their otherwise structureless bodies. By 
 the lashing of these flagella the spores (swarm-spores or 
 zoospores) are propelled through the water. The resem- 
 blance of these monadiform young (best called " flagel- 
 lulae ") to the adult forms known as Flagellata has led to 
 the suggestion that we have in them a case of recapitula- 
 tive development, and that the ancestors of the Gymno- 
 myxa were Protozoa similar to the Flagellata. Again the 
 Acinetaria produce spores which are uniformly clothed 
 with numerous vibratile cilia (Fig. XXVI.), although the 
 adults are entirely devoid of such structures ; this is 
 accounted for by the supposition that the Acinetaria 
 have been developed from ancestors like the Ciliata, whose 
 characters are thus perpetuated in their embryonic stages. 
 There can be little doubt that these embryological sugges- 
 tions are on the whole justified, and that the nucleated 
 Protozoa are the descendants of non-nucleated forms simi- 
 lar to the spores of Gymnomyxa and Sporozoa, whilst it 
 seems also extremely probable that the ancestral Protozoa 
 were neither exclusively amoeboid in the movement of 
 their protoplasm nor provided with permanent vibratile 
 filaments (flagella and cilia) ; they were neither Myxopods 
 nor Mastigopods (to use the terms which have been intro- 
 duced to express this difference in the character of the 
 locomotor processes), but the same individuals were capable 
 of throwing out their protoplasm sometimes in the form 
 of flowing lobes and networks, sometimes in the form of 
 vibratile flagella. A few such undifferentiated forms exist 
 at the present day among the Proteomyxa and in a little 
 more advanced condition among the lowest Flagellata, e.g., 
 Ciliophrys. 
 
 Death. It results from the constitution of the Proto- 
 zoon body as a single cell and its method of multiplication 
 by fission that death has no place as a natural recurrent 
 phenomenon among these organisms. Among the Entero- 
 zoa certain cells are separated from the rest of the consti- 
 tuent units of the body as egg-cells and sperm-cells ; these 
 conjugate and continue to live, whilst the remaining cells, 
 the mere carriers as it were of the immortal reproductive 
 cells, die and disintegrate. There being no carrying cells 
 which surround, feed, and nurse the reproductive cells of 
 Protozoa, but the reproductive cell being itself and alone 
 the individual Protozoon, there is nothing to die, nothing 
 to be cast off by the reproductive cell when entering on a 
 new career of fission. The bodies of the higher animals 
 which die may from this point of view be regarded as 
 something temporary and non-essential, destined merely to 
 carry for a time, to nurse, and to nourish the more import- 
 ant and deathless fission-products of the unicellular egg. 
 Some of these fission-products of the new individual de- 
 veloped from an egg-cell namely, the egg-cells and sperm- 
 cellsare as immortal as the unicellular Protozoon. This 
 method of comparing the unicellular and the multicellular 
 organism is exceedingly suggestive, and the conception we 
 thus gain of the individuality of the Enterozoon throws 
 light upon the phenomena of reproduction and heredity in 
 those higher organisms. 
 
 Experiment and observation in this matter are extremely 
 
PROTOZOA 
 
 difficult ; but we have no reason to suppose that there is 
 any inherent limit to the process of nutrition, growth, and 
 fission, by which continuously the Protozoa are propagated. 
 The act of conjugation from time to time confers upon 
 the protoplasm of a given line of descent new properties, 
 and apparently new vigour. Where it is not followed by 
 a breaking up of the conjugated cells into spores, but by 
 separation and renewed binary fission (Ciliata), the result 
 is described simply as " rejuvenescence." The protoplasm 
 originated by the successive division of substance traceable 
 to one parent cell has become specialized, and in fact too 
 closely adapted to one series of life-conditions ; a fusion 
 of substance with another mass of protoplasm equally 
 specialized, but by experience of a somewhat differing 
 character, imparts to the resulting mixture a new com- 
 bination of properties, and the conjugated individuals on 
 separation start once more on their deathless career with 
 renewed youth. 
 
 CLASSIFICATION OF THE PROTOZOA. 
 
 In attempting a scheme of classification it would be most in 
 accordance with the accepted probabilities of the ancestral history 
 of the Protozoa to separate altogether those forms devoid of a 
 nucleus from those which possess one, and to regard them as a 
 lower " grade " of evolution or differentiation of structure. 
 
 By some systematists, notably Biitschli (9), the presence or 
 absence of a nucleus has not been admitted as a basis of classifica- 
 tory distinction, whilst on the other hand both Haeckel (1) and 
 Huxley (10) have insisted on its importance. 
 
 The fact is that during recent years many of those Protozoa 
 which were at one time supposed to be devoid of nucleus even in a 
 rudimentary form, and furnished therefore the tangible basis for a 
 lowest group of "Protozoa Homogenea" or "Monera," have been 
 shown by the application of improved methods of microscopic 
 investigation to possess a nucleus, that is to say, a differentiated 
 corpuscle of denser protoplasm lying within the general protoplasm, 
 and capable when the organism is killed by alcohol or weak acids 
 of taking up the colour of various dyes (such as carmine and 
 hsematoxylin) more readily and permanently than is the general 
 protoplasm. In such cases the nucleus may be very small and 
 exhibit none of the typical structure of larger nuclei. It is usually 
 surrounded by a clear (i.e., non -granular) halo of the general 
 
 Erotoplasm which assists the observer in its detection. Nuclei 
 ave been discovered in many Reticularia (Foraminifera), a group 
 in which they were supposed to be wanting, by Schultze (11) and 
 the Hertwigs (12) and more recently in the Mycetozoa and in 
 Vampyrella and Protomonas (Zopf, 13), where so excellent an 
 observer as Cienkowski had missed them. 
 
 It seems therefore not improbable that a nucleus is present 
 though not observed in Protomyxa, Myxastrum, and other similar 
 forms which have been by Haeckel and others classed as " Monera " 
 or " Homogenea." The recently described (14) Archerina (Fig. II. 
 8, 11) certainly possesses no nucleus in the usual sense of that term, 
 but it is possible that the chlorophyll-coloured corpuscles of that 
 organism should be considered as actually representing the nucleus. 
 Whilst then refraining from asserting that there are no existing 
 Protozoa devoid of nucleus corresponding in this character with 
 non-nucleate Protophyta, such as the Bacteria, we shall not in our 
 scheme of classification institute a group of Homogenea, but shall 
 leave the taking of that step until it has been shown after critical 
 examination that those forms now regarded by some observers as 
 Homogenea are really so. In the meantime these forms will find 
 their places alongside of the Nueleata most nearly allied to them 
 in other characters. 
 
 The Protozoa with a definite permanent cortical substance of 
 differentiated protoplasm are undoubtedly to be regarded as evolved 
 from forms devoid of such differentiation of their substance, and 
 we accordingly take this feature as the indication of a primary 
 division of the Protozoa. 1 The lower grade, the Gj-mnomyxa, 
 afford in other respects evidence of their being nearly related to 
 the ancestral forms from which the Corticata (the higher grade) 
 have developed. The Gymnomyxa all or nearly all, whilst 
 exhibiting amoeboid movement and the flowing of their protoplasm 
 into " pseudopodia " of very varied shapes, produce spores which 
 swim by means of one or two flagella of vibratile protoplasm 
 (monadiform young or flagellulae). These flagellate young forms 
 
 1 The " exoplasm " and "endoplasm" described in Amoeba, &c., 
 by some authors are not distinct layers but one and the same con- 
 tinuous substance what was internal at one moment becoming ex- 
 ternal at another, no really structural difference existing between 
 them. 
 
 are closely related to the Flagellata, a group of the Corticata from 
 which it seems probable that the Dinoflagellata, the Ciliata, and 
 the Acinetaria have been derived. The Gymnomyxa themselves 
 cannot, on account of the small number of structural features 
 which they offer as indications of affinity and divergence in genetic 
 relationships inter se, be classified with anything like confidence in 
 a genealogical system. We are obliged frankly to abandon the 
 attempt to associate some of the simpler forms with their nearest 
 genetic allies and to content ourselves with a more or less artificial 
 system, which is not, however, artificial in so far as its main 
 groups are concerned. Thus the genetic solidarity of each of the 
 large classes Heliozoa, Reticularia, Mycetozoa, and Radiolaria is 
 not open to question. The Lobosa on the other hand appear to 
 be a more artificial assemblage, and it is difficult to say that 
 genetically there is any wide separation between them and the 
 Mycetozoa or between the Mycetozoa and some of the simpler 
 forms which we bring together under the class Proteomyxa. 
 The scheme of classification which we adopt is the following : 
 
 PROTOZOA. 
 
 GRADE A. GYMNOMYXA. 
 Class I. PROTEOMYXA. 
 
 Ex. Vampyrella, Protomyxa, Archerina. 
 Class II. MYCETOZOA. 
 
 Ex. The Eu-mycetozoa of Zopf. 
 Class III. LOBOSA. 
 
 Ex. Amteba, Arcella, Pelomyxa. 
 'Class IV. LABYRINTHULIDEA. 
 
 Ex. Labyrinthula, Chlamydomyxa. 
 Class V. HELIOZOA. 
 
 Ex. Actinophrys, Raphidiophrys, Clathrulina, 
 Class VI. RETICULARIA. 
 
 Ex. Gromia, Lituola, Astrorhiza, Globigerina. 
 Class VII. RADIOLARIA. 
 . Ex. Thalassicolla, Eucyrtidium, AcantJwmetra. 
 
 GRADE B. CORTICATA. 
 ( Class I. SPOROZOA. 
 J Ex. Gregarina, Coccidium. 
 Class II. FLAGELLATA. 
 
 Ex. Monas, Salpingoeca, Euglena, Volmx. 
 Class III. DINOFLAGELLATA. 
 
 Ex. Prorocentrum, Ceratium. 
 Class IV. RHYNCHOFLAGELLATA. 
 
 Ex. Noctiluca. 
 Class V. CILIATA. 
 
 Ex. Vortieella, Paramcecium, Slentor. 
 Class VI. ACINETARIA. 
 . Ex. Acineta, Dendrosoma. 
 The genetic relationships which probably obtain among these 
 groups may be indicated by the following diagram : 
 
 Claa Aciuetaria. 
 
 CUM 
 
 Rbjncho-flagellata. 
 Claw 
 Dino-flagellata. 
 
 Sections. 
 Proteana. 
 
 Plasmodiata. 
 Lobosa. 
 
 Filosa. 
 
 Lipostoma. 
 
 Stomato- 
 phora. 
 
 CUas 
 Beliozoa. 
 
 rowomyra. / 
 
 / 
 / 
 
 Literature. Certain works of an older date dealing with micro- 
 scopic organisms, and therefore including many Protozoa, have 
 historical interest. Among these we may cite O. F. Muller, 
 Animalcula Infusoria, 1786; Ehrenberg, Infusionsthierchen, 1838; 
 
 B 
 
10 
 
 PROTOZOA 
 
 Dujardin, Histoire naturelle des Infusoires, 1841 ; Pritchard, In- 
 fusoria, 1857. 
 
 The general questions relating to protoplasm and to the consti- 
 tution of the Protozoon body as a single cell are dealt with in the 
 following more recent treatises : Max Schultze, Ueber den Organ- 
 ismus der Polythalamien, 1854, and Ueber das Protoplasma der 
 Rhizopoden und Pfianzenzellen, 1863; and Engelmann, article "Pro- 
 toplasma" in Hermann's Handworterbuch der Physiologic, 1880. 
 
 Special works of recent date in which the whole or large groups 
 of Protozoa are dealt with in a systematic manner with illustra- 
 tions of the chief known forms are the following : Biitschli, "Pro- 
 tozoa," in Bronn's Classen und Ordnungen des Thierreichs, a 
 comprehensive and richly illustrated treatise now in course of 
 publication, forming the most exhaustive account of the subject 
 matter of the present article which has been attempted (the writer 
 desires to express his obligation to this work, from the plates of 
 which a large proportion of the woodcut figures here introduced 
 have been selected); W. S. Kent, Manual of the Infusoria, 1882 
 an exhaustive treatise including figures and descriptions of all 
 species of Flagellata, Dinoflagellata, Ciliata, and Acinetaria ; Stein, 
 Der Organismus der Infusionsthiere, 1867-1882; Haeckel, Die 
 Eadiolarien, 1862; Archer, "Resume of recent contributions to 
 our knowledge of freshwater Rhizopoda," Quart. Jour, of Micro- 
 scopical Science, 1876-77; Zopf, " Pilzthiere " (Mycetozoa), in 
 Encyklopiidie der Natuneissenschaften, Breslau, 1884. 
 
 We shall now proceed to consider the classes and orders of 
 Protozoa in detail. 
 
 PKOTOZOA. 
 
 Characters. Organisms consisting of a single cell or of a group 
 of cells not differentiated into two or more tissues ; incapable of 
 assimilating nitrogen in its diffusible compounds (ammonia or 
 nitrates) or carbon in the form of carbonates, except in special 
 instances which there is reason to regard as directly derived from 
 allied forms not possessing this capacity. The food of the Protozoa 
 is in consequence as a rule taken in the form of particles into the 
 protoplasm either by a specialized mouth or by any part of the 
 naked cell-substance, there to be digested and rendered diffusible. 
 
 GRADE A. GYMNOMYXA, Lankester, 1878 (64). 
 
 Characters. Protozoa in which the cell-protoplasm is entirely or 
 partially exposed to the surrounding medium, during the active 
 vegetative phase of the life-history, as a naked undifferentiated 
 slime or viscous fluid, which throws itself into processes or 
 " pseudopodia " of various form either rapidly changing or 
 relatively constant. Food can be taken into the protoplasm in the 
 form of solid particles at any point of its surface or at any point 
 of a large exposed area. The distinction into so-called "exoplasm" 
 and "endoplasm" recognized by some authors, is not founded on a 
 permanent differentiation of substance corresponding to the cortical 
 and medullary substance of Corticata, but is merely due to the 
 centripetal aggregation of granules lying in a uniform undiffer- 
 entiated protoplasm. The cell-individual exhibits itself under 
 four phases of growth and development (1) as a swarm-spore 
 (monadiform young or flagellula) ; (2) as an amoeba form ; (3) as 
 constituent of a plasmodium or cell-fusion or conjugation ; (4) as a 
 cyst, which may be a flagellula(Schwiirme)-producing cyst, an 
 amcebula-producing cyst, a covered-spore(chlamydospore) -producing 
 cyst (sporocyst sens, stric., Zopf), or a simple resting cyst which 
 does not exhibit any fission of its contents (hypnocyst). Any one 
 of these phases may be greatly predominant and specialized whilst 
 the others are relatively unimportant and rapidly passed through. 
 
 CLASS I. PEOTEOMTXA, Lankester. 
 
 Characters. Gymnomyxa which exhibit in the amoeba phase 
 various forms of pseudopodia often changing in the same individual, 
 and do not produce elaborate spore cysts; hence they are not re- 
 ferable to any one of the subsequent six classes. Mostly minute 
 forms, with small inconspicuous nucleus (absent in some ?). 
 
 A division into orders and families is not desirable, the group 
 being confessedly an assemblage of negatively characterized or 
 insufficiently known forms. 
 
 Genera. Vampyrella, Cienkowski (15); Vampyrellidium, Zopf 
 (13) ; Spirophora, Zopf ( = Amoeba radiosa, Perty) ; Haplococcus, 
 Zopf ; Leptophrys, Hertwig and Lesser (16) ; Endyoinena, Zopf ; 
 Bursulla, Sorokin (17) ; Myxastrum, Haeckel (1) ; Entcromyxa, 
 Cienkowski (18) ; Colpodella, Cienkowski (19); Pseudospora, Cien- 
 kowski (20) ; Protomonas, Cienkowski (15) ; Diplophijsalis, Zopf 
 (13) ; Oymnococcus, Zopf ; Aphelidium, Zopf ; Pseudosporidium, 
 Zopf ; Protomyxa, Haeckel (1) ; Plasmodiojihora, Woronin (21) ; 
 Tetramyxa, Gobel (22) ; Gloidium, Sorokin (23) ; Gymnophrys, 
 Cienkowski (24) ; Myxodictyum, Haeckel (1) ; Boderia, Wright 
 (25) ; Biomyxa, Leidy (92) ; Protogcnes, Haeckel (1) ; Prolamosb'a, 
 Haeckel (1); Nuclearia, Cienkowski (26); Monobia, Aim. Schneider 
 (27) ; Archerina, Lankester (14). 
 
 The forms here brought together include several genera (the 
 
 first nineteen) referred by Zopf to the Mycetozoa, some again 
 (Vampyrella, Myxastrum, Nuclearia, Monobia) which are by 
 Biitschli associated with the Heliozoa, others (Protamceba, Gloidium) 
 referred by the same authority to the Lobosa (Amcebsea) and others 
 (Colpodella, Protomonas) which might be grouped with the lower 
 Flagellata. By grouping them in the manner here adopted we 
 are enabled to characterize those higher groups more satisfactorily 
 and to give a just expression to our present Want of that knowledge 
 of the life-history both of these forms and of the higher Gymnomyxa 
 which when it is obtained may enable us to disperse this hetero- 
 geneous class of Proteomyxa. The group has the same function 
 in relation to the other classes of Gymnomyxa which the group 
 Vermes has been made to discharge in relation to the better defined 
 phyla of the Metazoa ; it is a lumber-room in which obscure, lowly- 
 developed, and insufficiently known forms may be kept until they 
 can be otherwise dealt with. 
 
 It is true that, thanks to the researches of Continental botanists 
 (especially Cienkowski and Zopf), we know the life-history of 
 several of these organisms; but we are none the less unable to con- 
 nect them by tangible characteristics with other Gymnomyxa. 
 
 Nearly all of the above-named genera are parasitic rather than 
 "voracious," that is to say, they feed on the organized products of 
 larger organisms both plants and animals (Haplococcus is parasitic 
 in the muscles of the pig), into whose tissues they penetrate, and 
 do not, except in a few cases (Protomyxa, Vampyrella), engulph 
 whole organisms, such as Diatoms, &c. , in their protoplasm. Many 
 live upon and among the putrefying debris of other organisms 
 (e.g. , rotting vegetable stems and leaves, excrements of animals), 
 and like the Mycetozoa exert a digestive action upon the substances 
 with which they come in contact comparable to the putrefying and 
 fermentative activity of the Schizomycetes (Bacteria). 
 
 Fig. II. illustrates four chief genera of Proteomyxa. 
 
 Protomyxa aurantiaca was described by Haeckel (1), who found 
 it on shells of Spirula on the coast of the Canary Islands, in the 
 form of orange yellow flakes consisting of branching and reticular 
 protoplasm nourishing itself by the ingestion of Diatoms and 
 Peridiuia. This condition is not a simple amoeba phase but a 
 "plasmodiiim" formed by the union of several young amoebae. The 
 plasmodium under certain conditions draws itself together into a 
 spherical form and secretes a clear membranous cyst around itself, 
 and then breaks up into some hundreds of flagellulee or swarm- 
 spores (Fig. II. 2). The diameter of the cyst is '12 to '2 millimetre. 
 The flagellulaa subsequently escape (Fig. II. 3) and swim by the 
 vibratile movement of one end which is drawn out in the form of a 
 coarse flagellum. The swarm-spore now passes into the amoeba 
 phase (Fig. II. 4). Several of the small amoebae creeping on the 
 surface of the spirula-shell then unite with one another and form 
 a plasmodium which continues to nourish itself by "voracious" 
 inception of Diatoms and other small organisms. The plasmodia 
 may attain a diameter of one millimetre and be visible by the 
 naked eye. 
 
 A nucleus was not observed by Haeckel in the spores nor in the 
 amoeba phase, nor scattered nuclei in the plasmodium, but it is not 
 improbable that they exist and escaped detection in the living con- 
 dition, in consequence of their not being searched for by methods 
 of staining, &c. , which have since come into use. A contractile 
 vacuole does not exist. 
 
 Vampyrella spirogyrse, Cienkowski (Fig. II. 5, 6, 7), is one of 
 several species assigned to the genus Vampyrella, all of which 
 feed upon the living cells of plants. The nucleus previously stated 
 to be absent has been detected by Zopf (13). There is no con- 
 tractile vacuole. The amoeba phase has an actinophryd character 
 (i.e., exhibits fine radiating pseudopodia resembling those of the 
 sun-animalcule, Actinophrys, one of the Heliozoa). This species 
 feeds exclusively upon the contents of the cells of Spirogyra, effect- 
 ing an entrance through the cell-wall (Fig. II. 5), sucking out the 
 contents, and then creeping on to the next cell. In some species 
 of Vampyrella as many as four amceba-individuals have been 
 observed to fuse to form a small plasmodium. Cysts are formed 
 which enclose in this species a single amoeba-individual. The cyst 
 often acquires a second or third inner cyst membrane by the 
 shrinking of the protoplasmic body after the first encystment and 
 the subsequent formation of a new membrane. The encysted pro- 
 toplasm sometimes merely divides into four parts each of which 
 creeps out of the cyst as an Actinophrys-like amoeba (Fig. II. 7) ; in 
 other instances it forms a dense spore, the product of which is not 
 known. 
 
 Protogcnes primordialis is the name given by Haeckel to a 
 very simple form with radiating filamentous pseudopodia which 
 he observed in sea-water. It appears to be the same organism as 
 that described and figured by Max Schultze as Amosba pomcta. 
 Schultze's figure is copied in Fig. II. 12. No nucleus and no con- 
 tractile vacuole is observed in this form. It feeds voraciously on 
 smaller organisms. Its life-history has not been followed over even 
 a few steps. Hence we must for the present doubt altogether as to 
 its true affinities. Possibly it is only a detached portion of the 
 protoplasm of a larger nucleate Gymnomyxon, The same kind of 
 
PROTOZOA 
 
 11 
 
 doubt is justified in regard to Haeckel's Protanueba primitiru, which 
 was observed by him in pond water and differs from Protogenes in 
 having lobose pseudopodia, whilst agreeing with it in absence of 
 nuclei, contractile vacuoles, and other differentiation of structure. 
 
 FIG. II. Various Proteomyxa. 1. Protomyxa aurantiaea, Haeckel, plas- 
 modium phase. The naked protoplasm shows branched, reticulate processes 
 (pseudopodia), and numerous non-contractile vacuoles. It is in the act of en- 
 gulphing a Ceratium. Shells of engulphed Ciliata (Tmtinnabola) are embedded 
 deeply in the protoplasm a. 2. Cyst phase of Protomyia. a, transparent cyst- 
 wall ; 6, protoplasm broken up into spores. 3. Flagellula phase of Protomyxa, 
 the form assumed by the spores on their escape from the cyst. 4. Amoebula 
 phase of the same, the form assumed after a short period by the flagellula?. 5. 
 Vampyrella spirogyrx, Cienk., amo3ba phase penetrating a cell of Spirogyra b, 
 by a process of its protoplasm r, and taking up the substance of the Spirogyra 
 cell, some of which is seen within the Vampyrella a. 6. Large individual of 
 Vampyrella, showing pseudopodia e, and food particles a. The nucleus (though 
 present) is not shown in this drawing. 7. Cyst phase of Vampyrella. The 
 contents of the cyst have divided into four equal parts, of whii-h three are 
 visible. One is commencing to break its way through the cyst-wall /; a, food 
 panicles. 8. Archerina Bottoni, Lankester, showing lobose and filamentous 
 protoplasm, and three groups of chlorophyll corpuscles. The protoplasm g is 
 engulphing a Bacterium i. 9. Cjst phase of Archerina. a, spinous cyst-wall ; 
 b, green-coloured contents. 10. Chlorophyll corpuscle of Archerina showing 
 tetraschistic division. 11. Actinophryd form of Archerina. 6. chlorophyll cor- 
 puscles. 12. Protagmu pnmordialit, H jeckel (.Amoeba porrecta, M. Schultze), 
 from Schultze's figure. 
 
 The structureless protoplasmic network described by Haeckel 
 
 from spirit-preserved specimens of Atlantic ooze and identified by 
 him with Huxley's (28) Bathybius, as also the similar network 
 described by Bessels (29) as Protobathybius, must be regarded for 
 the present as insufficiently known. 
 
 It is possible that these appearances observed in the ooze dredged 
 from great depths in the Atlantic are really due to simple Protozoa. 
 On the other hand it has been asserted by Sir Wyville Thomson, 
 who at one time believed in the independent organic nature of 
 Bathybius, that the substance taken for protoplasm by both Huxley 
 and Haeckel is in reality a gelatinous precipitate of calcium 
 sulphate thrown down by the action of alcohol upon sea-water. 
 Other naturalists have pointed to the possibility of the protoplasmic 
 network which Bessels studied in the living condition on board 
 ship being detached portions of the protoplasm of Reticularia and 
 Radiolaria. The matter is one which requires further investigation. 
 
 Archerina, Boltani is the name given by Lankester (14) to a very 
 simple Gymnomyxon inhabiting freshwater ponds in company 
 with Desmids and other simple green Algse (Fig. II. 8 to 11). 
 Archerina exhibits an amoeba phase in which the protoplasm is 
 thrown into long stiff filaments ( Fig. II. 11), surrounding a spherical 
 central mass about Wroth inch in diameter (actinophryd form). 
 A large vacnole (non-contractile) is present, or two or three small 
 ones. No nucleus can be detected by careful use of reagents in 
 this or other phases. The protoplasm has been seen to ingest solid 
 food particles (Bacteria) and to assume a lobose form. The most 
 striking characteristic of Archerina is the possession of chlorophyll 
 corpuscles. In the actinophryd form two oval green-coloured 
 bodies (6, b) are seen. As the protoplasm increases by nutrition the 
 chlorophyll corpuscles multiply by quaternary division (Fig. II. 10) 
 and form groups of four or of four sets of four symmetrically 
 arranged. The division of the chlorophyll corpuscles is not 
 necessarily followed by that of the protoplasm, and accordingly 
 specimens are found with many chlorophyll corpuscles embedded 
 in a large growth of protoplasm (Fig. II. 8) ; the growth may increase 
 to a considerable size, numbering some hundreds of chlorophyll 
 corpuscles, and a proportionate development of protoplasm. Such 
 a growth is not a plasmodium, that is to say, is uot formed by 
 fusion of independent amoaba forms, but is due to continuous 
 growth. When nutrition fails the individual chlorophyll corpuscles 
 separate, each carrying with it an investment of protoplasm, and 
 then each such amceba form forms a cyst around itself which is 
 covered with short spines (Fig. II. 9). The cysts are not known 
 to give rise to spores, but appear to be merely hypnocysts. 
 
 The domination of the protoplasm by the chlorophyll corpuscles 
 is very remarkable and unlike anything known in any other 
 organism. Possibly the chlorophyll corpuscles are to be regarded 
 as nuclei, since it is known that there are distinct points of affinity 
 between the dense protoplasm of ordinary nuclei and the similarly 
 dense protoplasm of normal chlorophyll corpuscles. 
 
 CLASS II. MYCETOZOA, De Bary. 
 
 Characters. Gymnomyxa which, as an exception to all other 
 Protozoa, are not inhabitants of water but occur on damp surfaces 
 exposed to the air. They are never parasitic, as are some of the 
 Proteomyxa most nearly allied to them (Plasmodiophora, &c.), but 
 feed on organic debris. They are structurally characterized by the 
 fact that the amceba forms, which develop either directly or through 
 flagellulse from their spores, always form large, sometimes very 
 large, i.e., of several square inches area, fusion plasmodia (or 
 rarely aggregation plasmodia), and that the spores are always 
 chlamydospores (i.e., provided with a coat) and are formed either 
 in naked groups of definite shape (sori) or on the surface of peculiar 
 columns (conidiophors) or in large fruit-like cysts which enclose the 
 whole or a part of the plasmodium and develop besides the spores 
 definite sustentacular structures (capillitium) holding the spores in 
 a mesh-work. 
 
 Three orders of Mycetozoa are distinguishable according to the 
 arrangement of the spores in more or less complex spore-fruits. 
 
 ORDER 1. SOROPHORA, Zopf. 
 
 Characters. Mycetozoa which never exhibit a vibratile (monadi- 
 form) swarmspore or flagellula phase, but hatch from the spore as 
 amffibae. A true fusion plasmodium is not formed, but an aggrega- 
 gation plasmodium bv the contact without fusion of numerous 
 amoeba forms. The spore fruit is a naked aggregation of definitely 
 arranged encysted amoeba called a sorus, not enclosed in a common 
 capsule ; each encysted amreba has the value of a single spore and 
 sets free on germination a single amcebula. They inhabit the dung 
 of various animals. 
 
 Genera. Copromyxa, Zopf ; Cynthulina, Cienk. ; IHctyoslelium, 
 Brefeld ; Acrasis, Van Tieghem ; Polyspondylium, Brefeld. 
 
 ORDER 2. EXDOSPOREA, Zopf. 
 
 Characters. Mycetozoa always passing through the flagellula 
 phase and alwavs forming true plasmodia by fusion of amoeba 
 forms. The spore-fruit is in the form of a large cyst which encloses 
 a quantity of the plasmodium ; the latter then breaks up into (a) 
 
12 
 
 PROTOZOA 
 
 spores (one corresponding to each nucleus of the enclosed plas- 
 modium) each of which has a cellulose coat, and (b) a capillitium 
 of threads which hold the spores together. Each spore (chlamydo- 
 spore) liberates on germination a single nucleated flagellula, which 
 develops into an amoebula, which in turn fuses with other amoebulae 
 to form the plasmodium. The Endosporea are essentially dwellers 
 on rotten wood and such vegetable refuse. 
 
 Fio. III. MycetOZOa (after De Bary). 1-6. Germination of spore (1) of Trichea 
 varia, showing the emerging "flagellula" (4, 5), and its conversion into an 
 "amcebula" (6). 7-18. Series leading from spore to plasmodium phase of 
 Chondrioderma difforme:!, spore; 10, flagellula; 12, amoebula; 14, apposi- 
 tion of two amoebulse ; 15-17, fusions ; 18, plasmodium. 19, 20, Spore-fruit 
 (cyst) of Physarum leucopkxum, Fr. ( x 25), the f ormer from the surface, the 
 latter in section with the spores removed to show the sustentacular network or 
 capillitium. 21. Section of the spore-cyst of Didymium squamulosum,vhh the 
 spores removed to show the radiating capillitium x and the stalk. 
 
 Sub-order 1. PERITRICHEA, Zopf. 
 
 Fam. 1. CLATHROPTYCHIACE.E, Eostafinski. 
 
 Genera. Clalhroptychium, Rost. ; Enteridium, Ehr. 
 Fam. 2. CRIBRARIACE.E. 
 
 Genera. Dictydium, Pers. ; Cribraria, Pers. 
 
 Sub-order 2. ENDOTRICHEA, Zopf. 
 
 Fam. 1. PJIYSAREA. 
 
 Genera. Physarum, Pers. ; Craterium, Trentepol ; Badhamia, 
 Berkeley ; Leocarpus, Link. ; Tilmadoche, Fr. ; Fuligo 
 (sEOialium), Hall ; Jtthaliopsis, Z. 
 Fam. 2. DIDYMIACE^B. 
 
 Genera. Didymium; Lepidoderma, De Bary. 
 Fam. 3. SPUMARIACE.H. 
 
 Genera. Spumaria, Pers. ; Diachea, Fries. 
 Fam. 4. STEMONITEA. 
 
 Genera. Stemonitis, Gleditsch ; Comatricha, Preuss ; Lam- 
 
 proderma, Rost. 
 Fam. 5. ENERTHENEMEA. 
 
 Genera. Encrthema, Bowman. 
 Fam. 6. RETICULARIACE^E, Zopf. 
 
 Genera. Amaurochsete, Rost. ; Reticularia, Bull. 
 Fam. 7. TRICHINACE.SI. 
 
 Genera. Hemiarcyria, Rost. ; Trichia, Hall. 
 Fam. 8. ARCYRIACE.E. 
 
 Genera. Arcyria, Hall ; Cornuvia, Rost. ; Lycogala, Ehr. 
 Fam. 9. PERICH^NACE^E. 
 
 Genera. Perichsena, Fries. ; Lachnobolus, Flies. ' 
 Fam. 10. LICEACE.S. 
 
 Genera. Licea, Schrader ; Tubulina, Pers. ; Lindbladia, 
 Fries. ; Tubulifera, Zopf. 
 
 ORDER 3. EXOSPOREA, Zopf. 
 
 Characters. The chlamydospore liberates an amcebula iu the 
 first instance, which develops into a flagellula. This subsequently 
 returns to the amceba form, and by fusion with other amoebulK it 
 forms a true fusion plasmodium. The spores are not produced 
 within a cyst but upon the surface of column-like up-growths of the 
 plasmodium, each spore (conidum) forming as a little spherical out- 
 growth attached to the column (conidiophor) by a distinct pedicle. 
 
 Sole Genus. Ceratium. [This name must be changed, since it 
 was already applied to a genus of Dinoflagellata, when Famintzin 
 and Woronin gave it to this Mycetozoon.] 
 
 Further Remarks on Mycetozoa. About two hundred species of 
 Mycetozoa have been described. Botanists, and especially those who 
 occupy themselves with Fungi, have accumulated the very large 
 
 mass of facts now known in reference to these organisms ; never- 
 theless the most eminent botanist who has done more than any 
 other to advance our knowledge of Mycetozoa, namely, De Bary, has 
 expressed the view that they are to be regarded rather as animals 
 than as plants. The fact is that, once the question is raised, it 
 becomes as reasonable to relegate all the Gymnomyxa without 
 exception to the vegetable kingdom as to do so with the Mycetozoa. 
 Whatever course we take with the latter, we must take also with 
 the Heliozoa, the Radiolaria, and the Reticularia. 
 
 The formation of plasmodia, for which the Mycetozoa are conspicu- 
 ous, appears to be a particular instance of the general phenomenon 
 of cell-conjugation. Small plasmodia are formed by some of the 
 Proteomyxa ; but among the other Gymnomyxa, excepting Myceto- 
 zoa, and among Corticate Protozoa, the fusion of two individuals 
 (conjugation sensu stricto) is more usual than the fusion of several. 
 Zopf (13) has attempted to distinguish arbitrarily between conjuga- 
 tion and plasmodium formation by asserting that in the former 
 the nuclei of the cells which fuse are also fused, whereas in the 
 latter process the nuclei retain their independence. Both state- 
 ments are questionable. What happens to the nucleus in such 
 conjugations as those of the Gregarince has not yet been made out, 
 whilst it is only quite recently that Strasburger (30) has shown 
 that the plasmodia of Mycetozoa contain numerous scattered nuclei, 
 and it is not known that fusion does not occur between some of 
 these. There is no doubt that the nuclei of plasmodia multiply 
 by fission, though we have no detailed account of the process. 
 
 The Sorophora are exceptional in that the amcebse which unite to 
 form a cell-colony in their case do not actually fuse but only remain 
 in close contact ; with this goes the fact that there are no large 
 spore-cysts, but an identification of spore and spore-cyst. The 
 amcebje arrange themselves in stalked clusters (sori), and each be- 
 comes encysted : one may, in this case, consider the cyst equally as 
 a spore or as a spore-cyst which produces but a single spore. The 
 amoebaj described by various writers as inhabiting the alimentary 
 canal and the dung of higher animals (including man) belong to 
 this group. The form described by Cunningham in the Quart. 
 Jour. Micr. Sci., 1881, as Protomyxomyces coprinarius is appa- 
 rently related to the Copromyxa (Guttulina) pi-otea of Fayod (31). 
 
 The spore-fruits of the Endosporese occur in various degrees of 
 elaboration. Usually they are (1) spherical or pear-shaped cysts 
 with or without an obvious stalk (Fig. III. 19, 20, 21), and often 
 have a brilliant colour, and are of a size readily observed by the 
 naked eye, the plasmodia which give rise to them being by no 
 means microscopic. But they may present themselves (2) as 
 irregular ridges growing up from the plasmodium, when they are 
 termed serpula forms. Lastly, the cysts may be united side by 
 side in larger or smaller groups instead of forming at various sepa- 
 rate points of the plasmodium. These composite bodies are termed 
 "fruit-cakes" or aethalia," in view of the fact that the spore-cysts 
 of Fuligo, also called jEthalium the well-known "flowers of tan" 
 form a cake of this description. 
 
 The capillitium or network of threads which lies between the 
 spores in the spore-cysts of Endosporeie is a remarkable structure 
 which exhibits special elaborations in detail in different genera, here 
 not to be noticed for want of space. Although definite in form and 
 structure, these threads are not built up by cells but are formed 
 by a residual protoplasm (cf. Sporozoa) which is left in the cyst 
 after the spores have been segregated and enclosed each in its 
 special coat. They are often impregnated by calcium carbonate, 
 and exhibit crystalline masses of it, as does also the cyst-wall. 
 
 The spores of the Mycetozoa are as a rule about the TrVoth " lcn 
 in diameter. They are produced by millions in the large fruit- 
 cakes of such forms as Fuligo. Often the spore-coat is coloured ; it 
 always consists of a substance which gives the cellulose reaction 
 with iodine and sulphuric acid. This has been sometimes con- 
 sidered an indication of the vegetable nature of the Mycetozoa, but 
 cannot be so regarded since many animals (especially the Tunicata 
 and various Protozoa) produce substances giving this same reaction. 
 
 Dryness, low temperature, and want of nutriment lead to a dor- 
 mant condition of the protoplasm of the plasmodium of many 
 Mycetozoa and to its enclosure in cyst-like growths known as 
 "selerotia," which do not give rise to spores, but from which the 
 protoplasm creeps forth unaltered when temperature, nutrition, and 
 moisture are again favourable. The selerotia are similar in nature 
 to the hypnocysts of other Protozoa. 
 
 The physiological properties chemical composition, digestive 
 action, reaction to moisture, heat, light, and other physical influ- 
 ences of the plasmodia of Mycetozoa have been made the subject 
 of important investigations ; they furnish the largest masses of 
 undifferentiated protoplasm available for such study. The reader 
 is referred to Zopf's admirable treatise (13) as to these matters, and 
 also for a detailed account of the genera and species. 
 
 CLASS III. LOBOSA, Carpenter. 
 
 Characters. Gymnomyxa in which (as in the succeeding four 
 classes) the amceba-phase predominates over the others in perma- 
 nence, size attained, and physiological importance. The pseudo- 
 
PROTOZOA 
 
 13 
 
 podia are lobose, ranging in form from mere wave-like bulgings 
 of the surface to blunt finger-like processes, but never having the 
 character of filaments either simple, arborescent, or reticulate. 
 Fusions of two individuals (conjugation) have been observed in a 
 
 Kio. IV. Various Lobosa. 1, 2, 3. Dactylotphxra (Amoeba) polypodia, M. 
 Schultze, in three successive stages of division; tbe changes indicated 
 occupied fifteen minutes, a, nucleus ; b, contractile racuole (copied from 
 F. E. Schultze, in Architf. Mikrotk. Anat.). 4. Amoeba pritutpg, Ehr. 
 
 (after Auerbach). a, nucleus ; 6, c, vacuoles (one or more contractile ; the 
 shaded granules are food-particles). 5. Pelomyxa patustris, Greeff 
 
 (after GreefT), an example with comparatively few food-particles (natural 
 size ^,th inch in length). 6. Portion of a Pelomyxa more highly magni- 
 fied, o, clear superficial zone of protoplasm (so-called " exoplasni ") ; b, 
 vacuoles, extremely numerous ; e, lobose pseudopodium ; d, a similar 
 pseudopodium ; , nuclei ;/, "refractive bodies "(reproductive?) ; scattered 
 about in the protoplasm are seen numerous cylindrical crystals. 7. 
 
 ArceUa vulgarif. Ehr. a, shell; 6, protoplasm within the shell ; c, extended 
 protoplasm in the form of lobose pseudopodia ; d, nuclei ; t, contractile 
 vacuole ; the dark bodies unlettered are gas vacuoles. 8, Codilio- 
 
 podium ptUucidum, Hert. and Less, a, nucleus surrounded by a hyaline 
 halo sometimes mistaken for the nucleus, whilst the latter is termed 
 nncleolus. 
 
 few cases, hut not fusions of many individuals so as to form 
 plasmodia ; nevertheless the size attained by the naked protoplasm 
 by pure growth is in some cases considerable, forming masses readily 
 visible by the naked eye (Pelomyxa). The presence of more than 
 
 one nucleus is a frequent character. A contractile vacuole may or 
 may not be present The formation of sporocysts and of chlamydo- 
 spores (coated spores) has not been observed in any species, but 
 naked spores (Hagellulae or amcebulae) have been with more or 
 less certainty observed as the product of the breaking up of some 
 species (Amoeba ? Pelomyxa). The cyst phase is not unusual, but 
 the cyst appears usually to be a hypnocyst and not a sporocyst 
 In the best observed case of spore-production (Pelomyxa) the spores 
 were apparently produced without the formation of a cyst. Repro- 
 duction is undoubtedly most freely effected by simple fission 
 (Amoeba) and by a modified kind of bud-fission (Arcella). Fresh- 
 water and marine. Two orders of the Lobosa are distinguished in 
 accordance with the presence or absence of a shell. 
 
 ORDER 1. NUDA. 
 
 Characters. Lobosa devoid of a shell. 
 
 Genera. Amceba, Auct (Fig. IV. 4) ; Ouranueba, Leidy (with a 
 villons tuft at one end, Wallich's A. villosa) ; Corycia, Dnj. (low, 
 ridge-like pseudopodia); Lithamceba, Lankester (Fig. V.); Dina- 
 macba, Leidy (92) (covered with short stiff processes) ; Eyalodiseus, 
 H. and L. ; Plakopus, F. E. Schultze ; Dactyloxph&ra, H. and L. 
 (Fig. IV. 1, 2, 3); Pelomyxa, Greeff (Fig. IV. 5, 6) ; Amphizonella, 
 Greeff (forms a gelatinous case which is broken through by the 
 pseudopodia). 
 
 ORDER 2. TESTACEA. 
 
 Characters. Lobosa which secrete a shell provided with an 
 aperture from which the naked protoplasm can be protruded. The 
 shell is either soft and membranous, or strengthened by the in- 
 clusion of sand-particles, or is hard and firm. 
 
 Genera. CocMiopodium (Fig. IV. 8), H. and L. ; Pyxidictila, 
 Ehr. ; Arcella, Ehr. (Fig. IV. 7) ; Hyalosphenia, Stein ; Quad- 
 rula, F. E. Schultze (shell membraneous, areolated) ; Difflugia, 
 Leclerc (shell with adventitious particles). 
 
 Further remarks on the Lobosa. The Lobosa do not form a very 
 numerous nor a very natural assemblage. Undoubtedly some of 
 the forms which have been described as species of Amoeba are 
 amoeba forms of Mycetozoa ; this appears to be most probably the 
 case in parasitic and stercoricolous forms. But when these are 
 removed, as also those Proteomyxa which have pseudopodia of 
 varying character, at one time lobose and at another filamentous, 
 we have left a certain small number of independent lobose 
 Gymnomyxa which it is most convenient to associate in a 
 separate group. We know very little of the production of spores 
 (whether it actually obtains or not) or of developmental phases 
 among these Lobosa. The common Amoeba are referable to the 
 species A. princeps, A. lobosa, Daetylosphsera polypodia, Ouramasba 
 rillosa. Of none of these do we know certainly any reproductive 
 phenomena excepting that of fission (see Fig. IV. 1, 2, 3). Various 
 statements have been made pointing to a peculiar change in the 
 nucleus and a production of spores having the form of minute 
 Amoebas, arising from that body ; but they cannot be considered 
 as established. Whilst the observed cases of supposed reproduc- 
 tive phenomena are very few, it must be remembered that we have 
 always to guard (as the history of the Ciliata has shown, see 
 below) against the liability to mistake parasitic amabnlae and 
 flagellnlae for the young forms of organisms in which they are 
 merely parasitic. The remarkable Pelomytn palustris of Greeff (32) 
 was seen by him to set free (without forming a cyst) a number of 
 amoebulae which he considers as probably its young. Mr Weldon 
 of St John's College, Cambridge, has observed the same pheno- 
 menon in specimens of Pelomyxa which made their appearance in 
 abundance in an aquarium in the Morphological Laboratory, 
 Cambridge. It seems probable that the amcebulse in this case are 
 not parasites but spore-like young, and this is the best observed 
 case of such reproduction as yet recorded in the group. 
 
 Arcella is remarkable for the production of bud-spores, which 
 may be considered as a process intermediate between simple fission 
 and the complete breaking up of the parent body into spores. As 
 many as nine globular processes are simultaneously pinched off from 
 the protoplasm extruded from the shell of the Arcella ; the nuclei 
 (present in the parent Arcella to the number of two or three) have 
 not been traced in connexion with this process. The bnds then be- 
 come nipped off, and acquire a shell and a contractile vacuole (33). 
 
 The presence of more than one nucleus is not unusual in Lobosa, 
 and is not due to a fusion of two or more uninuclear individuals, 
 but to a multiplication of the original nucleus. This has been 
 observed in some Amoebae (A. princepsl) as well as Arcella. 
 Pelomyxa (Fig. IV. 6) has a great number of nuclei like the Helio- 
 zoon, Actinosphaerium (Fig. VIII.). 
 
 Pelomyxa is the most highly differentiated of the Lobosa. The 
 highly vacuolated character of its protoplasm is exhibited in a less 
 degree by Lithamceba and resembles that of Heliozoa and Radiolaria. 
 Besides the numerous nuclei there are scattered in the protoplasm 
 strongly refringent bodies (Fig. IV. 6, /), the significance of which 
 has not been ascertained. The superficial protoplasm is free from 
 vacnoles, hyaline, and extremely mobile. Occasionally it is drawn 
 
14 
 
 PKOTOZOA 
 
 out into very short fine filaments. Scattered in the protoplasm are 
 a number of minute cylindrical crystals, of unascertained composi- 
 tion. Pelomyxa is of very large size for a Protozoon, attaining a 
 diameter of T \th of an inch. It takes into its substance a quantity 
 of foreign particles, both nutrient organic matter such as Rotifera 
 and Diatoms and sand particles. It occurs not uncommonly in old 
 
 FIG. V. Litham&ba discus, Lank, (after Lankester, 34). A, quiescent ; B, 
 throwing out pseudopodia. c.v., contractile vacuole, overlying which the 
 vacuolated protoplasm is seen ; cone, concretions insoluble in dilute HC1 
 and dilute KHO, but soluble tn strong HC1 ; n, nucleus. 
 
 muddy ponds (such as duck-ponds), creeping upon the bottom, and 
 has a white appearance to the naked eye. Lithamceba (Fig. V.) is 
 distinguished by its large size, disk-like form, the disk-like shape of 
 its pseudopodia, the presence of specific concretions, the vacuolation 
 of its protoplasm, and the block-like form and peculiar tessellated 
 appearance of its large nucleus, which has a very definite capsule. 
 In Lithamceba it is easy to recognize a distinct pellicle or temporary 
 cuticle which is formed upon the surface of the protoplasm, and 
 bursts when a pseudopodiuin is formed. In fact it is the rupture of 
 this pellicle which appears to be the proximate cause of the outflow 
 of protoplasm as a pseudopodium. Probably a still more delicate 
 pellicle always forms on the surface of naked protoplasm, and in the 
 way just indicated determines by its rupture the form and the 
 direction of the "flow" of protoplasm which is described as the "pro- 
 trusion" of a pseudopodium. 
 
 The shells of Lobosa Testacea are not very complex. That of 
 Arcella is remarkable for its hexagonal areolation, dark colour, and 
 firm consistence ; it consists of a substance resembling chitin. 
 That of Difflugia has a delicate membranous basis, but includes 
 foreign particles, so as to resemble the built-up case of a Caddis 
 worm. 
 
 Arcella is remarkable among all Protozoa for its power of secret- 
 ing gas-vacuoles (observed also in an Amoeba by Biitschli), which 
 serve a hydrostatic function, causing the Arcella to float. The gas 
 can be rapidly absorbed by the protoplasm, when the vacuole neces- 
 sarily disappears and the Arcella sinks. 
 
 CLASS IV. LABYRINTHULIDEA. 
 
 Characters. Gymnomyxa forming irregular heaps of ovoid 
 nucleated cells, the protoplasm of which extends itself as a branching 
 network or labyrinth of fine threads. The oval (spindle-shaped) 
 corpuscles, consisting of dense protoplasm, and possessing each a 
 well-marked nucleus (not observed in Chlamydomyxa), travel regu- 
 larly and continuously along the network of filaments. The oval 
 corpuscles multiply by fission ; they also occasionally become 
 encysted and divide into four spherical spores. The young forms 
 developed from these spores presumably develop into colonies, but 
 have not been observed. 
 
 Genera. Two genera only of Labyrinthulidea are known : 
 liabyrinthula, Cienkowski ; Chlamydomyxa, Archer. 
 
 Cienkowski (35) discovered Labyrinthula on green Algae growing 
 on wooden piles in the harbour of Odessa (marine). It has an 
 orange colour and forms patches visible to the naked eye. Chlamy- 
 domyxa was discovered by Archer of Dublin (36) in the cells of 
 Sphagnum and crawling on its surface ; hence it is a freshwater 
 form. Unlike Labyrinthula, the latter forms a laminated shell of 
 cellulose (Fig. VI. 2, c), in which it is frequently completely 
 enclosed, and indeed has rarely been seen in the expanded 
 labyrinthine condition. The laminated cellulose shells are very 
 freely secreted, the organism frequently deserting one and forming 
 another within or adherent to that previously occupied. The 
 network of Chlamydomyxa appears to consist of hyaline threads of 
 streaming protoplasm, whilst that of Labyrinthula has a more 
 horny consistence, and is not regarded by Cienkowski as protoplasm. 
 
 The spindle-shaped cells are much alike in form and size in the 
 two genera; but no nucleus was detected by Archer in those of 
 Chlamydomyxa. The encysting of the spindle-cells and their 
 fission into spores has been seen only in Labyrinthula. Chlamy- 
 domyxa is often of a brilliant green colour owing to the presence of 
 chlorophyll corpuscles, and may exhibit a red or mottled red and 
 green appearance owing to the chemical change of the chlorophyll. 
 
 It has been observed to take in solid nourishment, though Labyrin- 
 thula has not. 
 
 The Labyrinthulidea present strong resemblances to the Myceto- 
 zoa. The genus Dactylostelium (Sorophora) would come very close 
 to Labyriiithula were the amceba? of its aggregation plasmodium 
 
 FIG. VI. Labyrinthulidea. 1. A colony or "cell-heap" of Lalnjrinthula 
 vitellina, Cienk., crawling upon an Alga. 2. A colony or "cell-heap" 
 
 of Chlamydomyxa labyrinthuloides, Archer, with fully expanded network 
 of threads on which the oat-shaped corpuscles (cells) are moving, o is an 
 ingested food particle ; at c a portion of the general protoplasm has 
 detached itself and become encysted. 3. A portion of the network of 
 
 Labyrinthula vitellina, Cienk., more highly magnified, p, protoplasmic 
 mass apparently produced by fusion of several filaments ; p', fusion of 
 several cells which have lost their definite spindle-shaped contour; s, 
 corpuscles which have become spherical and are no longer moving (perhaps 
 about to be encysted). 4. A single spindle cell and threads of Laby- 
 
 rinthula macrocystis, Cienk. n, nucleus. 5. A group of encysted cells 
 
 of L. macrocystis, embedded in a tough secretion. 6, 7. Encysted cells 
 
 of L. macrocystis, with enclosed protoplasm divided into four spores. 
 8. 9. Transverse division of a non-encysted spindle-cell of L. macrocystis. 
 
 set upon a network of threads. Such a network, whether in the 
 condition of soft protoplasm or hardened and horny, is represented 
 in the higher Mycetozoa by the capillitium of the sporocysts. 
 
 The most important difference between Archer's Chlamydomyxa 
 and Cienkowski's Labvrinthula is that in the former the threads 
 
PROTOZOA 
 
 15 
 
 of the network appear to consist of contractile protoplasm, whilst 
 in the latter they are described as firm horny threads exuded by 
 the spindle-cells. Neither form has been re-examined since its 
 discovery ; and it is possible that this apparent difference will be 
 removed by further study. 
 
 FIG. YIT. Helioioa. 1. Actinophryg sol, Ehrb. ; x 800. a. food-particle 
 lying in a large food-racuole ; b, deep-lying finely granular protoplasm ; c, 
 axial filament of a pseudopodium extended inwards to the nucleus ; d, the 
 central nucleus; t, contractile vacuole; /, superficial much-vacuolated 
 protoplasm. 2. Clathrulina elegant, Cienk. ; x 200. 3. Beter- 
 
 ophryi marina, H. and L. x 660. a, nucleus: *, clearer protoplasm 
 surrounding the nucleus ; t, the peculiar felted envelope. 4. Saplii- 
 
 diophriis pattida, F. E. Schultze ; x 430. a, food-particle ; b, the nucleus ; 
 e, contractile vacuole ; d, central granule in which all the axis-filaments of 
 the pseudopodia meet. The tangentially disposed spicules are seen 
 arranged in masses on the surface. 5. Acanthoeystii turfatea, Carter ; 
 
 x 240. a, probably the central nucleus ; 6, clear protoplasm around the 
 nucleus ; c, more superficial protoplasm with vacuoles and chlorophyll 
 corpuscles ; d , coarser siliceous spicules ; e , finer forked siliceous spicules : 
 /, finely granular layer of protoplasm. The long pseudopodia reaching 
 beyond the spicules are not lettered. 6. Bi-flagellate "flagellnla" of 
 
 Acanthncytlit acultaia. a, nucleus. 7. Ditto of Clathrulina elegant, 
 
 a, nucleus. 8. Astrodiseulus ruber, Greeff; x 320. a, red-coloured 
 
 central sphere (? nucleus) ; b, peripheral homogeneous envelope. 
 
 CLASS V. HELIOZOA. Haeckel, 1866. 
 
 Characters. Gymnomyxa in which the dominating amceba phase 
 has the form of a spherical body from the surface of which radiate 
 
 numerous isolated filamentous pseudopodia which exhibit very little 
 movement or change of form, except when engaged in the inception 
 of food-particles. The protoplasm of the spherical body is richly 
 vacuolated ; it may exhibit one or more contractile vacuoles and 
 either a single central nucleus or many nuclei (Nuclearia, Actino- 
 sphserium). Skeletal products may or may not be present Flagel- 
 lulae have been observed as the young forms of some species (Acan- 
 thocystis, Clathrulina), but very little has been as yet ascertained 
 as to spore-formation or conjugation in this group, though isolated 
 facts of importance have been observed. Mostly freshwater forms. 
 
 11 
 
 FIG. YITI. Heliozoa. 1. Aelinoiphierium Eidihornii, Ehr. : X 200. a, 
 nuclei ; fr, deeper protoplasm with smaller vacuoles and numerous nuclei ; 
 
 c, contractile vacuoles ; d, peripheral protoplasm with larger vacuoles. 
 2. A portion of the same specimen more highly magnified and seen in 
 optical section, a, nuclei ; b, deeper protoplasm (so-called endosarc); 
 
 d, peripheral protoplasm (so-called ectosarc); e, pseudopodia showing the 
 granular protoplasm streaming over the stiff axial filament ; /, food- 
 particle in a food-vacnole. 3, 4. Nuclei of Actinosphaerium in the 
 resting condition. 5-13. Successive stages in the division of a 
 nucleus of Actinosphorium. showing fibrillation, and in 7 and 8 formation 
 of an equatorial plate of chromatin substance (after Hertwig). 14. 
 Cyst-phase of Actinofphterium Eifhhoinii, showing the protoplasm 
 divided into twelve chlamydospores, each of which has a siliceous coat ; 
 a, nucleus of the spore ; g, gelatinous wall of the cyst ; A, siliceous coat of 
 tht spore. 
 
16 
 
 PROTOZOA 
 
 ORDER 1. APHROTHORACA, Hcrtwig (56). 
 
 Characters. Heliozoa devoid of a spicular or gelatinous envelope, 
 excepting in some a temporary membranous cyst. 
 
 Genera. Nuclearia, Cienk. (37) (many nuclei ; many contractile 
 vacuoles ; body not permanently spherical, but amoeboid) ; Actin- 
 ophrys, Ehr. (Fig. VII. 1 ; body spherical ; pseudopodia with an 
 axial skeletal filament ; central nucleus ; one large contractile 
 vaeuole; often forming colonies ; A. sol, the Suu -animalcule); 
 Actinosphserium, Stein (Fig. VIII. ; spherical body ; pseudopodia 
 with axial filament ; nuclei very numerous ; contractile vacuoles 2 
 to 14) ; Actinolophus, F. E. Schulze (stalked). 
 
 ORDEK 2. CHLAMYDOPHORA, Archer (57). 
 
 Characters. Heliozoa with a soft jelly-like or felted fibrous 
 envelope. 
 
 Genera. Heterophrys, Archer (Fie. VII. 3); Sphamastrum, 
 Greeff; Astrodisculus, Greeff(Fig. VII. 8). 
 
 ORDER 3. CHALAROTHORACA, Hertw. and Lesser (58). 
 
 Characters. Heliozoa with a loose envelope consisting of isolated 
 siliceous spicules. 
 
 Genera. Raphidiophrys, Archer (Fig. VII. 4 ; skeleton in the 
 form of numerous slightly curved spicules placed tangentially in 
 the superficial protoplasm) ; Pompholyxophrys, Archer ; Pinacocystis 
 H. and L. ; Pinaciophora, Greeff ; Acanthocystis, Carter (skeleton 
 in the form of radially disposed siliceous needles ; encysted con- 
 dition observed, and flagellula young, Fig. VII. 6) ; Wagnerella, 
 Meresch. 
 
 ORDER 4. DESMOTHORACA, Hertw. and Less. 
 
 Characters. Heliozoa with a skeletal envelope in the form of a 
 spherical or nearly spherical shell of silica preforated by numerous 
 large holes. 
 
 Genera. Orbulinella, Entz (without a stalk) ; Clathrulina, 
 Cienk. (with a stalk, Fig. VII. 2). 
 
 Further remarks on the Heliozoa. The Sun-animalcules, Actino- 
 phrys and ActinosphEerium, were the only known members of this 
 
 froup when Carter discovered in 1863 Acanthocystis. Our further 
 nowledge of them is chiefly due to Archer of Dublin, who dis- 
 covered the most important forms, and figured them in the Quart. 
 Jour. Micr. Sci. in 1867. 
 
 Some of the Proteomyxa (e.g., Vampyrella) exhibit " heliozoon- 
 like " or " actinophryd " forms, but are separated from the true 
 Heliozoa by the fact that their radiant pseudopodia are not main- 
 tained for long in the stiff isolated condition characteristic of this 
 group. It is questionable whether Nuclearia should not be relegated 
 to the Proteomyxa on account of the mobility of its body, which in 
 all other Heliozoa has a constant spherical form. 
 
 Actinophrys sol is often seen to form groups or colonies (by 
 fission), and so also is Raphidiophrys. It is probable from the 
 little that is known that reproduction takes place not only by 
 simple fission but by multiple fission, producing flagellate spores 
 which may or may not be preceded by encystment. Only Clath- 
 rulina, Acanthocystis, Actinosphserium, and Actinophrys have 
 been observed in the encysted state, and only the first two have 
 been credited with the production of flagellated young. The two 
 latter genera form covered spores within their cysts, those of Actino- 
 sphaerium being remarkable for their siliceous coats (Fig. VIII. 
 14), but their further development has not been seen. 
 
 CLASS VI. RETICULAEIA, Carpenter, 1862. 
 (Foraminifera, Auct., Thalamophora, Hertwig). 
 
 Characters. Gymnomyxa in which the dominating amoeba- 
 phase, often of great size (an inch in diameter), has an irregular 
 form, and a tendency to throw out great trunks of branching and 
 often anastomosing filamentous pseudopodia, and an equally strong 
 tendency to form a shell of secreted membrane or secreted lime or of 
 agglutinated sand particles (only in one genus of secreted silex) into 
 which the protoplasm (not in all ?) can be drawn and out of and 
 over which it usually streams in widely spreading lobes and 
 branches. One nucleus is present, or there arc many. A contrac- 
 tile vaeuole is sometimes, but not as a rule, present (or at any rate 
 not described). Reproduction is by fission and (as in some other 
 Protozoa) by the formation of peculiar bud-spores which remain 
 for a time after their formation embedded in the parental proto- 
 plasm. No multiple breaking up into spores after or independent 
 of the formation of a cyst is known. Marine and freshwater. 
 
 The Reticularia are divisible into several orders. The marked 
 peculiarity of the shell structure in certain of these orders is only 
 fitly emphasized by grouping them together as a sub-class Per- 
 forata, in contrast to which the remaining orders stand as a 
 sub-class Imperforata. The distinction, however, is not an ab- 
 solute one, for a few of the Lituolidea are perforate, that is, are 
 sandy isomorphs of perforate genera such as Globigerina and 
 Rotalia. 
 
 Fro. IX. Gromiidea (Reticularia membranosa). 1. 
 Archeri, Barker, a, nucleus ; &, contractile vacuoles ; c, the yellow oil-like 
 body. Moor pools, Ireland. 2. Gromia oviformis, Duj. a, the 
 
 numerous nuclei ; near these the elongated bodies represent ingested 
 Diatoms. Freshwater. 3. Shepheardella tieniifonnis, Siddall (Quart. 
 Jour. Micr. Sci., 1880); X 30 diameters. Marine. The protoplasm is 
 retracted at both ends into the tubular case, a, nucleus. 5. Shep- 
 
 heardella tseniiformis ; x 15 ; with pseudopodia fully expanded. 
 6-10. Varying appearance of the nucleus as it is carried along in the 
 streaming protoplasm within the tube. 11. Amphitrema Wrightianum, 
 Archer, showing membranous shell encrusted with foreign particles. 
 Moor pools, Ireland. 12. Diaphorophodon mobile, Archer, a, nucleus. 
 
 Moor pools, Ireland. 
 
 SUB-CLASS A. Imperforata. 
 
 Characters. Shell-substance not perforated by numerous aper- 
 tures through which the protoplasm can issue, but provided with 
 only one or two large apertures, or in branched forms with a few 
 such apertures. 
 
 ORDER 1. GROMIIDEA, Brady. 
 
 Characters. Shell or test membranous, in the form of a simple 
 sac with a pseudopodial aperture either at one extremity or at both. 
 Pseudopodia thread-like, long, branching, reticulated. Marine and 
 freshwater. 
 
 Fam. 1. MONOSTOMINA, with a single aperture to the shell. 
 
PROTOZOA 
 
 17 
 
 Genera. Lieberkiihnia, Clap, and Lach. ; Gromia, Dnj. (Fig. 
 IX. 2) ; MUcrogromia, Hertw. ; Euglypha, Dnj. (shell built np of 
 hexagonal siliceous plates) ; Diaphorophodcm, Archer (38) (many 
 foreign particles cemented to form shell ; small pseudopodia issue 
 between these, hence resembling Perforata, and large long ones from 
 the proper mouth of the shell, Fig. IX. 12). 
 
 FlO. X. Imperforata. 1. Spiroloailina planulata, Lamarck, showing five 
 "coils"; porcellanous. 2. Yonng ditto, with shell dissolved and 
 
 protoplasm stained so as to show the seven nuclei n. 3. Spirolina (Pene- 
 roplis); a sculptured imperfectly colled shell; porcellanous. 4. 
 
 Vertebralina, a simple shell consisting of chambers succeeding one another 
 in a straight line ; porcellanous. 5, 6. Thurammina papiOata, Brady, a 
 sandy form. 5 is broken open so as to show an inner chamber ; recent, 
 x 25. 7. Lituola (Baplophragmium) canariemis, a sandy form ; 
 
 recent. 8. Nucleated reproductive bodies (bud-spores) of Haliphysema. 
 9. Sf-tammulina Ixrii, M. Schultze; X 40; a simple porcellanons 
 Miliolide. 10. Protoplasmic core removed after treatment with weak 
 chromic acid from the shell of Haliphysema Tumanoritzii, Bow. n, 
 vesicular nuclei, stained with hsematoxylin (after Lankester). 11. 
 
 Haliphystma Tumanoritzii ; x 25 diam. ; living specimen, showing the 
 wine-alass-shaped shell built up of sand-grains and sponee-spicules, and 
 the abundant protoplasm p, issuing from the mouth of the shell and 
 spreading partly over its projecting constituents. 12. Shell of Astro- 
 
 rhiza limimla. Sand.; x f ; showing the branching of the test on some of 
 the rays usually broken away in preserved specimens (original). 13. 
 
 Section of the shell of Marsipella, showing thick walls bunt of sand- 
 grains. 
 
 Fam. 2. AMPHISTOMIXA, with an aperture at each end of the shell. 
 
 Genera. Diplophrys, Barker (Fig. IX. 1); Ditrema, Archer; 
 Amphitrema, Archer (Fig. IX. 11); Shepheardella, Siddall (39) 
 (membranous shell very long and cylindrical so as to be actually 
 tubular, narrowed to a spout at each end, Fig. IX. 3 ; protoplasm 
 extended from either aperture, Fig. IX. 5, and rapidly circulating 
 within the tubular test during life, carrying with it the nucleus 
 which itself exhibits peculiar movements of rotation, Fig. IX. 6, 7, 
 8, 9, 10). 
 
 ORDER 2. ASTRORHIZIDEA, Brady. 
 
 Characters. Test invariably consisting of foreign particles ; it is 
 usually of large size and single-chambered, often branched or radiate 
 with a pseudopodial aperture to each branch, the test often con- 
 tinued on to the finer branches of the pseudopodia (Fig. X. 12) ; 
 never symmetrical. All marine. 
 
 Fam. 1. AJSTRORHIZINA, Brady. Walls thick, composed of loose 
 sand or mud very slightly cemented. 
 
 Genera. Astrorhiza, Saudahl (Fig. X. 12, very little enlarged); 
 Pelosina, Brady ; Storthosph&ra, Brady ; Dendrophrya, St Wright ; 
 Syringammina, Brady. 
 
 Fam. 2. PILULIXIXA. Test single-chambered ; walls thick, 
 composed chiefly of felted sponge-spicules and fine sand, without 
 calcareous or other cement. 
 
 Genera. Pilulina, Carpenter; Technitella, Norman ; Bothy - 
 siphon, Sars. 
 
 Fam. 3. SACCAMMIXIXA. Chambers nearly spherical ; walls thin, 
 composed of firmly cemented sand grains. 
 
 Genera. Psammosphsera, Schultze; Sorosphsera, Brady ; Saccam- 
 mina, M. Sars. 
 
 Fam. 4. KHABDAMMIXIXA. Test composed of firmly cemented 
 sand - grains, often with sponge - spicules intermixed ; tubular ; 
 straight, radiate, branched or irregular ; free or adherent ; with one, 
 two, or more apertures ; rarely segmented. 
 
 Genera. Jacidella, Brady; Marsipdla, Norman (Fig. X. 13) ; 
 Rhabdammina, M. Sars ; Aschemondla, Brady ; Rhizammina, 
 Brady ; Sagenella, Brady ; BotMina, Carp. ; Haliphysema, Bower- 
 bank (test wine-glass-shaped, rarely branched, attached by a disk- 
 like base ; generally beset with sponge-spicules, Fig. X. 11 ; psendo- 
 podial aperture at the free extremity). This and Astrorhiza are 
 the only members of this order in which the living protoplasm has 
 been observed ; in. the latter it has the appearance of a yellowish 
 cream, and its microscopic structure is imperfectly unknown (61). 
 In Haliphysema the network of expanded pseudopodia has been 
 observed by Saville Kent as drawn in Fig. X. 11. Lankester (59) 
 discovered numerous vesicular nuclei scattered in the protoplasm 
 (Fig. X. 10, n), and also near the mouth of the shell reproductive 
 bodies (probably bud-spores) embedded in the protoplasm (Fig. X. 
 8). Hahphysema was described by Bowerbank as a Sponge, and mis- 
 taken by Haeckel (60) for a very simple two-cell-layered animal 
 (Enterozoon), to which he assigned the class name of Physemaria. 
 
 ORDERS. MILIOLIDEA, Brady. 
 
 Characters. Test imperforate ; normally calcareous and porcel- 
 lanous, sometimes encrusted with sand ; under starved conditions 
 (e.g., in brackish water) becoming chitinous or chitino-arenaceous ; 
 at abyssal depths occasionally consisting of a thin homogeneous, 
 imperforate, siliceous film. The test has usually a chambered 
 structure, being divided by septa (each with a hole in it) into a 
 series of locnli which may follow one another in a straight line 
 (Fig. X. 4) or the series may be variously coiled (Fig. X. 1 and 3). 
 The chambering of the test does not express a corresponding cell- 
 segmentation of the protoplasm ; the latter, although growing in 
 volume as the new shell-ehambers are formed, remains one continuous 
 cell-unit with many irregularly scattered nuclei (Fig. X. 2). The 
 chambered and septate structure results in this group and in the other 
 orders from the fact that the protoplasm, expanded beyond the 
 last-formed chamber, forms a new test upon itself whilst it lies and 
 rests upon the surface of the old test. The variations in such a 
 formation are shown in Fig. XII. 1, 2, 3, 4. 
 
 Fam. 1. NUBECULARIXA. Test free or adherent, taking various 
 irregular asymmetrical forms, with variable aperture or apertures. 
 
 Genera. Squammulina, Schultze (Fig. X. 9, showing the ex- 
 panded psendopodia) ; Nvtieciilaria, Defrance. 
 
 Fam. 2. HILIOLIXA. Shell coiled" on an elongated axis, either 
 symmetrically or in a single plane or ineqnilaterally ; two cham- 
 bers in each convolution. Shell aperture alternately during growth 
 (addition of new chambers) at either end of the shell. 
 
 Genera. Biloculina, D'Orb. ; Fabularia, Defrance ; Spirolocu- 
 lina, D'Orb. (Fig. X. 1, 2) ; Miliolina, Williamson (Fig. XL). 
 
 Fam. 3. HAUERIXINA. Shell dimorphous ; chambers partly 
 milioline, partly spiral or rectilinear. 
 
 Genera. Articulina, D'Orb.; Verttbralina,, D'Orb. (Fig X. 4); 
 Ophthalmidium, Knbler ; Hauerina, D'Orb. ; Planispirina, Seguenza. 
 
 Fam. 4. PENEROPLTOIXA. Shell planospiral or cyclical, some- 
 times crosier-shaped, bilaterally symmetrical. 
 
 Genera. Carnuspira, Schultze; Peneroplis, Montfort (Fig. X. 3); 
 
 c 
 
18 
 
 PROTOZOA 
 
 Orliculina, Lamarck ; OrUtolites, Lamarck (by a division of the 
 chambers regularly into chamberlets, and a cyclical mode of growth 
 which results in shells of the size of a shilling, a very elaborate- 
 looking structure is produced which has been admirably analysed 
 by Carpenter (40), to whose memoir the reader is specially referred). 
 
 FIO. XI. JTtKoKiuc (Triloculina) tenera. Young living animal with ex- 
 panded pseudopodia (after Max Sclmltze). A single nucleus is seen iu the 
 innermost chamber. 
 
 Fam. 5. ALVEOLININA. Shell spiral, elongated in the line of 
 the axis of the convolution ; chambers divided into chamberlets. 
 
 Genus. Aheolina, D'Orb. 
 
 Fam. 6. KERAMOSPH.SRINA. Shell spherical ; chambers in con- 
 centric layers. 
 
 Genus. Keramosphxra, Brady. 
 
 ORDER .4. LITUOLIDEA, Brady. 
 
 Characters. Test arenaceous, usually regular in contour ; septa- 
 tion of the many-chambered forms often imperfect, the cavity being 
 labyrinthic. This order consists of sandy isomorphs of the simpler 
 Miliolidea, and also of the simpler Perforata (Lagena, Nodosaria, 
 Cristellaria, Globigerina, Rotalia, Nonionina, &c. ) ; it also contains 
 some peculiar adherent species. 
 
 Fam. 1. LITUOLINA. Test composed of coarse sand-grains, rough 
 externally ; often labyrinthic. 
 
 Genera. Seophax, Montfort ; Haplophragmium, Eeuss (Fig. 
 X. 7) ; Coskinolina, Stache ; Placopsilina, D'Orb. ; Haplostiche, 
 Reuss ; Lituola, Lamarck ; Bdelloidina, Carter. 
 
 Fam. 2. TROCHAMMININA. Test thin, composed of minute 
 sand-grains incorporated with calcareous and other organic cement, 
 or embedded in a chitinous membrane ; exterior smooth, often 
 polished ; interior smooth or rarely reticulated ; never labyrinthic. 
 
 Genera. Thurammina, Brady (test consisting typically of a 
 single spherical chamber with several mammillate apertures, Fig. 
 X. 5, 6) ; Hippocrepina, Parker ; Hormosina, Brady ; Ammo- 
 discus, Reuss ; Trochammina, Parker and Jones ; Carterina, 
 Brady; Webbina, D'Orb. 
 
 Fam. 3. ENDOTHYRINA. Test more calcareous and less sandy 
 than in the other groups of Lituolidea ; sometimes perforate ; 
 septation distinct. 
 
 Genera. Nodonnella, Brady ; Polyphragma, Reuss ; Involutina, 
 Terq. ; Endothyra, Phillips ; Bradyina, Moll. ; Stacheia, Brady. 
 
 Fam. 4. LOFTUSINA. Test of relatively large size ; lenticular, 
 spherical, or fusiform ; constructed either on a spiral plan or in 
 concentric layers, the chamber cavities occupied to a large extent 
 by the excessive development of the finely arenaceous cancellated 
 walls. 
 
 Genera. Cyclammina, Brady; Loftusia, Brady; Parkeria, 
 Carpenter. 
 
 SUB-CLASS B. Perforata. 
 
 Characters. Shell substance perforated by numerous minute 
 apertures, through which as well as from the main aperture the 
 protoplasm can issue. 
 
 ORDERS. TEXTULARIDEA, Brady. 
 
 Characters. Tests of the larger species arenaceous, either with 
 or without a perforate calcareous basis ; smaller forms hyaline and 
 conspicuously perforated. Chambers arranged in two or more 
 alternating series, or spiral or confused ; often dimorphous. 
 
 Fam. 1. TEXTULARINA. Typically bi- or tri-serial ; often bi- 
 rarely tri-morphous. 
 
 Genera. Textularia Defrance ; Cuneolina, D'Orb. ; Vemeiul- 
 ina, D'Orb.; Tritaxia, Reuss; Chrysalidina, D'Orb.; Bigenerina, 
 D'Orb. ; Pavonina, D'Orb. ; Spiroplecta, Ehr. ; Qaudryina, D'Orb. ; 
 Valmlina, D'Orb.; Clavulina, D'Orb. 
 
 Fam. 2. BULTMININA. Typically spiral ; weaker forms more or 
 less regularly biserial; aperture oblique, comma-shaped or some 
 modification of that form. 
 
 Genera.- Bulimina, D'Orb. ; Virgulina, D'Orb. ; Bifarina, 
 Parker and Jones ; Bolivina, D'Orb. ; Pleurostomella, Reuss. 
 
 Fam. 3. CASSIDULINA. Test consisting of a Textularia-like series 
 of alternating segments more or less coiled upon itself. 
 
 Genera. Cassidulina, D'Orb.; Ehrenbergina, Reuss. 
 
 ORDERS. CHILOSTOMELLIDEA, Brady. 
 
 Characters. Test calcareous, finely perforate, many-chambered. 
 Segments following each other from the same end of the long axis, 
 or alternately at the two ends, or in cycles of three, more or less 
 embracing. Aperture a curved slit at the end or margin of the final 
 segment. 
 
 Genera. Ellipsoidina, Seguenza; Chilostomella, Reuss; Alia- 
 morphina, Reuss. 
 
 ORDER?. LAGENIDEA, Brady. 
 
 Characters. Test calcareous, very finely perforated ; either 
 single-chambered, or consisting of a number of chambers joined in 
 a straight, curved, spiral, alternating, or (rarely) branching series. 
 Aperture simple or radiate, terminal. No interseptal skeleton nor 
 canal system. 
 
 Fam. 1. LAGENINA. Shell single-chambered. 
 
 Genera. Lagena, Walker and Boys; Nodosaria, Lamk. ; Lin- 
 gulina, D'Orb. ; Frondicularia, Defrance ; Rhabdogonium, Reuss ; 
 Marginulina, D'Orb. ; Vaginulina, D'Orb. ; Rimulina, D'Orb. ; 
 Cristellaria,'Lamk. ; Amphicoryne,Sc]\\nm\). ; Lingulinopsis, Reuss ; 
 Flabellina, D'Orb. ; Amphimorphina, Neugeb. ; Dentalinopsis, 
 Reuss. 
 
 Fam. 2. POLYMORPHININA. Segments arranged spirally or 
 irregularly around the long axis ; rarely biserial and alternate. 
 
 Genera. Polymorphina, D'Orb. ; Dimorphina, D'Orb. ; Uviger- 
 ina, D'Orb. ; Sagrina, P. and J. 
 
 Fam. 3. RAMULININA. Shell branching, composed of spherical 
 or pyriform chambers connected by long stoloniferous tubes. 
 
 Genus. Bamulina, Rupert Jones. 
 
 ORDERS. GLOBIGERINIDEA, Brady. 
 
 Characters. Test free, calcareous, perforate ; chambers few, 
 inflated, arranged spirally ; aperture single or multiple, con- 
 spicuous. No supplementary skeleton nor canal system. All the 
 larger species pelagic in habit. 
 
 Geneva.. Globigerina, D'Orb. (Fig. XII. 6) : Orbulina, D'Orb 
 (Fig. XII. 8) ; Hastigerina, Wy. Thomson (Fig. XII. 5) ; Pul- 
 lenia, P. and J. ; Sphxroidina, D'Orb. ; Candeina, D'Orb. 
 
 ORDER 9. ROTALIDEA, Brady. 
 
 Characters. Test calcareous, perforate; free or adherent. Typi- 
 cally spiral and "rotaliform" (Fig. XII. 2), that is to say, coiled 
 in such a manner that the whole of the segments are visible on the 
 superior surface, those of the last convolution only on the Inferior 
 or apertural side, sometimes one face being more convex sometimes 
 the other. Aberrant forms evolute, outspread, acervuline, or 
 irregular. Some of the higher modifications with double chamber- 
 walls, supplemental skeleton, and a system of canals. The nature 
 of this supplemental skeleton is shown in Fig. XII. 2 and 10. 
 
 Fam. 1. SPIRILLININA. Test a complanate, planospiral, non- 
 septate tube ; free or attached. 
 
 Genus. Spirillina, Ehr. 
 
 Fam. 2. ROTALINA. Test spiral, rotaliform, rarely evolute, very 
 rarely irregular or acervuline. 
 
 Genera. Patellina, Williamson ; Cymbalopora, Hay ; Discorbina, 
 P. and J. ; Planorbulina, D'Orb. ; Truncatulina, D'Orb. ; Anomal- 
 ina, P. and J. ; Carpentcria, Gray (adherent) ; Eupertia, 
 Wallick ; Pulvinulina, P. and J. ; Rotalia, Lamk. ; Calcarina, 
 D'Orb. [Shell rotaliform ; periphery furnished with radiating 
 spines ; supplemental skeleton and canal system largely developed. 
 This form is shown in a dissected condition in Fig. XII. 10. Outside 
 and between the successive chambers with finely perforated walls 
 a 2 , a 3 , a* & secondary shell-substance is deposited by the proto- 
 plasm which has a different structure. Whilst the successive 
 chambers with their finely perforate walls (resembling dentine in 
 structure) are formed by the mass of protoplasm issuing from the 
 mouth of the last-formed chamber, the secondary or supplemental- 
 shell substance is formed by the protoplasm which issues through 
 the fine perforations of the primary shell substance ; it is not 
 finely canaliculated, but is of denser substance than the primary 
 shell and traversed by coarse canals (occupied by the protoplasm) 
 which make their way to the surface of the test (c' , c'). In Cal- 
 carina a large bulk of this secondary shell-substance is deposited 
 around each chamber and also forms the heavy club-like spines.] 
 
 Fam. 3. TINOPORINA. Test consisting of irregularly heaped 
 chambers with (or sometimes without) a more or less distinctly 
 spiral primordial portion ; for the most part without any general 
 pseudopodial aperture. 
 
PROTOZOA 
 
 19 
 
 Genera. Tiiwporus, Carpenter; Gypsina, Carter; Aphrosina, 
 Carter ; Thalamopara, Roemer ; Polytrema, Risso. [Shell para- 
 sitic, encrusting, or arborescent ; surface areolated, coloured pink 
 or white, Fig. XIL 9. Interior partly occupied by small chambers, 
 arranged in more or less regular layers, and partly by non- 
 segmented canal-like spaces, often crowded with sponge-spicules 
 No true canal system. This is one of the most important types as 
 exhibiting the arborescent and encrusting form of growth. It is 
 fairly abundant] 
 
 04 
 
 FlO. XII. Perforata. 1- Spiral arrangement of simple chambers of a 
 Eeticularian shell. 2. Ditto, with double septal walls, aud supple- 
 mental shell-substance (shaded). 3. Diagram to show the mode in 
 which successively-formed chambers may completely embrace their pre- 
 decessors. 4. Diagram of a simple straight series of non-embracing 
 chambers. 5. Hastigerina (Globigerina) Murrayi, Wyv. Thomson, 
 a, bubbly (vacuolated) protoplasm, enclosing ft, the perforated Globi- 
 gerina-like shell (conf. central capsule of Kacliolaria). From the peripheral 
 protoplasm project, not only fine pseudopodia, but hollow spines of 
 calcareous matter, which are set on the shell, and have an aiis of active 
 protoplasm. Pelagic ; drawn in the living state. 6. Globigerina 
 buttoidet, D'Orb., showing the punctiform perforations of the shell and 
 the main aperture. 7. Fragment of the shell of Globigerina, seen 
 from within, and highly magnified, o, fine perforations in the inner shell 
 substances ; b, outer (secondary) shell substance. Two coarser perfora- 
 tions are seen in section, and one lying among the smaller. 8. Or- 
 bulina Uniterm, D'Orb. Pelagic example, with adherent radiating 
 
 calcareous spines (hollow), and internally a Email Globigerina shell. It is 
 uncertain whether Orbulina is merely a developmental phase of Globi- 
 gerina. a, Orbulina shell ; ft, Globigerina shell. 9. Polytrema minia- 
 ceum, Lin. ; x 12. Mediterranean. Example of a branched adherent cal- 
 careous perforate Beticularian. 10. Calcarina Sptngleri, Gmel. ; x 10. 
 Tertiary, Sicily. Shell dissected so as to show the spiral arrangement of 
 the chambers, and the copious secondary shell substance, a-, a', a*, 
 chambers of three successive coils in section, showing the thin primary 
 wall (finely tubulate) of each ; ft, ft, ft, 6, perforate surfaces of the primary 
 wall of four tiers of chambers, from which the secondary shell substance 
 has been cleared away ; c'. c', secondary or intermediate shell substance 
 in section, showing coarse canals ; d, section of secondary shell substance 
 at right angles to c' ; e, tubercles of secondary shell substance on the 
 surface ; /,/, club-like processes of secondary shell substance. 
 
 ORDEK 10. NUMMULINIDEA, Brady. 
 
 Characters. Test calcareous and finely tubulated ; typically 
 free, many-chambered, and symmetrically spiral. The higher 
 modifications all possess a supplemental skeleton, and canal system 
 of greater or less complexity. 
 
 Fam. 1. FUSULINIXA. Shell bilaterally symmetrical ; chambers 
 extending from pole to pole; each convolution completely enclosing 
 the previous whorls. Shell-wall finely tubulated. Septa single or 
 rarely double ; no true interseptal canals. Aperture a single 
 elongated slit, or a row of small rounded pores, at the inner edge 
 of the final segment. 
 
 Genera. Fusulina, Fischer ; Schwagmna, Holler. 
 
 Fam. 2. POLYSTOMELLINA. Shell bilaterally symmetrical, nauti- 
 loid. Lower forms without supplemental skeleton or interseptal 
 canals ; higher types with canals opening at regular intervals along 
 the external septal depressions. 
 
 Genera. Nanionina, D'Orb. ; Polystomella, Lamarck. 
 
 Fam. 3. NtrMMCXiTlXA. Shell lenticular or complanate ; lower 
 forms with thickened and finely tubulated shell-wall, but no inter- 
 mediate skeleton ; higher forms with interseptal skeleton and com- 
 plex canal system. 
 
 Genera. Archseodiscus, Brady ; Amphistegina, D'Orb. ; Oper- 
 eiUina, D'Orb. ; Heterostegina, D'Orb. ; Nvmmulites, Lamarck ; 
 Assilina, D'Orb. 
 
 Fam. i. CYCLOCLYPEIXA. Shell complanate, with thickened 
 centre, or lenticular ; consisting of a disk of chambers arranged 
 in concentric annuli, with more or less lateral thickening of lami- 
 nated shell substance, or acervnline layers of chamberlets. Septa 
 double and furnished with a system of interseptal canals. 
 
 Genera. Cyclodypeus, Carpenter ; Orbitaides, D'Orb. 
 
 Fam. 5. FXJZOONINA. Test forming irregular, adherent, acervu- 
 liue masses. 
 
 Genus. Eozoon Dawson. 
 
 further remarks on the Reticularia. The name Thalamophora, 
 pointing to the peculiar tendency which the larger members of 
 the group have to form chamber after chamber and so to build up 
 a complex shell, has been proposed by Hertwig (56) and adopted by 
 many writers. The old name Foraminifera (which did not refer 
 to the fine perforations of the Perforata but to the large pseudo- 
 podial aperture leading from chamber to chamber) has also been 
 extended bv some so as to include the simpler Gromia-like forms. 
 On the whole Carpenter's term Reticularia (62) seems most suitable 
 for the group, since they all present the character indicated. It 
 has been objected that the Radiolaria are also reticular in their 
 pseudopodia, but if we except the pelagic forms of Reticularia 
 (Globigerina, Orbulina, &c.), we find that the Radiolaria are really 
 distinguishable by their staffer, straighter, radiating pseudopodia. 
 No doubt the Labyrinthulid Chlamydomyxa and the plasmodia of 
 some Mycetozoa are as retieular in their pseudopodia as the 
 Reticnlaria, but they possess other distinctive features which 
 serve, at any rate in an artificial system, to separate them. 
 
 The protoplasm of the majority of the Reticnlaria is unknown, 
 or only very superficially observed ; hence we have made a point of 
 introducing among our figures as many as possible which show this 
 essential part of the organism. It is only recently (1876) that 
 nuclei have been detected in the calcareous-shelled members of the 
 group, and they have only been seen in a few cases. 
 
 The protoplasm of the larger shell-making forms is known to be 
 often strongly coloured, opaque, and creamy, but its minute struc- 
 ture remains for future investigation. Referring the reader to the 
 figures and their explanation, we would draw especial attention to 
 the structure of the protoplasmic body of Hastigerina (one of the 
 Globigerinidea) as detected by the "Challenger" naturalists. It 
 will be seen from Fig. XII. 5 that the protoplasm extends as a rela- 
 tively enormous ' ' bubbly " mass around the shell which is sunk 
 within it ; from the surface of this " bubbly " (vacuolated or alveol- 
 ated) mass the pseudopodia radiate. 
 
 The reader is requested to compare this with Fig. XIII., repre- 
 senting the "bubbly " protoplasmic body of Thalassicolla. It then 
 becomes obvious that the perforated central capsule CK. of the latter 
 holds the same relation to the mass of the protoplasm as does the 
 central perforated shell of Globigerina (Hastigerina). The extreme 
 vacuolation of the protoplasm in both cases (the vacuoles being 
 
20 
 
 PROTOZOA 
 
 filled with sea- water accumulated by endosmosis) and the stiff radiat- 
 ing pseudopodia are directly correlated with the floating pelagic life of 
 the two organisms. All the Radiolaria are pelagic, and many exhibit 
 this vacuolation ; only a few of the Reticularia are so, and their struc- 
 tural correlation to that habit has only lately been ascertained. 
 
 The Reticularia are almost exclusively known by their shells, 
 which offer a most interesting field for study on account of the very 
 great complexity of form attained by some of them, notwithstand- 
 ing the fact that the animal which produces them is a simple uni- 
 cellular Protozoon. Space does not permit the exposition here of 
 the results obtained by Carpenter in the study of the complex shells 
 of Orbitolites, Operculina, Nummulites, &c. ; it is essential that his 
 work Introduction to the Study of the Foraminifera (Ray Society, 
 1862) should be consulted, and in reference to the sandy-shelled 
 forms the monograph by Brady, in the Challenger Eeports, vol. ix., 
 1883 ; and it must be sufficient here to point out the general prin- 
 ciples of the shell-architecture of the Reticularia. Let us suppose 
 that we have an ever-growing protoplasmic body which tends to 
 produce a calcareous shell on its surface, leaving an aperture for the 
 exit of its pseudopodia. It will grow too large for its shell and 
 accumulate outside the shell. The accumulated external mass may 
 then secrete a second chamber, resting on the first as chamber 1 
 rests on chamber in Fig. XII. 4. By further growth a new 
 chamber is necessitated, anil so is produced a series following one 
 another in a straight line, each chamber communicating with the 
 newer one in front of it by the narrow pseudopodial aperture 
 (a, a 1 , a-, a 3 }. Now it is possible for these chambers to be very 
 variously arranged instead of simply as in Fig. XII. 4. For instance, 
 each new chamber may completely enclose the last, as in Fig. XII. 
 3, supposing the protoplasm to spread all over the outside of the 
 old chamber before making a new deposit. Again the chambers 
 need not succeed one another in a straight line, but may be dis- 
 posed in a spiral (Fig. XII. 1). And this spiral may be a flat coil, 
 or it may be a heliciue spiral with a rising axis ; further it may be 
 close or open. All these forms in various degrees of elaboration 
 are exhibited by Miliolidea and various Perforata. 
 
 But the Perforata in virtue of their perforate shell-walls introduce 
 a new complication. The protoplasm issues not only from the 
 mouth of the last-formed chamber, but from the numerous pores in 
 the wall itself. This latter protoplasm exerts its lime-secreting 
 functions ; it gathers itself into coarse branching threads which 
 remain uncalcified, whilst all around a dense deposit of secondary 
 or supplemental shell-substance is thrown down, thus producing a 
 coarsely canalicular structure. The thickness and amount of this 
 secondary shell and the position it may occupy between and around 
 the chambers of primitive shell-substance vary necessarily in dif- 
 ferent genera according to the mode in which the primitive cham- 
 bers are arranged and connected with one another. Calcarina is a 
 fairly typical instance of an abundant secondary shell-deposit (Fig. 
 XII. 10), audit is the existence of structure resembling the chambers 
 of Calcarina with their surrounding primary and secondary shell- 
 substances which has rendered it necessary to regard Eozoon (41) as 
 the metamorphosed encrusting shell of a pre-Cambrian Reticularian. 
 
 The division of the Reticularia into Imperforata and Perforata 
 which is here maintained has no longer the significance which was 
 once attributed to it. It appears, according to the researches of 
 Brady, that it is not possible to draw a sharp line between these 
 sub-classes, since there are sandy forms which it is difficult to 
 separate from imperforate Lituolidea and are nevertheless perforate, 
 in fact are " sandy isomorphs of Lagena, Nodosaria, Globigerina, 
 and liotalia." It does not appear to the present writer that there 
 can bo any insurmountable difficulty in separating the Lituolidea 
 into two groups those which are sandy isomorphs of the porcel- 
 lanous Miliolidea, and those which are sandy isomorphs of the 
 hyaline Perforata. The two groups of Lituolidea thus formed 
 might be placed in their natural association respectively with the 
 Imperforata and the Perforata. 
 
 The attempt to do this has not been made here, but the classifi- 
 cation of Brady has been adopted. In Biitschli's large work on the 
 Protozoa (9) the breaking up of the Lituolidea is darned out to a 
 logical conclusion, and its members dispersed among the Miliolidea 
 on the one hand and the various orders of Perforata on the other hand. 
 
 The calcareous shell-substance of the Miliolidea being opaque 
 and white has led to their being called " Porcellana," whilst the 
 transparent calcareous shells of the smaller Perforata has gained 
 for that group the synonym of "Hyalina." 
 
 The shells of the calcareous Reticularia and of some of the 
 larger arenaceous forms are found in stratified rocks, from the 
 Palfeozoic strata onwards. The Chalk is in places largely com- 
 posed of their shells, and the Eocene Nummulitic limestone is 
 mainly a cemented mass of the shells of Nummulites often as 
 large each as a shilling. The Atlantic ooze is a chalky deposit 
 consisting largely of the shells of Globigerina, &c. 
 
 CLASS VII. EADIOLABIA, Haeckel, 1862 (63) (Polycystina, Ehr.). 
 
 Characters. Gymnomyxa in which the protoplasmic body of 
 
 the dominant amoeba phase has the form of a sphere or cone from 
 
 the surface of which radiate filamentous pseudopodia, occasionally 
 anastomosing, and encloses a spherical (homaxonic) or cone-shaped 
 (monaxonic) perforated shell of membranous consistence known as 
 the central capsule, and probably homologous with the perforated 
 shell of a Globigerina. The protoplasm within the capsule (intra- 
 capsular protoplasm) is continuous through the pores or apertures 
 of the capsule with the outer protoplasm. Embedded in the former 
 lies the large and specialized nucleus (one or more). Gelatinous 
 substance is frequently formed peripherally by the extracapsular 
 protoplasm, constituting a kind of soft mantle which is penetrated 
 by the pseudopodia. A contractile vacuole is never present. 
 
 Usually an abundant skeleton, consisting of spicules of silica or 
 of a peculiar substance called acanthin arranged radially or tangen- 
 tially, loose or united into a basket-work, is present. Oil globules, 
 pigment, and crystals are found in greater or less abundance in 
 the protoplasm. 
 
 In most but not all Radiolaria peculiar nucleated yellow cor- 
 puscles are abundantly present, usually regarded as parasitic Algse. 
 Reproduction by fission has been observed, and also in some few 
 species a peculiar formation of swarm-spores (flagpllula 1 ) within the 
 central capsule, in which the nucleus takes an important part. 
 All the Radiolaria are marine. The Radiolaria are divided into 
 two sub-classes according to the chemical nature of their spicular 
 skeleton, and into orders according to the nature and the disposi- 
 tion of the apertures in the wall of the central capsule. 
 
 EP 
 
 ..0.1 
 
 al 
 
 FIG. XIII. Thalassicollapelagica, Haeckel; x 25. CK, central capsule ; 
 EP, extracapsular protoplasm ; al, alveoli, liquid-holding vacuoles in the 
 protoplasm similar to those of Heliozoa, Pelomyxa, Hastigerina, <fcc.; pi, 
 pseudopodia. The minute unlettered dots are the "yellow cells." 
 
 SUB-CLASS I. Silico-Skeleta, Lankester. 
 
 Characters. A more or less elaborate basket-work of tangential 
 and radial elements consisting of secreted silica is present ; in rare 
 exceptions no skeleton is developed. 
 
 ORDER 1. PERIPYL.EA, Hertwig. 
 
 Characters. Silico-skeletal Radiolaria in which the central cap- 
 sule is uniformly perforated all over by fine pore-canals ; its form is 
 that of a sphere (homaxonie), and to this form the siliceous skeleton 
 primarily conforms, though it may become discoid, rhabdoid, or 
 irregular. The nucleus is usually single, but numerous nuclei are 
 present in each central capsule of the Polycyttaria. 
 
 Fain. 1. SPH^ERIDA, Haeck. Spherical Peripylaea with a spheri- 
 cal basket-work skeleton, sometimes surrounded by a spongy outer 
 skeleton, sometimes simple, sometimes composed of many concentric 
 spheres (never discoid, flattened, or irregular). The central capsule 
 sometimes encloses a part of the spherical skeleton, and often is 
 penetrated by radiating elements. 
 
 Genera (selected). Ethmosphsera, Haeck. ; Xiphosj>hsera, Haeck. ; 
 Staurosphtera, Haeck. ; Heliosphtera, Haeck. (Fig. XIV. 14) ; As- 
 tromma, Haeck. ; Haliomma, Haeck. ; Actinomma, Haeck. (Fig. 
 XIV. 17; note the sphere within sphere, the smallest lying in the 
 nucleus, and the whole series of spherical shells connected by radial 
 spines) ; Arachnosphxra, Haeck. ; Plcgmosph&ra, Haeck. ; Sponyo- 
 sph&ra, Haeck. (Fig. XVI. 8). 
 
 Fam. 2. DISCIDA, Haeck. Discoid PeripyloBa ; both skeleton 
 and central capsule flattened. 
 
 Genera (selected). Pliseodiscus, Haeck. ; Hdiodiscus, Haeck. ; 
 Spongodiscus, Haeck. ; Spongurus, Haeck. 
 
PROTOZOA 
 
 21 
 
 Fam. 3. THALASSICOLLIDA. Peripylsea devoid of a skeleton, or 
 with a skeleton composed of loose siliceous spicules only. Nucleus 
 single ; central capsule and general protoplasm spherical. 
 
 Genera (selected). Thalassicolla, Huxley (Fig. XIII., Fig. 
 XIV. 1) ; Thalassosph&ra, Haeck. ; Physematium, Haeck. 
 
 Fam. 4. POLYCYTTARIA. Peripylaea consisting of colonies of 
 many central capsules united by their extracapsular protoplasm. 
 Central capsules multiplying by fission. Nuclei in each central 
 capsule numerous. Siliceous skeleton either absent, or of loose 
 spicules, or having the form of a spherical fenestrated shell sur- 
 rounding each central capsule. 
 
 Genera (selected). Collosphsera, Miiller (with fenestrated globular 
 skeleton) ; Sphserozoum, Haeck. (skeleton of numerous loose spicules 
 which are branched) ; Saphidolioum, Haeck. (spicules simple) ; Col- 
 lozoum, Miiller (devoid of skeleton, Fig. XIV. 2, 3, 4, 5). 
 
 Fro. XIV. Radiolaria. 1. Central capsule of Thalassicolla nucleata, 
 Huxley, in radial section, a, the large nucleus (Binnenblaschen); b, 
 corpuscular structures of the intracapsular protoplasm containing con- 
 cretions ; c, wall of the capsule (membranous shell), showing the fine 
 radial pore-canals ; d, nucleolar fibres (chromatin substance) of the 
 nucleus. 2, 3. Collozoum inerme, J. Miiller, two different forms of 
 
 colonies, of the natural size. 4. Central capsule from a colony of 
 
 Collozoum inerme, showing the intracapsular protoplasm and nucleus, 
 broken up into a number of spores, the germs of swarm-spores or flagelluls? ; 
 
 each encloses a crystalline rod. c, yellow cells lying in the extracapsular 
 protoplasm. 5. A small colony of Collozoum inerme, magnified 25 
 
 diameters, a, alveoli (vacuoles) of the extracapsular protoplasm ; b, 
 central capsules, each containing besides protoplasm a large oil-globule. 
 6-13. Yellow cells of various Eadiolaria : 6, normal yellow cell; 7, 8. 
 division with formation of transverse septum ; 9, a modified condition 
 according to Brandt ; 10, division of a yellow cell into four ; 11, amceboid 
 condition of a yellow cell from the body of a dead Sphserozoon ; 12, a 
 similar cell in process of division ; 13, a yellow cell the protoplasm of 
 which is creeping out of its cellulose envelope. 14. Heliosphxra 
 
 inerinis, Haeck., living example; x 400. a, nucleus; b, central capsule ; 
 c, siliceous basket-work skeleton. 15. Two swarm-spores (tta^ellula?) 
 
 of Collozoum inerme, set free from such a central capsule as that drawn in 
 4 ; each contains a crystal 6 and a nucleus a. 16. Two swarm-spores 
 
 of Collozoum inerme, of the second kind, viz., devoid of crystals, and of 
 two sizes, a macrospore and a microspore. They have been set free 
 from central capsules with contents of a different appearance from that 
 drawn in 4. a, nucleus. 17. Actinomma asteraeanthion, Haeck ; x .260; 
 one of the Peripytea. Entire animal in optical section, a, nucleus; 
 b, wall of the central capsule ; c, innermost siliceous shell enclosed in the 
 nucleus ; c 1 , middle shell lying within the central capsule ; c 2 , outer shell 
 lying in the extracapsular protoplasm. Four radial siliceous spines, hold- 
 ing the three spherical shells together are seen. The radial fibrillation of 
 the protoplasm and the fine extracapsular pseudopodia are to be noted 
 18. A i/iphilonche messanensii, Haeck; x 200; one of the Acanthometridea. 
 Entire animal as seen living. 
 
 ORDER 2. MOXOPYL^A, Hertwig. 
 
 Characters. Silico-skeletal Radiolaria in which the central cap- 
 sule is not spherical but monaxonic (cone-shaped), with a single per- 
 forate area (pore-plate) placed on the basal face of the cone ; the 
 membrane of the capsule is simple, the nucleus single ; the skeleton 
 is extracapsular, and forms a scaffold-like or bee-hive-like structure 
 of monaxonic form. 
 
 Flfl. X.V.Eucyrtidium cranioides, Haeck ; x!50 ; one of the Monopykea. 
 Entire animal as seen in the living condition. The central capsule is 
 hidden by the bee-hive-shaped siliceous shell within which it is lodged. 
 
 Fam. 1. PLECTIDA, Haeck. Skeleton formed of siliceous spines 
 loosely conjoined. 
 
 Genera (selected). PTagiacantha, Haeck. ; Plegmatium, Haeck. 
 
 Fam. 2. CTRTIDA, Haeck. Skeleton a monaxonic or triradiate 
 shell, or continuous piece (bee-hive-shaped). 
 
 Genera (selected). Halicalyptra, Haeck. ; Eucyrtidium, Haeck. 
 (Fig. XV.); Carpocanium, Haeck. (Fig. XVI. 3). 
 
 Fam. 3. BOTRIDA, Haeck. Irregular forms ; the shell composed 
 of several chambers agglomerated without definite order ; a single 
 central capsule. 
 
 Genera. Botryocyrtis, Haeck. ; Lithdbotrys, Haeck. 
 
 Fam. 4. SPYRIDA, Haeck. Gemminate forms, with shell con- 
 sisting of two conjoined chambers ; a single central capsule. 
 
 Fam. 5. STEPHIDA, Haeck. Skeleton cricoid, forming a single 
 siliceous ring or several conjoined rings. 
 
 Genera (selected). Acanthodesmia, Haeck. ; Zygostephanus, 
 Haeck. ; Lithocircus, Haeck. (Fig. XVI. 1). 
 
 ORDERS. PH.EODARIA, Haeck. (Tripyltea, Hertwig). 
 Characters. Silico-skeletal Radiolaria in which the central 
 
22 
 
 PROTOZOA 
 
 FIG. XVI. Eadiolaria. 1. Lithotirms annularis, Hertwig ; one of the 
 Monopylsea. Whole animal in the living state (optical section), a, nucleus ; 
 b, wall of the central capsule ; c, yellow cells ; d, perforated area of the 
 central capsule (Monopylsea). 2. Cystidium inerme., Hertwig ; one of the 
 Monopylcea. Living animal. An example of a Monopylocon destitute of 
 skeleton, a, nucleus ; b, capsule-wall ; c, yellow cells in the extracapsular 
 protoplasm. 3. Carpncanium diadema, Haeck. ; optical section of the bee- 
 hive-shaped shell to show the form and position of the protoplasmic body. 
 a, the tri-lobed nucleus ; ft, the siliceous shell ; c, oil-globules ; d, the per- 
 forate area (pore-plate) of the central capsule. 4. Ccelodendrum 
 gracillimum, Haeck. ; living animal, complete ; one of the Tripytea. a, the 
 characteristic dark pigment (phaeodium) surrounding the central capsule 6. 
 The peculiar branched siliceous skeleton, consisting of hollow fibres, and 
 the expanded pseudopodia are seen. 5. Central capsule of one of the 
 Tripytea, isolated, showing a, the nucleus ; b,c, the inner and the outer 
 laminae of the capsule-wall; d, the chief or polar aperture; e,e, the two 
 
 secondary apertures. 
 
 6, 7. Acanthometra Claparedei, Haeck. 7 shows 
 
 the animal in optical section, so as to exhibit the characteristic meeting of 
 the spines at the central point as in all Acanthometridea ; 6 shows the 
 transition from the uninuclear to the multinuclear condition by the 
 breaking up of the large nucleus, a, small nuclei ; 6, large fragments of 
 the single nucleus ; c, wall of the central capsule ; d, extracapsular jelly 
 (not protoplasm) ; e, peculiar intracapsular yellow cells. 8. Spongo- 
 
 ephsera, streptacantha, Haeck. ; one of the Peripytea. Siliceous skeleton 
 not quite completely drawn on the right side, a, the spherical extra- 
 capsular shell (compare Fig. XIV. 17), supporting very large radial spines 
 which are connected by a spongy network of siliceous fibres. ' 9. 
 Aulosphxra elegantissima, Haeck.; one of the Phajodaria. Half of the 
 spherical siliceous skeleton. 
 
 capsule has a double membrane and more than one perforate area, 
 viz., one chief " polar aperture," and one, two, or more accessory 
 apertures (Fig. XVI. 5). The nucleus is single. Around the 
 central capsule is an abundant dark brown pigment (phaeodium of 
 Haeckel). The siliceous skeleton exhibits various shapes regular 
 and irregular, but is often remarkable for the fact that it is built 
 up of hollow tubes. 
 
 Fam. 1. PH^OCYSTIDA, Haeck. The siliceous skeleton is either 
 entirely absent or consists of hollow needles which are disposed 
 outside the central capsule, regularly or irregularly. 
 
 Genera (selected). Aulacantha, Haeck. ; Thalassoplancta, Haeck. 
 
 Fam. 2. PJMOGROMIDA, Haeck. The siliceous skeleton consists 
 of a single fenestrated shell, which may be spherical, ovoid, or often 
 dipleuric, but always has one or more large openings. 
 
 Genera (selected). Challcngeria. Wy. Thomson; Lithogrmnia, 
 Haeck. 
 
 Fam. 3. PH^OSPH^RIDA. The siliceous skeleton consists of 
 numerous hollow tubes which are united in a peculiar way to form 
 a large spherical or polyhedral basket-work. 
 
 Genera (selected). Aulosphasra, Haeck. (Fig. XVI. 9); Aulo- 
 plegma, Haeck. ; Cannacantha, Haeck. 
 
 Fam. 4. PH.SMCONCHIDA. The siliceous skeleton consists of two 
 separate fenestrated valves, similar to a mussel's shells ; often there 
 are attached to the valves simple or branched hollow tubes of silex. 
 
 Genera (selected). Conchidium, Haeck. ; Calodendrum, Haeck. 
 (Fig. XVI. 4). 
 
 SUB-CLASS II. Acanthometridea, Lankester ( = Acanthino-skclcta). 
 
 Characters. Radiolaria in which the skeleton is composed of a 
 peculiar horny substance known as acanthin (rarely of silica). 
 The central capsule is uniformly perforate (Peripylsea type). A 
 divided or multiple nucleus is present in the capsule ; the capsule- 
 wall is single. The skeleton always has the form of spines which 
 radiate from a central point within the capsule where they are all 
 fitted to one another. Karely a fenestrated tangential skeleton is 
 also formed. 
 
 Fam. I. ACANTHONIDA, Haeck. Skeleton consisting of twenty 
 spines of acanthin disposed in five parallel zones of four spines each, 
 meeting one another at the central point of the organism ; never 
 forming a fenestrated shell. 
 
 Genera (selected). Acanthometra, J. Miiller (Fig. XVI. 6, 7) ; 
 Astrolonclie, Haeck. ; Amphilonche, Haeck. (Fig. XIV. 18). 
 
 Fam. 2. DIPLOCONIDA, Haeck. Skeleton a double cone. 
 
 Genus unicum.- Diploconus, Haeck. 
 
 Fam. 3. DORATASPIDA, Haeck. The twenty acanthin spines of 
 the skeleton form by transverse outgrowths a spherical fenestrated 
 shell. 
 
 Genera (selected). Stauraspis, Haeck. ; Dorataspis, Haeck. 
 
 Fam. 4. SPH^ROCAPSIDA, Haeck. The twenty acanthin spines 
 are joined together at their free apices by a simple perforate shell 
 of acanthin. 
 
 Genus unicum. Sphxrocapsa. 
 
 Fam. 5. LITHOLOPHIDA. Skeleton of many needles of acanthin 
 radiating from a single point without definite number or order. 
 
 Genera. Litholophus, Haeck. ; Aslrolophus, Haeck. 
 
 Further remarks on the Sadiolaria. It has not been possible in 
 the systematic summary above given to enumerate the immense 
 number of genera which have been distinguished by Haeckel (42) as 
 the result of the study of the skeletons of this group. The important 
 differences in the structure of the central capsule of different Eadio- 
 laria were first shown by Hertwig, who also discovered that the spines 
 of the Acanthometridea consist not of silica but of an organic com- 
 pound. In view of this latter fact and of the peculiar numerical 
 and architectural features of the Acanthometrid skeleton, it seems 
 proper to separate them altogether from the other Radiolaria. The 
 Peripylaja may be regarded as the starting point of the Radiolarian 
 pedigree, and have given rise, on the one hand to the Acantho- 
 metridea, which retain the archaic structure of the central capsule 
 whilst developing a peculiar skeleton, and on the other hand to 
 the Monopylrea and Phaiodaria which have modified the capsule 
 but retained the siliceous skeleton. 
 
 Phajodaria. 
 
 Peripylsea. 
 
 MonopylaBa. 
 
 Acanthometridea. 
 
 Archi-peripylaea. 
 RADIOLAEIA. 
 
 The occasional total absence of any siliceous or acanthinous 
 skeleton docs not appear to be a matter of classificatory importance, 
 since skeletal elements occur in close allies of those very few forms 
 
PROTOZOA 
 
 23 
 
 which are totally devoid of skeleton. Similarly it does not appear 
 to be a matter of great significance that some forms (Polycyttaria) 
 form colonies, instead of the central capsules separating from one 
 another after fission has occurred. 
 
 It is important to note that the skeleton of silex or acanthin 
 does not correspond to the shell of other Gymnomyxa, which 
 appears rather to be represented by the membranous central cap- 
 sule. The skeleton does, however, appear to correspond to the 
 spicules of Heliozoa, and there is an undeniable affinity between 
 such a form as Clathrulina (Fig. VII. 2) and the Sphserid Peripylaea 
 (such as Heliosphsera, Fig. XIV. 14). The Radiolaria are, however, 
 a very strongly marked group, definitely separated from all other 
 Gymnomyxa by the membranous central capsule sunk in their proto- 
 plasm. Their differences inter~se do not affect their essential struc- 
 ture. The variations in the chemical composition of the skeleton and 
 in the perforation of the capsule do not appear superficially. The 
 most obvious features in which they differ from one another relate to 
 the form and complexity of the skeleton, a part of the organism so 
 little characteristic of the group that it may be wanting altogether. 
 It is not known how far the form-species and form-genera which 
 have been distinguished in such profusion by Haeckel as the 
 result of a study of the skeletons are permanent (i.e., relatively 
 permanent) physiological species. There is no doubt that very 
 many are local and conditional varieties of a single Protean species. 
 The same remark applies to the species discriminated among the 
 shell-bearing Reticularia. It must not be supposed, however, that 
 less importance is to be attached to the distinguishing and record- 
 ing of such forms because we are not able to assert that they are 
 permanent species. 
 
 The yellow cells (of spherical form, '005 to 0'15 of a millimetre 
 in diameter) which occur very generally scattered in the extra- 
 capsular protoplasm of Radiolaria were at one time regarded as 
 essential components of the Radiolarian body. Their parasitic 
 nature is now rendered probable by the observations of Cien- 
 kowski (43), Brandt (44), and Geddes (45), who have established 
 that each cell has a cellulose wall and a nucleus (Fig. XIV. 6 to 13), 
 that the protoplasm is impregnated by chlorophyll which, as in 
 Diatoms, is obscured by the yellow pigment, and that a starch- 
 like substance is present (giving the violet reaction with iodine). 
 Further, Cienkowski showed, not only that the yellow cells multiply 
 by fission during the life of the Radiolarian, but that when isolated 
 they continue to live ; the cellulose envelope becomes softened ; 
 the protoplasm exhibits amoeboid movements and escapes from the 
 envelope altogether (Fig. XIV. 13) and multiplies by fission. 
 Brandt has given the name Zooxanthella nutricola to the parasitic 
 unicellular Alga thus indicated. He and Geddes have shown that a 
 similar organism infests the endoderm cells of Anthozoa and of 
 some Siphonophora in enormous quantities, and the former has been 
 led, it seems erroneously, to regard the chlorophyll corpuscles of 
 Hydra viridis, Spongilla, and Ciliata as also parasitic Algae, for 
 which he has coined the name Zoochlorella. The same arguments 
 which Brandt has used to justify this view as to animal chlorophyll 
 would warrant the creation of a genus " Phytochlorella " for the 
 hypothetical Alga which has hitherto been described as the 
 "chlorophyll corpuscles" of the cells of ordinary green plants. 
 
 Zooxanthella nutricola does not, for some unknown reason, infest 
 the Acanthometridea, and it is by no means so universally present 
 in the bodies of the Silico-skeleta as was supposed before its 
 parasitic nature was recognized. 
 
 The streaming of the granules of the protoplasm has been observed 
 in the pseudopodia of Radiolaria as in those of Heliozoa and 
 Reticularia ; it has also been seen in the deeper protoplasm ; and 
 granules have been definitely seen to pass through the pores of the 
 central capsule from the intracapsular to the extracapsular pro- 
 toplasm. A feeble vibrating movement of the pseudopodia has 
 been occasionally noticed. 
 
 The production of swarm-spores has been observed only in 
 Acanthometra and in the Polycyttaria and Thalassicollida;, and 
 only in the two latter groups have any detailed observations been 
 made. Two distinct processes of swarm-spore production have 
 been observed by Cienkowski (43), confirmed by Hertwig (46) dis- 
 tinguished by the character of the resulting spores which are ] 
 called " crystalligerous " (Fig. XIV. 15) in the one case, and "di- 
 morphous" in the other (Fig. XIV. 16). In both processes the 
 nucleated protoplasm within the central capsule breaks up by a 
 more or less regular cell-division into small pieces, the details of 
 the process differing a little in the two cases. In those individuals 
 which produce erystalligerous swarm-spores, each spore encloses a 
 small crystal (Fig. XIV. 15). On the other hand, in those indi- 
 viduals which produce dimorphous swarm-spores, the contents of 
 the capsule (which in both instances are set free by its natural 
 rupture) are seen to consist of individuals of two sizes " macro- 
 spores" and " microstores," neither of which contain crystals 
 (Fig. XIV. 16). The further development of the spores has not 
 been observed in either ease. Both processes have been observed 
 in the same species, and it is suggested that there is an alternation 
 of sexual and asexual generations, the crystalligerous spores 
 
 developing directly into adults, which in their turn produce in 
 their central capsules dimorphous swarm-spores (macrospores and 
 mierospores), which in a manner analagous to that observed in the 
 Volvocinean Flagellata copulate (permanently fuse) with one 
 another (the larger with the smaller) before proceeding to develop. 
 The adults resulting from this process would, it is suggested, pro- 
 duce in their turn crystalligerous swarm-spores. Unfortunately 
 we have no observations to support this hypothetical scheme of a 
 life-history. 
 
 Fusion or conjugation of adult Radiolaria, whether preliminary 
 to swarm-spore-production or independently of it, has not been 
 observed this affording a distinction between them and Heliozoa, 
 and an agreement, though of a negative character, with the Reticu- 
 laria. 
 
 Simple fission of the central capsule of adult individuals and 
 subsequently of the whole protoplasmic mass has been observed in 
 several instances, and is probably a general method of reproduction 
 in the group. 
 
 The siliceous shells of the Radiolaria are found abundantly in 
 certain rocks. They furnish, together with Diatoms and Sponge- 
 spicules, the silica which has been segregated as flint in the Chalk 
 formation. They are present in quantity (as much as 10 per cent) 
 in the Atlantic ooze, and in the celebrated "Barbados earth" (a 
 Tertiary deposit) are the chief components. 
 
 GRADE B. CORTICATA, Lankester, 1878(64). 
 
 Characters. Protozoa in which the protoplasm of the cell-body, 
 in its adult condition, is permanently differentiated into two layers, 
 an outer denser cortical substance and an inner more fluid medul- 
 lary substance (not to be confused with the merely temporary 
 distinction of exoplasm and endoplasm sometimes noted in 
 Gymnomyxa, which is not structural but due to the gravitation and 
 self-attraction of the coarser granules often embedded in the 
 uniformly fluid protoplasm). 
 
 Since the Corticata have developed from simple Gymnomyxa 
 exhibiting both amoeboid and flagellate phases of form and activity, 
 it results (1) that the forms of the body of many Corticata are 
 traceable to modifications of these primitive forms ; (2) that the 
 young stages of the Corticata are in the lower classes of that group 
 typical flagellulse or amoebulse ; and (3) that there are certain 
 archaic forms included in those lower classes whose position there 
 is doubtful, and which might be with almost equal propriety assigned 
 to the Gymnomyxa, since they are transitional from that lower grade 
 to the higher grade of Corticata. 
 
 CLASS I. SPOROZOA, Leuckart (47); Syn. Gregarinida, Auct. 
 
 Characters. Corticata parasitic in almost all classes and orders of 
 animals, imbibing nutriment from the diffusible albuminoids of 
 their hosts and therefore mouthless. In typical cases there is 
 hatched from a chlamydospore one or more modified nucleate or 
 non-nucleate flagellulse (falciform young, drepanidium phase). 
 The flagellula increases in size and differentiates cortical and 
 medullary substance. Fission is common in the younger stages of 
 growth. The movements now become neither vibratile nor amoe- 
 boid but definitely restrained, and are best described as "eugle- 
 noid" (cf. Flagellata, Fig. XX. 27, 28). The nucleus is single, 
 large, and spherical. No contractile vacuole and rarely any vacuole 
 is present. A size of yVth inch may be attained in this phase, 
 which may be definitely spoken of as the euglena phase corre- 
 sponding to the amoeba phase of Gymnomyxa. It is usually of 
 oblong form, with sac-like contractile wall of cortical substance, 
 but may be spherical (Coccidiidea) or even amoeboid (Myxosporidia). 
 
 Conjugation, followed directly or after an interval by spornlation, 
 may now ensue. The conjugated individuals (two), or sometimes a 
 single individual, become encysted. The contents of the cysts now 
 rapidly divide (by a process the details of which are unknown) into 
 minute ovoid nucleated (?) bodies ; sometimes a portion of the 
 protoplasm is not converted into spores but may form sporodncts 
 (cf. capillitium of Mycetozoa). Each piece acquires a special 
 chitin-like colourless coat, and is then a chlamydospore. Rarely 
 one spore only is formed from the whole contents of a cyst The 
 spore-coat is usually thick, and remarkable for processes and other 
 accessory developments. The included protoplasm of the chlamydo- 
 spore frequently divides into several pieces before hatching. These 
 usually, when set free from the spore-coat, have the form of modified 
 nucleated flagellulse, i.e., flagellul* in which the protoplasm is not 
 drawn out into a thread-like flagellum but exhibits an elongate form, 
 uniformly endowed with vibratile activity. With few (if any) excep- 
 tions, the falciform young thus characterized penetrates a cell of some 
 tissue of its host and there undergoes the first stages of its growth 
 (hence called Cytozoa). In some forms the pre-cystic phase never 
 escapes from its cell host. In other cases it remains connected with 
 the hospitable cell long after it has by growth exceeded by many 
 hundred times the bulk of its quondam entertainer ; often it loses 
 all connexion with its cell host and is carried away to some other 
 part of the infested animal before completing its growth and 
 encysting. 
 
24 
 
 PROTOZOA 
 
 The Sporozoa are divided into four sub-classes, differing from one 
 another according to the form and development attained by the 
 euglena phase. We shall place the most highly developed first, not 
 only because our knowledge about it is most complete, but because 
 it is possible that one at least of the other sub-classes is derived by 
 degeneration from it. 
 
 SUB-CLASS I. Gregarinidea, Butschli (9). 
 
 Characters. Sporozoa in which the euglena phase is dominant, 
 being relatively of large size, elongate in form, definitely shaped, 
 having contractile but not viscid cortex, and exhibiting often active 
 nutritional and locomotor phenomena. Though usually if not 
 invariably cell-parasites in early youth, they become free before 
 attaining adult growth, and inhabit either the body-cavity or the 
 intestine of their hosts. Many spores are produced in the encysted 
 phase. The spores have an oblong, sometimes caudate coat, and 
 produce each one or several falciform young. At present only 
 known as parasites of Invertebrata. 
 
 Flo. XVII. Sporozoa. i, 2. Monocystis ayMs, Stem ; x 250 ; from the testis 
 of the Earthworm. Two phases of movement a ring-like contraction 
 passing along the body from one end to the other. 3. Individual of the 
 
 same species which has penetrated in the young stage a sperm-cell of the 
 Earthworm, and is now clothed as it were with spermatoblasts. 4. 
 
 Monocystis magna, A. Schmidt, from the testis of the Earthworm (L. terres- 
 
 trig, L.). Two individuals, which are implanted by one extremity at 6 in 
 two epithelial cells of the rosette of the spermatic duct, a, nucleus of the 
 Monocystis. 5. Tailed chlamydospores of Monocystis sxnuridis, 
 
 Koll. 6. Two M. agilis encysted, spores forming on the surface of the 
 
 protoplasm. 7. A similar cyst f urtheradvanced in spore-formation (see 
 
 Fig. XVIII.). 8. Spore of M. agilis, now elongated but still naked. 
 a, nucleus. X 1400. 9. The spore has now encased itself in a navicula- 
 
 shaped coat, a, nucleus. 10. The spore protoplasm has now divided 
 
 into several falciform swarm-spores, leaving a portion of the protoplasm 
 unused, b, Schneider's residual core. 11. Optical transverse section of 
 a completed spore, b, Schneider's residual core. 12. Chlamydospore 
 
 of Klossia chitonis, nov. sp., from the liver of Chiton (original.) 13, 
 
 14. Chlamydospore of Monocystis nemertis, Koll., liberating falciform 
 young, b, Schneider's residue. 15. Monocystis pellucida, Koll. (from 
 
 Nereis) ; x 150 ; to show the very thick cortical substance and its fibrilla- 
 tion (after Lankester, 54). 16. Monocystis ssenuridis, Koll., two indivi- 
 duals adhering to one another (a syzygium). For spores see 5. 17. Mono- 
 cystii aphroditx, Lankester (55) ; x 60 ; remarkable among Monocystids 
 for its long proboscis resembling the epimerite of some Septata. 18. 
 
 Klossia helicina, Aim. Sclm., from the kidney of Helix hortensis. A. single 
 cell of the renal epithelium in which a full-grown Klossia is embedded. 
 a, nucleus of the Klossia ; a', nncleus of the renal cell. 19. Cyst of 
 
 Klossia helicina, the contents broken up into spherical chlamydo- 
 spores. 20. Single spore from the last, showing falciform young and a 
 Schneider's residue i. 21. The contents of the same spore. 22. A small 
 renal cell of Helix containing two of the youngest stage of Klossia. 23. 
 Monocystis saf/ittata, Leuck., from the intestine of Capitella capitata', 
 x 100. 24 to 31. Coccidium ovtforme, Leuck., from the liver of the Rabbit: 
 24, adult individual encysted ; 25, the protoplasm contracted a, 
 nucleus ; 26, 27, division into four spores, as yet naked ; 28, 29, the spores 
 have acquired acovering, i.e., are chlamydospores, and each contains a single 
 falciform young ; 30, 31, two views of a Chlamydospore more highly magni- 
 fied so as to show the single falciform young (from Leuckart). 32. Klossia 
 octopiana, Aim. Schn., from Cephalopoda, a, nucleus; b, cyst-membrane, 
 x 200 diam. 33. Single spherical spore of the same ; x 1400 diam ; 
 showing numerous falciform young, and b, Schneider's residue. 34. 
 Myxidium Lieberkutmii, BUtschli, one of the Myxosporidia, from the 
 bladder of the Pike (Esox); creeping euglena phase, showing strongly 
 lobed amoeboid character (pseudopodia and undifferentiated (?) cortex) ; 
 X 60 diam. 35-39. Eimeria falcijormis, Eimer sp., from the Mouse : 
 35, an adult non-encysted individual inhabiting an epithelial cell of the 
 intestine of the mouse ; 36, encysted phase ; 37, clear corpuscles appear 
 in the encysted protoplasm ; 38, the protoplasm now forms a single 
 spore containing several falciform young ; 6, Schneider's residue ; 39, 
 isolated spore showing falciform young, and b, Schneider's residue. 
 40. Chlamydospore of Myxobolus Mulleri, Butschli, one of the Myxo- 
 sporidia from the gills of Cyprinoid Fishes, a, nucleus ; b, refringent 
 corpuscle ; c, polar body or thread-capsule. 41. A similar Chlamydo- 
 spore which has ejected the filaments from its thread capsules. 42. 
 Chlamydospore of a Myxosporidium infesting the kidney of Lota wtlgaris. 
 c, polar body (psorosperm of authors). 43, 44. Chlamydospores of 
 a Myxosporidium from the gills of Perca (psorosperm of authors). 
 Compare with the tailed Chlamydospore of Monocystis sxnuridis, 5. 45 
 -47. Drepanidiutn ranarum, Lankester, the falciform young of an 
 unascertained Coccidiide infesting the Frog (supposed by Gaule to be pro- 
 duced by the blood corpuscles) : 45, specimen stained by iodine ; 46, red- 
 blood corpuscle of Frog, showing b, two contained Drepanidia, and a, the 
 nucleus of the blood corpuscle ; 47, living Drepanidium. 48. Chlamy- 
 dospore of Lieberkiihn's Coccidium of the Frog's kidney, perhaps belong- 
 ing to the life-cycle of Drepanidium ranarum. The spore contains 
 two falciform young (Drepanidia?) and a Schneider's residue. 49. 
 Chlamydospore of Monocystis thalassemie, Lankester, containing nume- 
 rous falciform young. 50, 51. Sarcocystis Miescheri, Lankester: 50, 
 falciform young escaped from chlamydospores ; 61, adult euglena phase 
 inhabiting a striated muscle fibre of the Pig. 
 
 ORDER 1. HAPLOCYTA, Lankester. 
 
 Characters. Gregarinidea in which there is never at any time a 
 partition of the medullary substance into two or more chambers. 
 The euglenoid is always a single contractile sac with, one mass of 
 medullary substance in w r hich Heats the large vesicular transparent 
 nucleus. Spores larger than in the next group, each producing 
 several falciform young. 
 
 Genus unicum. Monocyslis, Stein, 1848. The various generic 
 subdivisions proposed by Aim. Schneider (48), and accepted by 
 Butschli, appear to the present writer to have insufficient characters, 
 and serve to complicate rather than to organize our knowledge of 
 the subject. We do not yet know enough of the sporulation and 
 subsequent development of the various monocystic Gregarinides to 
 justify the erection of distinct genera. 
 
 Monocystis agilis, Stein, Fig. XVII. 1, 2, 3, 6, 7, 8, 9, 10, 11, 
 and Fig. XVIII. is the type. The other species of Monocystis 
 occur chiefly (and very commonly) in marine Annelids, Platyhel- 
 minthes, Gephyrsea, and Tunicata ; not in Arthropoda, Mollusca, 
 nor Vertebrata. The only definite differences which they present 
 of possibly more than specific worth, as compared with M. agilis, 
 are in the form of the chlamydospores, which are sometimes tailed, 
 as in M. s&nuridis (Fig. XVII. 5), and in M. nemertis (Fig. XVII. 
 13) and M. sipunculi, and further also certain differences in the 
 
 fneral form, as for instance the anchor-like M. sagittata (Fig. 
 VII. 23), and the proboscidifcrous M. aphroditie (Fig. XVII. 17). 
 The fine parallel striation of the cuticule in some species (M. 
 scrpulse, &c.) might also be made the basis of a generic or sub- 
 generic group. 
 
 On the whole it seems best to leave all the species for the present 
 in the one genus Monocystis, pending further knowledge. It seems 
 probable that more than one species (at least two, M. agilis and M. 
 magna] infest the common Earthworm. 
 
 ORDER 2. SEPTATA, Lankester. 
 
 Characters. Gregarinidea in which in the adult the medullary 
 substance is separated into two chambers a smaller anterior (the 
 
PROTOZOA 
 
 25 
 
 protomerite) and a larger posterior (the deutomerite), in which lies 
 the nucleus. There is frequently if not always present, either in 
 early growth or more persistent!)', an anterior proboscis-like appen- 
 dage (the epimerite) growing from the protomerite. The epimerite 
 serves to attach the parasite to its host, and may for that purpose 
 carry booklets. It is always shed sooner or later. The phase in 
 which it is present is called a "cephalont," the phase after it has 
 broken off a"sporont" (see Fig. XIX. 22, 23). The spores are 
 smaller than in the preceding group, often very minute, and some- 
 times the cyst is complicated by the formation of sporoducts, and 
 by a kind of " capillitium " of residual protoplasm (Fig. XIX. 2). 
 Spores producing each only a single (?) falciform young. 
 
 Genera. Gregarina, Dufour ; Hoplorhynchus, Von Cams. 
 
 [The numerous genera which have been proposed at different 
 times by Hammerschmidt and others, and more recently by Aime 
 Schneider, appear to the present writer to be unserviceable, owing 
 to the fact that our knowledge is as yet very incomplete. A 
 good basis for generic or family distinctions might probably be 
 found in the greater or less elaboration of the cyst and the forma- 
 tion or not of sporoducts. But of the majority of Septata we do 
 not know the cysts or the history of sporulation ; we merely know 
 that some have simple cysts with complete sporulation leaving no 
 residue of protoplasm, and that others form cysts with double walls 
 and elaborate tubular ducts, whilst a part of the protoplasm is not 
 sporulated but forms a capillitium (Fig. XIX. 2). 
 
 Another possible basis for generic division of the Septata may 
 be found in the characters of the epimerite. This may be present 
 or absent altogether. It may exist only in the young condition or 
 persist until growth is completed. It may be simple, short, 
 elongate, or provided with booklets. The presence of booklets on 
 the epimerite is the only character which at present seems to serve 
 conveniently for generic distinction. With regard to the other 
 points mentioned we are not sufficiently informed, since we know 
 the complete history of development from the young form set free 
 from the spore in only one or two cases.] 
 
 The Septata are found exclusively in the alimentary canals of 
 Arthropoda (Insects, llyriapods, Crustacea, not Arachnida). See 
 Fig. XIX. for various examples of the group. 
 
 FIG. XVIII. Cyst of Monocystis affilis, the common Gregarinide of the 
 Earthworm ; X 750 diam. ; showing ripe chlamydospores and complete 
 absence of any residual protoplasm or other material in the cyst 
 (original). 
 
 SUB-CLASS II. Coccidiidea, Butschli (9). 
 
 Sporozoa in which the euglena phase remains of relatively 
 minute size, of spherical shape and simple egg-cell-like structure. 
 It is not locomotive, but continues, until the cyst is formed, to 
 inhabit a single cell of the host. Many, few, or one single chlamy- 
 dospore are formed in the cyst. One or more falciform young 
 escape from each spore, and exhibit active movements (flagellula- 
 like) leading to a penetration of a tissue-cell by the young form as 
 in Gregarinidea. Many are parasites of Yertebrata. 
 
 OKDER 1. MOJTOSPOREA, Aim. Suhn. 
 
 Characters. The whole content of the cyst forms but a single 
 spore. 
 
 Genus unicum. Ei-meria (in the intestinal epithelium of Triton, 
 Frog, Sparrow, Mouse, and the Myriapods Lithobius and Glomeris, 
 Fig. XVII. 35 to 39). 
 
 2 3 
 
 FIG. XIX. Sporozoa (Septata). 1. Gregarina blattarvm, Siebold, from 
 the intestine of Blatta orientalis ; X 80. A syzygium of two individuals. 
 Each animal consists of a small anterior chamber, the protomerite, and a 
 large posterior chamber, the deutomerite, in which is the nucleus a. 2. 
 Over-ripe cyst of Gregarina blattarum, with thick gelatinous envelope e, 
 and projecting sporoducts d. The spores have been nearly all discharged, 
 but a mass of them still lies in the centre of the cyst 6. The specimen has 
 been treated with dilute KHO, and the granular contents of the cyst 
 dissolved. Around the central mass of spores is rendered visible the net- 
 work of protoplasmic origin in which the ejected spores were embedded. 
 This distinctly resembles in origin and function the capillitium of 
 Mycetozoa (Fig. III.), a, the plasmatic channels leading to the everted 
 sporoducts ; b, the still remaining spores ; c, the proper cyst-wall ; d, the 
 everted sporoducts ; e, the gelatinous envelope. 3. A ripe spore 
 
 (chlamydo spore) of Gregarina blattarum, a long time after its escape 
 from the cyst ; x 1600 diam. 4. Commencing encystment of a syzy- 
 
 gium of G. blattarum. a, protomerite of one individual ; 6, gelatinous 
 envelope ; c, protomerite of the second individual. 5. Three epithelial 
 cells of the mid-gut of Blatta orientalis, into the end of each of which an 
 extremely young Gregarina blattarum has made its way. 6. Further 
 
 development of the young Gregarina ; only the epimerite a is now buried 
 in the substance of the epithelial cell, and this will soon break off and set the 
 Gregarina free. It is now a " cephalont "; it will then become a " sporont." 
 7. Basal part of an everted sporoduct of Gregarina blattarum. a, granu- 
 lar-fibrous mass investing the base of the duct ; b, commencement of the 
 plasmatic channel in the interior of which the sporoduct was produced as 
 an invaginated cuticular formation before its eversion. 8. Gregarina 
 
 gigantea, . Van Ben., from the intestine of the Lobster ; X 150. a, nucleus. 
 
26 
 
 PROTOZOA 
 
 9 Anterior end of the same more highly magnified, a, protomerite ; 6, layer 
 of circular flbrillie lying below the cuticle ; c, cortical substance of the 
 deutomerite ; d, medullary substance of the deutomerite. 10. Two 
 spores of Grcgarina gigantea (after Butschli), showing the very thick coat of 
 the spore. 11-15. Stages in the development of Gregarina gigantea: 11, 
 recently escaped from the spore-coat, no nucleus; 12, still no nucleus, 
 one vibratile and one motionless process; 13, the two processes have 
 divided; one here drawn has developed a nucleus; 14, further growth; 
 15, the deutomerite commences to develop. 16. Cysts of Gregarina, 
 
 gigantea, from the rectum of the Lobster. The double contents are 
 believed by Ed. Van Beneden to be due not to conjugation previous to 
 encystment but to subsequent fission. 17, 18. Gregarina longicoUis, 
 
 Stein, from the intestine of Blaps mortisaga : 17, cephalont phase, with a 
 long proboscis-like epimerite a, attached to the protomerite b; 18, 
 sporont phase, the epimerite having been cast preliminarily to syzygy and 
 encystment. 19. Gregarina Manieri, Aim. Schneider, from the 
 
 intestine of Timarcha tenebricosa, to show the network of anastomosing 
 fibres beneath the cuticle, similar to the annular flbrilla; of G. gigantea 
 shown in 9. 20. Gregarina (Hoplorhynchus) obliyacanthus, Stein, 
 
 from the intestine of the larva of Agrion. Oephalont with spine-crowned 
 epimerite a. 21. Spores of Gregarina oligacantlms. 22, 23. Grega- 
 
 rina (Hoplorhynchus) Dujardini, Aim. Schneider, from the intestine of 
 Lithobiusforficatus : 22, specimen with epimerite a, therefore a " cepha- 
 lont " ; 23, specimen losing its epimerite by rupture and becoming a 
 "sporont." 
 
 ORDER 2. OLIGOSPOREA, Aim. Schn. 
 
 Characters. The cyst-content develops itself into a definite and 
 constant but small number of spores. 
 
 Genus unicum. Coccidium, Leuck. (in intestinal epithelium and 
 liver of Mammals, and some Invertebrates, Figs. XVII. 24 to 31). 
 
 ORDERS. POLYSPOBEA. 
 
 Characters. The cyst-content develops itself into a great num- 
 ber of spores (sixty or more). 
 
 Genus uniuum. Klossia, Aim. Schn. Three species of Klossia 
 are found in Mollusca viz., in Helix, in Cephalopods, and in 
 Chiton. Schneider's genus, Adelea, from Lithobius, appears to 
 belong here. Kloss (49) discovered the parasite of the renal cells of 
 Helix hortemis represented in Fig. XVII. 18, 19, 20, 21, and 22; 
 Schneider that of Cephalopods, Fig. XVII. 32, 33. In Chiton Dr 
 Tovey has discovered a third species with very remarkable spores, 
 which are here figured for the first time (Fig. XVII. 12). 
 
 The Drepanidium Banarum (Fig. XVII. 45, 46, 47), discovered 
 by Lankester (50) in the Frog's blood, is probably the falciform young 
 of a Coccidium parasitic in the Frog's kidney, and discovered there 
 by Lieberkiihn (51). A spore of this Coccidium is shown in Fig. 
 XVII. 48; whilst in 46 two Drepanidia which have penetrated a 
 red-blood corpuscle of the Frog are represented. 
 
 The Polysporous Coccidiidea come very close to the GregariniJe 
 genus Monocystis, from which they may be considered as being 
 derived by an arrest of development. The spores and falciform 
 young of the Coccidiidea are closely similar to those of Monocystis, 
 and the young in both cases penetrate the tissue-cells of their host ; 
 but in Monocystis this is only a temporary condition, and growth 
 leads to the cessation of such "cell-parasitism." On the other 
 hand, growth is arrested in the Coccidiidea, and the organism is 
 permanently a cell-parasite. 
 
 Since the parasitism is more developed in the case of a cell-para- 
 site than in the case of a parasite which wanders in the body cavity, 
 it seems probable that the Coccidiidea have been derived from the 
 Gregarinidea rather than that the reverse process has taken place. 
 
 SUB-CLASS III. Myxoeporidia, Butschli. 
 
 Characters. Sporozoa in which the euglena-phase is a large 
 multinucleate amoeba-like organism (Fig. XVII. 34). The cysts 
 are imperfectly known, but appear to be simple ; some attain a 
 diameter of two lines. The spores are highly characteristic, having 
 each a thick coat which is usually provided with a bifurcate process 
 or may have thread capsules (like nematocysts) in its substance 
 (Fig. XVII. 40, 41, 42, 43, 44). 
 
 The spores contain a single nucleus, and are not known to produce 
 falciform young, but in one case have been seen to liberate an 
 amoebula. The further development is unknown. The Myxo- 
 sporidia are parasitic beneath the epidermis of the gills and fins, and 
 in the gall-bladder and urinary bladder of Fishes, both freshwater 
 and marine. 
 
 Genera. Myxidium, Butschli (Pike, Fig. XVII. 34); Myxobolus, 
 Butschli (Cyprinoids) ; Lithocystis, Giard (the Lamellibranch Echino- 
 cardium). 
 
 The Myxosporidia are very imperfectly known. They present 
 very close affinities to the Mycetozoa, and are to be regarded as a 
 connecting link between the lower Gymnomyxa and the typical 
 Sporozoa. Possibly their large multinucleate amceba phase is a 
 plasmodium formed by fusion of amcebula? set free from spores, 
 though it is possible that the many nuclei are the result of a division 
 of an original single nucleus, preparatory to sporulation. 
 
 Their spores are more elaborate in structure than those of any 
 other Protozoa, and are more nearly paralleled by those of some 
 species of Monocystis than by those of Mycetozoa. The thread- 
 capsules of the spores are identical in structure with those of 
 Hydrozoa, and probably serve as organs of attachment, as do the 
 furcate processes of the spore-case. It is not certain that a definite 
 
 cyst is always or ever formed, but as occurs rarely in some Gregari- 
 nidea, the spores may be formed in a non encysted amoeba form. 
 
 Although pseudopodia, sometimes short and thread-like, have been 
 observed in the amosba phase, yet it is also stated that a distinction 
 of cortical and medullary substance obtains. 
 
 The " psorosperms " of J. Miiller are the spores of Myxosporidia. 
 
 SUB-CLASS IV. Sarcocystidia, Butschli. 
 
 (This division is formed by Butschli for the reception of Sarco- 
 cystis, parasitic in the muscular fibres of Mammals, and of Amcebi- 
 dium, parasitic in Crustacea. Both are very insufficiently known, 
 but have the form of tubular protoplasmic bodies in which numer- 
 ous ovoid spores are formed from wnich falciform young escape. ) 
 
 Genera. Sarcocystis, Lankester ; Amcebidium, Cienkowski (52). 
 Sarcocystis(Fig. XVII. 50, 51, S. Miescheri, Lank.), was first observed 
 by Miescher in the striated muscle-fibres of the Mouse ; then by 
 Rainey in a similar position in the Pig, and taken by him for the 
 youngest stage in the development of the cysts of Ttenia solium ; 
 subsequently studied by Beale and others in connexion with the 
 cattle-plague epidemic, and erroneously supposed to have a causal 
 connexion with that disease. It is common in healthy butcher's 
 meat. See Leuckart (47). 
 
 Further remarks on the Sporozoa. The Sporozoa contrast 
 strongly with the large classes of Gymnomyxa, the Heliozoa, 
 Reticularia, and Radiolaria, as also with the Ciliate and Tentaculi- 
 ferous Corticata, by their abundant and rapidly recurrent forma- 
 tion of spores, and agree in this respect with some Proteomyxa, 
 with Mycetozoa, and some Flagellata. Their spores are remark- 
 able for the firm, chitin-like spore-coat and its varied shapes, 
 contrasting with the cellulose spherical spore-coat of Mycetozoa 
 and with the naked spores of Radiolaria and Flagellata. 
 
 The protoplasm of the more highly developed forms (Gregarini- 
 dea) in the euglenoid phase exhibits considerable differentiation. 
 Externally a distinct cuticle may be present, marked by parallel 
 rugie (Monocystis serpulse) or by fine tubercles (Monocystis sipun- 
 culi). A circlet of hooks may be formed by the cuticle at one end 
 of the body. Below the cuticle is sometimes developed a layer of 
 fibrils running transversely to the long axis of the body (Fig. 
 XIX. 9 and 19), which have been regarded as contractile, but are 
 probably cuticular. The cortical layer of protoplasm below these 
 cuticular structures is dense and refringent and sometimes fibril- 
 lated (Monocystis pclludda, Fig. XVII. 15). It is the contractile 
 substance of the organism, and encloses the finely granular more 
 liquid medullary substance. The granules of the latter have been 
 shown by Butschli (9) to give a starch-like reaction with iodine, 
 &c. Probably the protoplasm in which they lie is finely reticulate 
 or vacuolar, and when the granules are few it is actually seen to be 
 so. No contractile vacuole is ever present. In Myxosporidia the 
 medullary protoplasm is coloured yellow by haematoidin derived 
 from the blood of its host or by absorbed bile-pigment, and also 
 contains small crystals. 
 
 The nucleus of the Gregarinidea is a large clear capsule, with a 
 few or no nucleolar granules. It Las never been seen in a state 
 of division, and it is not known what becomes of it during sporula- 
 tion, though sporulating Gregarinidea have been observed with 
 many minute nuclei scattered in their protoplasm, presumably 
 formed by a breaking up of the single nucleus. 
 
 The habit of attaching themselves in pairs which is common in 
 Gregarinidea is perhaps a reminiscence of a more extensive forma- 
 tion of aggregation plasmodia (compare Mycetozoa). The term 
 "syzygium" is applied to such a conjunction of two Gregarinidea ; 
 it is not accompanied by fusion of substance. The formation of 
 cysts is not connected with this pairing, since the latter occurs in 
 young individuals long before encystment. Also cysts are formed 
 by single Gregarinidea, as is always the case in the non-motile 
 Coccidiidea. 
 
 The encystment always leads to the formation of spores, but in 
 rare cases sporulation has been observed in unencysted Gregarini- 
 dea, and it occurs perhaps normally without true cyst-formation in 
 the Myxosporidia. 
 
 The cell-parasitism of the young Sporozoa, and their flagellula- 
 like (falciform) young and active vibratile movement, are points 
 indicating affinity with the lower Gymnomyxa, and especially with 
 those Proteomyxa, such as Vampyrella and" Plasmodiophora, which 
 are cell- parasites. Indeed it is probable that we have in this fact 
 of cell-parasitism, and especially of parasitism in animal cells, a 
 basis for the theoretical association of several unicellular organisms. 
 The Haplococcus of Zopf (regarded by him as a Mycetozoon) is 
 parasitic in the muscular cells of the Pig, and is probably related 
 to Sarcocystis. Recently Von Lendenfeld (53) has described in 
 Australia an amffiba-like organism as parasitic in the skin of Sheep, 
 which will probably be found to be either a Sporozoon or referable 
 to those parasitic spore-producing Proteomyxa which are separated 
 from Sporozoa only by their negative characters (see previous 
 remarks on the negative characters of Proteomyxa). 
 
 The application of the name "Gregarines" has sometimes been 
 
PROTOZOA 
 
 27 
 
 made erroneously to external parasitic organisms, which have 
 nothing in common with the Sporozoa, This was the case in regard 
 to a fungoid growth in human hair the so-called "chignon 
 Gregarine. " The Silk-worm disease known as "pebrine" has also 
 been attributed to a Gregarine. It seems probable that the parasitic 
 organism which causes that disease is (as is also the distinct parasite 
 causing the disease known as " flaccidezza " in the same animals) 
 one of the Sehizomycetes (Bacteria). No disease is known at 
 present as due to Sporozoa, although (e.g. , the Klossia chitonis) 
 they may lead to atrophy of the organs of the animals which they 
 infest, in consequence of their enormous numbers. Coccidia and 
 Sarcocystis are stated to occur in Man. 
 
 CLASS II. FLAGELLATA, 1 Ehrenberg. 
 
 Characters. Corticata in which the dominant phase in the life- 
 history is a corticate flagellula, that is, a nucleated cell-body pro- 
 vided with one or a few large processes of vibratile protoplasm. 
 Very commonly solid food particles are ingested through a distinct 
 cell mouth or aperture in the cortical protoplasm, though ill some 
 an imbibition of nutritive matter by the whole surface and a nutri- 
 tional process chemically resembling that of plants (holophytic), 
 chlorophyll being present, seems to occur. 
 
 Conjugation followed by a breaking up into very numerous minute 
 naked spores is frequent in some ; as also a division into small 
 individuals (microgonidia), which is followed by their conjugation 
 with one another or with big individuals (inacrogonidia) and subse- 
 quent normal growth and binary fission. 
 
 Many have a well-developed cuticle, which may form collar-like 
 outgrowths or stalk-like processes. Many produce either gelatinous 
 or chitin-like shells (cups or coanoecia), which are connected so as to 
 form spherical or arborescent colonies ; in these colonies the proto- 
 plasmic organisms themselves produce new individuals by fission, 
 which separate entirely from one another but are held together by 
 the continuity, with those already existing, of the new shells or 
 jelly-houses or stalk-like supports produced by the new individuals. 
 A single well-marked spherical nucleus, and one or more contractile 
 vacuoles, are always present in the full-grown form. 
 
 Often, besides ingested food-particles, the protoplasm contains 
 starch granules (amylon nucleus), paramylum corpuscles, chromato- 
 phors and chlorophyll corpuscles, some of which may be so abundant 
 as to obscure the nucleus. One or two pigment spots (stigmata or 
 so-called eye-spots) are often present at the anterior end of the body. 
 
 SUB-CLASS I. Lissoflagellata, Laukester. 
 
 Never provided with a collar-like outgrowth around the oral 
 pole. 
 
 ORDER 1. MONADIDEA, Biitschli. 
 
 Characters. Lissoflagellata of small or very small size and 
 simple structure ; often naked and more or less anueboid, sometimes 
 forming tests. Usually colourless, seldom with chromatophors. 
 With a single anterior large flugellum or sometimes with two 
 additional paraflagella. A special mouth-area is often wanting, 
 sometimes is present, but is never produced into a well-developed 
 pharynx. 
 
 Fam. 1. RHIZOMASTIGINA, Biitschli. Simple mouthless forms 
 with 1 to 2 fiagella; either permanently exhibiting a Gymnomyxa- 
 like development of pseudopodia or capable of passing suddenly 
 from a firm-walled into a Gymnomyxa-like condition, when the 
 flagclla may remain or be drawn in. Ingestion of food by aid of 
 the pseudopodia. 
 
 Genera. Mastigamcelia, F. E. Schultze; Ciliophrys, Cienkowski 
 (65) ; Dimorpha, Gruber ; Aclinomonas, Kent ; Trypanosoma, Gruby 
 (parasitic in the blood of Frogs and other Amphibia and Reptiles, 
 Fig. XX. 21, 22). The Rhizomastigina might all be assigned to 
 the Proteomyxa, with which they closely connect the group of 
 Flagellata. The choice of the position to be assigned to such a 
 form as Ciliophrys must be arbitrary. 
 
 Fam. 2. CERUOMONADINA, Kent. Minute oblong cell-body 
 which posteriorly may exhibit amceboid changes. One large 
 anterior flagellum. Mouth at the base of this organ. Reproduc- 
 tion by longitudinal fission and by multiple fission producing 
 spores in the encysted resting state. 
 
 Genera. Cercomonrts, Duj. (Fig. XX. 32, 33); Herpetomonas, S. 
 Kent; Oikomonas, Kent ( = Monas, James Clark; Pseitdospora, 
 Cienkowski, Fig. XX. 29, 30, 31) ; Ancyronwnas, S. K. 
 
 Fam. 3. CODON<ECINA, Kent. Small colourless monads similar 
 to Oikomonas in structure, which secrete a fixed gelatinous or 
 membranous envelope or cup. 
 
 Genera. Codonosca, James Clark; Platythceca, Stein. 
 
 Fam. 4. BIKCECINA, Stein. Distinguished from the last family 
 by the fact that the monad is fixed in its cup by a contractile 
 thread-like stalk ; cup usually raised on a delicate stalk. 
 
 Genera. Bicososca, 3. Cl. ; Poteriodendron, Stein. 
 
 i Butschli's wovk (9) has been pretty closely followed in the diagnosis of the 
 groups of Flagellata and the enumeration of genera here given. 
 
 FIG. XX. Flagellata. 1. Chlamydomonas pulvisculus, Ehr. (=Zygoselmis, 
 From.) ; one of the Phytomastigoda ; free-swimming individual, a, nucleus ; 
 b, contractile vacuole ; e, starch corpuscle ; d, cellulose investment ; 
 e, stigma (eye-spot). 2. nesting stage of the same, with fourfold 
 
 division of the cell-contents. Letters as before. 3. Breaking up of 
 
 the cell-contents into minute biflagellate swarm-spores, which escape, 
 and whose history is not further known. 4. Syncrypta volcox, Ehr. ; 
 
 one of the Phytomastigoda. A colony enclosed by a common gelatinous 
 test c. a, stigma; 6, vacuole (non-contractile). 5. Uroglena rolvox, 
 
 Ehr. ; one of the Monadidea. Half of a large colony, the flagellates 
 embedded in a common jelly. 6. Chlorogonium euchlorum, Ehr. ; 
 
 one of the Phytomastigoda. a, nucleus ; b, contractile vacuole ; c, starch 
 grain ; d, eye-spot. 7. Chlorogonium euchlorum, Ehr., one of the 
 
 Phytomastigoda. Copulation of two liberated microgonidia. a, nucleus; 
 6, contractile vacuole ; d, eye-spot (so-called). 8. Colony of Dinobryon 
 
 sertularia, Ehr. ; x 200 ; one of the Monadidea. 9. tlxmaio- 
 
 coccus palustris, Girod (= CMamydococms, Braun, Protococcus Cohn), 
 one of the Phytomastigoda ; ordinary individual with widely separated 
 test, a, nucleus ; &, contractile vacuole ; c, amylon nucleus (pyreuoid). 
 10. Dividing resting stage of the same, with eight fission products in 
 the common test e. 11. A microgonidium of the same. 12. 
 
 Phalansterittm consociatum, Cienk., one of the Choanoflagellata ; 
 x 325. Disk-like colony. 13. Euglena mridie, Ehr. ; x 300 ; one of 
 
 the Euglenoidea, a, pigment spot (stigma) ; b, clear space ; c, paramylum 
 granules; d, chromatophor (endochrome plate). 14. Goniumpectorale, 
 
 O. F. Muller ; one of the Phytomastigoda. Colony seen from the flat side, 
 x 300. a, nucleus : b, contractile vacuole ; c, amylon nucleus. 15. 
 
 Dinobryon sertularia, Ehr. ; one of the Monadidea. a, nucleus ; b, con- 
 
28 
 
 PROTOZOA 
 
 tractile vacuole ; e, amylon -nucleus ; d, free colourless flagellates, probably 
 not belonging to Dinobryon ; e, stigma (eye-spot); /, chromatophors. 
 16. Peranema trichophormn, Ehr., (one ol the Euglenoidea), creeping 
 individual seen from the back ; x 140. a, nucleus ; b, contractile 
 vacuoles ; c, pharynx ; d, mouth. 17. Anterior end of Euglena acus, 
 
 Ehr., in profile, a, mouth ; 6, contractile vacuoles ; c, pharynx ; d, stigma 
 (eye-spot); e, paramylum-body ; /, chlorophyll corpuscles. 18. Part of 
 
 the surface of a colony of Volvox globatur, L. (Phytomastigoda), showing 
 the intercellular connective fibrils, a, nucleus ; b, contractile vacuole ; 
 c, amylum granule. 19. Two microgonidia of Volvox globator, L. a, 
 
 nucleus ; b, contractile vacuole. 20. Ripe asexually produced 
 
 daughter-individual of Volvox minor, Stein, still enclosed in the cyst 
 of the partheno-gonidium. o, young parthenp-gonidia. 21, 22. 
 
 Trypanosoma tangwinii, Gruby ; one of the Rhizomastigina, from the 
 blood of Rana esmlenta. a, nucleus. X 500. 23-26. Repro- 
 
 duction of Bodo caudatus, Duj. (one of the Heteromastigoda), after Dallin- 
 ger and Drysdale : 23, fusion of several individuals (plasniodium) ; 24, 
 encysted fusion-product dividing into four ; 25, later into eight ; 26, cyst 
 filled with swarm-spores. 27. Astasia tenax, O. F. Mull. (Proteus) ; one of 
 the Euglenoidea ; x 440. Individual with the two fiagella, and strongly 
 contracting hinder region of the body, a, nucleus ; b, contractile vacuole, 
 close to the pharynx. 28. The same devoid of flagella. a, nucleus ; 
 
 c, c, the two dark pigment spots (so-called eyes) near the mouth. 29. 
 
 Oikomonas termo (Monas termo) Ehr. ; one of the Monadidea. a, nucleus ; 
 6, contractile vacuole ; c, food-ingesting vacuole ; d, food-particle. X 440. 
 30. The food-particle d has now been ingested by the vacuole. 81. 
 Oikomonas mutabilis, Kent (Monadidea), with adherent stalk, a, nucleus ; 
 b, contractile vacuole ; c, food-particle in food vacuole. 32, 33. Cerco- 
 monas crassicauda, Duj. (Monadidea), showing two conditions of the 
 pseudopodium-protruding tail, a, nucleus ; b, contractile vacuoles ; c, 
 mouth. 
 
 Fam. 5. HETERCMONADIN A, Butschli. Small colourless or green 
 monads which possess, besides one chief flagellum, one or two smaller 
 paraflagella attached near it, often forming colonies secreting a 
 common stalk. 
 
 Genera. Monas (Ehr.), Stein; Dendromonas. Stein: Cephalo- 
 thamnium, Stein ; Anthophysa, Bory d. Vine. (Fig. XXI. 12, 13); 
 Dinobryon, Ehr. (Fig. XX. 8 and 15) ; Epipyxis, Ehr. ; Uroglena, 
 Ehr. (Fig. XX. 5). 
 
 ORDER 2. EUGLENOIDEA, Butschli. 
 
 Characters. Generally somewhat large and highly developed 
 monoflagellate forms, of mouaxonic or slightly asymmetrical 
 build. Cuticle present ; cortical substance firm, contractile, and 
 elastic ; some forms quite stiff, others capable of definite annular 
 contraction and worm-like elongation. At the base of the flagellum 
 a small or large mouth leading into a more or less distinct 
 pharyngeal tube. Near this is always the contractile vacuole. 
 Rarely a pair of flagella instead of one. 
 
 Fam. 1. COZLOMONADINA. Coloured Euglenoidea, with numer- 
 ous small chlorophyll corpuscles or 1 to 2 large plate-like green or 
 brown chromatophors. Mouth and pharynx inconspicuous ; nutri- 
 tion probably largely vegetal (holophytic). 
 
 Genera. Caelomonas, Stein ; Gonyostomum, Dies. ; Vacuolaria, 
 Cienk. ; Microglena, Ehr. ; Chromuliiia, Cienk. ; Cryploglena, Ehr. 
 
 Fam. 2. EUGLENINA, Stein. Body monaxonic, elongated, hinder 
 end pointed. Spirally striated cuticle. A fine mouth-aperture 
 leads into the well-developed tubular pharynx. Flagellum usually 
 single, sometimes paired, often cast off. Near the pharynx is the 
 ' ' reservoir " of the contractile vacuoles and several of the latter. 
 A single (sometimes two) stigma or colour-speck near the same 
 spot. Chromatophors nearly always present, generally bright 
 green. A large nucleus in the middle of the body. Multiplication 
 by longitudinal fission. Encysted condition and attendant fission 
 imperfectly studied. Copulation doubtful. 
 
 Genera. (a) With flexible cuticle -.Euglena, Ehr. (Fig. XX. 13, 
 17 ; this is probably Priestley's "green matter," from which he 
 obtained oxygen gas ; though one of the very commonest of all 
 Protozoa, its life-history has yet to be worked out) ; Colacium, 
 Ehr. ; Eutreptia, Perty. 
 
 (b) With stiff, shell-like cuticle : Ascoglena, Stein ; Trachclo- 
 monas, Ehr. ; Lepocinclis, Perty ; Pliacus, Nitzsch. 
 
 Fam. 3. MEXOIDINA, Butschli. Similar to the Euglenina, but 
 devoid of chlorophyll, a deficiency connected with the saprophytic 
 mode of life. Stigma always absent. 
 
 Genera. (a) With flexible cuticle : Astasiopsis, Butschli ; Asta- 
 siodes, Biitschli. 
 
 (b) With stiff cuticle and non-contractile body : Monoidium, 
 Perty ; Alractonema, Stein ; Rhabdomonas, Fresenius. 
 
 Fam. 4. PEHANEMINA. Very contractile (metabolic) colourless 
 Euglenoids. Mouth and pharynx large ; inception of solid nutri- 
 ment certainly observed. 
 
 Genera. Peranema, Duj. (Fig. XX. 16) ; Urceolus, Meresch. 
 
 Fam. 5. PETALOMONADINA. Colourless, non-metabolic forms. 
 Mouth opening at the base of the single large flagellum. 
 
 Genera. Petalomonas, Stein. 
 
 Fam. 6. ASTASINA. Colourless, metabolic, or stiff Euglenoids, 
 differing from the rest in having a small or large paraflagellum in 
 addition to the chief one. Nutrition partly saprophytic partly 
 animal. 
 
 Genera. Astasia, Ehr. emend. Stein (Fig. XX. 27, 28) ; Eetero- 
 nema, Duj. ; Zygosdmis, Duj. ; Sphenomonas, Stein ; Tropido- 
 scyphus, Stein. 
 
 ORDER 3. HETEROMASTIGODA, Butschli. 
 
 Characters. Small and large monads. Naked and even amoeboid 
 or with stiff cuticle. Two flagella at the anterior end differing in 
 size : the smaller directed forwards subserves the usual locomotor 
 function ; the larger is directed backwards and trailed, without 
 movement. Sometimes two backwardly directed flagella are present. 
 Always a mouth and animal nutrition. Always colourless. 
 
 Fam. 1. BODONINA, Butschli. Size of the two flagella not very 
 different. 
 
 Genera. Bodo, Ehb. emend. Stein (Fig. XX. 23 to 26, and Fig. 
 XXI. 10 ; the hooked monad and the springing monad of Dai- 
 linger and Drysdale (66) ; ffeteromita of Dujardiu and Kent); 
 Phyllomitus, Stein ; Colponema, Stein ; Dallingeria, Kent ; Tri- 
 mastix, Kent. 
 
 Fam. 2. ANISONEMINA, Kent. Large forms with cuticle ; differ- 
 ence of the two flagella considerable. Mouth, pharynx, and animal 
 nutrition. 
 
 Genera. Anisonema, Duj. ; Entosiphon, Stein. 
 
 ORDER 4. ISOMASTIGODA, Butschli. 
 
 Characters. Small and middle-sized forms of monaxonic rarely 
 bilateral shape. Fore-end with 2, 4, or seldom 5 equal-sized and 
 similar flagella. Some are coloured, some colourless ; naked or 
 with strong cuticle or secreting an envelope. Mouth and pharynx 
 seldom observed ; nutrition generally holophytic (i.e., like a green 
 plant), but in some cases, nevertheless, holozoie (i.e., like a typical 
 animal). 
 
 Fam. 1. AMPHIMONADINA. Small, colourless, biflagellate Iso- 
 mastigoda. 
 
 Genera. Amphimonas, Duj. (? Pseudospora, Cienk.). 
 
 Fam. 2. SPONQOMONADINA, Stein. Small colourless oval forms 
 with two closely contiguous flagella. Chief character in the union 
 of numerous individuals in a common jelly or in branched gelatinous 
 tubes, the end of each of which is inhabited by a single and distinct 
 individual. 
 
 Genera. Spongomonas, Stein; Cladomonas, Stein; Shipido- 
 monas, Stein. 
 
 [Group Phytomastigoda, Butschli. The following three families, 
 viz., Chrysomonadina, Chlamydomonadina, and Volvocina, are so 
 closely related to one another as to warrant their union as a sub- 
 order. They are typical Isomastigoda, but have chlorophyll 
 corpuscles and holophytic nutrition with correlated deficient 
 mouth and pharynx. They are usually regarded by botanists as 
 belonging to the unicellular Algae.] 
 
 Fam. 3. CHRYSOMONADINA, Butschli. Single or colony-forming ; 
 seldom an envelope. Spherical free-swimming colonies may be 
 formed by grouping of numerous individuals around a centre. 
 With two or rarely one brown or greenish brown chromatophor; 
 a stigma (eye-speck) at the base of the flagella. 
 
 Genera. Slylochrysalis, Stein; Chrysopyxis, Stein; Nephrosel- 
 mis, Stein ; Synura, Ehr. ; Syncrypta, Ehr. (Fig. XX. 4). 
 
 Fam. 4. CHLAMYDOMONADINA. Fore-end of the body with two 
 or four (seldom five) flagella. Almost always green in consequence 
 of the presence of a very large single chromatophor. Generally a 
 delicate shell-like envelope of membranous consistence. 1 to 2 
 contractile vacuoles at the base of the flagella. Usually one eye- 
 speck. Division of the protoplasm within the envelope may pro- 
 duce four, eight, or more new individuals. This may occur in the 
 swimming or in a resting stage. Also by more continuous fission 
 microgonidia of various sizes are formed. Copulation is frequent. 
 
 Genera. Hymcnomonas, Stein ; Chlorangium, Stein ; Chloro- 
 goniuin, Ehr. (Fig. XX. 6, 7) ; Polytoina, Ehr. ; Chlamydomonas, 
 Ehr. (Fig. XX. 1, 2, 3); Hsemalococcus, Agardh ( = Chlamydo- 
 coccus, A. Braun, Stein ; Protococcus, Colin, Huxley and Martin ; 
 Chlainydonwnas, Cienkowski); Carteria, Diesing; Spondylomorum, 
 Ehr. ; Coccomonas, Stein ; Phacotus, Perty. 
 
 Fam. 5. VOLVOCINA. Colony-building Phytomastigoda, the cell- 
 individuals standing in structure between Chlamydomonas and 
 Haamatococcus, and always biflagellate. The number of individuals 
 united to form a colony varies very much, as does the shape of the 
 colony. Reproduction by the continuous division of all or of only 
 certain individuals of the colony, resulting in the production of a 
 daughter colony (from each such individual). In some, probably 
 in all, at certain times copulation of the individuals of distinct 
 sexual colonies takes place, without or with a differentiation of the 
 colonies and of the copulating cells as male and female. The 
 result of the copulation is a resting zygospore (also called zygote or 
 oo-spermospore or fertilized egg-cell), which after a time develops 
 itself into one or more new colonies. 
 
 Genera. Gonium, 0. F. Miiller (Fig. XX. 14) ; Stephanosphsera, 
 Cohn ; Pandorina, Bory de Vine. ; Eudorina, Ehr. ; Volvox, 
 Ehr. (Fig. XX. 18, 20). 
 
 [The sexual reproduction of the colonies of the Volvocina is one 
 of the most important phenomena presented by the Protozoa. In 
 some families of Flagellata full-grown individuals become amoeboid, 
 fuse, encyst, and then break up into flagellate spores which develop 
 
PROTOZOA 
 
 29 
 
 simply to the parental form (Fig. XX. 23 to 26). In the 
 Chlamydomonadina a single adult individual by division produces 
 small individuals, so-called "microgonidia." These copulate with 
 one another or with similar microgonidia formed by other adults 
 (as in Chlorogonium, Fig. XX. 7) ; or more rarely in certain 
 genera a microgonidium copulates with an ordinary individual 
 (maerogonidium). The result in either case is a " zygote," a cell 
 formed by fusion of two which divides in the usual way to produce 
 new individuals. The microgonidium in this case is the male 
 element and equivalent to a spermatozoon ; the maerogonidium is 
 the female and equivalent to an egg-cell. The zygote is a fertilized 
 egg-cell, or oo-spermospore. In the colony-building forms we find 
 that only certain cells produee by division microgonidia ; and, 
 regarding the colony as a multicellular individual, we may consider 
 these cells as testis-cells and their microgonidia as spermatozoa. 
 In some colony-building forms the microgonidia copulate with 
 ordinary cells of the colony which, when thus fertilized, become 
 encysted as zygotes, and subsequently separate and develop by 
 division into new colonies. In Volvox the macrogonidia are also 
 specially -formed cells (not merely any of the ordinary vegetative 
 cells), so that in a sexually ripe colony we can distinguish egg- 
 cells as well as sperm mother-cells. Not only so, but in some 
 instances (Eudorina and some species of Volvox) the colonies which 
 produce sexual cells can not merely be distinguished from the 
 asexual colonies (which reproduce parthenogenetically), but can be 
 distinguished also inter se into male colonies, which produce from 
 certain of their constituent cell-units spermatozoa or microgonidia 
 only, and female colonies which produce no male cells, but only 
 macrogonidia or egg-cells which are destined to be fertilized by 
 the microgouidia or spermatozoa of the male colonies. 
 
 The differentiation of the cell-units of the colony into neutral or 
 merely carrying cells of the general body on the one hand and 
 special sexual cells on the other is extremely important. It places 
 these cell-colonies on a level with the Enterozoa (Metazoa) in 
 regard to reproduction, and it cannot be doubted that the same 
 process of specialization of the reproductive function, at first com- 
 mon to all the cells of the cell-complex, has gone on in both 
 cases. The perishable body which carries the reproductive cells is 
 nevertheless essentially different in the two cases, in the Volvocina 
 being composed of equipollent units, in the Enterozoa being com- 
 posed of units distributed in two physiologically and morphologi- 
 cally distinct layers or tissues, the ectoderm and the endoderm. 
 
 The sexual reproduction of the Vorticellidse may be instructively 
 compared with that of the Phytomastigoda ; see below.] 
 
 Fam. 6. TETRAMITINA. Symmetrical, naked, colourless, some- 
 what ama;boid forms, with four flagella or three and an undulating 
 membrane. Nutrition animal, but mouth rarely seen. 
 
 Genera. Collodictyon, Carter ; Tetramitus, Perty (Fig. XXI. 
 11, 14 ; calycine monad of Dallinger and Drysdale (66)) ; Monocerco- 
 monaa, Grassi ; Trichomonas, Donne ; Trichomaslix, Blochmann. 
 
 Fam. 7. POLYMASTIOINA. Small, colourless, symmetrical forms. 
 Two flagella at the hinder end of the body and two or three on each 
 side in front. Nutrition animal or saprophytic. 
 
 Genera. Hexamitus, Duj. (Fig. XXI. 5) ; Megastoma, Grassi ; 
 Polymastix, Biitschli. 
 
 Fam. 8. TREPOMONADINA, Kent. As Polymastigina, but the 
 lateral anterior flagella are placed far back on the sides. 
 
 Genera. Trepomonas, Duj., described recently without name by 
 Dallinger (67). 
 
 Fam. 9. CRYPTOMOXADINA. Coloured or colourless, laterally 
 compressed, asymmetrical forms ; with two very long anterior 
 flagella, placed a little on one side springing from a deep atrium- 
 like groove or furrow (cf. Dinoflagellata and Noctiluca, to which 
 these forms lead). 
 
 Genera. Cyathomonas, From. ; Chilomonas, Ehr. ; Cryptommas, 
 Ehr. ; Oxyrrhis, Duj. 
 
 Fam. 10. LOPHOMONADINA. A tuft of numerous flagella anteriorly. 
 
 Genus. Lophomonas, Stein (Fig. XXI. 9, connects the Flagel- 
 lata with the Peritrichous Ciliata). 
 
 Sub-class II. Choanoflagellata, Saville Kent. 
 
 Flagellata provided with an upstanding collar surrounding the 
 anterior pole of the cell from which the single flagelium springs, 
 identical in essential structure with the "collared cells " of Sponges. 
 Single or colony-building. Individuals naked (Codosiga), or inhabit- 
 ing each a cup (Salpingceca), or embedded in a gelatinous common 
 investment (Proterospongia). 
 
 ORDER 1. NUDA, Lankester. 
 
 Cltaracters. Individuals naked, secreting neither a lorica (cup) 
 nor a gelatinous envelope. 
 
 Genera. Monosiga, S. Kent (solitary stalked or sessile) ; Codo- 
 siga, James Clark (united socially on a common stalk or pedicle, 
 Fig. XXI. 3, 4) ; Astrosiga, S. Kent ; Desmarella, S. Kent. 
 
 ORDER 2. LORICATA, Lankester. 
 
 Characters. Each individual collared-cell unit secretes a horny 
 cup or shell. 
 
 FIG. XXI. Flagellata. 1. Salpingaeca fusiformis, S. Kent ; one of the 
 Choanoflagellata. The protoplasmic body is drawn together within the 
 goblet-shaped shell, and divided into numerous spores, x 1500. 2. 
 Escape of the spores of the same as monoflagellate and swarm-spores. 
 3. Codosiga umbellata, Tatem ; one of the Choanoflagellata ; adult colony 
 formed by diehotomous growth ; x 625. 4. A single zooid of the same ; 
 x 1250. a, nucleus ; 6, contractile vacuole ; c, the characteristic " collar" 
 formed by cuticle on the inner face of which is a most delicate network of 
 naked streaming protoplasm. 5. Hexamita inflata, Duj. ; one of the 
 
 Isomastigoda ; x 650 ; normal adult; showing o, nucleus, and ft, contrac- 
 tile vacuole. 6, 7. Salpingoeca urceolata, S. Kent ; one of the Choano- 
 flagellata ; 6, with collar extended ; 7, with collar retracted within the 
 stalked cup. a, nucleus ; b, contractile vacuole. 8. Polytoma uvella, 
 Mull. sp. ; one of the Phytomastigoda. a, nucleus ; b, contractile vacuole. 
 x 800. 9. Lophomonas blattarttm, Stein ; one of the Isomagtigoda, 
 from the intestine of Blatta orientalis. a, nucleus. 10. Bodo lens, Mull. ; 
 one of the Heteromastigoda; x 800. a, nucleus; b, contractile vacuole ; 
 the wavy filament is a flagelium, the straight one is an immobile trailing 
 thread. 11. Tetramitus suhatus, Stein; one of the Isomastigoda ; X430. 
 a, nucleus; 6, contractile vacuole. 12. Anthophysa vegetans, O. F. 
 MUller ; one of the Monadidea ; x 300. A typical, erect, shortly-branching 
 colony stock with four terminal monad-clusters. 13. Monad cluster of 
 the same in optical section (x 800), showing the relation of the 
 individual monads or flagellate zooids to the stem a. 14. Tetramitus 
 rostratus, Perty ; one of the Isomastigoda ; x 1000. a, nucleus ; b, con- 
 tractile vacuole. 15. Proterospongia Haeckeli, Saville Kent ; one of 
 the Choanoflagellata; x 800. A social colony of about forty flagellate 
 zooids. a, nucleus; b, contractile vacuole; c, ambceifomi zooid sunk 
 
30 
 
 PROTOZOA 
 
 within the common jelly or test (compared by S. Kent to the mesoderm- 
 cells of a sponge-colony) ; d, similar zooid multiplying by transverse 
 fission ; e, normal zooids with their collars contracted ; /, hyaline mucila- 
 ginous common test or zoothecium ; g, individual contracted and dividing 
 into minute flagellate spores (microgonidia) comparable to the spermato- 
 zoa of a Sponge. 
 
 Genera. Salpingceca, James Clark (sedentary, Fig. XXI. 6, 7) ; 
 Lagenosca, S. Kent (free swimming) ; Polyosca, S. Kent (cups united 
 socially to form a branching zoeecium as in Dinobryon). 
 
 ORDER 3. GELATINIGERA, Lankester. 
 
 The cell-units secrete a copious gelatinous investment and form 
 large colonies. 
 
 Genera. Phalansteriwn, Cienk. (Fig. XX. 12) ; Proterospongia, 
 Saville Kent (Fig. XXI. 15). 
 
 [The Choanoflagel lata were practically discovered by the Ameri- 
 can naturalist James Clark (68), who also discovered that the ciliated 
 chambers of Sponges are lined by collared cells of the same peculiar 
 structure as the individual Choanoflagellata, and hence was led to 
 regard the Sponges as colonies of Choanoflagellata. Saville Kent 
 (69) has added much to our knowledge of the group, and by his 
 discovery of Proterospongia (see Fig. XXI. 15, and description) 
 has rendered the derivation of the Sponges from the Flagellata a 
 tenable hypothesis.] 
 
 Further remarks on the Flagellata. Increased attention has 
 been directed of late years to the Flagellata in consequence of the 
 researches of Cienkowski, Biitschli, James Clark, Saville Kent, and 
 Stein. They present a very wide range of structure, from the 
 simple amoeboid forms to the elaborate colonies of Volvox and 
 Proterospongia. By some they are regarded as the parent-group 
 of the whole of the Protozoa ; but, whilst not conceding to them 
 this position, but removing to the Proteomyxa those Flagellata 
 which would justify such a view, we hold it probable that they are 
 the ancestral group of the mouth-bearing Corticata, and that the 
 Ciliata and Dinonagellata have been derived from them. One 
 general topic of importance in relation to them may be touched on 
 here, and that is the nature of the flagellum and its movements. 
 Speaking roughly, a flagellum may be said to be an isolated filament 
 of vibratile protoplasm, whilst a cilium is one of many associated 
 filaments of the kind. The movement, however, of a flagellum is 
 not the same as that of any cilium ; and the movement of all 
 flagella is not identical. A cilium is simply bent and straightened 
 alternately, its substance probably containing, side by side, a con- 
 tractile and an elastic fibril. A flagellum exhibits lashing move- 
 ments to and fro, and is thrown into serpentine waves during these 
 movements. But two totally distinct kinds of flagella are to be 
 distinguished, viz., (a) the pulsellum, and (b) the tractellum. An 
 example of the pulsellum is seen in the tail of a spermatozoon which 
 drives the body in front of it, as does the tadpole's tail. Such 
 a "pulsellum" is the cause of the movement of the Bacteria. It 
 is never found in the Flagellata. So little attention has been paid 
 to this fact that affinities are declared by recent writers to exist 
 between Bacteria and Flagellata. The flagellum of the Flagellata 
 is totally distinct from the pulsellum of the Bacteria. It is carried 
 in front of the body and draws the body after it, being used as a 
 man uses his arm and hand when swimming on his side. Hence 
 it may be distinguished as a "tractellum. Its action may be 
 best studied in some of the large Euglenoidea, such as Astasia. 
 Here it is stiff at the base and is carried rigidly in front of the 
 animal, but its terminal third is reflected and exhibits in this 
 reflected condition swinging and undulatory movements tending to 
 propel the reflected part of the flagellum forward, and so exerting a 
 traction in that direction upon the whole animal. It is in this way 
 (by reflexion of its extremity) that the flagellum or tractellum of 
 the Flagellata also acts so as to impel food-particles against the base 
 of the flagellum where the oral aperture is situated. 
 
 Many of the Flagellata are parasitic (some hsematozoic, see Lewis, 
 70); the majority live in the midst of putrefying organic matter in 
 sea and fresh waters, but are not known to be active as agents of 
 putrefaction. Dallinger and Drysdale have shown that the spores 
 of Bodo and others will survive an exposure to a higher tempera- 
 ture than do any known Schizomycetes (Bacteria), viz., 250 to 
 300 Fahr., for ten minutes, although the adults are killed at 180. 
 
 CLASS III. DINOFLAGELLATA, Butschli. 
 Characters. Corticate Protozoaof a bilaterally asymmetricalform, 
 sometimes flattened from back to ventral surface (Diplopsalis, 
 Glenodinium), sometimes from the front to the hinder region 
 (Ceratium, Peridinium), sometimes from right to left (Dinophysis, 
 Amphidinium, Prorocentrum) the anterior region and ventral 
 surface being determined by the presence of a longitudinal groove 
 and a large flagellum projecting from it. In all except the genus 
 Prorocentrum (Fig. XXII. 6) there is as well as a longitudinal 
 groove a transverse groove (hence Diuifera) in which lies horizon- 
 tally a second flagellum (Klebs and Butschli), hitherto mistaken for 
 a girdle of cilia. The transverse groove lies either at the anterior 
 end of the body (Dinophysis, Fig. XXII. 3, 4 ; Amphidinium) or 
 
 at the middle. In Gymnodinium it takes a spiral course. In 
 Polykrikos (a compound metameric form) there are eight indepen- 
 dent transverse grooves. 
 
 The Dinoflagellata are either enclosed in a cuticular shell 
 (Ceratium, Peridinium, Dinophysis, Diplopsalis, Glenodinium, 
 Prorocentrum, &c. ) or are naked (Gymnodinium and Polykrikos). 
 The cuticular membrane (or shell) consists of cellulose or of a 
 similar substance (cf. Labyrinthulidea) and not, as has been sup- 
 posed, of silica, nor of chitin-like substance ; it is cither a simple 
 cyst or perforated by pores, and may be built up of separate plates 
 (Fig. XXII. 10). 
 
 The cortical protoplasm contains trichocysts in Polykrikos. 
 
 The medullary protoplasm contains often chlorophyll and also 
 diatomin and starch or other amyloid substance. In these cases 
 (Ceratium, some species of Peridinium, Glenodinium, Prorocentrum, 
 Dinophysis acuta) nutrition appears to be holophytic. But in 
 others (Gymnodinium and Polykrikos) these substances are absent 
 and food-particles are found in the medullary protoplasm which 
 have been taken in from the exterior through a mouth ; in these 
 nutrition is holozoic. In others which are devoid of chlorophyll 
 and diatomin, &c., there is found a vesicle and an orifice connected 
 with the exterior near the base of the flagellum (cf. Flagellata) by 
 which water and dissolved or minutely granular food-matter is 
 introduced into the medullary protoplasm (Protojieridinium pellu- 
 cidmn, Peridinium divergens, Diplopsalis lenticula, Dinophysis 
 lasvis). It is important to note that these divergent methods of 
 nutrition are exhibited by different species of one and the same 
 genus, and possibly by individuals of one species in successive 
 phases of growth (?). 
 
 No contractile vacuole has been observed in Dinoflagellata. 
 
 The nucleus is usually single and very large, and has a peculiar 
 labyrinthine arrangement of chromatin substance. 
 
 Transverse binary fission is the only reproductive process as yet 
 ascertained. It occurs cither in the free condition (Fig. XXII. 2) 
 or in peculiar horned cysts (Fig. XXII. 8). Conjugation has been 
 observed in some cases (by Stein in Gymnodinium). 
 
 Mostly marine, some freshwater. Many are phosphorescent. 
 
 The Dinoflagellata are divisible into two orders, according to the 
 presence or absence of the transverse groove. 
 
 ORDEII 1. ADINIDA, Bergh. 
 
 Characters. Body compressed laterally; both longitudinal and 
 transverse flagellum placed at the anterior pole ; a transverse groove 
 is wanting ; a cuticular shell is present. 
 
 Genera. Prorocentrum, Ehr. (Fig. XXII. 6, 7); Exuviella, 
 C\Gi\\i.(Dinopyxis, Stein; Cryptomonas, Ehr.). 
 
 ORDER 2. DINIFERA, Bergh. 
 
 Characters. A transverse groove is present and usually a longi- 
 tudinal groove. The animals are either naked or loricate. 
 
 Fam. 1. DINOPHYIDA, Bergh. Body compressed ; the transverse 
 groove at the anterior pole ; the longitudinal groove present ; 
 longitudinal flagellum directed backwards ; loricate. 
 
 Genera. Dinophysis, Ehr. (Fig. XXII. 3, 4) ; Amphidinium, 
 Cl. & L. ; Amphisolenia, Stein ; Histioneis, Stein ; Citharistes, 
 Stein ; Ornithocercus, Stein. 
 
 Fam. 2. PERIDINIDA, Bergh. Body cither globular or flattened ; 
 transverse groove nearly equatorial ; longitudinal groove narrow or 
 broad ; loricate. 
 
 Genera. Protoperidinium, Bergh; Peridinium (Ehr.), Stein 
 (Fig. XXII. 1, 2); Protoceratium, Bergh ; Ceratium, Schrank (Fig. 
 XXII. 15) ; Diplo2)salis, Bergh ; Glenodinium, Ehr. ; Ileterocapsa, 
 Stein ; Gonyaulax, Diesing ; Goniodoma, Stein ; Blepharocysta, 
 Ehr. ; Podolampas, Stein ; Amphidoma, Stein ; Oxytoxum, Stein ; 
 Plychodiscus, Stein ; Pyrophacus, Stein ; Ceratocorys, Stein. 
 
 Fam. 3. GYMNODINIDA, Bergh. As Peridinida but no lorica 
 (cuticular shell). 
 
 Genera. Gymnodinium (Fig. XXII. 5), Stein ; Hemidinium, 
 Bergh. 
 
 Fam. 4. POLYDINIDA, Butschli. As Gymnodinida, but with 
 several independent transverse grooves. 
 
 Genus. Polykrikos, Butschli. 
 
 Further Remarks on the Dinoflagellata. This small group is at 
 the moment of the printing of the present article receiving a large 
 amount of attention from Bergh (81), Klebs (83), and Biitschli (82), 
 and has recently been greatly extended by the discoveries of Stein 
 (80), the last work of the great illustrator of the Cilia te Protozoa 
 before his death. The constitution of the cell-wall or cuticle from 
 cellulose, as well as the presence of chlorophyll and diatomin, and 
 the holophytic nutrition of many forms recently demonstrated by 
 Bergh, has led to the suggestion that the Dinoflagellata are to be 
 regarded as plants, and allied to the Diatomacea? and Desmidiacere. 
 Physiological grounds of this kind have, however, as has been 
 pointed out above, little importance in determining the affinities 
 of Protozoa. Butschli (82) in a recent very important article has 
 shown in confirmation of Klebs that the Dinoflagellata do not 
 
PROTOZOA 
 
 31 
 
 possess a girdle of cilia as previously supposed, but that the struc- 
 ture mistaken for cilia is a second flagellum which lies horizontally 
 in the transverse groove. Hence the name Cilioflagellata is super- 
 seded by Dinoflagellata (Gr. dinos, the round area where oxen tread 
 out on a threshing floor). 
 
 19 
 
 FIG. XXII. Dinoflagellata and Rhynchodagellata. X.B. In all these 
 figures the apparent girdle of cilia is, accodring to Klebs and Butschli's 
 recent discovery, to be interpreted as an encircling flagellum lying in the 
 transverse groove. 1. Peridinium uberrimum, Allman ; x 300 (fresh- 
 
 water ponds, Dublin). Probably (according to Butschli) the processes on 
 the surface are not cilia nor flagellum. Both the longitudinal and the 
 transverse groove are well seen. 2. The same species in transverse 
 
 fission. 3. Dinophysis ovata, Cl. and L; x 350 (salt water, Norwegian 
 
 coast). 4. Dinophysis acuminata, Cl. and L. ; X350 (salt water, 
 
 Norwegian coast). 5. Gi/mnodinium, sp. ; x 600. 6. Prorocen- 
 
 trum micans, Ehr.; X300 (salt water). 7. Dorsal aspect of the 
 
 same species. 8, 9. Cysts of Peridinia ; the contents of 8 divided 
 
 into eight minute naked Peridinia; xSOO. 10. Empty cuirass of 
 
 Ceratium divergens. Cl. and L. ; x 500 ; showing the form and disposition 
 of its component plates. 11. The same species with the animal con- 
 
 tracted Into a spherical form. The transverse groove well seen. 12. 
 
 The same species in the normal state. The apparent girdle of cilia is 
 really an undulating flagellum lying in the transverse groove. 13, 14. 
 
 Young stages of Noctiluca miliaris. n, nucleus : s, the so-called spine 
 (superficial ridge of the adult); a, the big flagellum ; the unlettered filament 
 Is a flagellum which becomes the oral flagellum of the adult. 15. Cera- 
 
 tium tnpos, Mull. The transverse groove well seen. The cilia really are 
 a single horizontal flagellmn. 16, 17. Two stages in the transverse 
 
 fission of A octuuca miliaris, Suriray. n, nucleus ; N, food-particles -t the 
 muscular flagellum. 18. Noctiluca miliaris, viewed from the ab'oral 
 
 side (after Allman, Quart. Jour. Mic. Sci., 1872). a, the entrance to the 
 atrium or flagellar fossa (=longitudinal groove of Dinoflagellata) e the 
 superficial ridge; d, the big flagellum (= the flagellum of the transverse 
 groove of Dinoflagellata); h, the nucleus. 19. The animal acted upon 
 by iodine solution, showing the protoplasm like the " primordial utricle" 
 of a vegetable cell shrunk away from the structureless firm shell or 
 cuirass. 20. Lateral view of Noctiluca, showing a, the entrance to the 
 
 groove-like atrium or flagellar fossa in which 6 is placed ; c, the superficial 
 ridge ; d, the big flagellum ; e, the mouth and gullet, in which is seen 
 Krohn a oral flagellum (=the chief flagellum or flagellum of the longitu- 
 dinal groove of Dmo-nagellata) ; /, broad process of protoplasm extending 
 from the superficial ridge c to the central protoplasm ; g, duplicature of 
 the shell in connexion with the superficial ridge ; A, nucleus. 
 
 Butschli further suggests that the Dinoflagellata with their 
 two flagella and their i-shaped combination of longitudinal and 
 transverse grooves may be derived from the Cryptomonadina (see 
 p. 858). In the latter a groove-like recess is present in connexion 
 with the origin of the two flagella. Biitschli thinks the large pro- 
 boscis-like Hagellum of Noctiluca (Rhynehoflagellata) represents 
 the horizontal flagellum of Dinoflagellata, whilst the prominent 
 longitudinal flagellum of the Dinoflagellata is represented in that 
 animal by the small flagellum discovered by Krohn within the 
 gullet (see Fig. XXII. 20, e). The young form of Noetiluca (Fig. 
 XXII. 14) has the longitudinal flagellum still of large size. 
 
 The phosphorescence of many Dinoflagellata is a further point 
 of resemblance between them and Noctiluca. 
 
 Bergh has shown that there is a considerable range of form in 
 various species of Dinoflagellata (Ceratium, &c.), and has also drawn 
 attention to the curious fact that the mode of nutrition (whether 
 holophytic or holozoic) differs in allied species. Possibly it may be 
 found to differ according to the conditions of life in individuals of 
 one and the same species. 
 
 The drawings in Fig. XXII. were engraved before the publication 
 of Butschli's confirmation of Klebs's discovery as to the non-existence 
 of cilia in the transverse groove. The hair-like processes figured 
 by Allman (91) external to the transverse groove in his Peridinium 
 uberrimum (Fig. XXII. 1, 2) cannot, however, be explained as a 
 flagellum. Biitschli inclines to the opinion that their nature was 
 misinterpreted by Allman, although the latter especially calls 
 attention to them as cilia, and as rendering his P. uberrimum 
 unlike the Peridinium of Ehrenberg, in which the cilia (horizontal 
 flagellum) are confined to the transverse groove. 
 
 y.B. See Fig. XXVII., and esplanation, p. 37. 
 
 CLASS IV. BHYNCHOFLAGELLATA, Lankester. 
 
 Characters. Corticate Protozoa of large size (^V tn inch) and 
 globular or lenticular form, with a firm cuticular membrane and 
 highly vacuolated (reticular) protoplasm. In Noctiluca a deep 
 groove is formed on one side of the spherical body, from the bottom 
 of which springs the thick transversely striated proboscis or 
 "big flagellum." Near this is the oral aperture and a cylin- 
 drical pharynx in which is placed the second or smaller flagellum 
 (corresponding to the longitudinal flagellum of Dinoflagellata). 
 
 Nutrition is holozoic. No contractile vacuole is present ; granule- 
 streaming is observed in the protoplasm. An alimentary tract and 
 anus have been erroneously described. The nucleus is spherical 
 and not proportionately large (see for details Fig. XXII. 18 to 20). 
 
 Reproduction by transverse fission occurs, also conjugation and, 
 either subsequently to that process or independently of it, a forma- 
 tion of spores (Cienkowski, 87), the protoplasm gathering itself, 
 within the shell-like cuticular membrane, into a cake which divides 
 rapidly into numerous flagellated spores (flagellulse). These escape 
 and gradually develop into the adult form (Fig. XXII. 13, 14). 
 
 The proboscis-like large flagellum is transversely striated, and 
 exhibits energetic but not very rapid lashing movements. 
 
 Noctiluea is phosphorescent, the seat of phosphorescence being, 
 as determined by Allman (86), the cortical layer of protoplasm 
 underlying the cuticular shell or cell-wall as the primordial cuticle 
 of a vacuolated vegetable cell underlies the vegetable cell-wall. 
 
 Genera. Only two genera (both marine) are known : Noctiluca, 
 Suriray (90) (Fig. XXII. 17-20) ; Leptodiscus, Hertwig (88). 
 
 Further Remarks on the Khynchoflagellata. The peculiar and 
 characteristic feature of Noctiluca appears to be found in its large 
 transversely-striated flagellum, which, according to Butschli, is not 
 the same as the longitudinal flagellum of the Dinoflagellata, but 
 probably represents the horizontal flagellum of those organisms in 
 a modified condition ; hence the name here proposed Rhyncho- 
 flagellata. 
 
 Noctiluca is further remarkable for its large size and cyst-like 
 form, and the reticular arrangement of its protoplasm, like that of 
 a vegetable cell. This is paralleled in TracheJius ovum among the 
 Ciliata (Fig. XXIV. 14), where the same stiffening of the cuticle 
 allows the vacuolation of the subjacent protoplasm to take place. 
 The remarkable Leptodiscus medusoides of R. Hertwig (88) appears 
 to be closely related to Noctiluca. 
 
 It would no doubt be not unreasonable to associate the Dino- 
 
32 
 
 PROTOZOA 
 
 flagellata and the Rhynchoflagellata with the true Flagellata in one 
 class. But the peculiarities of 'the organization of the two former 
 groups is best emphasized by treating them as separate classes de- 
 rived from the Flagellata. Neither group leads on to the Ciliata or 
 to any other group, but they must be regarded as forming a lateral 
 branch of the family tree of Corticata. The relationship of Nocti- 
 luca to Peridinium was first insisted upon by Allman, but has quite 
 recently been put in a new light by Biitsehli, who identifies the 
 atrial recess of Noctiluca (Fig. XXII. 20, 6) with the longitudinal 
 furrow or groove of the Dinoflagellata, and the large and minute 
 flagella of the former with the transverse and longitudinal flagella 
 respectively of the latter. The superficial ridge c of Noctiluca 
 appears to represent the continuation of the longitudinal groove. _ 
 The phosphorescence of the sea, especially on northern coasts, is 
 largely caused by Noctiluca, but by no means exclusively, since 
 Medusas, Crustaceans, Annelids, and various Protozoa often take part 
 in the phenomenon. Not (infrequently, however, the phosphor- 
 escence on the British coasts seems to be solely due to Noctiluca, 
 which then occurs in millions in the littoral waters. 
 
 FlQ. XXIII. Ciliata. 1. Spiroitomum ambiguum, Ehr.; one of the Hetero- 
 tricha ; x 120. Observe on the right side the oral groove and special hetero- 
 trichous band of long cilia, a, moniliform nucleus ; b, contractile vacuole. 
 2. Stentor polymorphic, Stuller ; one of the Heterotricha ; x 50 ; group of 
 
 individuals with the area fringed by the heterotrichous cilia expanded 
 trumpet-wise. 3. Tintinnus lagenula, C. and L.; one of the Hetero- 
 
 tricha; x 300. 4. Strombidium Claparedii, S. K.; one of the Peritricha; 
 X 200. 5. Empty shell of Codonella campaiiella, Haeck.; one of the 
 
 Heterotricha ; x 180. 6, 1. Torguatella typica, Lankester. p, the supra- 
 oral lobe seen through the membranous collar. 8, 9. View of the 
 base and of the side of Trichodina pedicului, Ehr.; one of the Peritricha; 
 x 300. a, nucleus ; c, corneous collar ; d, mouth. 10. Spirochona, 
 gernmipara, Stein ; one of the Peritricha ; x 350. a, nucleus ; g, bud. 11. 
 Vorticella citrina, Ehr.; X 150 (Peritricha). At d multiple fission of an 
 individual cell to form "microgonidia." 12. Vorticella micro&toma, 
 Ehr. (Peritricha); x 300. At e eight "microgonidia" formed by fission 
 of a single normal individual. 13. Same species, binary fission, a, 
 elongated nucleus. 14. Vorticella nebitlifera, Ehr. ; free-swimming 
 zooid resulting from fission in the act of detaching itself and swimming 
 away, possessing a posterior circlet of cilia, e, ciliated disk ; /, 
 pharynx. 15. Vorticella microstoma, Ehr.; normal zooid with two 
 microgonidia (or microzooids) c,d, in the act of conjugation, a, nucleus ; 
 b, contractile vacuole ; e, ciliated disk ; /, pharynx. 16, Vorticella 
 microstoma, Ehr., with stalk contracted and body enclosed in a cyst, a, 
 nucleus. 17. Vorticella nebulifera, Ehr. a, nucleus ; b, contractile 
 vacuole ; c, muscular region of the body continuous with the muscle of the 
 stalk ; d, pharynx (the basal continuation of the oral vestibule which 
 receives at a higher point the fcecal excreta and the ejected liquid from 
 the contractile vacuole). 18. Carchesium ypectabile, Ehr. ; retractile 
 colony ; x 50. 19 Trichocysts of Epistylis flavicans, Ehr. , as figured 
 by Greeff. 20. Opercularia stenostoma, Stein ; x 260 ; a small colony. 
 Observe the ciliation of the oral vestibule and the upstanding ciliate disk 
 (opercular-like). 21, 22. Pyxicola afflnis, S. K. ; one of the stalked 
 loricate Peritricha, in expanded and retracted states, x, the true oper- 
 culum. 23, 24. Gyrocoria oxyura, Stein ; one of the free-swimming 
 Peritricha, with Bpii-al equatorial cilia-band; x 250. b, contractile 
 vacuole. 25, 26. Thuricola yalvata, Str. Wright ; one of the sessile 
 tubicolous Peritricha. Two individuals are as a result of flssion tempo- 
 rarily occupying one tube ; , the valve attached to the tube, like the door 
 of the trap-door spider's nest and the valve of the Gasteropod Clausilium. 
 
 CLASS V. CILIATA, Ehrenberg (Infusoria sensu stricto). 
 
 Characters. Corticata of relatively large size, provided with 
 either a single band of cilia surrounding the anteriorly placed oral 
 aperture or with cilia disposed more numerously over the whole 
 surface of the body. The cilia are distinguished from the flagella 
 of Flagellata by their smaller size and simple movements of 
 alternate flexion and erection ; they serve always at some period of 
 growth as locomotor organs, and also very usually as organs for 
 the introduction of food particles into the mouth. Besides one 
 larger oblong nucleus a second (the paranucleus) is invariably (?) 
 present (Fig. XXV. 2), or the nucleus may be dispersed in small 
 fragments. Conjugation of equal-sized individuals, not resulting 
 in permanent fusion, is frequent. The conjugated animals separate 
 and their nuclei and paranuclei undergo peculiar changes ; but no 
 formation of spores, either at this or other periods, has been de- 
 cisively observed (Fig. XXV. 8 to 15). Multiplication by transverse 
 fission is invariably observed in full-grown individuals (Fig. XXV. 
 16), and conjugation appears to take place merely as an interlude 
 in the fissiparous process ; consequently young or small Ciliata are 
 (with few exceptions) unknown. Possibly spore-formation may 
 hereafter be found to occur at rare intervals more generally than is 
 at present supposed (Fig. XXIV. 15, 18). A production of micro- 
 gonidia by rapid fission occurs in some Peritricha (Fig. XXIII. 
 11, 12, 14, 15), the liberated microgonidia conjugating with the 
 normal individuals, which also can conjugate with one another. 
 
 The Ciliata, with rare exceptions (parasites), possess one or more 
 contractile vacuoles (Fig. XXV. 3). They always possess a delicate 
 cuticle and a body-wall which, although constant, in form is elastic. 
 They may be naked and free-swimming, or they may form horny 
 (Fig. XXIII. 21, 25) or siliceous cup-like shells or gelatinous 
 envelopes, and may be stalked and form colonies like those of 
 Choanoflagellata, sometimes with organic connexion of the con- 
 stituent units of the colony by a branching muscular cord (Vorti- 
 cellidEe). Many are parasitic in higher animals, and of these some 
 are mouthless. All are holozoic in their nutrition, though some are 
 said to combine with this saprophytic and holophytic nutrition. 
 
 The Ciliata are divisible into four orders according to the 
 distribution and character of their cilia. The lowest group (the 
 Peritricha) may possibly be connected through some of its members, 
 such as Strombidium (Fig. XXIII. 4), with the Flagellata through 
 such a form as Lophomonas (Fig. XXI. 9). 
 
 In the following synopsis, chiefly derived from Saville Kent's 
 valuable treatise (71), the characters of the families and the names 
 of genera are not given at length owing to the limitation of our 
 
 ORDER 1. PERITRICHA, Stein (79). 
 Characters. Ciliata with the cilia arranged in one anterior 
 circlet or in two, an anterior and a posterior ; the general surface of 
 the body is destitute of cilia. 
 
 Sub-order 1. NATANTIA (animals never attached). 
 
 Fam. 1. TORQTJATELLIDJE. 
 
 G enus . Torquatella, Lankester, like StromUdium, but the cilia 
 adherent so as to form a vibratile membranous collar (Fig. XXIII. 
 
 6, 7). 
 
 Fam. 2. DICTYOCYSTID.E. Animals loricate. 
 
 Fam. 3. ACTINOBOLIDJE. llloricate, with retractile tentacula. 
 
PROTOZOA 
 
 33 
 
 Fam. 4. HALTERIID.E. 
 
 Genera. Strombidium, Cl. & L. (Fig. XXIII. 4) ; Haltena, 
 Dujard., with a sui)plemeutary girdle of springing hairs; Didinium, 
 Stein, (Fig. XXIV. 19). 
 
 Fam. 5. GYROCORID,E. 
 
 Genera. Oyrocoris, Stein, with an equatorial ciliary girdle spirally 
 disposed (Fig. XXIII. 23, 24); Urocentrum, Nitzsch, girdle annular. 
 
 FIG. XXIV. Ciliata- 1. Ophaltnopsis sepiolx, Foett. ; a parasitic Holo- 
 trichous mouthless Ciliate from the liver of the Squid, o, nuclei ; &, 
 vacuoles (aon-contractile). 2. A similar specimen treated with picro- 
 cannine, showing a remarkably branched and twisted nucleus; a, in 
 place of several nuclei. 3. Trichonympha agilii, Leidy ; parasitic 
 
 in the intestine of the Termites (White Ants): x 600. o, nucleus; 6, 
 granules (food?). 4. Opalina raiiarum, Purkinje ; a Holotrichoua 
 
 mouthless Ciliate parasitic in the Frog's rectum ; adult ; x 100. a, a, the 
 numerous regularly dispersed nuclei. 5. The same ; an individual in pro- 
 cess of binary fission, a, nuclei. 6. The same ; the process of fission has 
 now reduced the individuals to a relatively small size. 7. Smallest fission- 
 produced fragment encysted, expelled from the Frog in this state and 
 swallowed by Tadpoles. 8. Young iminucleate individual which has 
 
 emerged from the cyst within the Tadpole, and will now multiply its 
 nuclei and grow to full size before in turn undergoing retrogressive 
 fission. 9, Anoplopkrya naidos, Duj. ; a mouthless Holotrichous 
 
 Ciliate parasitic in the worm Nais; x 200. a, the large axial nucleus ; 6, 
 contractile vacunles. 10. Anoplophrt/a prnlffera, C. and L.jfrom the 
 
 intestine of Clitellio. EemarkaUe fur tlie adhesion in a nietameric series 
 
 of incomplete fission-products, a, nucleus. 11. Amphileptug gigai, 
 
 C. and L. ; one of the Holotricha; x 100. 6, contractile vacuoles ; e, tricho- 
 cysts (see Fig XXIII. 19) ; d, nucleus ; e, pharynx. 12, 13. Prorodon 
 
 nioeus, Ehr.; one of the Holotricha; x 75. a, nucleus; b, contractile 
 vacuole; e, pharynx with horny fascicular lining. 12. The fasciculate 
 cuticle of the pharynx isolated. 14. Tracheliut omm, Ehr. (Holo- 
 
 tricha) ; x 80 ; showing the reticulate arrangement of the medullary pro- 
 toplasm, b, contractile vacuoles; c, the cuticle-lined pharynx. 15, 16, 
 17, 18. Icthyophthiriui multi/llius, Fouquet ; one of the Holotricha ; 
 x 120. Free individual and successive stages of division to form spores, 
 o, nucleus ; b, contractile vacuoles. 19. Didinium nas-atum, Mull. ; 
 
 one of the Peritricha ; x 200. The pharynx is everted and has seized a 
 Paramcecium as food, a, nucleus; 6, contractile vacuole; c, everted 
 pharynx. 20. Euplotei eharon, Mull.; one of the Hypotricha ; lateral 
 view of the animal when using its great hypotrichous processes, x, as 
 ambulatory organs. 21. Euplotes harpa, Stein (Hypotricha); x 150. 
 
 h, mouth; x, hypotrichous processes (limbs). 22. Kyctotherux cardi- 
 
 formis, Stein ; a Heterotrichous Ciliate parasitic in the intestine of the 
 Frog, a, nucleus ; 6, contractile vacuole ; e, food particle ; d, anus ; e, 
 heterotrichous band of large cilia ; /, <j, mouth ; A, pharnyx ; i, small cilia. 
 
 Fam. 6. URCF.OLARIID.E. 
 
 Genera. Trichodina, Ehr. ; two ciliate girdles ; body shaped as a 
 pyramid with circular sucker-like base, on which is a toothed corneous 
 ring (Fig. XXIII. 8, 9); Limophora, Clap.; Cvdochxta, Hat. Jacks. 
 
 f 
 
 '** 
 
 8 . pit 9 P nS 10 
 
 -prf 
 
 ' N ]f JV 
 
 pn /v v x />n>?> ^-kpn* 
 
 N 
 
 e 
 
 W 
 
 pn-*r^j_ 
 
 pn+ M ^ 
 
 - -** v *^ 
 
 -rrfv 
 
 FIG. XXV. Ciliata (conjugation, Ac.). 1. Surface view of Holotrichous 
 Ciliate, showing the disposition of the cilia in longitudinal rows. 2. 
 
 
 
34 
 
 PROTOZOA 
 
 Diagrammatic optical section of a Ciliate Protozoon, showing all structures 
 except the contractile vacuoles. a, nucleus; b, paranucleus (so-called 
 nucleolus) ; c, cortical substance ; D, extremely delicate cuticle ; E, 
 medullary (more fluid) protoplasm ; /, cilia; y, trichocysts ; ft, filaments 
 ejected from the trichocysts ; ', oral aperture ; k, drop of water contain- 
 ing food-particles, about to sink into the medullary substance and form 
 a food-vacuole ; I, m, n, o, food-vacuoles, the successive order of their 
 formation corresponding to the alphabetical sequence of the letters ; the 
 arrows indicate the direction of the movement of rotation of the medul- 
 lary protoplasm ; p, pharynx. 3. Outline of a Ciliate (Paramcecium), to 
 show the form and position of the contractile vacuoles. 4-7. 
 
 Successive stages in the periodic formation of the contractile vacuoles. 
 The ray-like vacuoles discharge their contents into the central vacuole, 
 which then itself bursts to the exterior. 8-15. Diagrams of the changes 
 undergone by the nucleus and paranucleus of a typical Ciliate during 
 and immediately after conjugation : N, nucleus; pn, paranucleus; 8, 
 condition before conjugation ; 9, conjugation effected ; both nucleus 
 and paranucleus in each animal elongate and become fibrillated ; 10, 
 two spherical paranuclei pn* in each, two dividing or divided nuclei 
 N* ; 11, the spherical paranuclei have become fusiform ; 12, there 
 are now four paranuclei in each (pn* and pn 1 '), and & nucleus 
 broken into four or even more fragments ; 13, the two paranuclei 
 marked pn* in 12 have united in each animal to form the new nucleus 
 pn' ; the nuclear fragments are still numerous ; 14, after cessation 
 of conjugation the nuclear fragments N and the two unfused paranuclear 
 pieces pn* are still present ; 15, from a part or all of the fragments 
 the new paranucleus is in process of formation, the new nucleus (pn 1 = N) 
 is large and elongated. 10. Diagram of a Ciliate in process of trans- 
 
 verse fission. 17. Condition of the nucleus N, and of the paranucleus 
 
 pn in Paramcecium aurelia after cessation of conjugation as observed 
 by Butschli. 18. Stylonichia mytilus (one of the Hypotricha), 
 
 showing endoparasitic unicellular organisms 6, formerly mistaken for 
 spores ; a, nuclei (after conjugation and breaking up). 
 
 Fam. 7. 
 
 Genera. Astylozoon, Engelm. ; Ophnjoscolex, Stein. 
 
 Sub-order 2. SEDENTARIA, animals always attached or sedentary 
 during the chief part of the life-history. 
 
 Fam. 1. VORTICELLID^;. Animals ovate, campanulate, or sub- 
 cylindrical ; oral aperture terminal, eccentric, associated with a 
 spiral fringe of adoral cilia, the right limb of which descends into 
 the oral aperture, the left limb encircling a more or less elevated 
 protrusible and retractile ciliary disk. 
 
 Sub-family 1. Vorticelliuse : animalcules naked. 
 
 a. Solitary forms. 
 
 Genera. Gcrda, Cl. andL. ; Scyphidia, Dujnrd. ; Spirochona, Stein 
 (sessile with peristome in the form of a spirally convolute mem- 
 branous expansion, Fig. XXIII. 10) ; Pyxidium, Kent (with a 
 non-retractile stalk) ; Vorticella, Linn, (with a hollow stalk in 
 which is a contractile muscular filament). 
 
 j8. Forming dendriform colonies. 
 
 Genera. Carchesium, Ehr. (Fig. XXIII. 18, with contractile 
 stalks) ; Zoothammium, Ehr. (contractile stalks) ; Epistylis, Ehr. 
 (stalk rigid) ; Opercularia, Stein (stalk rigid, ciliated disk oblique ; 
 an elongated peristomial collar, Fig. XXIII. 20). 
 
 Sub-family 2. Vaginicolinae : animalcules secreting firm cup-like 
 or tube-like membranous shells. 
 
 Genera. Vaginicola, Lamarck (no internal valve); Thuricola, 
 Kent (with a door-like valve to the tube, Fig. XXIII. 25, 26) ; 
 Cothurina, Ehr. (lorica or shell pediculate ; no operculum); Pyxicola, 
 Kent (lorica pedunculate, animal carrying dorsally a horny oper- 
 culum, Fig. XXIII. 21, 22). 
 
 Sub-family 3. Ophrydina : animalcules secreting a soft gelatinous 
 envelope. 
 
 Genera. Ophionella, Kent; Ophrydium, Ehr. 
 
 ORDER 2. HETEROTRICHA, Stein. 
 
 Characters. A band or spiral or circlet of long cilia is 
 developed in relation to the mouth (the heterotrichous band) 
 corresponding to the adoral circlet of Peritricha; the rest of the 
 body is uniformly beset with short cilia. 
 
 a. Heterotrichal band circular. 
 
 Genera (selected). Tinlinnus, Schranck (Fig. XXIII. 3); Tri- 
 chodinopsis, Cl. and L. ; Codonella, Haeck. (with a peri-oral fringe 
 of lappet-like processes) ; Calccolus, Diesing. 
 
 j8. Heterotrichal band spiral. 
 
 Genera (selected). Stenlor, Oken (Fig. XXIII. 2) ; Blepharisma, 
 Perty (with an undulating membrane along the oral groove); 
 Spirostomum, Ehr. (oral groove linear and elongate, Fig. XXIII. 
 1); Leucophrys, Ehr. (oral groove very short). 
 
 y. Heterotrichal band in the form of a simple straight or oblique 
 adoral fringe of long cilia. 
 
 Genera (selected).- Sursaria, Miiller ; Nyctothcrus, Leidy (with 
 well-developed alimentary tract and anus, Fig. XXIV. 22) ; Balan- 
 tidium, Cl. and L. (B. colt parasitic in the human intestine). 
 
 ORDER 3. HOLOTKICHA, Stein. 
 
 Characters. There is no special adoral fringe of larger cilia, nor 
 a band-like arrangement of cilia upon any part of the body ; short 
 cilia of nearly equal size are uniformly disposed all over the surface. 
 The adoral cilia sometimes a little longer than the rest. 
 
 a. With no membraniform expansion of the body wall. 
 
 Genera. Paranuecium, Ehr. (Fig. XXV. 1, 2) ; Prorodon, Ehr. 
 
 (Fig. XXIV. 13); Coleps, Ehr. ; Enchelys, Ehr.; Trachelocerca, Ehr.; 
 Trachelius, Ehr. ; Amphileptus, Ehr. ; Icthyophthirius, Fouquet 
 (Fig. XXIV. 15). 
 
 j8. Body with a projecting membrane, often vibratile. 
 
 Genera. Ophryoglena, Ehr.; Colpidium, Stein; Lembus, Cohn ; 
 Trichonympha, Leidy (an exceptionally modified form, parasitic, 
 Fig. XXIV. 3). 
 
 y. Isolated parasitic forms, devoid of a mouth. 
 
 Genera. Opalina, Purkinje (nuclei numerous, no contractile 
 vaeuole, Fig. XXIV. 4 to 8) ; Bcnedenia, Foett. ; Opalinopsis, 
 Foett. (Fig. XXIV. 1, 2); Anoplophrya, Stein (large axial nucleus, 
 numerous contractile vaeuoles in two linear series, Fig. XXIV. 9 
 10) ; Haptophrya, Stein ; Hvplitvphrya, Stein. 
 
 ORDER 4. HYPOTRICHA, Stein. 
 
 Characters. Ciliata in which the body is flattened and the 
 locomotive cilia are confined to the ventral surface, and are often 
 modified and enlarged to the condition of muscular appendages 
 (setae so-called). Usually an adoral band of cilia, like that of 
 Heterotricha. Dorsal surface smooth or provided with tactile 
 hairs only. Mouth and anus conspicuously developed. 
 
 a. Cilia of the ventral surface uniform, fine, and vibratile. 
 
 Genera. Chilodon, Ehr. ; Loxodes, Ehr. ; Dysleria, Huxl. ; 
 Huxley a, Cl. and L. 
 
 /3. Cilia of the ventral surface variously modified as seta? 
 (muscular appendages), styles, or uncini. 
 
 Genera. Stylonichia, Ehr. (Fig. XXV. 18); Oxytricha, Ehr.; 
 Euplotes, Ehr. (Fig. XXIV. 20, 21). 
 
 Further remarks on the Ciliata. The Ciliata have recently 
 formed the subject of an exhaustive treatise by Mr Saville Kent (71) 
 which is accessible to English readers. On the other hand Prof. 
 Butschli has not yet dealt with them in his admirable critical 
 treatise on the Protozoa. Hence a large space has not been devoted 
 in this article to the systematic classification and enumeration of 
 their genera. See (79) and (93). 
 
 One of the most interesting features presented by the group is 
 the presence in many of a cell anus as well as a cell mouth (Fig. 
 XXIV. 22, d). In those devoid of an anus the undigested 
 remnants of food are expelled either by a temporary aperture on 
 the body-surface or by one opening into the base of the pharynx. 
 In many parasitic Ciliata, as in higher animal parasites, such as 
 the Cestoid worms, a mouth is dispensed with, nutriment being 
 taken by general imbibition and not in the solid form. Many 
 Ciliata develop chlorophyll corpuscles of definite biconcave shape, 
 and presumably have so far a capacity for vegetal nutrition. In 
 Vorticella vlridis the chlorophyll is uniformly diffused in the pro- 
 toplasm and is not in the form of corpuscles (72). 
 
 The formation of tubes or shells and in connexion therewith of 
 colonies is common among the Peritricha and Heterotricha. The 
 cuticle may give rise to structures of some solidity in the form of 
 hooks or tooth-like processes, or as a lining to the pharynx (Fig. 
 XXIV. 12). 
 
 The phenomena connected with conjugation and reproduction 
 are very remarkable, and have given rise to numerous misconcep- 
 tions. They are not yet sufficiently understood. It cannot be 
 surely asserted that any Ciliate is at the present time known to 
 break up, after encystment or otherwise, into a number of spores, 
 although this was at one time supposed to be the rule. Icthyoph- 
 thirius (Fig. XXIV. 15 to 18) and some Vorticella; (76) have been 
 stated, even recently, to present this phenomenon ; but it is not 
 impossible that the observations are defective. The only approach 
 to a rapid breaking up into spores is the multiple formation (eight) 
 of microgonidia or microzooids in Vorticellida; (Fig. XXIII. 11, 
 12); otherwise the result of the most recent observations appears to 
 be that the Ciliata multiply only by binary fission, which is very 
 frequent among them (longitudinal in the Peritricha, transverse 
 to the long axis in the others). 
 
 Several cases of supposed formation of spores within an adult 
 Ciliate and of the production endogenously of numerous "acineti- 
 form young " have been shown to be cases of parasitism, minute 
 unicellular parasites, e.g., parasitic Acineta; (such as Sphterophrya 
 described and figured in Fig. XXVI. ) being mistaken for the young. 
 
 The phenomenon of conjugation is frequent in the Ciliata, and is 
 either temporary, followed by a separation of the fused individuals, 
 as in most cases, or permanent, as in the case of the fertilization 
 of normal individuals by the microgonidia of Vorticellidae. 
 
 Since the process of conjugation or copulation is not followed 
 by a formation of spores, it is supposed to have merely a fertilizing 
 effect on the temporarily conjoined individuals, which nourish 
 themselves and multiply by binary fission more actively after the 
 process than before (hence termed "rejuvenescence)." 
 
 Remarkable changes have been from time to time observed in 
 the nuclei of Ciliata during or subsequently to conjugation, and 
 these were erroneously interpreted by Balbiani (73) as indicating 
 the formation of spermatozoa and ova. The nuclei exhibit at one 
 period great elongation and a distinct fibrillation, as in the dividing 
 
PROTOZOA 
 
 35 
 
 nuclei of tissue cells (compare Fig. I. and Fig. XXV. 9, 11, 17). 
 The fibrillse were supposed to be spennatozoids, and this erroneous 
 view was confirmed by the observation of rod-like Bacteria 
 (Schizomycetes) which in some instances infest the deeper proto- 
 plasm of large Ciliata. 
 
 The true history of the changes which occur in the nuclei of 
 conjugating Ciliata has been determined by Butschli (74) in some 
 typical instances, but the matter is by no means completely under- 
 stood. The phenomena present very great obstacles to satis- 
 factory examination on account of their not recurring very fre- 
 quently and passing very rapidly from one phase to another. 
 They have not been closely observed in a sufficiently varied 
 number of genera to warrant a secure generalization. The follow- 
 ing scheme of the changes passeil through by the nuclei must be 
 regarded as necessarily referring to only a few of the larger 
 Heterotricha, Holotricha, and Hypotricha, and is only probably 
 true in so far as details are concerned, even for them. It is at 
 the same time certain that some such series of changes occurs in 
 all Ciliata as the sequence of conjugation. 
 
 In most of the Ciliata by the side of the large oblong nucleus is a 
 second smaller body (or even two such bodies) which has been very 
 objectionably termed the nucleolus (Fig. XXV. 8), but is better 
 called the " paranucleus " since it has nothing to do with the nucle- 
 olus of a typical tissue-cell. When conjugation occurs and a 
 "syzygium" is formed, both nucleus and paranucleus in each con- 
 jugated animal elongate and show fibrillar structure (Fig. XXV. 
 10). Each nucleus and paranucleus now divides into two, so that 
 we get two nuclei and two paranuclei in each animal. Elongation 
 and fibrillation are then exhibited by each of these new elements 
 and subsequently fission, so that we get four nuclei and four para- 
 nuclei in each animal (11, 12). The fragments of the original 
 nucleus (marked N in the figures) now become more dispersed and 
 broken into further irregular fragments. Possibly some of them 
 are ejected (so-called " cell excrement "); possibly some pass over 
 from one animal to the other. Two of the pieces of the four-times- 
 divided paranucleus now reunite (Fig. XXV. 13), and form a 
 largish body which is the new nucleus. The remaining fragments 
 of paranucleus and the broken down nucleus now gradually dis- 
 appear, and probably as a remnant of them we get finally a few cor- 
 puscles which unite to form the new paranucleus (14, 15). The 
 conjugated animals which have separated from one another before 
 the later stages of this process are thus reconstituted as normal 
 Ciliata, each with its nucleus and paranucleus. They take food 
 and divide by binary fission until a new period of conjugation 
 arrives, when the same history is supposed to recur. 
 
 The significance of the phenomena is entirely obscure. It is not 
 known why there should be a paranucleus or what it may correspond 
 to in other cells whether it is to be regarded simply as a second 
 nucleus or as a structurally and locally differentiated part of an 
 ordinary cell-nucleus, the nucleus and the pavanucleus together 
 being the complete equivalent of such an ordinary nucleus. An 
 attempt has been made to draw a parallel between this process and 
 the essential features of the process of fertilization (fusion of the 
 spermatic and ovicell nuclei) in higher animals ; but it is the fact 
 that concerning neither of the phenomena compared have we as yet 
 sufficiently detailed knowledge to enable us to judge conclusively as 
 to how far any comparison is possible. Whilst there is no doubt 
 as to the temporary fusion and admixture of the protoplasm of the 
 conjugating Ciliata, it does not appear to be established that there 
 is any transference of nuclear or paranuclear matter from one indi- 
 vidual to the other in the form of solid formed particles. 
 
 Conjugation resulting merely in rejuvenescence and ordinary fis- 
 sive activity is observed in many Flagellata as well as in the Ciliata. 
 
 A noteworthy variation of the process of binary fission occurring 
 in the parasite Opalina deserves distinct notice here, since it is inter- 
 mediate in character between ordinary binary fission and that 
 multiple fission which so commonly in Protozoa is known as spore- 
 formation. In Opalina (Fig. XXIV. 4) the nucleus divides as the 
 animal grows ; and we find a great number of regularly disposed 
 separate nuclei in its protoplasm. (The nuclei of many other 
 Ciliata have recently been shown to exhibit extraordinary branched 
 and even "fragmented" forms; compare Fig. XXIV. 2.) Atacertain 
 stage of growth binary fission of the whole animal sets in, and growth 
 ceases. Consequently the products of fission become smaller and 
 smaller (Fig. XXIV. 6). At last the fragments contain each but 
 two, three, or four nuclei. Each fragment now becomes encased 
 in a spherical cyst (Fig. XXIV. 7). If this process had occurred 
 rapidly, we should have had a uninucleate Opalina breaking up 
 at once into fragments (as a Gregarina does), each fragment being 
 a spore and enclosing itself in a spore-case. The Opalina ranarum 
 lives in the rectum of the Frog, and the encysted spores are 
 formed in the early part of the year. They pass out into the 
 water and undergo no change unless swallowed by a Tadpole, in 
 the intestine of which they forthwith develop. From each spore- 
 case escapes a uninucleate embryo (Fig. XXIV. 8), which absorbs 
 nourishment and grows. As it grows its nucleus divides, and so 
 the large multinucleate form from which we started is reattained. 
 
 This history has important bearings, not only on the nature of 
 sporulation, but also on the question of the significance of the 
 multinucleate condition of cells. Here it would seem that the 
 formation of many nuclei is merely an anticipation of the retarded 
 fissive process. 
 
 It is questionable how far we are justified in closely associating 
 Opalina, in view of its peculiar nuclei, with the other Ciliata. It 
 seems certain that the worm-parasites sometimes called Opalinae, but 
 more correctly Anaplophrya, &c., have no special affinity with the 
 true Opalina. They not only differ from it in having one large 
 nucleus, but in having numerous very active contractile vacnoles 
 (75). 
 
 Recently it has been shown, more especially by Gruber (84), that 
 many Ciliata are multinucleate, and do not possess merely a single 
 nucleus and a paranucleus. In Oxytricha the nuclei are large and 
 numerous (about forty), scattered through the protoplasm, whilst 
 in other cases the nucleus is so finely divided as to appear like a 
 powder or dust diffused uniformly through the medullary proto- 
 plasm (Trachelocerca, Choenia). Carmine staining, after treatment 
 with absolute alcohol, has led to this remarkable discovery. The 
 condition described by Foettinger (85) in his Opalinopsis (Fig. 
 XXIV. 1 , 2) is an example of this pulverization of the nucleus. The 
 condition of pulverization had led in some cases to a total failure 
 to detect any nucleus in the living animal, and it was only by the 
 use of reagents that the actual state of the case was revealed. 
 Curiously enough, the pulverized nucleus appears periodically to 
 form itself by a union of the scattered particles into one solid 
 nucleus just before binary fission of the animal takes place ; and 
 on the completion of fission the nuclei in the two new individuals 
 break up into little fragments as before. The significance of this 
 observation in relation to the explanation of the proceedings of the 
 nuclei during conjugation cannot be overlooked. It also leads to 
 the suggestion that the animal cell may at one time in the history 
 of evolution have possessed not a single solid nucleus but a finely 
 molecular powder of chromatin-substance scattered uniformly 
 through its protoplasm, as we find actually in the living Trachelo- 
 cerca. 
 
 Some of the Ciliata (notably the common Vorticellae) have been 
 observed to enclose themselves in cysts ; but it does not appear that 
 these are anything more than " hypnocysts " from which the animal 
 emerges unchanged after a period of drought or deficiency of food. 
 At the same time there are observations which seem to indicate that 
 in some instances a process of spore-formation may occur within 
 such cysts (76). 
 
 The differentiation of the protoplasm into cortical and medul- 
 lary substance is very strongly marked in the larger Ciliata. 
 The food-particle is carried down the gullet by ciliary currents 
 and is forced together with an adherent drop of water into the 
 medullary protoplasm. Here a slow rotation of the successively 
 formed food- vacnoles is observed (Fig. XXV. 2, I, m, n, o), the 
 water being gradually removed as the vacuole advances in position. 
 It was the presence of numerous successively formed vacuoles which 
 led Ehrenberg to apply to the Ciliata the not altogether inappro- 
 priate name " Polygastrica. " The chemistry of the digestive pro- 
 cess has not been successfully studied, but A. G. Bourne (8) has 
 shown that, when particles stained with water-soluble aniliu blue 
 are introduced as food into a Vorticella, the colouring matter is 
 rapidly excreted by the contractile vacuole in a somewhat concen- 
 trated condition. 
 
 The differentiation of the protoplasm of Ciliata in some special 
 cases as "muscular" fibre cannot be denied. The contractile 
 filament in the stalk of Vorticella is a muscular fibre and not 
 simple undifferentiated contractile protoplasm ; that is to say, its 
 change ot dimensions is definite and recurrent, and is not rhythmic, 
 as is the flexion of a cilium. (Perhaps in ultimate analysis it is 
 impossible to draw a sharp line between the contraction of one side 
 of a cilium which causes its flexion and the rhythmical contraction 
 of some muscular fibres.) The movements of the so-called " setse " 
 of the Hypotricha are also entitled to be called "muscular," as 
 are also the general contractile movements of the cortical substance 
 of large Ciliata. Haeckel (77) has endeavoured to distinguish 
 various layers in the cortical substance; but, whilst admitting that, 
 as in the Gregarinre, there is sometimes a distinct fibrillation of 
 parts of this layer, we cannot assent to the general distinction of a 
 " myophane" layer as a component of the cortical substance. 
 
 Beneath the very delicate cuticle which, as a mere superficial 
 
 Eellicle of extreme tenuity, appears to exist in all Ciliata we 
 equently find a layer of minute oval sacs which contain a spiral 
 thread ; the threads are everted from the sacs when irritant 
 reagents are applied to the animal (Fig. XXV. 2, g, K). These 
 were discovered by Allman (78), and by him were termed " tricho- 
 cysts." They appear to be identical in structure and mode of 
 formation with the nematocysts of the Ccelentera and Platyhelmia. 
 Similar trichocysts (two only in number) are found in the spores 
 of the Myxosporidia (see ante, page 855). 
 
 The comparative forms of the nucleus and of the contractile 
 vacuoles, as well as of the general body-form, &c., of Ciliata may 
 
36 
 
 PROTOZOA 
 
 be learnt from an examination of Figs. XXIII., XXIV., XXV., 
 and the explanations appended to them. 
 
 CLASS VI. ACINETAEIA, Lankester (Tentaculifera, Huxley). 
 
 Characters. Highly specialized Corticate Protozoa, probably 
 derived from Ciliata, since their young forms are provided with a 
 more or less complete investment of cilia. They are distinguished 
 by having no vibratile processes on the surface of the body in the 
 adult condition, whilst they have few or many delicate but firm 
 
 lG. XXVI. Acinetaria. 1. Ithyncheta cydopitm, Zenker. a, nucleus; 
 b, contractile vacuole ; only a single tentacle, and that suctorial ; x 150. 
 Parasitic on Cyclops. 2. Sphxrophrya urostylm, Maupas ; normal 
 
 adult ; x 200. a, nucleus ; b, contractile vacuole. Parasitic in Urostyla. 
 3. The same dividing by transverse fission, the anterior moiety with tem- 
 porarily developed cilia, a, nucleus ; b, contractile vacuole. 4, 5, 6. 
 Sphxrophrya stentorea, Maupas ; x 200. Parasitic in Stentor, and at one 
 time mistaken for its young. 7. Trichophrya epistylidis, Cl. and L. ; 
 X 150. a, nucleus; 6, contractile vacuole. 8. Hemiophrya, gemmi- 
 para, Hertwig; x 400. Example with six Ijuds, into each of which a 
 branch of the nucleus a is extended. 9. The same species, showing 
 the two kinds of tentacles (the suctorial and the pointed), and the con- 
 tractile vacuoles b. 10. Ciliated embryo ol Podophrya Steinii, Cl. and 
 L.; x 300. 11. Acineta grandis, Saville Kent ; x 100 ; showing pedun- 
 culated lorica, and animal with two bunches of entirely suctorial tentacles. 
 a, nucleus. 12. Sphxrophrya magna, Maupas ; x 300. It has seized 
 
 with its tentacles, and Is in the act of sucking out the juices of six examples 
 of the ciliate Colpoda parvifrons. 13. Podophrya elongata, Cl. and L. ; 
 
 X 150. a, nucleus ; b, contractile vacuole. 14. Hemiophrya Benedenii, 
 Fraip. ; x 200 ; the suctorial tentacles retracted. 16. Vendrocometes 
 
 rradoxui, Stein ; x 350. Parasitic on Gammaruz pulex. a, nucleus ; 
 contractile vacuole ; c, captured prey. 16. A single tentacle of 
 
 Podophrya; x 800. (Saville Kent.) 17-20. Dendrosoma radians, Ehr.: 
 17, free-swimming ciliated embryo, x 600 ; 18, earliest fixed condition of 
 the embryo, X 600 ; 19, later stage, a single tentaculiferous process now 
 developed, x 600 ; 20, adult colony ; c, enclosed ciliated embryos ; d, 
 branching stolon ; e, more minute reproductive (V) bodies. 21. Ophryo- 
 
 dendron pedicellatum, Hincks ; x 300. 
 
 tentacle-like processes, which are either simply adhesive or tubular 
 and suctorial. In the latter case they are provided at their ex- 
 tremity with a sucker-disk and have contractile walls, whereas in 
 the former case they have more or less pointed extremities. The 
 Acinetaria are sedentary in habit, even if not, as is usual, per- 
 manently fixed by a stalk. The nucleus is frequently arboriform. 
 Reproduction is effected by simple binary fission, and by a modified 
 fission (bud-fission) by which (as in Reticularia and Arcella) a 
 number of small bud-like warts containing a portion of the branched 
 parental nucleus are nipped oft' from the parent, often simul- 
 taneously (Fig. XXVI. 8). These do not become altogether dis- 
 tinct, but are for a time enclosed by the parental cell each in a 
 sort of vacuole or brood-chamber, where the young Acinetarian 
 develops a coat or band of cilia and then escapes from the body of 
 its parent (Fig. XXVI. 10, 17). After a brief locomotive existence, 
 it becomes sedentary, develops its tentacles, and loses its cilia. 
 
 The Acinetaria have one or more contractile vacuoles. Their 
 nutrition is holozoic. 
 
 The surface of the body in some cases is covered only by a 
 delicate cuticle, but in other cases a definite membranous shell or cup 
 (often stalked) is produced. Freshwater and marine. See Fraipont 
 (89). 
 
 ORDER 1. SUCTORIA, Kent. 
 
 A greater or less proportion or often all of the tentacles are 
 suctorial and terminated with sucker-like expansions. 
 
 Genera. Rhyncheta, Zenker (stalkless, naked, with only one 
 tentacle ; epizoic on Cyclops ; Fig. XXVI. 1) ; Urnula, C. and L. ; 
 Sphserophrya, C. and L. (naked, spherical, with distinctly capitate 
 tentacles only ; never with a pedicle ; parasitic within Ciliata, 
 supposed young ; Fig. XXVI. 2-6, 12) ; Trichophrya, C. and L. (as 
 Spheeroptirya, but oblong and temporarily fixed without a pedicle); 
 Podophrya, Ehr. (naked, solitary, globose, ovate or elongate, fixed 
 by a pedicle ; tentacles all suctorial, united in fascicles or distri- 
 buted irregularly; Fig. XXVI. 10, 13, 16) ; Hemiophrya, S. Kent (as 
 Podophrya, but the tentacles are of the two kinds indicated in the 
 definition of the group ; Fig. XXVI. 8, 9, 14); Podocyathus, S. Kent 
 (secreting and inhabiting stalked membranous cups or loricse ; ten- 
 tacles of the two kinds) ; Solenophrya, C. and L. (with a sessile 1 
 lorica ; tentacles only suctorial) ; Acineta, Ehr. (as Solenophrya, 
 but the lorica is supported on a pedicle ; Fig. XXVI. 11) ; Dendro- 
 cometcs, Stein (cuticle indurated ; solitary, sessile, discoid ; tentacles 
 peculiar, viz., not contractile, more or less branched, root-like, and 
 perforated at the extremities and suctorial in function ; Fig. 
 XXVI. 15). Dendrosoma, Ehr. (forming colonies of intimatel)' 
 fused individuals, with a basal adherent protoplasmic stolon and 
 upstanding branches the termination of which bear numerous capi- 
 tate suctorial tentacles only ; Fig. XXVI. 17-20). 
 
 OEDER2. NON-SUCTORIA, Lankester ( = Actinaria, Kent). 
 
 Characters. Tentacles filiform, prehensile, not provided with a 
 sucker. 
 
 Genera. Ephclota, Str. Wright (solitary, naked, pedunculate, 
 with many flexible inversible tentacles) ; Actinocyathus, S. Kent ; 
 Ophryodendron, C. and L. (sessile, with a long, extensile, anterior 
 proboscis bearing numerous flexible tentacles at its distal extremity ; 
 Fig. XXVI. 21) ; Acinelopsis, Robin (ovate, solitary, secreting a 
 stalked lorica ; from the anterior extremity of the animal is deve- 
 loped a proboscis-like organ which does not bear tentacles). 
 
 Further remarks on the Acinetaria. The independence of the 
 Acinetaria was threatened some years ago by the erroneous view of 
 Stein (79) that they were phases in the life-history of Vorticellidae. 
 Small parasitic forms (Sphferophrya) were also until recently 
 regarded erroneously as the " acinetiform young" of Ciliata. 
 
 They now must be regarded as an extreme modification of the 
 Protozoon series, in which the differentiation of organs in a 
 unicellular animal reaches its highest point. The sucker-tentacles 
 of the Suctoria are very elaborately constructed organs (see Fig. 
 XXVI. 16). They are efficient means of seizing and extracting the 
 juices of another Protozoon which serves as food to the Acinetarian. 
 The structure of Dendrosoma is remarkable on account of its 
 multicellular character and the elaborate differentiation of the 
 reproductive bodies. 
 
 The ciliation of the embryos or young forms developed from the 
 buds of Acinetaria is an indication of their ancestral connexion 
 with the Ciliata. The cilia are differently disposed on the young 
 of the various genera (see Fig. XXVI. 10, 17). 
 
PROTOZOA 
 
 37 
 
 Bibliography. (1) HAKCKKI. (Protista), " Monographic der Moneren," 
 Jenaische Zeilschr., iv., 1868. (2) DUJARDIK (Sarcode), "Observations sur les 
 organismes inferieures," Annales des Sciences A'aturelles, 1835, 2d series, vol. 
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 BRANDT (chlorophyll In animals), Sitzungsbericht der Oetellsch. Ifaturforsch, 
 Freunde zu Berlin, No. 9, 1881. (6) MECZSIKOW (phagocytes), Arbeiten a. d. 
 Zoolog. Jnstit. Wien, 1883, and Biologisches Centra/Mail, November 1883, both 
 translated in Quart. Jour. Micr. Set., January 1884. (7) EXGELHANN (proto- 
 plasm) in Hermann's Handuorterb. der Physiologie, translated in the Quart. 
 Jour, of Micr. Set., July 1884. (8) BOURXK (excretion by contractile vacuole) 
 in translation of (7), Quart. Jour. Uicr. Set., 1884, p. 378. (9) BDTSCHLI (Pro- 
 tozoa), in Bronn's Classen u. Ordnungen des Tltierreichs (Protozoa, 1883, in pro- 
 gress). (10) HCXLET (classification of Protozoa), A Manual of the Anatomy of 
 Imertebrated Animals, 1877, p. 76. (11) SCHULTZK, F. E. (nuclei of Foraminifera) 
 "Rhizopodenstudien," Archie f. Mikros. Anat., 1874-75-77. (12) HEBTWIG, E. 
 (nuclei of Foraminifera), Jenaische Zeitschrift, x., 1876. (13) ZOPF (Mycetozoa), 
 Encylclop. der Natumissenseh., Abtheilung i., Licferung. 39-*l, 1884. (14) 
 LAXKESTER, E. RAT (Archerina), Quart. Jow. Micr. Sci., January 1885. (15) 
 CIKNKUWSKI (Vampyrella), Archie f. Mikrosk. Anatomie, vol. i. p. 218. (16) 
 HRRTWIG, R., and LESSER (Leptophrys) Archie f. Mikrosk. Anat., x., Supplement, 
 1874. (17) SOROKIX (Bursulla), Annales des Sciences Naturelles (Botanique), 
 1876, p. 40. (18) CIEXKOWSKI (Enteromyxa), cited in (13) by Zopf, p. 114. (19) 
 CIENKOWSKI (Colpodella), " Beitrage zur Kenntniss der Monaden," Archiv f. 
 Mikmk. Anal., vol. i. (20) CIESKOWSKI (Pseudospora), same as (18). (21) 
 \VOROXIX (Plasmodiophora), Pringshcim's Jahrbiicher, xi. 548. (22) GOBEL 
 (Tetramyxa), Flora, No. 23, 1884. (23) SOROKIS (Gloidium), tforphol. Jahrb., 
 vol. iv., 1878. (24) CIEXKOWSKI (Gymnophrys), Archie f. Mikrosk. Anatomie, 
 vol. xii., 1876. (25) WRIGHT (Boderia), Jour, of Anat. and Physiol^ vol. i., 
 
 1867. (26) CIESKOWSKI (Xuclearfa), Archie f. Mikrosk. Anatomie, vol. i. 
 1865. (27) SCHNEIDER, AIM. (Monobia), Archives d. Zoolog. Experimental^, 
 vol. vii., 1878. (28) HCXLET (Bathybius), Quart. Jour. Micr. Set., vol. viii., 
 
 1868. (29) BEJSELS (Protobathybins), Jenaische Zeitschrift, ix. ; also American 
 Naturalist, ix. (30) STRASBUKGKR (nuclei of MycetozoaX Zellbildung und 
 Zelltheilung, 3d ed., p. 79. (31) FAYOD (Copromyxa), Bolan. Ztitung, 1883, No. 
 11. (32) GREEFF (Pelomyxa=Pelobius), Archie f. Mikrosk. Anatomie, vi., 1870. 
 (33) BUCK (Arcella, spore-bud production), Zeitsch. unss. Zoologie, xxx. (34) 
 LAXKESTER, E. RAT (Lithamceba), Quart. Jour. Micr. Set., vol. xix., 1879. 
 (35) CIENKOWSKI (Labyrinthula). Archie f. Mikrosk. Anal., vol. iii., 1867, p. 
 
 vo;?; ,?iLiL/*ii-i. ^vjni. ^.n-<ti in. n.i^, ^un/.. Jour. Micr. Set., vol. xx., 1880, p. 130, 
 
 (40) CARPEXTER (shell of Orbitolites), Phil. Trans. Roy. Soc. London, 1883, part ii. 
 
 (41) CARPENTER (Eoozoon), Quart. Jour. Geol. Soc., vols. xxi. and xxii. ; and 
 Annals and Mag. Nat. Hist., xiii. (42) HAECKEL (Radioloria), Jenaische Zeitschr., 
 xv., 1881. (43) CIEXKOWSKI (yellow cells of Radiolaria), Archit f. Mikrosk. 
 Anat., vii., 1871. (44) BRAXDT (yellow cells of Radiolaria), Monatsber. d. Berlin 
 Acad., 1881, p. 388. (45) GEUDES (yellow cells of Radiolaria), Nature, voL xxv., 
 1S82, p. 303. (46) HERTWIG (Radiolarian reproduction), "Der Organismus der 
 Radiolarien," Jenaische Denkschriften, 1879 ; also Zur Jlistologie der Radiolarien, 
 
 Lelpslc, 187C. (47) LEUCKABT (Sporozoa), Die mensthlicnen Parasiten, 2d ed., 
 1879. (4a) SCHNEIDER, AIM. (Gregariniaea), Archives d. Zoologie jirperim., 1873, 
 p. S15, 1875, p. 432 and p. 493, 1881, p. 387. (49) KLOSS (Coccidiide of Helix), 
 Abhand. d. Senkenberg. naturf. Gesellsch., i., 1855. (50) LANKESTER, E. RAT 
 (Drepanidium), Quart. Jour. Micr. Sri, voL xxii, 1882, p. 53. (51) LIEBERKCHM 
 (Coccidium of Frog's kidney), Archiv f. Anat. and Physiolog., 1854. (52) 
 CIESKOWSKI (Amcebidium), Botan. Zeitung, 19 Jahrg., 1861, p. 169. (53) Vos 
 LEXDEXFELD (parasitic amceboid organism), in Proceedings of Linnean Society of 
 New South Wales, 1885. (54) LAXKESTER, E. RAT (Monoci/stis pellucida), Quart. 
 Jour. Micr. Sci. (new series), vol. vi., 1866. (55) LANKHSTER, E. RAT (Mono- 
 cystis aphroditx). Quart. Jour. Micr. Sci. (new series), vol. iii., 1863. (56) 
 HERTWIO, R. (AphrothoracaX in Organismus der Radiolarien, Jena, 1879. (57) 
 ARCHER (Chlamydophora), " Resume", <fec.," Quart. Jour. Micr. Set., vol. xvi. f 
 1876. (58) HERTWIG, R., and LESSER (Chalarothoraca), Archiv f. Uikrosk. Anat., 
 x., Supplement, 1874. (59) LANKESTER, E. RAT (Haliphysema), Quart. Jour. 
 Micr. Sci. (new series), vol. xix., 1879. (60) HAECKEL (Physmaria), Jenaische 
 Zeitschr., x, (61) BESSELS (Astrorhiza), Jenaische Zeitschr., ix. (62) CARPEJJTER 
 (classification of Reticularia), "Reseai'ches on the Foraminifera," Phil. Trans., 
 1856-59-60. (63) HAECKEL (Radiolaria), Die Radiolarien, Berlin, 1862. (64) 
 LANKESTER, E. RAT (term Corticata), Preface to the English editionof Gegenbaur's 
 Elements of Comparative Anatomy, 1878. (65) CIESKOWSKI (Ciiiophrys), Archia 
 f. Mikrosk. Anat., xii., 1876, p. 15-50. (66) UALLIXGER and DRTSDALK (hooked 
 and springing -Monads), a seriis of papers in the Monthly Microscopical Journal, 
 1873-74-75. (67) DALLISGKE (Trepomonas), President's Address, Jour, of the 
 Roy. Micr. Soc., April 1885. (68) JAMES CLARK (Choanoflagellata, Memoirs of 
 the Boston Society of Nat. Hist., 1867, vol. i. (69) SAVILLE KEHT (Choano- 
 flagellata), Monthly Microscopical Journal, vol. vi., 1871. (70) LEWIS, T. R. 
 (Hasmatozoic Flagellata), Quart. Jour. Micr. Set., vol. xxiv., 1884, and voL xix, 
 1879. (71) SAVILLE KENT, Manual of the Infusoria, London, 1882. (72) 
 SALLITT, J. (chlorophyll of Ciliata), Quart. Jour. Micr. Sci., 1884. (73) BALBIASI 
 (sexuality of Ciliata), Journal de la Physiologic, i., iii., and iv., and Archives de 
 Zool. Eiperim., ii., 1873. (74) BLTSCHLI (conjugation of Ciiiata), Abhand. d. 
 Senkenberg. naturf. Gesellschaft., x., 1876. (75) LANKESTER (Opalina=Anaplo- 
 phyra), Quart. Jour. Micr. Sci. (new series), vol. x., 1870. (76) ALLMAX (encysted 
 Vorticellse), Quart. Jour. Micr. Sci. (new series), vol. xii, 1872, p. 393. (77) 
 HAECKEL (structure of Ciliata), Zur Morphologie der Infusorien, Leipsic, 1873. 
 
 (78) ALLMAN (trichocysts of Ciliata), Quart. Jour. Micr. Sci., vol. iii., 1855. 
 
 (79) STEIS (relations of AcineUe to Ciliata) Der Organismus der Jnfusionsthiere, 
 Abth. L, Leipsic, 1859. (80) STEIS (Dinoflagellata). Der Organismus, &c., Abth. 
 iii., Uipsic, 1883. (81) BEEGH (Dinoflagellata), Morphotoy. Jahrb., vii., 1881. 
 (82) BBrscHLi (Dinoflagellata), Morpholog. Jahrb., x., 1885. (83) KLEBS (Dino- 
 flagellataX Botan. Ztitung, 1884, pp. 722, 737. (84) GRUBER (nuclei of Ciliata), 
 Zeitschr. f. wiss. Zoologie, xi., 1884. (85) FOETTINGER (Opalinopsis, Ac.), 
 Archives de Biologie, vol. ii. (86) ALLMAN (Noctilnca), Quart. Jour. Micr. Sci. 
 (new series), vol. xii., 1872, p. 326. (87) CIENKOWSKI (Xoctilnca spores), Arch, 
 f. Mikrosk. Anal., vii., 1871. (88) HERTWIG (LeptodiscusX Jenaische Zeitschr^ 
 xL, 1877. (89) FRAIPOST, " Recherches snr les Acineiiniens de la c6te d'Ostende," 
 Bulletins de C 'Acad. Roy. Bruxelles, 1877-78. (9O) SCRIRAT, Uagasin de Zoologie, 
 1836. (91) ALLMAS (PeridiniumX Quart. Jour. Micr. Set., iii., 1855. (92) LEIDT, 
 U.S. Geological Survey of the Territories, vol. xii. (93) CLAPAREDE and LACHMASN, 
 Etudes sur les Jnfusoires tt les Rhizopodes, Geneva, 1858-61. (E. Ii. L.) 
 
 FIG. XXVII. Dinoflagellata. This figure is not contained in the 
 article as published in the Encyclopsedia Britannica. It presents 
 the recent discoveries of Klebs, Butschli, and Stein. 
 
 1. Diagram of the Dinoflagellate Hemidinium. n, nucleus ; /, 
 
 flagellum of the transverse groove ; h, flagellum of the vertical 
 groove. 
 
 2. Diagram of the Cryptomonadine Ozyrrhis (to compare with the 
 
 preceding), n, nucleus ; g, the deep fossa or pit in which the 
 two flagella are affixed ; t, the origin of the flagellum which 
 corresponds with that of the transverse groove of Dino- 
 flagellata. The second flagellum is seen to be attached near 
 the mouth of the fossa. 
 
 3. Glenodinium cinctum, Ehr., seen from the ventral surface, a, 
 
 amyloid granules ; b, eye-spot ; c, chromatophores ; d, flagellum 
 of the transverse groove; e, flagellum of the vertical groove; 
 v, vacuole. 
 
 4. The same, seen from the hinder pole (letters as in 3). 
 
 5. Cuticle of Histioneis cymbalaria, Stein, from the Atlantic, i, 
 
 ventral process ; k, cuticular collar ; I, posterior process. 
 
 6. The same, seen from the dorsal surface, m, cephalic funnel (k 
 
 and I as in 5). 
 
 7. Cuticle of Amphisolenia gldbifera, Stein, from the Atlantic, seen 
 
 from the left side, i, narrow ventral processes ; m, cephalic 
 funnel ; o, the mouth ; p, pharynx ; q, the shrunken proto- 
 plasm. 
 
 8. Cuticle of OmUhocercus magnificMS, Stein, from the Atlantic. 
 
 mm,', the cephalic funnel ; rr', the two large ribs of the 
 cutieular collar (the collar itself similar to k in No. 5 is not 
 drawn); s, the two rows of dorsal cuticular teeth. 
 
 9. Cuticle of Ccratocorys horrida, Stein, from the Southern 
 
 Ocean, i, the large frontal plate ; pp' the outgrown margins 
 of the transverse groove ; v, f 2 , basal plates ; w, one of the 
 four frontal horns ; x, the dorsal horn ; y, the ventral horn. 
 
 XXVII. 
 
39 
 SPONGES 
 
 (By W, Johnson, Sollas, LL.D., F.R.S., Professor of Geology, Trinity College, Dublin.) 
 
 THE great advance which has been made during the 
 past fifteen years in our knowledge of the sponges 
 is due partly to the vivifying influence of the evolutional 
 hypothesis, but still more to the opportunities afforded by 
 novel methods of technique. To the strength and weak- 
 ness of the deductive method Haeckel's work on the Kalk- 
 schwdmme (6) J is a standing testimony, while the slow but 
 sure progress which accompanies the scientific method is 
 equally illustrated by the works of Schulze (20), who by 
 a masterly application of the new processes has more 
 than any one else reconstructed on a sure basis the general 
 morphology of the sponges. In the general progress the 
 fossil sponges have been involved, and the application of 
 Nicol's method of studying fossil organisms in thin slices 
 has led, in the hands of Zittel and others (24, jj), to a 
 complete overthrow of those older classifications which 
 relegated every obscure petrifaction to the fossil sponges, 
 and consigned them all to orders no longer existing. 
 But, whilst many problems have been solved, still more 
 have been suggested. An almost endless diversity in 
 details differentiates the sponges into a vast number 
 of specific forms ; the exclusive possession in common of 
 a few simple characters closely unites them into a compact 
 group, sharply marked off from the rest of the animal 
 kingdom. 2 
 
 1 These italic numbers refer to the bibliography which will be 
 found at page 54. 
 
 2 Since, this was written, in 1887, four large monographs, includ- 
 ing considerably over 2000 pages of letterpress, have been published on 
 the Sponges. Three of these, viz. : Schulze on the Hexactinellida, 
 Ridley and Dendy on the Alonaxonida, and Sollas on the Tetractind- 
 lida appear as Reports of the "Challenger" Expedition, the fourth by 
 Von Lendenfeld ou the "Horny Sponges" as a special volume issued 
 by the Royal Society. With this addition to our knowledge a longer 
 preface than this would be possible, but for the general student the 
 following amended classification of the Afonaxonida will probably be 
 found sufficient. 
 
 Order. Monaxonida. 
 Sub-order 1. ASEMOPHORA, Sollas. 
 
 Family 1. HOMORAPHID.E, Ridley and Dendy. Megascleres either 
 oxeas or strongyles. No microscleres. Ex. : UaHchondria. 
 
 Sub-order 2. MEXISCOPHORA, Sollas. 
 
 The microscleres when present are sigmaspires, sigmas, or cymbas. 
 
 Family 1. HETERORAPHID.S, Ridley and Dendy. Megascleres of 
 various forms, microscleres never cymbas. Ex. : Rhizochalina, 0. S. 
 
 Family 2. DESMACIDONID.E, O.S. Megascleres usually monactinal, 
 microscleres cymbas. Ex.: Desmacidtm, 0. S. 
 
 Sub-order 3. SPINTHAROPHORA, Sollas. 
 
 The microsclere when present is some form of aster. 
 
 Group 1. HOMOSCLERA. The spicules are all microscleres. 
 
 Family 1. ASTROPEPLUXE. The microscleres are microxeas and 
 asters. Ex. : Astropeplus, Soil. 
 
 Group 2. HETEROSCLERA, Soil. Megascleres are always present, 
 and sometimes microscleres. 
 
 DEMOS 1. CENTROSPINTHARA, Soil. The microsclere when present 
 is a euaster. 
 
 Family 1. AXINELLID.S, O.S. Non-corticate, mesoderm collen- 
 chymatous, chamber system eurypylous. The skeleton consists of 
 axial and radial spicular fibres. Ex.: Axinella, O.S. 
 
 Family 2. DORYPLERID^E, Soil. Non-corticate, mesoderm collen- 
 
 Structure and Form. 
 
 Description of a Simple Sponge. As an example of Simple 
 one of the simplest known sponges we select Ascetta, sponge. 
 primordialis (fig. 1), Haeckel. This is a hollow vase-like 
 sac closed at the lower end, by which it is attached, 
 opening above by a comparatively large aperture, the 
 osculum or vent, and at the sides by numerous smaller 
 apertures or pores, which perforate the walls. Except for 
 the absence of tentacles and the presence of pores it offers 
 a general resemblance to some simple form of Hydrozoon. 
 Historically, however, it presents considerable dif- 
 ferences, since, in addition to an endoderm and an 
 
 FIG. 1. Ascetta primordialis, llaeckel. 
 After Haeckel. 
 
 ectoderm, a third or mesodermic layer contributes to 
 the structure of the walls ; and the endoderm consists of 
 cells (see fig. 21, (?) each of which resembles in all essential 
 features those complicated unicellular organisms known 
 as choanoflagellate Infusoria (see PROTOZOA, vol. xix. p. 
 858). With this positive character is associated a nega- 
 tive one : nematocysts are entirely absent. The activity 
 
 chymatous. Skeleton consisting of oxeas arranged without order. 
 Ex. : Dorypleres, Soil. 
 
 Family 3. TETHYID.E, Vosm. Corticate. Skeleton consisting of 
 radially arranged oxeas. The microsclere is a spheraster. Ex.: 
 Tethya, Lam. 
 
 DEMCS 2. SPIRASPINTHARA, Soil. The microsclere is a spiraster. 
 
 Family 1. SCOLOPID.E, Soil. The cortex is thin and fibrous, with 
 radially arranged closely-packed microxeas and oxeas. The skeleton 
 consists of oxeas collected into radially disposed fibres. The micro- 
 sclere when present is an amphiaster. Ex. : Scolopus, SolL 
 
 Family 2. SUBERITIIXE, O.S. Cortex with a skeleton of radially 
 arranged styles. Microscleres usually absent. The megascleres are 
 tylostyles. Ex. : Suberites, Nardo . 
 
 Family 3. SPIRASTRELLID.E, Ridley and Dendy. The megascleres 
 are rhabdi or styles. The microscleres are spiraster-> or discasters. 
 Ex.: Spirastrella, O.S. 
 
40 
 
 SPONGES 
 
 of the Anre.Ua, as of all sponges, is most obviously mani- 
 fested, as Grant (5) first observed, by a rapid outflow of 
 water from the osculo and a gentle instreaming through 
 the pores, a movement brought about by the energetic 
 action of the flagolla of the 
 eiidodennic cells. The in- 
 streaming currents boar with 
 them into the cavity of the 
 sac (paragastric cavity) both 
 protoplasmic particles (such as 
 Infusoria, diatoms, and other 
 .small organisms) and dissolved 
 oxygon, which are ingested by 
 the flagellated colls of the on- 
 dodorm. The presence of one 
 or more contractile vacuolos in 
 thoso colls suggests that they 
 extricate water, urea, and car- 
 
 liiinic arid. Thr insolnlilii re 
 
 sidtio of the introduced food, 
 together with the fluid excreta, 
 is carried out through the os- 
 cule by the excurrent water, 
 Now individuals are produced 
 from the union of ova and 
 spermatozoa, winch develop 
 from wandering aiiuuboid colls 
 in the mesoderm. The walls 
 of Amrttd are strengthened by 
 calcareous scloros, more especi- 
 ally designated as spicules, Pm. 2 nomoOtrma upaitdra, Lfd. 
 
 wliii-li linvn tlin form nf tri- " lmlf rut awftv 1>v tt vortical 
 
 I,..-. I IIMI HOOtloil. Ml.'. V. I .,.!, 
 
 radiate needles. If wo make fuiu(xaixmt<)). 
 
 abstraction of tliose wo obtain an ideal sponge, which 
 
 Haockel has called Olynthus (6), and which may bo ro- 
 
 Canal System. Wo shall now trace the several modifi- 
 cations which the Olynthus has undergone as expressed in 
 the different types of canal system. 
 
 The simple paragaster of Ascetta may become compli- Acoa 
 cated in a variety of ways, such as by the budding off tyl 16 - 
 from a parent form of stolon-like extensions, which then 
 give rise to fresh individuals, or by the branching of the 
 Ascon sac and the subsequent anastomosis of the branches ; 
 but in no case, so long as the sponge remains within the 
 Ascon typo, does the endoderm become differentiated into 
 different histological elements. The most interesting 
 modification of the Ascon form occurs in J/omoderma sy- 
 candra (is), in which from tho walls of a simple Ascon 
 ctecal processes grow out radiately in close regular whorls, 
 each process reproducing tho structure of the parent 
 sponge (figs. 2, 3). From this it is but a short step to 
 the important departure which gives rise to the Sycons. 
 
 In tho simplest examples of this typo tho characters of Sycon 
 Homoderma sycandra are reproduced, with the important 
 exception that tho endoderm lining the paragastric cavity 
 of tho original Ascon form loses its primitive character, 
 
 
 Km. 3. Houuxlerma lyoumim, I.fd. Transvi<nw sort Ion, showing radial tubes opening 
 Into coutml iwrtigaslrlc cavity. Aftor V. LoiKlciifuia (x about 12). 
 
 gardod as tho ancestral form from which all other sponges 
 have been derived. To give greater exactness to our ab- 
 straction wo should perhaps stipulate for tho Olynthw a 
 somewhat thicker mesoderm and more spherical form than 
 a decalcified Ascon presents. 
 
 T 
 
 Fio. 4. Httenptgrnawxlut-gardll, Pol. Partofatransvorsoscctlon. Thostraight 
 lines incliiMti- s|iiriilrs : tin' pnrit'iTcms MirfiuT is uppermost ; the branching 
 milial tubes lire rendered dark by nunummM small circles representing 
 clioiinocytes. After FoleJaelT, " Challenger" Report (x 50). 
 
 and from a layer of flagellated cells becomes converted 
 into a pavement epithelium, not in any distinguishable 
 feature different from that of the ectoderm. Tho 
 flagellated cells are thus restricted to the cajcal 
 outgrowths or radial tubes. Concurrently with 
 this differentiation of tho endoderm a more abun- 
 dant development of mc.soderm occurs. In some 
 Sycons (Sycaltis, Hk.) the radial tubes remain 
 separate and free ; in others they lie close together 
 and are united by trabeculse, or by a traliecular 
 network, consisting of mesodermic strands sur- 
 rounded by ectoderm (tig. 4). The spaces between 
 the contiguous radial tubes thus become converted 
 into narrow canals, through which water passes 
 from the exterior to outer the pores in tho walls 
 of tho radial tubes. These canals are the " inter- 
 canals " of Haeckel, now generally known by their 
 oklor name of incurrent canals. Tho openings of 
 the incurrent canals to the exterior are called 
 pores, a term which vr lia\r also applied to the 
 openings which U-ad directly into the radial tubes 
 or paragastric cavity ; to avoid ambiguity we shall 
 for the future distinguish the latter kind of open- 
 ing as a ]>i\>s<>i>i/l?. The term "pore" will then bo 
 restricted to the sense in which it was originally used by 
 (3 rant. Tho mouth by which a radial tube opens into tho 
 paragastor is known as a gastric ostium. In the higher 
 forms of Sycons tho radial tubes no longer arise as simple out- 
 growths of the whole sponge-wall, but rather as outgrowths 
 
SPONGES 
 
 41 
 
 of the endoderm into the mesodenn, which, together with 
 the ectoderm, exhibits an independent growth of its own ; 
 and this results in the formation of a thick investment, 
 known as the cortex (fig. 5), to the whole exterior of the 
 
 Fio.5. UteArgenlea, Pot Part of a transverse section. The concentric circles, 
 indicating transverse sections of spicnles, lie within the cortex. After Pole- 
 jaeff, " Challenger " Report ( x 100). 
 
 sponge. The radial tubes may branch, Heteropegma (fig. 
 4). If the branches are given off regularly, as the radial 
 tubes were in the first plan, and if at the same time the 
 original radial tube exchanges its flagellated for a pave- 
 ment epithelium, a structure as shown in fig. 6 (Polejna 
 
 Rhagon 
 
 type. 
 
 Fio. 6. Polejna connexlva, Pol. Part of a transverse section. E, excnrrent 
 canals, into which the flagellated chambers open. After Polejaeff, " Challenger" 
 Report (x 50). 
 
 connexiva, Pol.) will result. This form might also be 
 brought about by unequal growth of the gastral endoderm 
 leading to a folding of the inner part of the sponge-wall. 
 Very little direct evidence exists as to which of these two 
 plans has actually been followed. Phylogenetically the 
 transition from a simple Ascon to the most complicated 
 Sycon can be traced step by step ; and ontogeny shows 
 that such a Sycon form as Grantia raphanus passes through 
 an Ascon phase in the course of its larval development. 
 
 Returning to the ancestral form of sponge, Olynthus, 
 let us conceive the endoderm growing out into a number 
 of approximately spherical chambers, each of which com- 
 municates with the exterior by a prosopyle and with the 
 paragastric cavity by a comparatively large aperture, which 
 we may term for distinction an apopyle; at the same time 
 let the endoderm lose its flagellated character and become 
 
 converted into a pavement epithelium, except in the 
 spherical chambers. Such a form, called by Haeckel 
 " dyssycus," may be more briefly named a Rimy on from 
 the grape-like form of its flagellated chambers, which differ 
 from those of a Sycon both by their form and their smaller 
 dimensions. The Ehagon occurs as a stage in the early 
 development of PlaJcina motvolopha (Schulze) and Reniera, 
 fertilis (9) (fig. 7) ; a calcareous sponge which appears to 
 
 Fio. 7. Vertical section of a Rliagon, partly diagrammatic, o, oscule ; p, 
 paragaster. After Keller (X about 100). 
 
 approach it somewhat is Leucopsis pedunculata, Lfd. By 
 the folding of the wall of a Ehagon, or by its outgrowth 
 into lobes, a complicated structure such as that of Plakina, 
 monolopha (20) (see fig. 26 /) results. This is character- 
 
 Fio. 8. Transverse section across an excurrent canal and surrounding choanc- 
 some of Cydonium eosaster, Soil, e, excurrent canal ; /, flagellated chambers 
 communicating with it by aphodal canals ; i, an incurrent canal cut across ; s, 
 a sterraster ; o, an oxea cut across. After Sollas, * ' Challenger " JKeport ( x 1 25). 
 
 ized by the chambers retaining their immediate communi- 
 cation with the incurrent and excurrent canals, opening 
 into the latter by the widely open apopyle and receiving 
 the former by one ( 
 several prosopyles. This J 
 may be termed the eury- \ 
 pylous type of Rhagon K 
 canal system. The fold- - 
 ing of the sponge-wall 
 may be simple, as in the 
 example given, or too 
 complex to unravel. In 
 higher forms of sponges ' 
 (Geodinidx, Stellettidx) 0' 
 the chambers cease to 
 open abruptly into the 
 excurrent canals : each is ; 
 prolonged into a narrow 
 canal, apkodus, or abitiis, 
 which usually directly, ; 
 sometimes after uniting 
 
 with one Or more of its Flo . 9 ._ D iplodal canal system in Corticium 
 
 fellows, Opens into an candelabrum, O.S. f, excurrent canal ; the 
 
 . ,, incurrent canal is shown on the left-hand 
 
 excurrent Canal. Ine side, near its commencement in the cortex. 
 
 prosopyles, now restrict- After F - E - Schulze <> 
 ed to one for each chamber, may remain unchanged in 
 character, or at the most be prolonged into very short 
 
 F 
 
42 
 
 SPONGES 
 
 tubes, each a prosodus or aditw (fig. 8). This may be 
 termed the aphodal or racemose type of Rhagon system, 
 since the chambers at the ends of the aphodi radiating 
 from the excurrent canal look like grapes on a bunch. As 
 Haeckel, however, has used "racemose" in a different sense, 
 \ve shall adopt here the alternative term. By the exten- 
 sion of the prosodal or adital canals into long tubes a still 
 higher differentiation is reached (fig. 9). This, which from 
 the marked presence of both prosodal and aphodal canals 
 may be termed the diplodal type of the Rhagon canal 
 system, occurs but rarely. Chondrosia is an example. 
 
 The following scheme will render clear the foregoing 
 distinctions : 
 
 1. Ascon type : simple, ex. Ascctta, Hk. ; strobiloid, ex. Hoino- 
 
 derma, Lfd. 
 
 2. Sycon type : simple radial tubes, ex. Syceltrt., Hk. ; branched 
 
 radial tubes (cylindrical chambers), ex. Ifetcropegma, Fl. ; 
 chamber-layer folded, ex. Polcjna, Pol. 
 
 3. Rhagon type : eurypylous, with several prosopyles to each 
 
 chamber, ex. Spongclia ; with a single prosopyle to each 
 chamber, ex. Oscarclla, Thenca ; aphodal, aphodal canals well 
 developed, ex. Gcodia, Lmk. ; diplodal, with both aphodal 
 and prosodal canals well developed, ex. Chondrosia, O.S. 
 
 In the case of the calcareous sponges Polejaeff has argued 
 forcibly that the eurypylous type arises directly from the 
 Sycon and not from the Rhagon. It is therefore doubtful 
 how far the Rhagon in other sponges is a primitive form 
 derived directly from an Olyntkm, or whether it may not 
 be a secondary larval state resulting from the abbreviated 
 development of a former Sycon predecessor. Whatever 
 may have been its past history, the Rhagon serves now at 
 all events as a starting-point for the development of the 
 higher forms of canal system. 
 
 Subder- In the higher Rhagons, as in the Sycons, further com- 
 ma l plications ensue, owing to an independent growth of the 
 cavities. ex t e mal ectoderm and the adjacent mesoderm. While the 
 endoderm, with its associated mesoderm, is growing out 
 or folding to form the excurrent canal system, the super- 
 ficial mesoderm increases in thickness, and the ectoderm, 
 extending laterally from the sides of the incurrent sinuses, 
 burrows into it, parallel to the surface of the sponge. 
 Thus it forms beneath the skin (i.e., the layer of superficial 
 mesoderm and investing ectoderm) cavities which may be 
 either simple and spacious or be broken up into a number 
 of labyrinthine passages by a network of mesoblastic 
 strands (invested with ectoderm) which extend irregularly 
 from roof to floor of the chamber. These cavities are 
 known as subdermal chambers. 
 
 With the appearance of subdermal chambers the sponge 
 Ecto- becomes differentiated into two almost independent regions, 
 some, an ou ter or ectosome and an inner or choanosome, which is 
 me" " cuarac terized by the presence of flagellated chambers. 
 The ectosome forms the roof and walls of the subdermal 
 chambers, and is in its simplest form merely an investing 
 skin ; but in a large number of sponges it acquires con- 
 siderable thickness and a very complicated histological 
 structure. It is then known as a cortex. The thickening 
 which gives rise to a cortex takes place chiefly beneath 
 those parts of the skin which are not furnished with pores. 
 Beneath the pores in this case collected into sieve-like 
 areas dome-like cavities are left in the cortex; they open 
 freely into the subdermal cavities below and their roof is 
 formed by the cribriform pore membrane above. In many 
 sponges (Geodia, Stelletta) the cortical domes are constricted 
 near their communication with the subdermal cavity (sub- 
 cortical crypt) by a transverse muscular sphincter, which 
 defines an outer division or ectochone from an inner or 
 endockone (fig. 10), the whole structure being a chone. 
 Chone. The endochone is frequently absent (fig. 10). The early 
 development of the cortex has scarcely yet been studied. 
 In Stelletta 2^hrissens (Soil.), one of the " Challenger " Stel- 
 
 lettidx, an early form of the sponge (fig. 11), shows the 
 choanosome already characteristically folded within the 
 cortex, which forms a com- 
 plete not-folded envelope 
 around it. The roots of 
 the incurrent sinuses form 
 widely open spaces imme- 
 diately beneath the cortex 
 and are the rudiments of 
 subcortical crypts. Again, 
 in some sponges a part of 
 the endoderm and asso- 1 
 ciated mesoderm may like- 
 wise develop independ- j 
 ently of the rest of the 
 sponge, as in the Hexac- 
 tinellida, where the choa- 
 nosome forms a middle 
 layer between a reticula- 
 tion of ectosome on the 
 one side and of endoderm 
 and mesoderm, i.e., endo- [ 
 some, on the other. Fin-U . , ... 
 
 nllv tVif> nttarViprl nr Imvpr Fl - 10. Section through the cortex of Cy- Endo- 
 ally, tlie attached lower d?nium eomst er, Soil., showing the pore- some, 
 half of a Rhagon may de- sieve overlying the chone, which com- 
 
 velop in an altogether dif- 
 ferent manner from the 
 other or upper half, the 
 endoderm not producing 
 
 any flagellated chambers. In this case the upper portion 
 alone is characterized by the flagellated chambers, which 
 are the distinctive mark of a sponge, and hence may be 
 
 municates through a sphinctrate aperture 
 W ith the sulwortical crypt, lying in the 
 choanosome with its nagellatod chambers. 
 The dotted circles in the cortex are sterr- 
 asters connected by fibrous strands. 
 
 After Jas '" ( 
 
 FIG. 11. Young sponge of Stelletta pTirissen-s, Soil. Longitudinal median sec- 
 tion, showing the choanosome folded within the cortex, o, oscule. After 
 Sollas, " Challenger" Report (x60). 
 
 called the spongomere ; the lower half, which consists of 
 all three fundamental layers, may be called the hypomere. 
 The form and general composition of sponges are ex- 
 ceedingly various and often difficult to analyse, presenting, 
 along with some important differences, a remarkable general 
 resemblance to the Ccelentera in these respects. Like Oscule, 
 them, some sponges are simple, and others, through 
 asexual multiplication, compound. The only criterion by 
 which the individual sponge can be recognized is the oscu- 
 lum ; and, as it is frequently difficult, and in many cases 
 impossible, to distinguish this from the gastric opening of 
 a large excurrent canal, there are many cases in which the 
 simple or compound nature of the sponge must remain 
 open to doubt. The oscule may also fail (lipostomosis), 
 and so may the paragastric cavity (lipogastrosis) ; the 
 problem then becomes insoluble. The loss of the oscule 
 
SPONGES 
 
 43 
 
 Mineral 
 spicules. 
 
 may in some cases be due to the continued growth of 
 several endodermal folds towards the exterior, with a 
 corresponding absorption of the mesodenn and ectoderm 
 which lie in the way, till the folds penetrate to the ecto- 
 derm and open at the exterior, thus giving rise to excurrent 
 openings, which are not readily distinguishable from pores. 
 At the same time the original osculum closes up and 
 entirely disappears. Lipogastrosis, on the other hand, 
 may be produced by the growing together of the roots of 
 the choanosomal folds, thus reducing the paragastric cavity 
 to a labyrinth of canals, -which may easily be confounded 
 with the usual form of excurrent canals. While in some 
 sponges the original oscule is lost, in others secondary 
 independent openings, deceptively like oscules, are added. 
 This pseudostomosis is due to a folding of the entire sponge, 
 so as to produce secondary canals or cavities, which may 
 be incurrent (testibular) or excurrent (cloacal), the opening 
 of the latter to the exterior being termed a false oscule 
 or pseudostome. The faulty use of the term oscule for 
 what is neither functionally nor morphologically a mouth 
 is here obvious, for in one sense the oscule is always a 
 pseudostome ; it would be better if the term pseudoproct 
 could be substituted. 
 
 Skeleton. Skeleton. All sponges, except three or four genera be- 
 longing to the Jfyxosponffise, possess some kind of skeletal 
 .structures. They may be either calcareous or silicious or 
 horny scleres, the latter usually having the form of fibres, 
 which sometimes enclose silicious needles (spicules) or 
 foreign bodies introduced from without. Foreign bodies 
 also contribute to the formation of the skeleton of some 
 silicious sponges, and occasionally form the entire skeleton, 
 no other hard parts being present. 
 
 Mineral scleres usually occur in the form of spicules. 
 The spiculeg of calcareous sponges consist of carbonate of 
 lime, having the crystalline structure and other properties 
 of calcite (29). Each spicule, so far as its mineral com- 
 ponent is concerned, is a single crystal, all the molecules 
 of calcite of which it is built up being similarly oriented. 
 On the other hand, its form and general structure are 
 purely organic. Its surfaces are always curved, and usually 
 it has the form of a cone or combination of cones, each of 
 which consists of concentric layers of calcite surrounding 
 an axial fibre of organic matter, probably of the same 
 nature as spongiolin or spongin, the chief constituent of 
 the fibres of horny sponges. A thin layer of organic matter, 
 known as the spkule sheath, forms an outer investment to 
 the spicule and is best rendered visible as a residue by 
 removing the calcite with weak acid. Silicious spicules 
 consist of colloid silica or opal, and hence can be distin- 
 guished from calcareous by having no influence upon polar- 
 ized light. Structurally the two kinds of spicules present 
 no important difference. The spicules of different sponges 
 differ greatly both in form and in size. They may be 
 conveniently divided into two groups, minute or flesh 
 spicules, which usually serve as the support of a single cell 
 only (microsderes), and larger or skeletal spicules, which 
 usually contribute to the formation of a more or less con- 
 sistent skeleton (megascleres). The distinction is not one 
 that can be exactly defined, and must so far be regarded 
 as of a provisional nature. There is usually but little diffi- 
 culty in applying it in practice, except in some doubtful 
 cases where large spicules do not form a continuous skeleton, 
 or in others where flesh spicules appear to be passing into 
 those of larger size. It is indeed highly probable that all 
 large spicules have originated from flesh spicules (12). 
 
 (1) Monaxon Eiradiate Type. (rhabdus). By far the 
 commonest form is the oxea, a needle-shaped form pointed 
 at both ends and produced by growth from a centre at the 
 same rate in opposite directions along the same axis. It 
 is therefore uniaxial and enuibiradiate (fig. 12 a). (2) Mon- 
 
 axon Uniradiate Type (stylus). By the suppression of one 
 of the rays of an oxea, an acuate spicule or stylus results 
 (fig. 12 b). (3) Triaxon Triradiate Type. Linear growth 
 
 scleres. 
 
 a 
 
 V 
 
 FIG. 12. Typical megascleres. a, rhabdus (monaxon diactine); d, stylus 
 (monaxon monactine) ; c, triod (triaxon triactine) ; d, calthrops (tetraxon 
 tetractine) ; f, triaxon hexactine ; /, desms of an anomocladine Lithistid 
 (polyaxon) ; g, sterraster (polvaxon) ; A, radial section through the outer 
 part of g, showing two actines soldered together by intervening silica, the 
 free ends terminating in recurved spines and the axis traversed by a central 
 fibre. 
 
 from a centre in three directions inclined at an angle of 
 120 to each other gives rise to the primitive form of tri- 
 radiate spicule so eminently characteristic of the calcareous 
 sponges, but by no means confined to them (fig. 1 2 <). (4) 
 Tetraxon Quadriradiate Type (Calthrops). Growth from a 
 centre in four directions inclined at about 110 to each 
 other produces the primitive quadriradiate form of the 
 Tetractinellida and of some calcareous sponges (fig. 12 d). 
 (5) Sexradiate Type. Growth in six directions along three 
 rectangular axes produces the primitive sexradiate spicule 
 of the Hexactinellida sponges (fig. 12 e). (6) Mtdtiradiate 
 Type. Extensions radiating in many directions from a 
 centre produce a stellate form (fig. 12/). (7) Spherical 
 Scleres. Concentric growth of silica about an organic 
 particle produces the sphere, which occurs as a reduction 
 of the rhabdus in some species of PaecUlastra, or as an 
 overgrown globule (flesh spicule) in Caminus. 
 
 Usually conical, the spicular rays often become cylindrical ; usu- Uniaxial 
 ally pointed (oxeate) at the ends, they are also frequently rounded type, 
 off (strongylate), or thickened into knobs (tylotaie), or branched 
 (dadosc}. Their growth is not always rigorously confined to a 
 
 FIG. 13. Modifications of monaxon type, a, strongyle ; 6, tylote ; c, oxea ; d, 
 tylotoxea; , tylostyle; /style; g, spined tylostyle; fc, sagittal triod (a 
 triaxon form derived from the monaxon) ; j, oxytylote ; I-, anatriscne ; I, pro- 
 triaene ; m, orthotriaaie ; n, dichotriaene ; o, centrotriaene ; p, amphitrisene 
 (this is trichocladose) ; q, crepidial strongyle (basis of Rhabdocrepid Lithistid 
 desma) ; r, young form of Rhabdocrepid desma, showing crepidial strongyle 
 coated with successive layers of silica ; s, Rhabdocrepid desma fully grown. 
 The dotted line through the upper figures marks the origin of the actines. 
 
 straight line : frequently they are curved or even undulating. They 
 are also liable to become spined, either by mere superficial thicken- 
 ing or by a definite outgrowth involving the axial fibre (fig. 13 g, h). 
 The rhabdus if pointed at both ends is known as an oxea (fig. 
 13 c) ; if rounded at both ends as a slrongylc (fig. 13 n) ; if knobbed 
 
44 
 
 SPONGES 
 
 Triradi- 
 ate type, 
 
 Quadri- 
 radiate 
 type. 
 
 at both cuds as a tylote (fig. 13 V) ; the tylote if pointed at one end 
 is a tylotoxca (fig. 13 d) ; the strongyle similarly becomes a strongyl- 
 oxea. These last two forms are with difficulty distinguished from 
 the stylus, which is usually pointed at the end, and strongylate (fig. 
 13/) or tylotate (fig. 13 c) about the origin. A particular case of 
 the cladose rhabdus, but one of the most frequent occurrence, is 
 the triiene ; in this form one ray of a rhabdus ends in three branches, 
 which diverge at equal angles from each other. The rhabdus then 
 becomes known as the shaft or rhabdomc, and the secondary rays 
 are the arms or cladi, collectively the head or cladome of the spicule. 
 The arms make different angles with the shaft : when recurved a 
 grapnel or anatriiene is produced (fig. 13 k), when projecting forwards 
 a protrisene (fig. 13 1), and when extended at right angles an ortho- 
 trisene (fig. 13 m). The arms of a triame may bifurcate (dichotrisene) 
 once (.fig. 13 n), twice, or oftener, or they may trifurcate. Again, 
 they may extend laterally into undulating lamellae, or unite to form 
 a disk, the trifene character of which is indicated by the included 
 axial fibre. The shaft may also become trifid at both ends, amphi- 
 trissne (fig. 13 p), and the resulting rays all bifurcate, or the cladome 
 may arise from the centre of the rhabdome, centrotrisene (fig. 13 d). 
 Amongst one group of Lithistid sponges (Rhabdocrepida) the normal 
 growth of a strongyle is arrested at an early stage ; it then serves 
 as a nucleus upon which further silica is deposited, and in such a 
 manner as to produce a very irregularly branching sclere or desma 
 (fig. 13 s), within which the fundamental strongyle can be seen en- 
 closed. In such a desma no axial fibre besides that of the enclosed 
 strongyle is formed. 
 
 The chief modification of the triradiate spicule is due to an elonga- 
 tion of one ray, distinguished as apical, the shorter paired rays 
 being termed basal, and the whole spicule a sagittal triradiate. The 
 angle included by the basal rays is usually over 120" (fig. 14 a}. 
 
 Some or all of the rays of the primitive calthrops (fig. 14 b) may 
 
 CV; 
 
 FIG. 14. Modifications of the triaxon and tetraxon types, n, sagittal triradiate 
 or triod ; b t calthrops ; c, candelabra (a polycladose microcalthrops) ; d, a 
 spiued microcalthrops ; e, Tetracladine Lithistid desma. 
 
 subdivide into a number of terminal spines candelabra (fig. 14 c) ; 
 
 or some or all of them may bifurcate once or twice and finally 
 
 terminate by subdividing into numerous variously shaped processes ; 
 
 such a tctracladine desma (fig. 14 e) characterizes one division of the 
 
 Lithistid sponges. 
 
 Sexradi- By the excess or defect of one or more rays a series of forms such 
 ate type, as are represented in fig. 15 arise. In the oxea, which results from 
 
 FIG. 15. Modifications of the triaxon hexactine type. , dagger ; b, c, two 
 varieties of pinnulus ; d, amphidisk ; c, pentactine ; /, staurus ; 0, dermal 
 rhabdus. After Sclmlze. 
 
 the suppression of all rays but two, the sexradiate character is some- 
 times preserved by the axial fibre, which gives off two or four pro- 
 cesses in the middle of the spicule where the defective arms would 
 arise. Let fig. 12 e represent a regular sexradiate spicule with its 
 four horizontal arms extended beneath the dermis of its sponge ; 
 the over-development of the proximal ray and a reduction of the 
 distal ray produce a form known as the dagger (fig. 15 a] ; the 
 suppression of the proximal ray and the development of spines pro- 
 jecting forwards on the distal ray produce the pinnulus (fig. 15 ft, c) ; 
 the suppression of both proximal and distal rays gives the staurus 
 (fig. 15/), and the suppression of two of the remaining horizontal 
 rays a dermal rhabdus (fig. 15<jr). The suppression of a distal ray, 
 excessive development of a proximal ray, and recurved growth of 
 the remaining rays produce an anchor. In Hyalonema (glass rope 
 sponge) anchors over a foot long occur, but their arms or teeth are 
 not restricted to four, and the axial fibre gives off its processes 
 before reaching the head of the spicule. Such a grapnel helps to 
 support the sponge in the ooze of the sea-bed. Other character- 
 
 Fia. 16. a, uncinaria ; 6, clavula ; c, scopularia. After Schulze. 
 
 istic spicules belonging to sponges distinguished by sexradiate 
 spicules are the following : the uncinaria (fig. 16 a), a spiuose 
 
 oxea with the spines all pointing one way ; the clavula, a tylotate 
 form with a toothed margin to the head (fig. 16 b); the scojndaria, 
 (fig. 16 c), a besom-shaped spieule with tylotate rays, which vary 
 in number from two to eight ; the amphidisk (fig. 15 d), a shaft 
 terminating at each end in a number of recurved rays. When the 
 sexradiate spicules of the llcxactinellida unite together in a manner 
 to be described later, the rays may be bent in a variety of ways 
 out of the triaxial type, so that the sexradiate character alone 
 remains. 
 
 Multiradiate Type. The rays of an aster as of other spicules Multi- 
 may be spined or tylotate. In one remarkable form known as a radiate 
 stcrraster (fig. 12 g, h), and characteristic of the family Oeodinidss, type, 
 the rays are almost infinite in number, and coalesced for the greater 
 part of their length ; the distal ends, however, remain separate, 
 and, becoming slightly tylotate, are produced into four or five re- 
 curved spines, which give attachment to connective tissue fibres 
 by which adjacent sterrasters are united together. 
 
 In one aberrant group of Lithistid sponges (Anomocladina) the 
 skeleton is formed of desmas, which are multiradiate, each present- 
 ing a massive centrum (with an included cavity) produced into a 
 variable number (4 to 8) of rays, which rays terminate in expanded 
 ends (fig. 12/). 
 
 It is doubtful whether a distinction between megascleres and Micro- 
 microscleres can be maintained in the calcareous sponges, unless scleres 
 the minute oxeas which occur in Eilhardia schulzci, Pol. (16), are 
 to be referred to this group. They are widely distributed through- 
 out the silicious sponges, and by their different forms afford charac- 
 ters of the highest importance in classification. 
 
 One of the simplest forms is the sigmaspire (fig. 17 a, b) ; it looks 
 like the letter C or 8, according to the direction in which it is 
 
 FIG. 17 Microscleres. a, 6, sigmaspire viewed in different directions, a, along 
 axis, and 6, obliquely ; c, toxaspire ; d, spiraster ; e, sanidaster ; /, amphi- 
 aster ; g, sigma or cymba ; h, cymha, with three ptera at eacli end, the central 
 one a proral pteron and the lateral, pleural ptera ; j, one end of another form 
 of cymba, showing seven ptera ; k, monopteral cymba, proral ptera only, 
 developed at ends, tropidial ptera much enlarged ; I, oocymba, in which proral 
 and pleural ptera have grown towards each other and coalesced ; m, spher- 
 aster ; n, oxyaster ; o, the same, with six actines ; p, the same, with four 
 actines ; 5, the same, with two actines (a ccntrotylote microxea); r, micro- 
 tylote ; s, microxea (7, r, and s are reduced asters) ; (, rosette. 
 
 viewed, its actual form being that of a single turn of a cylindrical 
 spiral. A turn and a part of a turn of a spiral of somewhat higher 
 pitch than that of a sigmaspire gives the toxaspire (fig. 17 c) ; a con- 
 tinued spiral growth through several revolutions gives the poly- 
 spire. The sigmaspire becoming spined produces the spiraster or 
 spinispirula (fig. 17 d) ; this, by losing its curvature, becomes the 
 sanidaster (figr 17 e), and by simultaneous concentration of its spines 
 into a whorl at each end, the amphiaster (fig. 17/). By reduction 
 of the spire the spiraster passes into the stellate or aster (fig. 17 n). 
 A thickening about the centre of the aster produces the sphcrastcr 
 (fig. 17m), allied to which is the sterraster. By a reduction in the 
 number of its rays the aster becomes a minute calthrops, from which, 
 by increased growth, the skeletal calthrops may very well be derived ; 
 by further reduction to two rays a little rhabdus or microrabd re- 
 sults, and of this numerous varieties exist, of which the oxeate 
 microrabd is the most interesting, since it only differs in size from 
 the commonest of all skeletal spicules, the oxeate or acerate rhab- 
 dus. The sigmaspire is formed as a superficial spiral thickening 
 in the wall of a spicule cell or scleroblast ; as superficial deposits 
 also the next group of spicules, the so-called anchorates, arise. 
 Take a hen's egg as the model of a scleroblast, draw round it a 
 broad meridional band, interrupted only on one side, for 30 above 
 and below the equator ; this will represent a truly C-shaped spicule, 
 which differs from a sigmaspire by the absence of spiral twist. 
 It may be termed a cymba (fig. 17 g). The back of the " C " is the 
 keel or tropis; the points are the prows or prorse. Now broaden out 
 the prora on the eggshell into oval lobes (proral ptercs) ; and from 
 each pole draw a lobe midway between the prora and the tropis 
 (pleural ptercs), and a common form of anchorate, the ptcrocymba 
 
SPONGES 
 
 45 
 
 results (fig. 17 h). The pterocymba is subject to considerable modi- 
 fications : the prows may be similar (homoproral) or dissimilar 
 (hetcroproral) ; the pteres may be lamellar or ungual ; additional 
 lamellae (tropidial pteres) may be produced by a lateral outgrowth 
 of the keel (fig. 17 k) ; and by growing towards the equator the 
 opposed proral and pleural pteres may conjoin, producing a spicule 
 of two meridional bands (oocymba ; fig. 17 I). A curious group of 
 flesh spicules are the trichites. In this group silica, instead of being 
 deposited in concentric coatings around an axial fibre, forms within 
 the scleroblast a sheaf of immeasurably fine fibrillse or trichites, 
 which may be straight (fig. 17 m) or twisted. The trichite sheaf 
 may be regarded as a fibrillated spicule. Trichite sheaves form in 
 some sponges, as Dragmastrd, (25), a dense accumulation within 
 the cortex. In Hexactinellid sponges the rays of the aster are 
 limited to six, arranged as in a primitive sexradiate spicule, but 
 divided at the ends into an indefinite number of slender filaments, 
 which may or may not be tylotate, rosettes (fig. 17 1). 
 Spongin Spongin is a horny substance, most similar to silk in 
 Bcleres. chemical composition, from which it differs in being in- 
 soluble in an ammoniacal solution of copper sulphate 
 (cuproso-ammonium sulphate). In Darwinella aurea, F. 
 Miiller, it occurs in forms somewhat resembling tri-, 
 quadri-, and sex-radiate spicules. But usually the spongin 
 skeleton takes the form of fibres, consisting of a central 
 core of soft granular substance around which the spongin 
 is disposed in concentric layers, forming a hollow cylinder 
 (fig. 23 b). The relative diameters of the soft core and 
 of the spongin cylinder differ greatly in different sponges. 
 The fibres branch so as to form antler-like twigs or bushy 
 tree-like growths, or anastomose to form a continuous net- 
 work, as in the bath sponge (Eusponyia offidnalis). The 
 detailed characters of the network differ with the species, 
 and are useful in classification. In Janthella certain cells 
 (sponginblasts) become included between the successive 
 layers of the spongin cylinder, and their deep violet colour, 
 contrasting with the amber tint of the spongin, renders 
 them very conspicuous. 
 
 Union of In some sponges the scleres are simply scattered through the 
 scleres mesoderm and do not give rise to a continuous skeleton, Cortirium, 
 into a Ch-ondrilla, Thrombus. In the Cakarea and many silicious sponges 
 skeleton, they are dispersed through the mesoderm, but so numerously that 
 by the overlapping of their rays a loosely felted skeleton is pro- 
 duced. In the calcareous sponges the spicules are frequently regu- 
 larly disposed ; and in the Sycons in particular a definite arrange- 
 
 Fio. 1& Articulate and inarticulate tubar skeletons of calcisponges. a, articu- 
 late ; b, inarticulate skeleton. After Haeckel. 
 
 ment, on two plans, the articulate and inarticulate, can be traced 
 in the skeleton of the radial tubes. On the latter plan the triradi- 
 ate or quadriradiate spicules, the apical rays of which are of con- 
 siderable length, are arranged in two sets, one having the basal 
 rays lying in the mesoderm of the paragastral wall and the other 
 with the corresponding rays in the dermal mesoderm. The apical 
 rays of each set lie in the mesoderm of the radial tubes parallel to 
 their length, but pointing in opposite directions (fig. 18 b). In the 
 articulate division numerous spicules, small in comparison with 
 the size of the radial tubes, form a series of rows round the tubes, 
 their basal rays lying parallel to the paragastric surface and the 
 apical pointing towards the ends of the radial tubes (fig. 18 a). 
 
 In the Silicispongia sheaves of long oxeate spicules radiate from 
 the base of the sponge if of a plate-like form, or from the centre if 
 globular, and extend to the surface. If trianes are present their 
 arms usually extend within the mesoderm immediately below the 
 
 dermal surface (fig. 19). Single spicules reach from centre to sur- 
 face only in small sponges. As the sponge increases in size the 
 spicules must either correspondingly lengthen, or fresh spicules 
 must be added, if a , / 
 
 continuous skeleton is 
 to be formed. The 
 latter is the plan fol- 
 lowed in fact : the ad- 
 ditional spicules over- 
 lap the ends of those 
 first formed like the 
 fusiform cells in a 
 woody fibre. "With the 
 formation of a fibre, 
 often strengthened by 
 spongin or bound to- 
 gether with connective 
 tissue, there appears to 
 be a tendency for the 
 constituent spicules to 
 diminish in size, and 
 the length of each iu 
 
 the most markedly Fio. 19. Mode of arrangement of spicules in a 
 fibrous sponges is in- 3g A^SouL SP ^' Dragmastm normini ' 
 significant when com- 
 pared with the length of the fibre. The spicular fibre thus 
 formed may be simple or echinated by spicules either similar to 
 those which form its mass or different. More usually they are 
 different, and generally styles, often spinose about their origin. 
 The spongin which sometimes cements together the spicules of a 
 fibre may progressively increase in quantity and the spicules di- 
 minish in number, till a horny fibre containing one or more rows 
 of small oxeas results. In an echinated fibre the axial spicules 
 may disappear and the eehinating spicules persist. Finally all 
 spicules may be suppressed and the norny fibre of the Ceratose 
 sponges results. The horny fibres may next acquire the habit of 
 embedding foreign bodies in their substance, though foreign en- 
 closures are not confined to the Ceratosa but occur in some Silici- 
 spongix as well. The included foreign bodies may increase in 
 quantity out of all proportion to the horny fibres ; and finally the 
 skeleton may consist of them alone, all spongin matter having 
 disappeared. 
 
 In the Lithistid sponges a skeleton is produced by the articula- 
 tion of desmas into a network. The rays of the desmas (figs. 12 f, 
 13 s, 14 e) terminate in apophyses, which apply themselves to some 
 part of adjacent desmas, either to the centrum, shaft, arms, or 
 similar apophyses, and then, growing round them like a saddle on 
 a horse's back, clasp them firmly without anchylosis. Thus they 
 give rise to a rigid network, in conjunction with which fibres com- 
 posed of rhabdus spicules may exist. In the Hexactinellida both 
 spicular felts and fibres occur, and in one division (Dictyonina) a rigid 
 network is produced, not, however, by a mere clasping of apophyses, 
 but by a true fusion. The rays of adjacent spicules overlap and a 
 common investment of silica grows over them. 
 
 Histology. 
 
 The ectoderm usually consists of simple pavement Ecto- 
 epithelial cells (pinnatocytts), the margins of which can derm, 
 be readily rendered visible by treatment with silver nitrate, 
 best by Harmer's method. 1 The nucleus and nucleolus 
 are usually visible in preparations made from spirit speci- 
 mens, the nucleus being often readily recognizable by its 
 characteristic bulging beyond the general surface. In some 
 sponges (Thecapliord) the epithelium may be replaced 
 locally by columnar epithelium, and the cells of both pave- 
 ment and columnar epithelium may bear flagella (Aplysilla 
 violacea, Oscarella lobularis). The endoderm presents the Endo- 
 same characters as the ectoderm, except in the Ascons and derm, 
 the flagellated chambers of all other sponges, where it is 
 formed of collared flagellated cells or choanocytes, cells 
 with a nearly spherical body in which a nucleus and nucleo- 
 lus can be distinguished and one or more contractile vacu- 
 oles. The endoderm extends distally in a cylindrical neck 
 or colhim, which terminates in a long flagellum surrounded 
 by a delicate protoplasmic frill or collar (fig. 21 g). In 
 Tetractinellida, and probably in many other sponges cer- 
 tainly in some the collars of contiguous choanocytes 
 coalesce at their margins so as to produce a fenestrated 
 membrane, which forms a second inner lining to the flagel- 
 
 1 S. F. Harmer, " On a Method for the Silver Staining of Marine 
 Objects," Mitth. Zoolog. Station zu Stopd, 1884, p. 445. 
 
46 
 
 SPONGES 
 
 lated chamber (fig. 20, ii.). The presence of this membrane 
 enables us readily to distinguish the excurrent from the 
 
 Fia. 20. Choanocytes with coalesced collars, (i.) Longitudinal section through 
 two flagellated chambers of Anthastra communis, Soil. ; i, prosopyles ; c, 
 aphodal canals leading from the flagellated chambers ; e, excurrent canal ; 
 the tissue surrounding the chambers is sarcenchyme (X360). (ii.) Diagram 
 showing the fenestrated membrane (?n) produced by coalesced collars of 
 choanocytes. After Sollas, "Challenger" Report. 
 
 incurrent face of the chamber, since its convex surface is 
 always turned towards the prosopyle. In sponges with an 
 
 FIG. 21. Histological elements, a, collencytes, from TTietim muricata; 
 chondrenchyme, from cortex of Corticium candelabrum (the unshaded bodies 
 are microscleres) ; c, cystenchyme, from Paehyniatisnut joknstoni (partly dia- 
 grammatic) ; d, desmacyte, from Dragmastm normani ; e, myocytes in con- 
 nexion with collencytes, from Cinachyra barbata ; /, thesocyte, from Thenea 
 murwata ; g, choanocyte, from Sycandm raphanus', h-n, scleroblasts A, of 
 a young oxea, from an embryo of Craniella cranium ; i, of a fully grown oxea, 
 from an adult C. cranium ; j, orthotriajne, with associated scleroblast from 
 Slellelta ; fc, of a tetracladine desma, from Theonclla swinhoei ; I, of a sigma- 
 spire, from Craniella cranium ; m, of an orthodragma, from. Disyringa dis- 
 similis', n t of a sterraster, from Geodia barretti. Figs. 6 and g after Schulze, 
 the others after Sollas. 
 
 aphodal canal system the flagellated chambers usually pass 
 gradually into the aphodal canal, but the incurrent canal 
 
 enters abruptly. This abrupt termination of the incurrent 
 canal appears to mark the termination of the ectoderm 
 and the commencement of the endoderm. The flagellated 
 chambers differ greatly in size in different sponges, and 
 evidently manifest a tendency to become smaller as the 
 canal system increases in complexity; thus Sycon are always 
 larger than Rhagon chambers, and eurypylous than aphodal 
 Rhagon chambers. In most sponges except the Ascons the 
 mesoderm is largely developed, and in many it undergoes Meso- 
 a highly complex histological differentiation. In its com- derm, 
 monest and simplest form it consists of a clear, colourless, 
 gelatinous matrix in which irregularly branching stellate 
 cells or connective tissue corpuscles are embedded ; these 
 may be termed colleiwytes (fig. 21 ) and the tissue collen- 
 chyme. In the higher sponges (Geodia, Stelletta) it consists 
 of small polygonal granular cells either closely contiguous 
 or separated by a very small quantity of structureless jelly, 
 and in this form may be termed sarcenchyme (fig. 20). 
 Collenchyme does not originate through the transformation 
 of sarcenchyme, as one might expect, for it precedes the 
 latter in development. Schulze (20), who has compared 
 collenchyme to the gelatinous tissue which forms the chief 
 part of the umbrella of "jelly-fish," describes it as becoming 
 granular immediately in the neighbourhood of the flagel- 
 lated chambers in the bath sponge, the granules becoming 
 more numerous in sponges in which the canal system 
 acquires a higher differentiation, till at length the collen- 
 cytes are concealed by them. According to this view, 
 sarcenchyme would appear to originate from a densely 
 granular collenchyme. Amoeboid wandering cells or archx- 
 ocytes (fig. 22) are scattered through the matrix of the 
 collenchyme. They evidently serve very different purposes : 
 some appear to act as carriers of nourishment or as 
 scavengers of useless or irritant foreign matter ; others 
 may possibly contribute to the formation of higher tissues, 
 some certainly becoming converted into sexual products. 
 Their parentage and early history are unknown. 
 
 A tissue (cystenchyme) which in some respects resembles certain 
 forms of vegetable parenchyma occurs in some sponges, particularly 
 Geodinidee and other Tetractincllida. It consists of closely ad- 
 jacent large oval cells, with thin well-defined walls and fluid 
 contents. Somewhere about the middle of the cell is the nucleus 
 with its nucleolus, supported by protoplasm, which extends from 
 it in fine threads to the inner side of the wall, where it spreads out 
 in a thin investing film (fig. 21 c). Cystenchyme very commonly 
 forms a layer just below the skin of some Gcodinidee, particularly of 
 Pachymatisnia, and, as on teasing the cortex of this sponge a large 
 number of refringent fluid globules immiscible with water are set 
 free, it is just possible that it is sometimes a fatty tissue, and if so 
 the contained oil must be soluble in alcohol, for alcoholic prepara- 
 tions show no trace of it. A tissue resembling cartilage, chondren- 
 chyme, occurs in Corlicidse (fig. 21 V). 
 
 Connective-tissue cells or desmacytes are present in most Desma, 
 sponges ; they are usually long fusiform bodies, consisting cytes. 
 of a clear, colourless, often minutely fibrillated sheath, 
 surrounding a highly refringent axial fibre, which stains 
 deeply with reagents (fig. 21 d). In other cases the des- 
 macyte is simply a fusiform granular cell, with a nucleus 
 in the interior and a fibrillated appearance towards the 
 ends. The desmacytes are gathered together, their ends 
 overlapping, into fibrous strands or felted sheets, which in 
 the ectosome of some sponges may acquire a considerable 
 thickness, often constituting the greater part of the cortex. 
 The spicules of the sponge often furnish them with a sur- 
 face of attachment, especially in the Geodinidse, where each 
 sterraster of the cortex is united to its neighbours by des- 
 macytes, in the manner shown in fig. 10. 
 
 Contractile fibre cells or myocytes occur in all the higher Myo- 
 sponges. They appear to be of more than one kind. Most c y tes - 
 usually they are fine granular fusiform cells with long 
 filiform terminations, and with an enclosed nucleus and 
 nucleolus (fig. 21 e). In the majority of sponges both ex- 
 current and incurrent canals are constricted at intervals 
 
SPONGES 
 
 47 
 
 by transverse diaphragms or vela, which contain myocytes 
 concentrically and sometimes radiately arranged. The 
 excessive development of myocytes in such a velum gives 
 rise to muscular sphincters such as those which close the 
 chones of many corticate sponges, such as Pachymatigma. 
 In this sponge, which occurs on the British shores, the 
 function of the oscular sphincters can be readily demon- 
 strated, since irritation of the margin of the oscule is 
 invariably followed after a short interval by a slow closure 
 of the sphincter. 
 
 Supposed sense -cells or sesthacytes (fig. 22) were first 
 observed by Stewart and have since been described by 
 Yon Lendenfeld (/-?). According to the latter, they are 
 spindle-shaped cells, - 01 mm. long by 0'002 thick ; the 
 distal end projects beyond the ectodermal epithelium in a 
 fine hair or jialpocil ; the body is granular and contains a 
 large oval nucleus ; and the inner end is produced into 
 fine threads, which extend into the collenchyme and are 
 supposed though this is not proved to become con- 
 tinuous with large multiradiate collencytes, which Yon 
 Lendenfeld regards as multipolar ganglion cells (fig. 22). 
 
 FIG. 22. Transverse section through the edge of a pore in Dtndrilla carernosa, 
 Lfd. ; cells in the middle to the right, archseocytes ; fusiform cells on 
 each side of them, myocj^tes ; g, above and below these, with processes 
 terminating against the epithelium, gland cells ; fusiform cells terminating 
 against the epithelium at i, *sthacytes ; at their inner ends these are con- 
 tinuous with ganglion cells. After Von Lendenfeld (x 800). 
 
 More recently he has described an arrangement of these 
 cells curiously suggestive of a sense-organ. Numerous 
 aesthacytes are collected over a small area, and at their 
 inner ends pass into a granular mass of cells with well- 
 marked nuclei, but with boundaries not so evident ; these 
 he regards as ganglion cells. From the sides of the gan- 
 glion other slender fusiform cells, which Yon Lendenfeld 
 regards as nerves, pass into the mesoderm, running tan- 
 gentially beneath the skin. The inner end of the ganglion 
 is in communication with a membrane formed of fusiform 
 cells which Yon Lendenfeld regards as muscular. If his 
 observations and inferences are confirmed, it is obvious 
 that we have here a complete apparatus for the conversion 
 of external impressions into muscular movements. 
 
 In most sponges a direct connexion can be traced by 
 means o f their branching processes between the collen- 
 c .V tes f the mesoderm and the cells of the ectodermal 
 and endodermal epithelium and the choanocytes of the 
 flagellated chambers. As the collencytes are also united 
 amongst themselves, they place the various histological 
 constituents of the sponge in true protoplasmic continuity. 
 Hence we may with considerable probability regard the 
 collencytes as furnishing a means for the transmission of 
 impulses : in other words, we may attribute to them a 
 rudimentary nervous function. In this case the modifica- 
 tion of some of the collencytes in communication with the 
 ectoderm might readily follow and special a;sthacytes arise. 
 Fusiform collencytes perpendicular to the ectoderm, and 
 with one end touching it, are common in a variety of 
 sponges ; but it is difficult to trace the inner end into 
 connexion with the stellate collencytes, so that precisely in 
 
 those cases in which it would be most interesting to find 
 such a connexion absolute proof of it is wanting. 
 
 The colour of sponges usually depends on the presence Pigment 
 of cells containing granules of pigment ; though dispersed ce] ^- 
 generally through the mesoderm, these cells are most richly 
 developed in the ectosome. Pigment granules also occur 
 in the choanocytes of some sponges, Oscarella lobulari* 
 and Aplysina aerophoba, for instance. In the latter the 
 pigment undergoes a remarkable change of colour when 
 the sponge is exposed to the air, and finally fades away. 
 In many cases sponges borrow their colours from parasitic 
 algae (Osctflatoria. and Nostoc) with which they are infested. 
 The colours of sponge-pigments are very various. They 
 have been examined by Krukenberg and Merejknovsky. 
 Zoonerythin, a red pigment of the lipochrome series, is one 
 of the most widely diffused ; it is regarded as having a 
 respiratory function. Eeserve cells or thtsocytts (fig. 2 If) 
 have been described in several sponges as well as amylin 
 and oil-bearing cells. 
 
 Each spicule of a sponge originates in a single cell Sclero- 
 (fig. 21 h-n), within which it probably remains enclosed blasts, 
 until it has completed its full growth ; the cell then prob- 
 ably atrophies. During its growth the spicule slowly 
 passes from the interior to the exterior of the sponge, and 
 is finally (in at least some sponges, Geodia, Stellttta) cast 
 out as an effete product. The sponge is thus constantly 
 producing and disengaging spicules; and in this way we 
 may account for the extraordinary profusion of these struc- 
 tures in some modern marine deposits and in the ancient 
 stratified rocks. Within the latter these deciduous spicules 
 have furnished silica for the formation of flints, which have 
 been produced by a silicious replacement of carbonate of 
 lime (26). 
 
 The horny fibres of the Ceratosa are produced as a 
 secretion of cells known as sponginblasts, which surround 
 as a continuous mantle the sides of each growing fibre, and 
 cover in a thick cap each growing point (fig. 23). The 
 
 FIG. 23 Section through the horny fibre and associated tissues of a horny 
 sponge (DendriUa). A, longitudinal section ; *, layers of spongin, surrounded 
 at the sides by the lateral mantle of sponginblasts, and at the ends by the 
 terminal cap. A desntachymatous sheath, a, surrounds the whole (xlSO). 
 B, transverse section ; in the centre is the soft core, surrounded by wavy 
 spongin layers, the outermost being surrounded by sponginblasts, and these 
 by a fibrous sheath ; i, part of an incurrent canal lined by flagellated epi- 
 thelium ; e, part of an excurrent canal; /, partofaflagellated'chamber(xl50). 
 After Von Lendenfeld. 
 
 lateral sponginblasts are elongated radially to the fibre ; 
 the terminal cells are polygonal and depressed. The latter 
 give rise to the soft granular core and the former to the 
 spongin -walls of the fibre. Cells similar to the lateral 
 sponginblasts, and regarded as homologous with them, 
 occur in a single layer just below the outer epithelium of 
 some horny sponges (Aplysilla and Dendrilla), and under 
 certain circumstances secrete a large quantity of slimy 
 mucus ff). 
 
48 
 
 SPONGES 
 
 Classification. 
 
 Classi- The phylum Parazoa or Spongix consists of two main 
 fication. branches, as follows : 
 
 Branch A. MEG A MASTIC'- Branch B. MICROMASTIC- 
 
 TORA. TOR A. 
 
 Class CALCAREA, Grant Class I. MYXOSPONGI.E, 
 
 Order 1. Homoccda, Pol. Haeckel. 
 
 Order 2. Heterocala, Pol. Order l.ffalisarcina. 
 
 Order 2. Chondrosina. 
 
 Class II. SILICISPONGI^E. 
 
 Sub-class i. HEXACTINELLIDA, 
 
 O. Schmidt. 
 
 Order 1. Lyssacina, Zittel. 
 Order 2. Diciyonina, Zittel. 
 
 Sub -class ii. DEMOSFONGIJE, 
 
 Sollas. 
 Tribe a. Monaxonida. 
 
 Order 1. Monaxona. 
 
 Order 2. Ceratosa, Grant. 
 
 Tribe b. Tetractinellida, 
 
 Marshall. 
 
 Order \.-Choristida, Sollas. 
 Order 2. Lithistida, O.S. 
 
 Position By the possession of both sexual elements and a complex histo- 
 
 in animal logical structure, and in the character of their embryological devt-1- 
 
 kingdoin. opment, the sponges are clearly separated from the Protozoa ; on 
 
 the other hand, the choanoflagellate character of the endoderm, 
 
 which it retains in the flagellated chambers throughout the group 
 
 without a single exception, as clearly marks them off from the 
 
 Metazoa. They may therefore be regarded as a separate phylum 
 
 derived from the choanoflagellate Infusoria, but pursuing for a 
 
 certain distance a course of development parallel with that of the 
 
 Metazoa. 
 
 Different views have been propounded by other authors. Savile 
 Kent regards the sponges as Protozoa (10} ; Balfour suggested that 
 they branched off from the Metazoan phylum at a point below the 
 Ccelentera, and considered them as intermediate between Protozoa 
 and Metazoa ; Schulze regards them as derived from a simple 
 ancestral form of Ccelentera (23) ; Marshall advocates the view that 
 they are degraded forms derived from Ccelenterates which were 
 already in possession of tentacles and mesenteric pouches (14). 
 
 As a phylum the Spongix are certainly divisible into two branches, 
 one including the Calcarea and the other the remaining sponges, 
 which Vosmaer has termed Non-Calcarea, and others Plethospongite. 
 Since, however, the choanocytes of the Calcarea are usually, if not 
 universally, larger than those of other sponges, we may make use 
 of this difference in our nomenclature, and distinguish one branch 
 as the Mcgamastidora (fiaaTlKTwp, "scourger") and the other as 
 the Micromastictora. 
 
 Branch A.MEGAMASTICTORA. 
 
 Sponges in which the choanocytes are of comparatively large 
 size, 0'005 to 0'009 mm. in diameter (Haeckel, 6). 
 
 Class CALCAREA. 
 
 Calcarea. Megamastietora in which the skeleton is composed of calcareous 
 spicules. 
 
 Order 1. HOMOOELA. Calcarea in which the endoderm consists 
 wholly of choanocytes. Examples : Leucosolenia, Bwk. ; Homo- 
 derma, Lfd. 
 
 Order 2. HETEROCOJLA. Calcarea in which the endoderm is dif- 
 ferentiated into pinnacocytes, which line the paragastric cavity 
 and excurrent canals, and choanocytes, which are restricted to special 
 recesses (radial tubes or flagellated chambers). Examples : Sycon, 
 O.S. ; Grautia, Fl. ; Leuconia, Bwk. 
 
 Branch B. MICROMASTICTORA. 
 
 (Non-Calcarea, Vosmaer; Plethospongise, Sollas.) Sponges in 
 which the choanocytes are comparatively small, 0'003 mm. in 
 diameter. 
 
 Class I. MYXOSPONGLS. 
 
 Myxo- Micromastictora in which a skeleton or scleres are absent. 
 
 spongiie. Order 1. HALISARCINA. Myxosponyiie in which the canal system 
 is simple, with simple or branched Sycon or eurypylous Rhagon 
 chambers. An ectosome sometimes and a cortex always absent. 
 Examples: Halisarca, Duj.; Oscarella, Vosm. ; Bajalus, Lfd. 
 
 Order 2. CHONDROSINA. Myxospongise, in which the canal 
 system is complicated, with diplodal Rhagon chambers and a 
 well-developed cortex. Example : Chondrosia, O.S. 
 
 The Halisarcina are evidently survivals from an ancient and 
 primitive type. The simplicity of the canal system is opposed to 
 the view that they are degraded forms ; we may therefore regard 
 the absence of scleres as a persistent primary and not a secondary 
 acquired character. They are as interesting, therefore, from one 
 
 Subdivi- 
 
 groups. 
 
 point of view (absence of scleres) as the Ascons are from another 
 (undifferentiated endoderm). With the Chondrosina the case is 
 different ; they differ only from Chondrilla and its allies by the 
 absence of asters ; these differ only from the Tethyidse by the 
 absence of strougyloxeas ; and we may very reasonably assume that 
 in these three groups we have a series due to loss of characters, the 
 Chondrillw being reduced Tcthyidee, and the Chondrosina reduced 
 Chondrillss. Still, as Huxley has well remarked, "classification 
 should express not assumptions but facts " ; and therefore till we 
 are in possession of more direct evidence it will be well to exclude 
 the Chondrosina from the SilicispongiK. 
 
 Class II. SlLICISPONGLffi. 
 
 Micromastictora possessing a skeleton or scleres which are not 
 calcareous. 
 
 Sub-class i. HEXACTINELLIDA. 
 
 Silicispongias characterized by sexradiate silicious spicules. Hexmti- 
 Canal system usually simple, with Sycon chambers. Sponge nettida. 
 differentiated into ecto-, choano-, and endo-some. 
 
 Order 1. LYSSACINA. Huxaclincllida in which the skeleton is 
 formed of separate spicules, or, if united, then by a subsequent not a 
 contemporaneous deposit of silica. Examples : Euplectella, Owen ; 
 Asconema, S. Kent ; Hyalonema, Gray ; Rossella, Crtr. 
 
 Order 2. DICTYONINA. Hexactincllida in which sexradiate 
 spicules are cemented together by a silicious deposit into a con- 
 tinuous network pan passu with their formation. Examples : 
 Farrea, Bwk. ; Eurete, Marshall ; Aphrocallistes, Gray ; Myliiisia, 
 Gray ; Dactylocalyx, Stutchbury. 
 
 The Hcxactinellida are a very sharply denned group, impressed 
 with marked archaic features. No other Silicispongise possess, so 
 far as is known, so simple a syconate canal system. The oldest 
 known fossil sponge is a member of the Lyssacina (7 and 24), viz., 
 Protospmigia, Salter, from the Menevian beds, Lower Cambrian, 
 St David's Head, Wales. The group is almost world- wide in distri- 
 bution, chiefly affecting deep water, from 100 to 300 fathoms, but 
 often extending into abyssal depths ; occasionally, however, though 
 rarely, it frequents shallow water (Cystispongia superstcs dredged off 
 Yucatan in 18 fathoms). 
 
 Sub-class ii. DEMOSPONGLffi. 
 
 Silicispongix in which sexradiate spicules are absent. Detno- 
 
 Tribea. MOXAXOXIDA. spongi* 
 
 Demospongise in which the skeleton consists either of silicious 
 spicules which are not quadriradiate, or of horny scleres or in- 
 cluded foreign bodies, or of one or more of these constituents in 
 conjunction. 
 
 Order 1. MONAXONA. The skeleton is characterized by either 
 uniaxial or polyaxial spicules. Examples : Amorphina, 0. S. 
 ("crumb of bread" sponge); Spongilla, Link, ("freshwater" 
 sponge) ; Chalina, Bwk. ; Tcthya, Lmk. 
 
 Order 2. CERATOSA. The skeleton consists of horny scleres 
 which never include "proper" spicules, or of introduced foreign 
 bodies, or of both these in conj unction. Examples : Darwinclla, 
 F. Miiller ; Euspongia, Bronn (the "bath" sponge). 
 
 Tribe 6. TETRACTINELLIDA. 
 
 Demospongix possessing quadriradiate or trisene spicules or 
 Lithistid scleres (desmas). 
 
 Order 1. CHORISTIDA. Tetractinellida with quadriradiate or 
 triame spicules, which are never articulated together into a rigid 
 network. Examples: Tetilla, 0. S. ; Thenea, Gray ; Geodia, Lmk. ; 
 Dcrcitus, Gray. 
 
 Order 2. LITHISTIDA. Tetractinellida with branching scleres 
 (desmas), which may or may not be modified tetrad spicules, arti- 
 culated together to form a rigid skeleton. Trisene spicules may or 
 may not be present in addition. Examples : Thconclla, Gray ; Coral- 
 listes, O.S. ; Azorica, Crtr.; Vetulina, O.S. 
 
 This large sub-class embracesthe great majority of existing sponges. 
 Its external boundaries are fairly well denned, its internal divisions 
 much less so, as its various orders and families pass into each other 
 at many points of contact. Although there does not appear to be 
 much resemblance between a Lithistid sponge, such as Theonella, 
 a Monaxonid such as Amorphina, and an ordinary "bath" sponge 
 (Euspongia), yet between these extremes a long series of inter- 
 mediate forms exists, so nicely graduated as to render their dis- 
 ruption into groups by no means an easy task. If the delimitation 
 of orders is difficult, that of genera is often impossible, so that 
 they are reduced to assemblages depending on the tact or taste of 
 the author. Thus Polejaeff states that with a single exception 
 " none of the genera of Ceratosa are separable by absolute charac- 
 ters." The chief spicules of Monaxona are uniaxial, often accom- 
 panied by characteristic microscleres. Although distinguished as a 
 group by the absence of quadriradiate or triune spicules, two ex- 
 ceptions are known in which these occur (Triccntrion, Ehlers, and 
 Acarnus, Gray) ; these, however, present unusual characters which 
 suggest an independent origin. The canal system of Monaxona has 
 not yet been fully investigated ; it appears usually to follow the 
 
SPONGES 
 
 49 
 
 eurypylons Rhagon type, but the aphodal is not unknown. The 
 Ceratosa contain all sponges with a horny skeleton, except those 
 in which the horny fibres are cored or spined with silicious spicules 
 secreted by the sponge ("proper" spicules) ; these are arbitrarily 
 assigned to the ifonaxona. There is convenience in this proceed- 
 ing, for horny matter is widely disseminated throughout the Demo- 
 spongise, occurring even in the Lithistida, and it frequently serves 
 to cement the oxeate spicules of the Monaxona into a fibre, without 
 at the same time forming a preponderant part of the skeleton. It 
 would be well nigh impossible to say where the line should be drawn 
 between a fibre composed of spicules cemented by spongin and one 
 consisting of spongin with embedded spicules, while there is com- 
 paratively no difficulty in distinguishing between fibres containing 
 spicules and fibres devoid of them. That the distinction, however, 
 is entirely artificial is shown by the fact that, after spieules have 
 disappeared from the horny fibre, they may still persist in the 
 mesoderm ; thus Yon Lendenfeld announces the discovery of micro- 
 scleres (cymba) in an Aplysillid sponge and of strongyles in a 
 Cacospongia, both horny sponges. (A form intermediate between 
 this Aplysillid and the Desmacidonidx would appear to be Toxo- 
 chalina, Ridley.) The Ceratosa frequently enclose sand, Fora- 
 minifera, deciduous spicules of other sponges and of compound 
 Aseidians, and other foreign bodies within the horny fibres of their 
 skeleton ; they also sometimes attach this material, probably by a 
 secretion of spongin, to their outer surface, and thus invest them- 
 selves in a thick protective crust. In some Ceratosa no other 
 skeleton than that provided by foreign enclosures is present The 
 canal system is syeonate or eurypylous in the simpler forms and 
 diplodal in the higher. The ilonaxonida make their earliest ap- 
 pearance in the Silurian rocks (Climacospongia, Hinde), and are 
 now found in all seas at all depths. The only sponges inhabiting 
 fresh water belong to this group. The Tetraetinellida adhere to 
 the ilonaxonida at more than one point, and one of these groups 
 has probably been a fruitful parent to the other, but which is 
 offspring and which parent is still a subject for discussion. The 
 Chorislida in its simplest forms presents a eurypylous Rhagon 
 system, in the higher an aphodal system. It is in this group that 
 the most highly complex cortex is met with ; in the Geodinidx, 
 for instance, it consists usually of at least five distinct layers. 
 Thus, proceeding outwards, next to the choanosome is a layer of 
 thickly felted desmachyme, passing into collenchyme on its inner 
 face ; then follows a thick stratum of sterrasters united together 
 by desmacytes ; this is succeeded by a layer of cystenchyme or 
 other tissue of variable thickness ; external to this is a single layer 
 of small granular cells and associated dermal asters ; and finally, 
 the surface is invested by a layer of pavement epithelium. The 
 Lithistida, like the Ceratosa, are possibly of polyphylitic origin ; 
 in one group (Tetradadina) the articulated scleres are evidently 
 modified calthrops spicnles (see fig. 14 e), and associated with them 
 are free trisenes, which support the dermis and resemble precisely 
 the trisenes of the Choristida. In another group (Shabdocrepida) 
 the scleres are moulded on a Monaxonid base (see fig. 13 q-s) ; but, 
 associated with them, trisenes sometimes occur similar to those of 
 the Tctracladina. Both these groups are in all probability derived 
 from the Chorislida, and a distinct passage can be traced from the 
 Tetracladose to the Rhabdocrepid group. In the Rhabdocrrpida 
 we find forms without trisenes ; these may possibly be degenerate 
 forms. The third group of Lithistids is derived from the Khabdo- 
 crepida, the Anomocladine desma being derivable from the Rhabdo- 
 crepid by a shortening of the main axis into a centrum. The 
 thick centrum, from which the arms, variable in number, ori- 
 ginate, is hollowed out by a cavity, which appears during life to 
 have been occupied by a large nucleus, like that of a scleroblast, 
 and it is quite conceivable that the scleroblast, which in the 
 Tetracladine Lithistids lies in an angle between the arms, may 
 have become enclosed in an overgrowth of silica, from which addi- 
 tional arms were produced. The constancy with which spicules 
 in other sponges maintain their independence is very striking. 
 When once a persistent character like this is disturbed, excessive 
 variability may be predicted, as in the Anomocladine scleres. 
 
 Classifi- The classification of the sponges into families is shown in the 
 
 cation in following scheme. 
 
 famUies - Class CALCAREA. 
 
 Order 1. HOMOCCELA, PoL 
 
 Family 1. ASCOXIDJE, Hk. ffomocasla which are simple or com- 
 posite, but never develop radial tubes. Examples: Ascetta, Hk. 
 (fig. 1) ; Leucosolenia, Bwk. 
 
 Family 2. HOMODERMID.E, Lfd. ffomocacla with radial tubes. 
 Example : Homodcrma, Lfd. (figs. 3, 4). 
 
 Order 2. HETEBOCCELA, Pol. 
 Tribe a, tSvcoxAEiA. 1 
 
 The flagellated chambers are either radial tubes or cylindrical 
 sacs. 
 
 Family 1. SYCONIDJB. The radial tubes open directly into the 
 paragastric cavity. 
 
 Sub-family a. Syconina. The radial tubes are free for their whole 
 length, or at least distally. Examples : Sycetta, Hk.; Sycon, O.S. 
 
 Sub-family b. Uteina, Lfd. The radial tubes are simple and 
 entirely united. The ectosome is differentiated from the choanosome 
 and sometimes develops into a cortex. Examples : Grantissa, Lfd. ; 
 Ute, O.S. (fig. 5); Sycortusa, Hk.; Amphoriscus, PoL 
 
 Sub-family c. Grantina, Lfd. The radial tubes are branched. 
 The incurrent canal system is consequently complicated. An ecto- 
 some is present Examples : Grantia, FL ; Heieropegma, PoL (fig. 
 4) ; Anamaxilla, Pol. 
 
 Family 2. STLLEIBID.E, Lfd. The choanosome is folded. The 
 flagellated chambers (which are partly rhagose in Vosmaeria) 
 communicate with the paragastric cavity by excurrent canals. 
 Examples : Polfjna, Lfd. (fig. 6) ; Vosmaeria, Lfd. 
 
 Family 3. TEICHOXELLID.E, Carter. Composite Sylleflridie with 
 the oscnles and pores occurring on different parts of the surface. 
 Example : TeichoneUa, Crtr. 
 
 Tribe 6. 
 
 The canal system belongs to the eurypylous Rhagon type. 
 
 Family 1. LEUCOXID.S, Hk. The outer surface is not differentiated 
 into osculiferous and poriferous areas. Examples : Ltucetta, Hk. ; 
 Lcutallis, Hk. ; Lewcortis, Hk. 
 
 Family 2. EILHABDIDJB, PoL Composite Leuconaria, with the 
 outer surface differentiated into special osculiferous and poriferous 
 areas. Example : EUhardia, Pol. 
 
 The arrangement adopted above is founded on Yon Lendenfeld's 
 revision (//) of the classification propounded by Polejaeff (j6), who 
 in a masterly survey has thrown an unexpected light on the struc- 
 ture and inter-relationships of a group which Haeckel has rendered 
 famous. It should not be overlooked that Yosmaer (j/) had pre- 
 viously explained the structure of the Leucones. However errone- 
 ous in detail, Haeckel's views are confirmed in their broad outlines, 
 and it was with true insight that he pronounced the Calcarea to 
 offer one of the most luminous expositions of the evolutional theory. 
 In this single group the development in general of the canal system 
 of the sponges is revealed from its starting-point in the simple 
 Ascon to its almost completed stage in the Leucon, with a complete- 
 ness that leaves little further to be hoped for, unless it be the re- 
 quisite physiological explanation. 
 
 Class IfYXOSPOXGIjE. 
 Order 1. HALJSABCTNA. 
 
 Family 1. HALISARCIDJB, Lfd. The flagellated chambers are 
 syeonate. Examples: Halisarca, Dnj. (with branched chambers); 
 Bajalus, Lfd. (with simple chambers). 
 
 Family 2. OSCAEELLIDJE, Lfd. The flagellated chambers are 
 enrypylous and rhagose. Example : Oscarella, Yosm. 
 
 Order 2. CHONDBOSTNA. 
 
 Family 1. CHOXDROSIID.B. With the characters of the order. 
 Example: Chondrosia, O.S. 
 
 Class SILICISPOXGI^E. 
 
 Sub-class I. HEXACTINELLIDA. 
 
 Order 1. -rLYSSACTHA, 
 
 Family 1. EtJPLECTELLiD.fi. The spicules of the dermal mem- 
 brane are "daggers" (fig. 15 a). Examples : Eupleciella, Owen; 
 Holasais, E. Sch. ; ffabrodictyum, W.T. 
 
 Family 2. ASCOXESCATIDJE. The dermal spicules are " pinnnli " 
 (fig. 15 b, c). Examples: Asconema, S. Kent; Sympagella, O.S.; 
 Cauloph&us, Schulze. 
 
 Family 3. HTALOXEMATID^. The dermal spicules are pinnuli 
 and amphidisks (fig. 15 rf). Example : Hyalonema, Gray. 
 
 Family 4. tRossELiD-E. The dermal spicules are gomphi, stauri 
 (fig. 15/), and oxeas. Examples: Rossflla, Crtr.; CraUromorpha, 
 Gray ; Aulochana, E. Sch. 
 
 Family 5. 'RECEPTACTTLIDJE, Hinde. The distal ray of the 
 dermal spicules is expanded horizontally into a polvgonal plate. 
 Example : 'Seeeptaeulites, Defr. 
 
 Order 2. tDlCTTONTKA. 
 Sub-order 1. VXCIXITARIA. 
 Untinate spicnles are present 
 
 Tribe a. CLAVTLARIA. 
 Clavulae (fig. 16 e) are present 
 
 Family 1. FAP.REIDJB. Characters those of the tribe. Example : 
 Farrca, Bwk. 
 
 Tribe b. SCOPULAEIA. 
 
 The dermal spicules are scopnlarue (fig. 16 b). 
 Family 1. tEvRETiD-E. Branched anastomosing tubes, or goblet- 
 shaped, with lateral outlets. Examples : Ewrete, Marshall ; Peri- 
 phragella, Marshall ; Lefroyella, Schulze. 
 
 Family 2. tMELLlTrosiDjE. Tubular or goblet-shaped, with 
 honeycomb-like walls. Example : Apkroeallistes, Gray. 
 
 1 An * indicates that the group is only known in the fossil state, a f that it 
 is both recent and fossil. 
 
 G 
 
50 
 
 SPONGES 
 
 Family 3. tCHONELASMATiD^;. Flat or beaker-shaped ; straight 
 funnel-shaped canals perforating the wall perpendicularly and 
 opeuinf laterally on each side. Example : Chonelasma, Schulze. 
 
 FamFly 4. tVoLVULiNiD^s. Tubular, goblet-shaped, or massive ; 
 crooked canals more or less irregular in their course. Examples : 
 Volmdina, Schulze ; Fieldingia, S. Kent. 
 
 Family 5. SCLEROTHAMNIMS. Arborescent body ; perforated at 
 the ends and sides by round narrow radiating canals. Example : 
 Sclerothamnus, Marshall. 
 
 Sub-order 2. INERMIA. 
 
 Dictyonina without uncinati, clavulse, or scopulariae. 
 
 Family 1. tMYLiusiDa:. Depressed cup -shaped; a complex 
 folding of the wall produces lateral excurrent tubes. Example : 
 Myliusia, Gray. 
 
 Family 2. tDACTYLOCALYClD*;. Goblet -shaped or pateriiorm, 
 with a thick wall consisting of numerous parallel anastomosing 
 tubes, of uniform breadth, which terminate at the same level 
 within and without. Examples : Dactylocalyx, Gray ; Scleroplegma, 
 O.S. ; MargarUella, O.S. 
 
 Family 3. tEuRYPLEGMATiD.*. Goblet-shaped or resembling 
 ear-shaped saucers ; the wall deeply folded longitudinally so as to 
 produce a number of dichotomously branched canals or covered-in 
 grooves. Example : Euryplegma, Schulze. 
 
 Family 4. tAuLOCYSTiD^E. Of massive rounded form, with an 
 axial cavity ; wall consisting of a system of obscurely radiating 
 anastomosing tubes and intervening inter-canals ; both inter-canals 
 and the external terminations of the tubes are covered by a thin 
 membrane, which is perforated by slit -like openings over the 
 lumina of the tubes, and thus assumes a sieve -like character. 
 Examples : Aulocystis, Schulze ; Cystispongia, Roemer. 
 
 This arrangement of the Hexactinellida is taken from the latest 
 work on the subject, Schulze's Preliminary Report on the "Challen- 
 ger " Hexactinellida. The reference of fossil forms to the families 
 here instituted is rendered difficult by the disappearance of the 
 requisite "guiding" spicules in the process of mineralization. A 
 revision of the fossil families to bring them into harmony with the 
 recent has certainly been rendered necessary, but this is too large 
 a task to undertake in this place. 
 
 Sub-class II. DEMOSPONGI.E. 
 Tribe a. MONAXONIDA. 
 Order 1. MONAXONA. 
 
 Family 1. TETHYID.E. Skeleton consisting of radiately arranged 
 strongyloxeas (except in the genus Chondrilla, which is without 
 megascleres) and large spherasters. The ectosome is a thick fibrous 
 cortex. Example: Tethya, Lmk. ; Chondrilla, O.S. 
 
 Family 2. POLYMASTIM. Skeleton consisting of styles radiately 
 arranged and cortical tylostyles. The oscules in many cases open 
 at the ends of long papillae. Examples : Polymastia, Bwk. ; Theca- 
 phora, O.S. ; Trichostemma, Sars. 
 
 Family 3. SUBERITID.E. Skeleton consisting of strongylate or 
 tylotate styles, arranged to form a felt. The flesh spicules when 
 present are usually microrabds or spirasters. Examples : Suberites, 
 Nardo ; Cliona, Grant ; Poterion, Schlegel. 
 
 Family 4. DESMACIDONID-E. The flesh spicules are cymbas. 
 
 Examples : Esperella, Vosm. ; Desmacidon, Bwk. ; Cladorhiza, Sars. 
 
 Family 5. tHALiCHONDRlM. The flesh spicules when present 
 
 are never cymbas. Examples : Halickondria, Fl. ; Rcniera, O.S. ; 
 
 Chalina, Bwk. ; * Pharetrospongia, Soil. 
 
 Family 6. ECTYONID.E. The skeleton consists of fibres echinated 
 by projecting spicules. Examples : Plocamia, 0. S. ; Ectyon, Gray ; 
 Clathria, O.S. 
 
 Family 7. tSpONGiLLiD*. Halichondridas which are reproduced 
 both sexually and by statoblasts. Habitat freshwater. Examples : 
 Spongilla, Lmk. ; Ephydatia, Lmk. ; Parmula, Crtr. ; Potamolepis, 
 Marshall. 1 
 
 The foregoing classification is purely provisional ; the group re- 
 quires a complete revision. 
 
 Order 2. CERATOSA. 
 
 Family 1. DARWINELLID.SI. Canal system of the eurypylons 
 Rhagon type. Flagellated chambers, pouch-shaped, large ; the sur- 
 rounding collenchyme not granular. Horny fibres with a thick 
 core. Examples : Darwinella, Fritz Miiller ; Aplysilla, F.E.S. ; 
 lanthella, Gray. 
 
 Family 2.. SPONGELID^E. Canal system as in the Darwincllidse, 
 but the flagellated chambers more or less spherical. Horny fibres 
 with a thin core, and usually containing foreign enclosures. 
 Examples : Velinea, Vosm. ; Spongelia, Nardo ; Psammoclema, 
 Marshall ; Psammopemma, Marshall. 
 
 Family 3. SPOXGID.E. Canal system aphodal. Chambers small 
 and spherical ; surrounding collenchyme granular. Fibres with a 
 thin core. Examples : Euspongia, Bronn ; Coscinodcrma, Crtr. ; 
 Phyllospongia, Ehlers. 
 
 1 Freshwater sponges without statoblasts are excluded from this family, and 
 left for distribution amongst allied marine genera. 
 
 Family 4. APLYSINID.E. Canal system diplodal ; collenchyme 
 surrounding the flagellated chambers densely granular. Fibres 
 with a thick core. Examples : Luffaria, Duch. and Mich. ; Verrni- 
 gia, Bwk. ; Aplysina, Nardo. 
 
 The species of sponge in common use are three, Euspongia 
 offic.ina.lis (Linn.), the tine Turkey or Levant sponge ; E. zimocca 
 'O.S.), the hard Zimocca sponge ; and Hippospongia equina (O.S.), 
 the horse sponge or common bath sponge. The genus Euspongia. 
 is distinguished by the regular development of the skeletal network 
 throughout the body, its narrow meshes, scarcely or not at all 
 visible to the naked eye, and the regular radiate arrangement of 
 its chief fibres. Hippospongia is distinguished by the thinness of 
 its fibres and the labyrinthic character of the choanosome beneath 
 the skin. As a consequence its chief fibres have no regular radiate 
 arrangement. The species of Euspongia are distinguished as fol- 
 lows. In E. officinalis the chief fibres are of different thicknesses, 
 irregularly swollen at intervals, without exception cored by sand 
 grains ; in E. zimocca they are thinner, more regular, and almost 
 A-ee from sand. In E. officinalis, again, the uniting fibres are soft, 
 thin, and elastic ; whilst in E. zimocca they are denser and thicker, 
 to which difference the latter sponge owes its characteristic hard- 
 ness. Finally, the skeleton of E. officinalis is of a lighter colour than 
 that of E. zimocca. The common bath sponge (Hippospongia, 
 equina) has almost always a thick cake-like form ; but its specific 
 characters are not yet further defined. 
 
 Tribe b. TETKACTINELLIDA. 
 
 Order 1. CHORISTIDA. 
 Sub-order 1. SIGMATOPHORA. 
 
 The microsclere is a sigmaspire. 
 
 Family 1. TETILLID^E. The characteristic megasclere is a pro- 
 triaene. Canal system in the lower forms eurypylous, in the higher 
 aphodal. The ectosome in the simpler forms is a dermal membrane, 
 in the higher a highly differentiated cortex. Examples : Tetilla, 
 O.S.; Craniclla, O.S. (fig. 2.1 h, I). 
 
 Family 2. SAMID^E. The characteristic megasclere is an amphi- 
 trisene. Example : Samus, Gray. 
 
 Sub-order 2. ASTEROPHORA. 
 
 The microsclere is an aster. 
 
 Group 1. SPIRASTROSA. A spiraster is usually present. 
 
 Family 1. THENEID.E, Carter. The flesh spicule is a spiraster. 
 Canal system eurypylous. Ectosome not differentiated to form a 
 cortex. Examples : Thenea, Gray (fig. 21 a, /) ; Paxillastra (Nor- 
 mania), Bwk. 
 
 Family 2. tPACHASTRELL!D,E. Canal system eurypylous in the 
 lower, aphodal in the higher forms. Examples : Plakortis, F.E.S.; 
 Dercitus, Gray. 
 
 Group 2. EtTASTROSA. Spirasters are absent. 
 
 Family 1. tSTELLETTiD*. Canal system aphodal, but approach- 
 ing the eurypylous in the lower forms. The cortex chiefly consists 
 of collenchyme in the lower forms ; in the higher it is highly differ- 
 entiated. Example: Stelletta, O.S. (fig. 11); Ancorina, O.S. ; 
 Myriastra, Soil. 
 
 Family 2. TETHYID/E. Although this family has been placed 
 in the Monaxonida, this seems to be its more natural position. 
 
 Group 3. STERRASTROSA. A sterraster is present, usually in 
 addition to a simple aster. 
 
 Family 1. tGEODlNiD^E. The megascleres are partly trisenes. 
 Canal system always aphodal. Cortex highly differentiated. Ex- 
 amples : Gcodia, Lmk. (fig. 21 n) ; Pachymatisma, Bwk. (fig. 21 c) ; 
 Cydonium, Miiller (fig. 10) ; Erylus, Gray. 
 
 Family 2. PLACOSPONGIM. The megasclere is a tylostyle. 
 Triffines are absent. Example : Placospongia, Gray. 
 Sub-order 3. MICROSCLEROPHORA. 
 
 Microscleres only are present. 
 
 Family 1. PLAKINID.E, Schulze. Canal system very simple, 
 belonging to eurypylous Rhagon type. Characteristic spicules 
 candelabra. Examples: Plakina., F.E.S. (fig. 26). 
 
 Family 2. CORTICIM;. Canal system aphodal or diplodal. 
 Mesoderm a collenchyme crowded with oval granular cells ; the 
 spicules either candelabra, amphitrisenes, or triaenes irregularly 
 dispersed in it. Example : Corticium, O.S. (figs. 9, 21 b). 
 
 Family 3. THROMBID^G. Canal system diplodal. Spicules tricho- 
 triaenes. Example : Thrombus, Soil. 
 
 The Pacliastrettidx or the Corticidx are probably the families 
 from which the Tetracladine Lithistids have been derived. In the 
 Tetillidie the characteristic microsclere may occasionally fail, but 
 there is never any difficulty in identifying the sponge in this case, 
 as the trisenes are of a very characteristic form : the arms of the 
 protrifenes are slender, simple, and directed very much forwards, 
 making a very large angle with the shaft. Microscleres, having the 
 form of little'globules, are sometimes present with the sigmaspires. 
 
 Order 2. LITHISTIDA, O.S. 
 Sub-order 1. TETRACLADINA, Zittel. 
 The desmas are modified calthrops spicules. 
 
SPONGES 
 
 51 
 
 Family 1. TETEACLADID^ With the characters of the sub- 
 order. Examples : Theonella, Gray (fig. 21 ic) ; Discodcrmia, Bocage; 
 *Siphonia, Parkinson. 
 
 Sub-order 2. RHABDOCREPIDA. 
 
 The desmas are of various forms, produced by the growth of silica 
 over a uniaxial spicule. 
 
 Family 1. HEGAMORINID.E. The desmas are comparatively 
 large. Triaenes, usually dichotriaenes, help to support the ecto- 
 some. Microscleres usually spirasters. Examples : Corallistes, 
 O.S.; *Hyalotragos, Zittel ; Lyidium, O.S.; * Dorydermia, Zittel. 
 
 Family 2. MICKOMOKIXID.Z. The desmas are comparatively 
 small. Trisenes and microscleres are both absent. Examples : 
 Azorica, Crtr.; *Verruclina, Zittel. 
 
 Sub-order 3. AXOJfOCLADIXA. 
 
 Desmas with a massive nucleated centrum, from which a variable 
 number of arms (28) extend radiately (see fig. 12/). Examples : 
 Vctulina, O.S.; Astylospongia, Boemer. 
 
 Reproduction and Embryology. 
 
 Fresh individuals arise by asexual gemmation, both 
 external and internal, by fission, and by true sexual repro- 
 duction. 
 
 Asexual Fission is probably one of the processes by which com- 
 multipli- pound sponges are produced from simple individuals. 
 catlon - Artificial fission has been practised with success in the 
 cultivation of commercial sponges for the market. Ex- 
 ternal gemmation has been observed in Thenea, Tethya, 
 Pol ymastia, and Oscarella. A mass of indifferent sponge- 
 cells accumulates at some point beneath the skin, bulges 
 out, drops off, and gives rise to a new individuaL Internal 
 gemmation, which results in the formation of a statoblast, 
 is only known to occur in the freshwater Spongillidz. 
 The statoblasts consist of a mass of yolk -bearing 
 mesoderm cells, invested by a capsule, which in 
 Ephydatia fluviatilis is composed of an inner 
 cuticle of spongin separated from a similar outer 
 layer by an intermediate zone of amphidisks and 
 interspersed protoplasmic cells. On one side of 
 the capsule is a hilum which leads into the interior. 
 Their development has recently been studied by Gotte, 
 with results that confirm the conclusions of Carter (j) 
 and Lieberkiihn (/j). The process commences with an 
 accumulation of amoeboid cells within the mesodenn to 
 form a globular cluster ; yolk granules develop within 
 them, especially in those that lie nearer the centre. The 
 external cells give rise to the investing capsule ; some 
 resemble sponginblasts and secrete the inner and outer 
 horny cuticle ; others give rise to the amphidisks and 
 interspersed cells of the middle layer. Under favourable 
 conditions the interior cells creep out through the pore 
 of the capsule, and form a spreading heap, which by 
 subsequent differentiation gives rise to a young Sponyilla. 
 Since the freshwater sponges can only be regarded as 
 modified descendants of ancient marine species (prob- 
 ably of the family Halichondridx), we may consider the 
 internal gemmules, like the similar statoblasts of the 
 freshwater Polyzoa, as special adaptations to a changed 
 mode of life. They appear primarily to serve a protective 
 purpose, ensuring the persistence of the race, since they 
 only appear in extreme climates on the approach of 
 drought, and in cold ones on the approach of winter. 
 As a secondary function they serve for the dispersal of 
 the species ; some are light enough to float down a 
 stream, but not too far, so that there is no danger of 
 their being carried to sea ; others, which are character- 
 ized by large air-chambers, are possibly distributed by 
 the wind. 
 
 Semal Both sexual elements may be formed in the 
 repro- game individual, e.g., Oscarella lobularis, Grantia 
 auction, raphan^ an( i many others ; but even in herm- 
 aphrodites one or other element usually occurs to 
 excess in different individuals, so that some are F , G 
 predominantly male and others predominantly 
 
 granules ; at first they exhibit lively amoeboid movements, 
 but later pass into a resting stage. The cavity of the 
 mesodenn within which they are situated becomes lined 
 
 FIG. 24. Spermatozoa, a-*, Development of spermatozoa in Sycatidra raj*- 
 anus, highly magnified ; Ik, mature spermatozoa. After Polejaeff (x7S>2). j, 
 A sperm ball in Osmrdla Mnlarii (x 500) ; 1; an isolated mature spermatozoon. 
 After Schnlze(xSOO). 
 
 by a layer of epithelium, which may not appear, however, 
 till a late stage of segmentation. In Eutpongia qfficinalii 
 the ova occur congregated in groups within the mesodenn, 
 thus presenting an early form of ovary. The spermatozoa, 
 which also develop from wandering amoeboid cells, are 
 minute bodies with an oval or pear-shaped head and a 
 long vibratile tail (fig. 24 ). Each amoeboid cell produces 
 a large number of spermatozoa, which occur in spherical 
 clusters or sperm-balls. The heads of the spermatozoa, 
 as in the Metazoa, are produced from the nucleus of the 
 mother-cell, the tails from the surrounding protoplasm. 
 The development in detail is upon two plans. In Grantia 
 
 b, c, ovum seg- 
 
 i. Development of a calcareous sponge (Syoandro raplantu). 
 
 mented, b, as seen from above, e, lateral view ; d, blastosphere ; , amphiblastula ; /, com. 
 
 t _ i r < mencement of the invagination of the flagellated cells of the amphiblastula ; a, eastrala 
 
 lemale. rolejaetf lOUnd only One SUCh male lOrm attached by its oral face ; , j, young sponge (Ascon stage),-*, lateral view, ;, as seen from 
 
 to 100 female forms in Grantia rapkanus. In above - After Schuize. 
 
 Other sponges Reniera fertilis, Evspongia officinalis the j raphanus (rj) the nucleus of the mother-cell divides into two 
 sexes are distinct. The ova develop from archajocytes or (%, 24 *) one , c **"> resulting daughter nuclei undergoes no 
 ,., ,, 1.-1- j further change, but with a small quantity of peripheral protoplMB 
 
 wandering amoeboid cells, which increase m size and ac- forms a " covlr-cell" to the other or primitiveVperm nucleus and its 
 quire a store of reserve nourishment in the form of yolk | associated protoplasm. The sperm nucleus repeatedly divides, with- 
 
SPONGES 
 
 Ccelenteratc history as exemplified in the last two events will furnish 
 an explanation of the remarkable divergencies which distinguish 
 the two phyla. The history of the second or planula type has been 
 thoroughly worked out by Schulze (20) in a little incrusting Tetrac- 
 tinellid sponge (Plakina monolopha, Schulze). The ovum by regu- 
 lar segmentation produces a blastosphere, the blastomeres of which 
 
 out involving the surrounding protoplasm (fig. 24 c-/). The result- 
 ing nuclei at length cease to exhibit a nueleolus, and become directly 
 transformed into the heads of spermatozoa; the tails are appropriated 
 by each head from the common protoplasmic residue. The mother- 
 cell in this case undergoes no increase in volume as development 
 proceeds, and it is not enclosed within an " endothelial " layer. In 
 the second and apparently more usual case (20) no "cover- 
 cell " is formed, but the mother-cell divides and subdivides, 
 protoplasm as well as nuclei, till a vast number of minute 
 cells results ; the nucleus of each becomes the head of a 
 spermatozoon and the protoplasm its tail. In this case the 
 sperm-ball does increase in bulk : it grows as it develops, 
 and the cavity containing it becomes lined by epithelium, 
 or so-called " endothelium " (fig. 24/). No doubt (75) the 
 development of the epithelium stands in direct physiological 
 connexion with the growth of the sperm-ball. 
 
 Embryo- Obscure as are the details of this subject, suffi- 
 lgy- cient is known to enable us to make out two chief 
 types of development. One, common amongst the 
 calcareous sponges, and possibly occurring in a single 
 genus (Gummina) of the Micromastictora, is char- 
 acterized by what is known as the " amphiblastula " 
 stage; the other, widely spread amongst the 
 Micromastictora (Reniera, Desmacidon, Euspongia, 
 Spongelia, Aplysilla, Oscarella), is characterized by 
 a " planula " stage. 
 
 The first has been most thoroughly investigated in 
 Orantia raphanus by Schulze (20). The ovum by repeated 
 segmentation gives rise to a hollow vesicle, the wall of 
 which is formed by a single layer of cells blastosphere 
 (fig. 25 d). Eight cells at one pole of the blastosphere 
 now become differentiated from the rest; they remain 
 rounded in form, comparatively large, and become filled 
 with granules (stored nutriment), while the others, rapidly 
 multiplying by division, become small, clear, columnar, 
 and flagellated. By further change the embryo becomes 
 egg-shaped; the granular cells, now increased in number 
 to thirty-two, form the broader end, and the numerous 
 small flagellated cells the smaller end. Of the granular 
 cells sixteen are arranged in an equatorial girdle adjoin- 
 ing the flagellate cells. A blastosphere thus differen- 
 tiated into two halves composed of different cells is 
 known as an amphiblastula. The amphiblastula (fig. 25 c} 
 
 now perforates the maternal tissue, and is borne along an - 
 
 excurrent canal to the oscule, where it is discharged to FIG. 26. Development of & Demospongia. (Platenn. monolopha). a, planula (the central part 
 
 the exterior and swims about in a whirling lively dance. 
 It then assumes a more spherical form, a change premoni- 
 tory of the next most remarkable phase of its career. In 
 this the flagellated layer becomes flattened, depressed, and 
 finally invaginated within the hemisphere of granular colls, 
 to the inner face of which it applies itself, thus entirely obliterating 
 the cleavage cavity, but by the same process originating another 
 (the invagination cavity) at its expense (fig. 25/). The two-layered 
 sac thus produced is a paragastrula ; its outer layer, known as the 
 epiblast, gives rise to the ectoderm, the inner layer or hypoblast to 
 the endoderm. The paragastrula next becomes somewhat beehive- 
 shaped, and the mouth of the paragastric cavity is diminished in 
 size by an ingrowth of the granular cells around its margin. The 
 larva now settles mouth downwards on some fixed object, and ex- 
 changes a free for a fixed and stationary existence (fig. 25 </). The 
 granular cells completely obliterate the original mouth, and grow 
 along their outer edge over the surface of attachment in irregular 
 pseudopodial processes, which secure the young sponge firmly to 
 its seat (fig. 25 h}. The granular cells now become almost trans- 
 parent, owing to the exhaustion of the yolk granules, and allow 
 the hypoblast within to be readily seen ; a layer of jelly-like 
 material, the rudimentary mesoderm, is also to be discerned between 
 the two layers. The spicules then become visible ; slender oxeas 
 appear first, and afterwards tri- and quadri-radiate spicules. The 
 larva now elongates into a somewhat cylindrical form ; the distal 
 end flattens ; and an oscule opens in its midst. Pores open in the 
 walls ; the endodermal cells, which had temporarily lost their 
 flagella, reacquire them, at the same time extending the character- 
 istic collar. In this stage (fig. 25 h, j) the young sponge corresponds 
 to a true Ascon, no trace of radial tubes being visible ; but as they 
 characterize the parent sponge they must arise later, and thus we 
 have clear evidence through ontogeny of the development of a 
 Sycon sponge from an Ascon. 
 
 The three most striking features in the history of this larva are, 
 first, the amphiblastula stage ; next the invagination of the flagel- 
 late cells within the granular, instead of invagination in the reverse 
 order ; and third the attachment of the larva by the oral instead of 
 the aboral surface. Should Schulze be correct in deriving the 
 sponges from the Cceleutcra, it is probable that the reversal of the 
 
 should be shaded). 6, Section through side of planula ; ec, flagellated cells ; fl, their 
 flagella ; col, coenoblast. c, Attached gastrula (the paragaster is formed by fission), d, 
 Section across the foregoing, e, Young sponge (Ehagon). /, Part of a section through 
 fully grown sponge ; the attached basal layer is the hypomere ; the spongomere is folded 
 so as to produce incurrent and excurrent canals ; the canal system is eurypylous ; on, ova 
 (a segmented ovum lies between two of them) ; U, blastospheres. After Schulze. 
 
 increase in number by further subdivision till they become con- 
 verted into hyaline cylindrical flagellated cells (fig. 26/). Thus a 
 
 blastosphere is produced eonsistingwholly of similar flagellated cells. 
 It becomes egg-shaped, and, hitherto colourless, assumes a rose-red 
 tint, which is deepest over the smaller end. The larva (now a 
 planula, fig. 26 a, by the filling in of the central cavity) escapes from 
 the parent and swims about broad end foremost. In this stage 
 thin sections show that the cleavage cavity is obliterated, its place 
 being occupied by a mass of granular gelatinous material contain- 
 ing nuclei (fig. 26 b). In from one to three days after hatching the 
 larva becomes attached. It then spreads out into a convex mass, 
 and a cavity is produced within it by the splitting of the central 
 jelly (fig. 26 c, d ; compare Eucope and others amongst the Crclen- 
 terates). This cavity becomes lined by short cylindrical cells (endo- 
 derm), while the flagellated cells of the exterior lose their flagc-lla 
 and become converted into pinnacocytes (ectoderm). The gelatin- 
 ous material left between the two layers now formed acquires the 
 characters of true collenchyme and thus becomes the mesoderm. 
 The endoderm then sends off into the mesoderm, as buds, rounded 
 chambers, which communicate with the paragastric cavity by a 
 wide mouth and with the exterior by small pores (fig. 26 c). An 
 oscule is formed later, and the sponge enters upon the Hhagon phase. 
 Subsequent foldings of the sponge-wall give rise to a very simple 
 canal system (fig. 26/). In addition to these two well-ascertained 
 modes of development others have been described which at present 
 appear aberrant. In OscarcUa lobularis, O. S. (?/), a curious series 
 of early developmental changes results in the formation of an 
 irregular paragastrula, the walls of which become folded (while still 
 within the parent sponge) in a complex fashion, so as to produce a 
 form in which the incurrent and excurrent canals appear to be 
 already sketched out before the flagellated chambers are differenti- 
 ated off. In Spongi/la Gbtte describes the ectoderm as becoming 
 entirely lost on the attachment of the larva, so that the future 
 sponge proceeds from the endoderm alone. As Sjxmgilla, however, 
 
SPONGES 
 
 53 
 
 is a freshwater form, anomalies in its development (which remind 
 us of those in the development of the freshwater Hydra) might 
 almost be expected. 
 
 Probably in no other single group is the doctrine of 
 homoplasy enunciated by Lankester more tellingly illus- 
 trated than in the sponges. The independent develop- 
 ment of similar types of canal system in different groups, 
 sometimes within the limits of a single family, is a remark- 
 able fact. In the following table the sign x shows inde- 
 pendent evolution of similar types of canal system in 
 different groups: 
 
 Physio- 
 logy. 
 
 
 
 
 
 Rhagon. 
 
 
 
 Ascon. 
 
 ~; : ~. 
 
 Eary- 
 pylous. 
 
 AphodaL 
 
 Diplodal 
 
 Class Calcarea 
 
 X 
 
 X 
 
 X 
 
 
 
 Order ffalisarcina 
 
 
 X 
 
 X 
 X 
 
 X 
 
 ... 
 
 
 
 X 
 
 X 
 
 X 
 
 X 
 
 Sub-order Microsclcro- 
 
 
 
 X 
 
 X 
 
 X 
 
 Order Oh/oristida 
 
 
 
 X 
 
 X 
 
 
 Family Tetillidx 
 
 
 
 X 
 
 X 
 
 
 
 
 
 
 
 
 In the gross anatomy of the canal system similar homo- 
 plasy obtains; thus, to cite one case amongst many, a 
 peculiar type of canal system characteristic of Siphonia 
 (Lithistid) occurs also in^;n7>/CKa(Hexactinellid),cAnM#f'a 
 (Monaxonid), and other apparently unrelated genera. The 
 development of a cortex has likewise taken place inde- 
 pendently, but on parallel lines, in the Syconidx, Leu- 
 conidse, Jfonaxona, Tetillidx, and Stellettidx. Calcareous 
 and silicious spicules have evidently an independent his- 
 tory, and yet all the chief forms of the former are repeated 
 in the latter. Quite as remarkable is the similarity of 
 the independently evolved horny spicules of DarwineUa 
 aurea to the quadri- and sex-radiate silicious spicules. We 
 have now sufficient knowledge of the morphology and evolu- 
 tion of the sponge to furnish the physicist with data for an 
 explanation of the skeleton, at least in its main outlines. 
 The obvious conclusion from this is that variation does not 
 depend upon accident, but on the operation of physical 
 laws as mechanical in their action here as in the mineral 
 world. Another important consequence follows : if homo- 
 plasy i.e., the independent evolution of similar structures 
 is of such certain and quite common occurrence in the 
 case of the sponges, it is also to be looked for in other 
 groups, and polyphylitic origin, so far from being improb- 
 able, is as likely an occurrence as monophylitic origin. 
 
 Physiology and ^Etiology. 
 
 Under the head of "physiology" we have almost a 
 blank. At present we do not even know what cells of the 
 sponge are primarily concerned in the ingestion of food. 
 If a living sponge, such as Spongilla, be fed with carmine 
 for a few minutes, then immersed in dilute osmic acid, and 
 examined in thin sections, its flagellated chambers are 
 found to be all marked out as red circular patches, and a 
 closer investigation shows that the choanocytes, and they 
 alone, have ingested the carmine. In this way we con- 
 firm the earlier observations of Carter made by teasing 
 carmine -fed sponges. This might be thought to decide 
 the question ; but, though it effectually disposes of Pole- 
 jaeff's argument that the choanocytes do not ingest nutri- 
 ment because mechanical disadvantages (conceived a priori) 
 make it impossible, it has not proved a final solution. Yon 
 Lendenfeld, by feeding sponges such as Aplysilla with 
 carmine for a longer interval a quarter of an hour finds 
 that amoeboid cells crowd about the sides and particularly 
 the floor of the subdermal cavities, and are soon loaded 
 with carmine granules ; after a time they wander away to 
 the flagellated chambers and there cast out into the ex- 
 currerit canals the carmine they have absorbed, apparently 
 
 in an altered state. On the other hand, the choanocytes, 
 though they at first absorb the carmine", soon thrust it out, 
 apparently in an unaltered state. Hence Von Lendenfeld 
 concludes that it is the epithelium of the subdermal cavities 
 which is charged with the function of ingestion, and that 
 the amoeboid cells subsequently digest and distribute it, 
 and finally cast out the worthless residues. There may be 
 much truth in this view, but it requires to be supported 
 by further evidence. (1) Sufficient proof is not adduced 
 to show that the carmine granules expelled from the amoe- 
 boid cells are really more decomposed than those rejected 
 by the choanocytes. (2) There is at present no proof that 
 carmine is a food, or that if it is sponges will readily feed 
 upon it. In either case one would expect the amoeboid 
 cells to play the part which they perform in other organisms 
 and to remove as soon as possible useless or irritant matter 
 from the surface which it encumbers ; at the same time 
 the choanocytes, not having found the food to their liking, 
 would naturally eject it. (3) If the choanocytes do not 
 ingest food, how does the Ascon feed, since in this sponge 
 all the pinnacocytes are external ? It is, however, a very 
 noticeable fact that, as the organization of a sponge 
 increases in complexity, the choanocytal layers become 
 reduced in volume relative to the whole bulk of the 
 individual; and it is quite possible that as histological 
 differentiation proceeds it may be accompanied by physio- 
 logical differentiation which relieves the choanocytes to 
 some extent of the ingestive part of their labours. 
 
 The origin of the sponges is to be sought for among JStio- 
 the choanoflagellate Infusoria ; and Savile Kent has de- lo gJ'- 
 scribed a colonial form of this group which is suggestively 
 similar to a sponge. Its differences, however, are as 
 marked as its resemblances, and have been sufficiently 
 pointed out by Schulze (23). Kent has called this form 
 Protospongia, a name already made use of, and fortunately, 
 as the organism is not in any sense a true sponge ; the 
 present writer proposes, therefore, to call it Savillia, in 
 honour of its discoverer. It consists of choanoflagellate 
 Infusoria (see PBOTOZOA, vol. xix. p. 858, fig. XXI., 15), 
 half projecting from and half embedded in a structureless 
 jelly or blastema, within which other cells of an amoeboid 
 character and reproductive function are immersed. Pro- 
 fessor Haddon arrives at the generalization that conjuga- 
 tion amongst the Protozoa always takes place between 
 individuals of the same order : flagellate cells conjugate 
 with flagellate, amoeboid with amoeboid, but never with 
 flagellate ; while in true sexual reproduction the conjuga- 
 tion occurs between two individual cells in different stages 
 of their life cycle : a flagellate cell conjugates with a resting 
 amoeboid cell. Now Savillia would appear to be extremely 
 near such a true sexual process, since the simultaneous 
 coexistence of cells in two different stages of life and 
 within easy reach of each other a necessary preliminary, 
 one would think, to the union has already been brought 
 about. That coalescence between two different histological 
 elements should result in products similarly histologically 
 differentiated (compare amphiblastula stage of Calcarea) 
 has in it a certain fitness, which, however, has still to be 
 explained. The mode by which an organism like Savillia 
 might become transformed into an Ascon cannot be sug- 
 gestively outlined with any satisfactory results till our 
 knowledge of the embryology of sponges is more advanced. 
 The minute characters of the flagellate cells of the amphi- 
 blastula and other sponge larvae are still a subject for 
 research. They often possess a neck or colluni ; but the 
 existence of a frill or collar is disputed. Kent asserts 
 that it is present in several embryos which he figures; 
 and Barrois makes the same assertion in respect to the 
 larva of Oscarella, and illustrates his description with a 
 figure. On the other hand, Schulze and Marshall both 
 
54 
 
 SPONGES 
 
 deny its existence, and the former attributes Kent's 
 observations to error. One constant character they do 
 possess : they are provided with flagella at some stage of 
 their existence, but never with cilia. Ciliated cells, in- 
 deed, are unknown amongst the sponges, and, when pinna- 
 cocytes exceptionally acquire vibratile filaments, as in 
 Oscarella and other sponges, these are invariably flagella, 
 never cilia. An Ascon stage having been reached at some 
 point in the history of the sponges, the Sycon tubes and 
 Rhagon chambers would arise from it by the active pro- 
 liferation of choanocytes about regularly distributed centres, 
 possibly as a result of generous feeding. Vosmaer recog- 
 nized as the physiological cause of Sycon an extension of 
 the choanocytal layer. Polejaeff, relying on Von Lenden- 
 feld's experiments, which seem to prove that it is the 
 pinnacocytes and not the choanocytes which are concerned 
 in the ingestion of nutriment, argues that, as in Sycon 
 the pinnacocytal layer is increased relatively to the choano- 
 cytal, we have in this a true explanation of the transition. 
 The existence of ffomoderma, Lfd., however, shows that 
 in the first stage there was not a replacement of choano- 
 cytes by pinnacocytes, but that this was a secondary 
 change, following the development of radial tubes, and 
 therefore cannot be relied upon to explain them. The 
 radial tubes having been formed by a proliferation of 
 choanocytal cells, the reduction of those lining the para- 
 gastric cavity to pinnacocytes would follow in consequence 
 of the poisonous character of the water delivered from the 
 radial tubes to the central cavity, since this water not 
 only parts with its dissolved oxygen to the choanocytes 
 it first encounters, but receives from them in exchange 
 urea, carbonic acid, and faecal residues. The development 
 of subdermal cavities is explicable on Von Lendenfeld's 
 hypothesis. 
 
 Distribution. 
 
 Distribu- Our knowledge of this subject is at present but frag- 
 tiou in rnentary ; we await fuller information in the remaining 
 space, reports on the sponges obtained by the " Challenger." The 
 sponges are widely distributed through existing seas, and 
 freshwater forms are found in the rivers and lakes of all 
 continents except Australia, and in numerous islands, in- 
 cluding New Zealand. Many genera and several species 
 are cosmopolitan, and so are most orders. 
 
 As instances of the same species occurring in widely remote 
 localities we take the following from Polejaeff : Sycon arcticum is 
 found at the Bermudas and in the Philippine Islands, as also are 
 Leuconia multiformis and Leucilla utcr ; Sycon raphanus occurs at 
 Tristan da Cunha and the Philippines ; Hcteropcgma nodus-gordli 
 and Lcuconia dura at the Bermudas and Torres Straits. We do not 
 know, however, whether these species are isolated in their distribu- 
 tion or connected by intermediate localities. Of the Calcarca about 
 eighty-one species have been obtained from the Atlantic, twenty- 
 two from the Pacific, and twenty-two from the Indian Ocean ; but 
 these numbers no doubt depend largely on the extent to which the 
 several oceans have been investigated, for the largest number of 
 species has been found in the ocean nearest home. Schulze states 
 that the Hcxactincllida brought home by the "Challenger" were 
 obtained at seventeen Atlantic stations, twenty-seven Pacific, and 
 nineteen in the South Seas. In the last the number of species 
 was greatest, in the Atlantic least. They nourish best on a 
 bottom of diatomaceous mud. The Calcarea and Ceratosa are 
 most abundant in shallow water and down to 40 fathoms, but 
 they descend to from 400 to 450 fathoms. The ffcxactincttida are 
 most numerous over continental depths, i.e., 100 to 200 fathoms; 
 but they extend downwards to over 2500 fathoms and upwards 
 into shallow water (10 to 20 fathoms). The Lithistida are not such 
 deep-water forms as the ffexadincllida, being most numerous from 
 10 to 150 fathoms. Only one or two species have been dredged 
 from depths greater than 400 fathoms, and none from 1000 fathoms. 
 The Churistida range from shallow water to abyssal depths. A 
 characteristic deep-sea Choristid genus is Thcnca, Gray ( = Wyvillc 
 Thompsonia, Wright ; Donrillia, Kent). This is most frequently 
 dredged from depths of from 1000 to 2000 fathoms ; but it extends 
 to 2700 fathoms on the one hand and to 100 on the other. 
 in time. Until about 1876 one of the chief obstacles to the inter- 
 
 pretation of fossil sponges arose from a singular mineral 
 replacement which most of them have undergone, leading 
 to the substitution of calcite for the silica of which their 
 skeletons were originally composed. This change was de- 
 monstrated by Zittel (jj) and Sollas (24), and, though it 
 was at first pronounced impossible, owing to objections 
 founded on the chemical nature of silica, it has since be- 
 come generally recognized. These observers also showed 
 that the fossil sponges do not belong to extinct types, but 
 are assignable to existing orders. Zittel in addition sub- 
 jected large collections to a careful analysis and marshalled 
 them into order with remarkable success. Since then 
 several palaeontologists have worked at the subject, Pocta, 
 Dunikowski, and Hinde (7), who has published a Cata- 
 logue which is much more than a catalogue of the 
 sponges preserved in the British Museum. The result of 
 their labours is in general terms as follows. Fossil sponges 
 are chiefly such as from the coarseness or consistency of 
 their skeletons would be capable of preservation in a miner- 
 alized state. Thus the majority are Hexactinellida, chiefly 
 Dictyonina ; Tetractinellida, chiefly Lithistida ; and Cal- 
 carea, chiefly Leuconaria. Monaxonid sponges rarely occur ; 
 the most ancient is Climacospongia, Hinde, found in Sil- 
 urian rocks. A very common Halichondroid sponge of this 
 group (Pliaretrospongia strahani, Soil.) occurs in the Cam- 
 bridge greensand ; it owes its preservation to the collection 
 of its small oxeate spicules into dense fibres. The C/ioristida, 
 though not so common as the Lithistids, are commoner 
 than the Monaxonids, particularly in Mesozoic strata. 
 
 The distribution of fossil sponges in the stratified systems may 
 be summarized as follows. CALCAREA. Homoccela, none. Hetcro- 
 cosla, a Syconid, in the Jurassic system. Numerous Leuconaria 
 from the Devonian upwards. MYXOSPONGI.E. None ; not fitted 
 for preservation. HEXACTINELLIDA. Lyssacina, from the Lower 
 Cambrian upwards. Dictyonina, commencing in the Silnrian ; most 
 numerous in the Mesozoic group ; still existing. MONAXONIDA. 
 Monaxona, from the Silurian upwards. Ceratosa, none ; few are 
 fitted for preservation. TETRACTINELLIDA. Choristida, from the 
 Carboniferous upwards ; most numerous in the Cretaceous system. 
 Lithistida, from the Silurian upwards ; most numerous in the 
 Mesozoic group. In ancient times the Hexactinellids and Lithistids 
 seem not to have been so comparatively uncommon in shallow 
 water as they are at the present day. Thus, in the Lower Jurassic 
 strata of the south-west of England we find Dictyonine Hexactinel- 
 lids, Lithistids, and Leuconarian Calcarca associated together in a 
 shelly breccia and in company with littoral shells, such as Patella 
 and Trochus. Several Palaeozoic Hexactinellids actually occur in a 
 fine-grained sandstone. Of the Chalk, which is the great mine of 
 fossil sponges, we must speak with caution, owing to the insufficient 
 evidence as to the depth at which it was deposited. 
 
 As shown by Protospongia, the phylum of the sponges was in 
 existence in very early Cambrian times, and probably much earlier. 
 Before the end of the Silurian period its main branches had spread 
 themselves out, and, developing fresh shoots since then, they have 
 extended to the present day. Of the offshoots none of higher value 
 than families are known to have become extinct, and of these 
 decayed branches there are very fe\v. The existence in modern 
 seas of the Asconidse, which must surely have brauch'ed off very 
 near the base of the stem, is another curious instance of the per- 
 sistence of simple types, which would thus appear not to be so vastly 
 worse off in the struggle for existence than their more highly 
 organized descendants. 
 
 Bibliography. A fairly complete list of works on sponges published before 
 1882 will be found in Vosmaer's article "Porifenc," in Bronn's Klassen und 
 Ordnungen, vol. ii. D'Arcy Thompson's Catalogue of Papers on Protozoa and 
 Codenterata, a still more complete list, extends to 1884. 
 
 The following is a list of works, including those referred to in the preceding 
 pages : (/) C. Barrois, Embryologie d. quelr/ues Sponges d. I. Manche, Paris, 1876. 
 (?) Bowerbank, A Monograph of British Spongiadrc, vols. i.-iv., 1864-82 (vol. 
 iv. is posthumous, edited by Dr Norman). (3) Carter, a series of papers in the 
 Ann. and Mag. Nat. Hist., from 1847 to the present time (1887). (4) 3. Clark, On 
 the Spongia" ciliatse as Infusoria flagellata, 1865. (j) Grant, Eili-n. Phil. Jmirn., 
 1825. (6) Haeckel, Monngraphie d. Kalkschicammt, 1871. (7) Hinde, A Cata- 
 logue of theSjionges in the British Museum, 1883. (f) Id., "On the Ktceptaculitidm," 
 in Quart. Jo-urn. Geol. Soc., xl. 795, 1884. (<j) Keller, "Studien ii. Organisation 
 u. Entwickelung rl. Chalineen," in Ztschr. f. wiss. Zool., xxxiii., 1879. (/o) Kent, 
 "Notes on the Embryology of the Sponges," in Ann. and Mag. Nat. Hist., 
 1878, ii. 139. (//) Von Lendenfeld, "On Aplusinidai," in Ztschr. f. wiss. Zool., 
 xxxviii. (12) Id., "A Monograph of Australian Sponges," in Proc. Linn. Soc., N.S. 
 Wales, vols. ix., x. (other papers by Von Lendenfeld will be found under this 
 reference, and also in the Zool. Anzeiger). (/?) Lieberkiihn, " Developmental 
 History of Spongilla," in Mull. Archiv, 1856. (14) Marshall, Jenaische Ztschr., 
 xviii., 1885 (translated in Ann. and Mag. Nat. Hist.), (rj) Polejaeff, " On Sperma 
 and Spermatogenesis in Sycandra raphanus," in Sits.-Ber. Acad. wiss. Zool., 
 
SPONGES 
 
 55 
 
 Intl. d. UnioertOdt Gnu. (16) Id., "CTtaticnger" Report m the Calcarea, 1883. 
 (/7) Id., Ditto on the Ceratosa, 1884. (ig) Ridley, On Ou Zool. Collection of the 
 "Alert," 1884. (/o) Schmidt, Sponges of the Adriatic Sea, 1862, with Supple- 
 ment 1 in 1864, and Supplement 2 in 1 S06 ; Sponges of the Coast of Algiers, 1868; 
 Sponge-Fa-una of Ou Atlantic, 1870 ; Sponges of the Gulf of Mexico, 1879. (20) 
 F. E. Schulze, investigations into the structure and development of sponges, 
 in Ztschr. f. Kiss. Zool.," On Halisaraa," voL xxviii., 1877 ; " On Chondnsidx," 
 nil., 1877; "On Aplysinidtc," DOL, 1878; "On Metamorphosis of Sycandra 
 raphanus," ixxi., 1S7S; "On Spongdia," xxxii., 1878; "On Spongidx," ib. ; 
 "On Hircinia and Oligoceras," xiniL, 1879; "On Plakinida," ixiiv., 1880; 
 "On Corticium candelabrum," mv., 1881. (*/) Id., "On Soft Parts of 
 Euplectella. aspergillum," in Trans. Boy. Sac. Edin., xxix., 1880. (.?->) Id., 
 Preliminary Report on the "Challenger" Heiactinellida. (23) Id., "On the 
 Relationship of the Sponges to the ChoanoJIagellata," in SUz.-Ber. d. k.-preuss. 
 
 the Triininingham Chalk," i*., vL, 1879. (*7) Id., " Development of Halisarca 
 tobularis," in Quart. Jour*. Micr. Sci., nriv., 1884. (28) Id., " On Vetulina and 
 the Anomadadina," in Proe. R. Irish Acad., iv., 1885. (19) Id., "Physical 
 Characters of Sponge-Spienles," in Proc. R. Dub. Soc., 1885. 'jo) Vejdovsky, 
 " The Freshwater Sponges of Bohemia," in Abk. d. k. Bdhm. Akad, d. Wiss.. ser. 
 Ti., voL ni., 1883. (31) Vosmaer, OTI Leucandra aspera (doctor's diss., Leyden, 
 1SSO). (32) Id., "On the Desmacidinid*;," in Sates from Ou Leyden Museum, 
 vol. ii. (S3) Sponges of the H'illem Barents Expedition, 1884. (?*) " Poriferse," in 
 Bronn's Klasstn und Ordnungen, vol. ii., 1882, and still in progress, (jy) Zittel, 
 studies of fossil sponges, in Abh. d. k. buyer. Akad.,-Hetactinellida, 1877; 
 Li&istida, 1878 ; Monactintllida and Calcarea, 1878. 
 
 Commerce. 
 
 When the living matter is removed from a Ceratose 
 sponge a network of elastic horny fibres, the skeleton of 
 the animal, remains behind. This is the sponge of com- 
 merce. Of such sponges the softest, finest in texture, and 
 most valued is the Turkey or Levant sponge, Euspongia 
 ojfirinalis, Lin. The other two varieties are the Hippo- 
 fpongia equina, O. Schmidt, and the Zimocca sponge, 
 Euspongia zimocca, O.S., which is not so soft as the others 
 Distribu- (see p. 423 above). All three species are found at from 2 
 tion. to 100 fathoms along the whole Mediterranean coast, includ- 
 ing its bays, gulfs, and islands, except the western half of 
 its northern shores as far as Venice and the Balearic Isles, 
 Corsica, Sardinia, and Sicily. Bath sponges occur around 
 the shores of the Bahamas, and less abundantly on the north 
 coast of Cuba. They are of several kinds, one not dis- 
 tinguishable from the fine Levant sponge ; others, the 
 "yellow" and "hardhead" varieties, resemble the Zimocca 
 sponge ; and of horse sponges there appear to be several 
 varieties, such as the " lamb's-wool " and the "velvet" 
 sponge (Hippospongia gossypina. and H. meandriformis). 
 The fine bath sponge occurs on the shores of Australia 
 (Torres Straits, the west coast, and Port Phillip on the 
 south coast). A sponge eminently adapted for bathing 
 purposes (Coscinoderma lanuginosum, Crtr. ; Euspongia 
 mathewsii, Lfd.), but not yet brought into the market, 
 occurs about the South Caroline Islands, where it is actu- 
 ally in use, and at Port Phillip in Australia. The fine 
 bath sponge occurs in the North Pacific, South Atlantic, 
 and Indian Oceans, so that its distribution is world-wide. 
 Fishing. The methods employed to get sponges from the bottom 
 of the sea, where they grow attached to rocks, stones, and 
 other objects, depend on the depths from which they are 
 to be brought. In comparatively shallow water they may 
 be loosened and hooked up by a harpoon ; at greater 
 depths, down to 30 or 40 fathoms, they are dived for; and 
 at depths of from 50 to 100 fathoms they are dredged 
 with a net. The method of harpooning was the earliest 
 practised, and is still carried on in probably its most 
 primitive form by the Dalmatian fishermen. Small boats 
 are used, manned by a single harpooner with a boy to 
 steer ; when, however, the expedition is to extend over 
 night the crew is doubled. The harpoon is a five-pronged 
 fork with a long wooden handle, and if this is not long 
 enough another harpoon is lashed on to it. The Greek 
 fishers use a large boat furnished with two or three smaller 
 ones, from which the actual harpooning is carried on ; the 
 crew numbers seven or eight. One of the chief difficulties 
 is to see the bottom distinctly through a troubled surface. 
 The Dalmatian fishers throw a smooth stone dipped in oil 
 
 a yard or so in front of the boat ; the stone scatters drops 
 of oil as it flies and so makes a smooth track for the " look- 
 out." The Greeks use a zinc-plate cylinder about 1 J feet 
 long and 1 foot wide, closed at the lower end by a plate of 
 glass, which is immersed below the surface of the sea ; on 
 looking through this the bottom may be clearly seen even 
 in 30 fathoms. This plan is also adopted in the Bahamas, 
 where harpooning carried on after the Greek system gives 
 employment to over 5000 men and boys. 
 
 The primitive method of diving with no other apparatus 
 than a slab of stone to serve as a sinker and a cord to 
 communicate with the surface is still practised in the 
 Mediterranean. The diver carries a net round his neck 
 to hold the sponges. On reaching the bottom he hastily 
 snatches up whatever sponge he sees. After staying down 
 as long as he is able an interval which varies from two 
 to at the most three minutes he tugs violently at the 
 cord and is rapidly drawn up. On entering the boat from 
 depths of 25 fathoms he quickly recovers from the effects 
 of his plunge after a few powerful respirations ; but after 
 working at depths of 30 to 40 fathoms or more he reaches 
 the surface in a swooning state. At the beginning of the 
 season blood usually flows from the mouth and nose after a 
 descent ; this is regarded as a symptom of good condition ; 
 should it be wanting the diver will scarcely venture a second 
 plunge for the rest of the season. The work is severe, and 
 frequently the diver returns empty-handed to the boat. 
 Diving is usually carried on in the summer months; in 
 winter it is too cold, at all events without a diving-dress. 
 The ordinary diver's dress with pumping apparatus is 
 largely used by the Greeks. The diving is carried on 
 from a ship manned by eight or nine men, including one, 
 or rarely two, divers. At a depth of from 10 to 15 fathoms 
 the diver can remain under for an hour, at greater depths 
 up to 20 fathoms only a few minutes ; the consequences of 
 a longer stay are palsy of the lower extremities, stricture, 
 and other complaints. Dredging is chiefly carried on along 
 the west coast of Asia Minor, principally in winter after 
 the autumn storms have torn up the seaweeds covering 
 the bottom. The mouth of the dredge is 6 yards wide 
 and 1 yard high ; the net is made of camel-hair cords of 
 the thickness of a finger, with meshes 4 inches square. It 
 is drawn along the bottom by a tow-line attached to the 
 bowsprit of a sailing vessel or hauled in from the shore. 
 
 Prompted by a suggestion made by Oscar Schmidt, that Cnltiva- 
 sponges might be artificially propagated from cuttings, 
 the Italian Government supplied funds for experiments to 
 determine the feasibility of cultivating sponges as an in- 
 dustrial pursuit. A station was established on the island 
 of Lesina, off the Dalmatian coast, and experiments were 
 carried on there for six years (1867-72) under the super- 
 intendence of Von Buceich. The results were on the whole 
 successful, but all expectations of creating a new source 
 of income for the sponge-fishers of Dalmatia were defeated 
 by the hostility of the fishers themselves. 
 
 The details of the method of sponge-farming as practised 
 by Von Buceich are briefly as follows. The selected speci- 
 mens, which should be obtained in as uninjured a state as 
 possible, are placed on a board moistened with sea water 
 and cut with a knife or fine saw into pieces about 1 inch 
 square, care being taken to preserve the outer skin as in- 
 tact as possible. The operation is best performed in winter, 
 as exposure to the air is then far less fatal than in summer. 
 The sponge cuttings are then trepanned and skewered on 
 bamboo rods ; the rods, each bearing three cuttings, are 
 secured in an upright position between two parallel boards, 
 which are then sunk to the bottom of the sea and weighted 
 with stones. In choosing a spot for the sponge-farm the 
 mouths of rivers and proximity to submarine springs must 
 be avoided ; mud in this case, as in that of reef-building 
 
56 
 
 SPONGES 
 
 Prepara- 
 tion for 
 market. 
 
 corals, is fatal. A favourable situation is a sheltered bay 
 with a rocky bottom overgrown with green seaweed and 
 freshened by gentle waves and currents. So favoured, 
 the cuttings grow to a sponge two or three times their 
 original size in one year, and at the end of five to seven 
 years are large enough for the market. Similar experi- 
 ments with similar results have more recently been carried 
 on in Florida. The chief drawback to successful sponge- 
 farming w^uld appear to be the long interval which the 
 cultivator has to wait for his first crop. 
 
 After the sponge has been taken from the sea, it is 
 exposed to the air till signs of decomposition set in, and 
 then without delay either beaten with a thick stick or 
 trodden by the feet in a stream of flowing water till the 
 skin and other soft tissues are completely removed. If 
 this process is postponed for only a few hours after the 
 sponge has been exposed a whole day to the air it is almost 
 impossible to completely purify it. After cleaning it is 
 hung up in the air to dry, and then with others finally 
 pressed into bales. If not completely dried before pack- 
 ing the sponges " heat," orange yellow spots appearing on 
 the parts attacked. The only remedy for this is to unpack 
 the bale and remove the affected sponges. The orange- 
 coloured spots produced by this "pest," or "cholera" as 
 the Levant fishermen term it, must not be confounded 
 with the brownish red colour which many sponges natu- 
 rally possess, especially near their base. The sponges on 
 reaching the wholesale houses are cut to a symmetrical 
 shape and further cleaned. The light-coloured sponges 
 often seen in chemists' shops have been bleached by 
 chemical means which impair their durability. Sponges 
 are sold by weight ; sand is used as au " adulteration." 
 
 It is difficult to obtain recent statistics as to the extent 
 of the sponge trade ; the following tables gives a summary 
 of the sponges sold in Trieste, the great European sponge 
 market, in the year 1871 : 
 
 TABLE I. 
 
 Description of Sponge. 
 
 For Export. 
 
 Value in . 
 
 Moan price 
 per pound. 
 
 Horse sponge 
 
 60,000 
 20,000 
 20,000 
 2,000 
 
 6s. 
 6s. 
 14s. 
 
 8s. 
 
 Zimocca sponge 
 
 
 Fine Dalmatian sponge 
 
 
 TABLE II. 
 
 Description cf Sponge. 
 
 For Home Consumption. 
 
 Value in . 
 
 Mean price 
 per pound. 
 
 Horse sponge 
 
 4400 
 550 
 950 
 
 6s. 
 6s. 
 14s. 
 
 
 Fine Levant sponge 
 
 Fine Dalmatian sponge 
 
 
 (W. J. S.) 
 
HYDROZOA 
 
 FTIHE HYDROZOA form one of the three classes into 
 J_ which the Codentera nematophora (distinguished from 
 the Codentera porifera, or Sponges) have been divided. 
 It results from observations made by Ernst Haeckel that 
 the Ctenopkora should not be regarded as a class equi- 
 
 valent to the Hydrozoa and Actinozoa, nor as a subdivision 
 of the latter class, but that they must be considered as a 
 peculiar modification of the medusiform Hydrozoa (see 
 final paragraph). If this conclusion be accepted, it will 
 be necessary to divide the Hydrozoa into two primary 
 
 Scyphomedusas from the Deep Sea. (After Haeckel, Challtager Rtportt, vol. iv. 1882). 
 
 A. feriphflla minM2tt, Haeck., one of the Peromednsse, one-third the natural size, a, one of the font interradial tentaculocysts (sensory organs) sunk 
 
 between its lappets ; 6, one of the sixteen snbradial coronal lobes. The twelve tentacles (four perradial. eight ail radial) are seen. 
 
 B. Perradial section through Luarnaria baUiyphila, Haeck., nat. size, a, perradial gastral poach ; 6, gasrral aiial cavity ; c, ovary (four); d, gasrral filaments; 
 
 e, perradial gastral pouch ; /, manubrium and mouth ; 0, the bunches of tentacles (eipht, adradial). 
 
 The eight principal tentacles (four perradial and four interradial) are not in this species converted Into adhesive anchors as In L. auricula, but are 
 altogether suppressed. 
 
 groups or grades, for which the names Polypomorpha and 
 Ctenophora are proposed. 
 
 The Hydrozoa correspond to the Linnsean genera Hydra, 
 Tulndaria, Sertularia, and Medusa. The name was applied 
 by Huxley in 1856 to a group corresponding to that termed 
 Hydromedusx by Vogt (1851) and Htdusx by Leuckart 
 (1853), and embracing the forms placed by Gegenbaur in 
 his Elements of Ccmparative Anatomy (1878) in four classes, 
 viz., Hydromedusx, Calycozoa, Thecomedusx, and Medusx. 
 Our knowledge of the structure and life-history of the 
 Hydrozoa, many of which, on account of their delicacy and 
 oceanic habits, are excessively difficult to obtain in a state 
 fit for investigation, has greatly extended within the last 
 five years. Whilst in the two decades preceding this period 
 the admirable researches of Huxley, Gegenbaur, Agassiz, 
 and Allman had brought to light and systematized a vast 
 mass of information with regard to these organisms, the 
 later observations of Claus, the Hertwigs, Haeckel, and 
 Metschnikoff, have corrected, extended, and added to 
 their history, especially in respect of embryological and 
 histological detail. An epitome of the present condition 
 of our knowledge of the group is afforded by the subjoined 
 tabular classification of its families, orders, and sub-classes. 
 
 The definition and synonymy of the divisions recognized 
 
 will be entered into, after a sketch has been given of the 
 common structural features of typical Hydrozoa. 
 
 CLASS HYDROZOA. 
 
 Sub-Class I. Scyphomedusse (syn. Ephyromedmae). 
 
 Order 1. LCCEENABLK. Example*. 
 
 Fam. 1. Eleutherocarpid* .......... { 
 
 Order 2. DISCOMEDUS.S (Haeckel). 
 Sub-Order 1. Cuoostoma?. 
 Fam. 1. Protephyridje. 
 i. Nausithoid*. 
 *. EphyrellidK. 
 4. AtoUids. 
 i. Cyclorchidas. 
 Sub-Order 2. Semostomae. 
 
 Kausithoe. 
 
 Fam. 1. 
 
 2. Cvanjeidae ............... 
 
 \ 3. Sthenonidje 
 4. Anrelidn 
 Sub-Order 3. Rhizostnmse. 
 
 Fam. 1. TetragamelUe 
 
 2. Monogamelix ... 
 
 Order 3. CoxoMEcrsiai (Haeckel). 
 
 Fam. 1. Charybdeidse 
 
 2. Bursarida 1 . 
 3. Chiropsahnidff. 
 Order 4. PEEOMEDUS^ (Haeckel). 
 Fam. 1. Periphyllidw. 
 ,, 2. Pericn-ptidie. 
 
 Sthenonhu 
 
 Aurelia (figs. 26-31). 
 
 ICephea. 
 Cassiopeia. 
 
 Rhizostoma (fig. 24, a). 
 
 Cliarybdaea (figs. 20-23). 
 
58 
 
 HYDROZOA 
 
 Sab-Class II. Hydromedusae. 
 
 Order 1. GTMNOBLASTEA-ANTHOMEDUS*. 
 
 ( Tuliularia (flg. 35). 
 Fam. 1. Tubularidas .................. J. Hybocodon. 
 
 ( Corymorpha (flg. 34). 
 
 - 2-Pennarid* ..................... { Sta. 
 
 ( BongainviUia (figs. 36, 37). 
 3. Eudendridai .................. \ Pevigonium. 
 
 (. Lizzia (flg. 44). 
 
 t Cladonema. 
 
 4. Cladonemida: 
 
 Clavatella. 
 
 6. Dicorj'nidse .................. Dicoryne (flg. 40). 
 
 1 Sarsiadse (flg. 45). 
 
 7. Corynidse .................... \ Coryne. 
 
 ( Syncoryne (flgs. 41, 46). 
 
 3-Hydractin.d* ............... { > * 39 >' 
 
 . 10.Hy.Wd. ........................ 
 
 Order 2. CALYPTOBLASTKA-LEPTOMEDUS.*. 
 Fam. !. Plumularida, .................. 
 
 / Eucopidse. 
 
 3. 
 
 4. Thaumantiad*.. 
 
 ( Obelia. 
 /Thaumantias. 
 
 ] iwSrtmi. 
 
 (Tima. 
 f jEquorea. 
 5. .ajquoridaB ..................... < Zygodaetyla. 
 
 ( Rhegmatodes. 
 Order 3. TRACHOMEDUS.S (Haeckel). 
 
 Fam. 1. Petasidae ..................... Petasus. 
 
 2. Trachynemidffi ............... Rhopalonema. 
 
 3. Aglauridae ..................... Aglaura. 
 
 4.Geryonid ..................... { Car'marina (flgs. 48, 49). 
 
 Order 4. NARCOMEDUS^: (FTaeckcl). 
 
 Fam. 1. Cunanthtdae .................. Cunina {figs. 50, 51). 
 
 ,, 2. Peganthidse .................. Polyxenia. 
 
 Order 5. HTDROCORALLIS-.S (Moseley). 
 
 Fam. 1. Milleporidce ................... Mlllepora (figs. 52, 53). 
 
 f Sporadopora, 
 2. Stylasteridse .................. ( Distichopora. 
 
 ( Astylus (flg. 54). 
 Order 6. S;PHONOPHORA. 
 
 Sub-Order 1. Physophoridse. 
 
 Fara. 1. Athorybiada; ................. Athorybia, 
 
 2. Physoplioridse ............... Physophora (fig. 57, C). 
 
 (ForskalHa. 
 Halistemma. 
 Agalma (flg, 57, E). 
 4. Apolemiadffi .................. Apolemia. 
 
 5. Rhizophysidse .............. Rhizophysa. 
 
 Sub-Order 2. Physalidse. 
 
 Fam 1. Physalidte ..................... Physalia. 
 
 Sub-Order 3. Calycophoridae. 
 
 Fam. 1. Hippopodiidx ................ Gleba. 
 
 ( Praya. 
 2.Diphyidas ..................... 1 Diphyes (flg. 67, A). 
 
 (Abyla. 
 3. Monophyidse .................. Sphseronectes. 
 
 Sub-Order 4. Discoideaa. 
 
 ram.l.Velemd 
 
 The Hydrozoa present a greater simplicity of ultimate 
 structure than do any animal organisms possessed of as 
 great a complexity of external form. As in all Metazoa or 
 Enterozoa, the life cycle of a hydrozoon starts with an egg 
 which is at first a single cell or unit of protoplasm, but 
 proceeds after fertilization to multiply by transverse fission 
 in such a way that the resulting cells or units are arranged 
 in two layers, each one cell deep, disposed around a central 
 cavity the enteron or archenteron. The sac thus formed 
 is known as a diblastula (figs. 1, 2, and 25). By the forma- 
 tion 1 of a mouth to the sac, the enteron acquires the functions 
 of a digestive retort in which food matters taken in at 
 the mouth are brought into a chemical condition suitable 
 for the nutrition of the surrounding cells. The two layers 
 of cells (of which the outer only acquires additional layers 2 
 
 1 In HydromeduscE the inner layer of cells forms by delamination, 
 in Scyphomedusce by invagination. In the latter case the sac closes 
 up, and the mouth is formed by a new opening. 
 
 2 It is probable that the numerous rows of cells described in the 
 endoderm of Tvindaria and Corymorpha by Allman, in his great mono- 
 graph of the Tubularian Hydroids, are due to a plication of the 
 
 by the division of the primary cells, and that by no 
 means in all cases) received from Allman (Phil. Trans., 
 1855) the names respectively of the & 
 
 ectoderm and the endoderm, having 
 previously been shown by Huxley 
 (1849) to be the fundamental mem- 
 branous constituents of which the 
 most varied parts of the more com- 
 plex Hydrozoa, such as tentacles, 
 swimming bells, and air-bladders 
 are built up in the adult condition. 
 Huxley also pointed out the iden- 
 tity of these membranes with the 
 two primary layers of the vertebrate 
 embryo. The endoderm and the 
 ectoderm, which present themselves, 
 as is now known, in the diblastula (or 
 gastrula) phase of all Enterozoa, re- 
 main in Hydrozoa (and also in the allied 
 groups of Caelentera) as permanently distinguishable ele- 
 ments of structure. This important disposition is associ- 
 ated with and dependent on the simple character which the 
 archenteron or primitive digestive space retains. Into what- 
 ever lobes or processes the sac-like body may be, so to 
 
 FIG. 1. Diagram of a Di- 
 blastula. a, orifice of in- 
 vagination (blastopore) ; 
 b, archenteric cavity ; c t 
 endoderm ; d, ectoderm. 
 (From Gegenbaur's Ele- 
 ments of Comparative 
 Anatomy.) 
 
 FIG. 2. Formation of the Diblasrnla of Eucope (one of the Calyptoblastic Hydro- 
 medusa) by delamination. (From Balfour, after Kowalewsky.) A, B, C, three 
 successive stages, ep, ectoderm; hy, endoderm; a/, enteric cavity. 
 
 speak, moulded, whether tentacles 3 or broader expansions, 
 into these the cavity of the archenteron is extended in the 
 first instance ; and where the actual cavity is obliterated 
 the endodermic cell-layer remains to represent it (Gefass- 
 platte or endoderm-lamella, see figs. 7 and 16). 
 
 Conversely, whatever canals or spaces are discovered in 
 the substance of a hydrozoon (excepting only the cavity of 
 ectodermal otocysts) are simple and direct continuations 
 of the one original enteric cavity of the diblastula, and all 
 such spaces are permanently in free communication with 
 one another. 4 
 
 The whole of the Hydrozoa seem to present a lower grade 
 of structure than the Actinozoa, in so far as the latter, 
 whilst retaining permanently free communication between 
 ! all parts of the archenteric space, yet exhibit a differentia- 
 tion of this space into an axial and a periaxial portion a 
 digestive tube and a body cavity. The differentiation has 
 only to proceed a step further, namely, to the closure or 
 shutting off of the axial from the periaxial portion of 
 the archenteric space, and we obtain the condition which 
 characterizes the adult forms of the Caelomata, or animals 
 
 original endodermal cell-layer. The two kinds of cells in two layers 
 figured by the same authority in the endoderm of Gemmellaria imjilexa, 
 pi. vii. fig. 5, cannot, however, be thus explained. 
 
 3 Some solid tentacles, with a single axial row of endodermal cells, 
 form an exception to this statement. 
 
 4 The observations of Eilhard Schulze cited in the article COXENTERA 
 do not form any real exception to this statement. 
 
HYDROZOA 
 
 59 
 
 with blood-lymph space distinct from digestive canal 1 
 With the attainment of the coclomate condition, the two 
 fundamental cell-layers, ectoderm and endoderm, which still 
 appear in the embryo, become so far interwoven, and their 
 products so highly differentiated, that it is no longer possible 
 to recognize them as anatomical structures in the adult. 
 
 The only deep-seated distinction between Hydrozoa and 
 Anthozoa (the Actinozoa being thus termed when the 
 Ctenophora are detached from them) appears to be the 
 particular differentiation of the archenteric space m Anthozoa 
 which has just been noted. It is no longer possible to 
 separate the two groups from one another as Exoarii and 
 Endoarii, as was proposed by Kapp (Ueber die Polypen im 
 Allgemeinen mid die Actinien insbesondere, Weimar, 1&29) 
 the first term indicating the Hydrozoa as possessed of 
 external generative organs, whilst by the hitter term the 
 Anthozoa are pointed to as having internal generative 
 organs. 2 This distinction breaks down completely in the 
 case of Lucernaria, and even in that of the so-called phanero- 
 carpous and some other medusae which discharge their 
 genital products by the mouth, and quite rarely by rupture of 
 the Outer body-wall The tendency to form calcareous 
 deposits in the deep layers of the ectoderm, or mesoderm, 
 as it has been termed, exhibited almost universally by the 
 Anthozoa (whence the name Coralligena applied to them), 
 is distinctive of them, though it has been shown first by 
 Louis Agassiz, and more fully and recently by Moseley, to 
 be paralleled among Hydrozoa, by the external calcareous 
 deposits of the abundant and widely distributed Millepores 
 and Stylasterids. A minute distinction between Hydrozoa 
 and Anthozoa, which does not, however, hold good uni- 
 versally, is found in the form of the barbed threads ejected 
 by the nematocysts. Instead of the complicated forms 
 present in the latter group, the Hydrozoa are usually pro- 
 vided with either an unbarbed thread or one in which the 
 barbs are confined to three at the base and a few minute 
 barblets (tig. 5). 
 
 Fundamental Forms of the Hydrozoa. The diblastula 
 derived from the egg of a hydrozoon, when provided with 
 a mouth, may be spoken of (as are the equivalent forms 
 in other animals groups) as a person. Either this person 
 elongates and develops tentacles in a circlet around or near 
 the mouth, and usually becomes fixed by the aboral pole of 
 the sac-like body, or the sac gradually assumes the form 
 of a clapper-bell or of an umbrella with greatly thickened 
 handle, the mouth being placed at the free end of the handle 
 or of the clapper, and the animal freely swimming by the 
 contractions and expansions of the dome of the bell (disc 
 of the umbrella). The two forms of persons are known, 
 the former as the " hydriform " (2, 3 in fig. 16), the 
 latter as the " medusiform " (4, 5, 6 in fig. 16). 
 
 The HYDRIFORM PERSONS usually occur as fixed branching 
 colonies or trees (figs. 36 and 37) produced by lateral budding 
 from an original hydra-form developed from a diblastula. 
 
 The hydriform person in its most fully developed state 
 is seen in the colonies of Tubularia. In such a colony a 
 number of hydriform persons are united like the flowers of 
 a plant on its branches (whence Allman's terms hydranth, 
 hydrophyton). Each hydriform person (fig. 35) has an 
 elongated body with oral and aboral pole. The mouth is 
 placed centrally at the oral pole, which is somewhat enlarged 
 and conical At the apex of the cone, immediately around 
 the mouth, is a circlet of small tentacles ; at the base of 
 the cone is a second circlet of larger tentacles ; the surface 
 of the oral cone is termed the hypostome. In other genera 
 
 1 The Enterozoa or Ifetazoa admit of division into two grades (1) 
 tlie Coslentera, including sponges, polyps, jelly-fish, and corals, and 
 (2) the Calotnata, including all remaining forms. 
 
 * See, however, note to the paragraph headed Definition of the 
 Hydrozoa, p. 555. 
 
 (e.ff., Hydra, fig. 42) the smaller circle of tentacles is 
 wanting ; in others, again, the tentacles are irregularly 
 placed and not concentrated into one circlet (fig. 38). 
 We regard the former as the typical condition. In the 
 hydriform persons of the Scyphomedusae (figs. 26 and 27) 
 the vertical axis is much shortened, the hypostome is flat, 
 and the whole body cup-like or hemispherical 
 
 The tentacles of the hydriform person are sometimes 
 hollow (Hydra, Garveia nutans, Hydrocorallina), being 
 mere prolongations of the sac-like body ; but usually, 
 though the endodermal cell-layer is continued into them, 
 they are solid (2 in fig. 16). Very generally the tentacles 
 of the hydra-form are indefinite in number, but in those 
 belonging to the group of Scyphomedusae a primary series 
 indicating four radii (perradial) can be distinguished, to 
 which are added four intermediate to these, marking four 
 secondary radii (interradial), whilst eight more placed 
 between the eight of the perradial and interradial series 
 are known as adradial tentacles. The surface of the hydra- 
 form may be entirely naked, or encased in a horny tube 
 (perisarc) formed by the ectoderm : this may be confined 
 to the aboral portion of the hydranth and to the common 
 stem which unites the persons of a colony, or it may rise 
 up and form a cup (or hydrotheca) around the oral region 
 of the hydranth (tigs. 32 and 33). 
 
 The bodies of all hydriform persons, as well as the ten- 
 tacles, are excessively contractile, and when hydrothecae are 
 present can be withdrawn into them. 
 
 The ectoderm or outer cell-layer furnishes the protective 
 and contractile tissues of the hydra-form. Very usually 
 it is not more than one or 
 two cells deep, and is sepa- 
 rated from the endoderm by 
 a structureless lamella of 
 firm consistence. In Hydra 
 large cells of the ectoderm 
 
 (neuro-muscular Cells of 
 Kleinenberg) bound the 
 
 external surface (fig. 3) and give off horizontal muscular 
 processes which lie side by side on the structureless lamella 
 forming thus a deep muscular coat, the fibrous elements of 
 
 m, muscular -fibre processes. 
 K>einenberg, from Gegentaur.) 
 
 (After 
 
 FIG. 4> Portion of the body-wall of Hydra, showing ectoderm cells above, 
 separated by "structureless lamella" from three flagellate endoderm cells 
 below. The latter are vacnolated, and contain each a nucleus and several dark 
 granules. In the middle ectoderm cell are seen a nucleus and three nemato- 
 cvstt, with trigger hairs projecting beyond the cuticle. A large neraatocyst, 
 with everted thread, is seen in the right-hand ectcdermal cell. (After F. E. 
 Schnlze.) 
 
 which are not independent cells. In larger species some of 
 the fibres may become separated from the tegumentary or 
 superficial cells, and acquire the character of independent 
 nucleated corpuscles (Hydrattinia, Van Beneden). No 
 nervous elements nor sense-organs occur in any hydra-form 
 (except perhaps the Lucernarue). In Antennularia some 
 ectoderm cells are amoebiform, and project processes which 
 change shape (nematophors). Tactile hairs (palpocils), 
 
60 
 
 HYDROZOA 
 
 however, occur on the ectodermal cells, and the solid ten- 
 tacles are essentially tactile organs. Placed in and between 
 the large cells of the ecto- 
 derm (Hydra, Cordylophora, 
 Allman, Kleinenberg, F. E. 
 Schulze) are small nucleated 
 cells which become con- 
 verted into vesicles contain- 
 ing a three-barbed (figs. 4 
 and 5) or simple filament 
 (nematocysts). These are 
 frequently grouped on the 
 surface in wart-like pro- 
 cesses or " batteries." Ne- 
 matocysts also are found in 
 the endoderm; but it is prob- 
 able that their presence 
 there is due to their having 
 been swallowed. 
 
 The endoderm is usually 
 
 but One cell deep, and lines FIG. S. Xematocyst of Hydra, showing 
 
 the entire cavity of the body S^S S*T$ 
 starting from the margin of F. E. scimize.) 
 the mouth. In the region of the body proper, and in hollow 
 tentacles, the cells are ciliated (fig. 4). In this region they are 
 concerned in the secretion of digestive fluids and in absorp- 
 tion, and sometimes contain coloured granules (hepatic?). All- 
 man found in Myriothela (Phil. Trans., 1875) that the endo- 
 dsrrn cells project processes 
 like the pseudopodia of Pro- 
 tozoa, and suggests that solid 
 food particles are incepted 
 by them. T. J. Parker has 
 published similar observa- 
 tion on Hydra (1880). In 
 the solid tentacles the en- 
 dodermal cells are greatly 
 modified, forming a kind 
 of skeletal tissue, each cell recalling by its vacuolation 
 and firm cell-wall the characters of vegetable parenchyma 
 (fig. 6). In the stems of Siphonophora endoderm cells give 
 origin to muscular processes like those of the ectoderm 
 (Glaus). This latter fact has a morphological significance 
 which cannot be too gravely estimated. 
 
 Generative products are not developed by any hydriform 
 persons (excepting the Lucernarice), the sexual process being 
 carried on by a distinct set of buds developed on the sides 
 of hydriform persons. These buds either become medusi- 
 form persons, or are degenerated representatives of such 
 persons (sporosacs) (figs. 17 and 18). Even the fresh-water 
 Hydra (fig. 42) does not appear to be an exception to this 
 generalization. The single egg-cell of Hydra projects at 
 the breeding season in an ectodermal covering, as a wart, 
 from the lower part of the body. A conical eminence or 
 two nearer the mouth contains the spermatozoa. Each 
 ovarium and each spermarium represents an aborted gene- 
 rative person. According to Kleinenberg the egg-cell and 
 the sperm-cells are both derived from the ectoderm. The 
 Lucernarice develop internal generative organs (fig. 19) 
 which correspond closely with those of the medusiform 
 persons of the group Scyphomedusce (see below), with which 
 they are classified. Both ova and testis are endodermal in 
 origin in Lucernaria and in the medusiform persons of the 
 Scyphomedusce, whilst they appear to be ectodermal in 
 origin in the complete medusiform persons of Hydro- 
 medusce, though in the degenerate medusiform persons 
 known as sporosacs they may either or both have an 
 endodermal origin. 
 
 MEDUSIFORM PERSONS usually present themselves as 
 isolated free-swimming individuals, but like hydriform 
 
 FIG. 6. Vacuolated endoderm cells of carti- 
 laginous consistence from tlie axis of the 
 tentacle of a Medusa (Cunina), (From 
 Gegenbaur's Elements of Comparative 
 Anatomy.) 
 
 persons they have the power of producing new persons by 
 budding (figs. 44, 45, and 46), which may become detached 
 or may remain connected with the primary person (fig. 57) 
 to form a freely swimming colony (Siphonophora) compar- 
 able to the fixed colonies of hydriform persons. Medusi- 
 form persons are often produced as the immediate result of 
 the development of the diblastula without any intermediate 
 hydriform phase (Pelagia among Scyphomedusce, Tracho- 
 medusce, Narcomedusce, and probably someAnthomedusa; and 
 Leptomedusoe), but quite as frequently originate as lateral 
 buds upon the body-walls of hydriform persons (figs. 34. 
 37, and 43), or of other medusiform persons (see below), or 
 as metameric fission-products of hydra-forms. The typical 
 medusa-form is a hemispherical cup (the nectocalyx, or 
 umbrella, or disc), from the centre of which rises up a 
 cylindrical or conical process (the manubrium, erroneously 
 polypite) at the summit of which is the mouth (4, 5 in fig. 
 16). Four perradial (see above for use of this term) ten- 
 tacle-like lobes very commonly surround the mouth, or 
 numerous small tentacles (fig. 58), whilst the margin of 
 the disc is beset with tentacles four in number, or a mul- 
 tiple of four (sometimes six, or one only, or indefinite). 
 Theaboral pole is dome-like, aud is never attached except 
 in those forms which take their origin as buds on a hydri- 
 form colony when the connexion exists at this point. The 
 tentacles are, as in the hydriform persons, some solid, some 
 hollow : both occur in the same individual. 
 
 d 
 m 
 il 
 
 FIG. 7. Portions of sections through the disc of medusa;, the upper one of Lizzia, 
 the lower of Aurelia. el, endoderm lamella, or vascular lamella; m, muscular 
 processes of the ectoderm cells in cross section ; d, ectoderm; en, endoderm 
 lining the enteric cavity ; e, wandering endoderm cells of the gelatinous sub- 
 stance. (After Hertwig.) 
 
 The body is not so completely hollowed out as in the 
 hydriform persons. The mouth leads into a straight tube 
 (the stomach) which occupies the axis of the manubrium, 
 and expands at its insertion into the disc. The disc, even 
 when thick and fleshy, is not fully excavated by the enteric 
 cavity. In young forms the cavity does occupy it right up 
 to the margin, but gradually the lumen disappears (fig. 29), 
 leaving a series of canals and a continuous plate of endo- 
 derm (fig. 7) formed by the coalesced walls of the space (the 
 endoderm-lamella of the Hertwigs, see Organisnnis der 
 Medusen, 1878; the vascular-lamella of Glaus, "Polypen 
 und Quallen der Adria," Wiener Denksch., 1878). The 
 peripheral portion of the lumen of the original enteric cavity 
 forms the ring-canal, which runs all round the margin of 
 the disc, and is continued into the hollow tentacles. The 
 lumen is further retained at intervals in the form of radiat- 
 ing canals connecting the axial enteric cavity with the ring- 
 canal. These may be perradial, interradial, and adradial 
 (see above as to tentacles of hydra-form), and may branch 
 dichotomously in the disc or form networks. 
 
 The medusae are thicker and more fleshy to the touch 
 than are the hydra-forms, and are at the same time trans- 
 parent. This is entirely due to the enormous development 
 of a structureless substance between ectoderm and endoderm, 
 corresponding to the " Stutz-lamella" or structureless lamella 
 of the hydra-forms. (See figs. 49 and 51, representing 
 sections of Carmarina and of Cunina,) 
 
HYDROZOA 
 
 61 
 
 The remarkable development of this substance in a hyaline con- 
 dition has led to the description of canals and spaces where none 
 exist the supposed spaces being really occupied by this hyaline 
 substance. F. E. Schulze's statements as to extra-enteric spaces in 
 Sarsia are thus explained and more decidedly the supposed circular 
 and longitudinal canals attributed by some authors to the scyphi- 
 stoma phase of Discomediisx. In the same manner (according to 
 Claus) Allman's observations on Stephanoscyphus are reconciled 
 with those of F. E. Schulze on Spongicola clearly the same form. 
 Stepluinoscyphus is devoid of either circular or longitudinal canals, 
 and though it has four remarkable ridges on the enteric wall like 
 those of the scyphistoma of Scyphomedusce (see fig. 26) stands in all 
 probability very close indeed to the Tubularian genus, Perigonimus. 
 
 In a large number of medusa-forms the hyaline gelatinous 
 substance is structureless, but in many of the larger Scy- 
 phowedusce it is occupied by in-wanderingamoeboid cells de- 
 rived from the endoderm and by fibrous trabeculse (fig. 8). 
 
 < 
 
 Fio. 8. Gelatinous substance of the disc of Aurelia, showing a, fibrous tra- 
 becute, and 6, wandering endoderm cells, with amoeboid movements. (From 
 Gegenbaur.) 
 
 The wandering eudodermal cells are nutrient in function, 
 and represent so far isolated elements of the enteric canal 
 system. 
 
 The medusiform person is fundamentally adapted to 
 swimming movements. The muscular fibres are mostly 
 transversely striated, and are as a rule outgrowths of super- 
 
 FIG. 9. Muscular cells of mcdusre (Lizzia). The uppermost is a purely muscular 
 cell fiom the sub-umbrella; the two lower are epidermo-mnscular cells from 
 the base of a tentacle; the upstanding nucleated portion forms part of the 
 epidermal mosaic on the free surface of the body. (After Hertwig.) 
 
 ficial ectoderm cells as in Hydra (fig. 9), (though in some 
 cases distinct cells) ; they are confined to a sheet spread on 
 the oral face only of the disc or swimming-bell (sometimes 
 called sub-umbrella), to the extensile manubrium and 
 tentacles, and to an inwardly directed flap of the margin of 
 the disc known as the velum (Ve in 4 of fig. 16), which is 
 present in those medusae that are not flattened but conical 
 (bell-like). The muscular fibres on the oral face of the disc 
 and on the velum have a circular direction, interrupted 
 in some cases by radial tracts. The direction of the swim- 
 ming movements is obvious from this arrangement. 
 
 The velum is not a constant element in the medusa's 
 disc ; it serves to contract the space by which water is 
 expelled from beneath the bell in the act of swimming. 
 
 All fully-developed Hydromedusae possess the velum, but 
 only a few of the Scyphomediisce (CharybdcKa). In the 
 former the endoderm plate (vascular lamella) is not con- 
 tinued into it; in the latter vessels of the enteric system are 
 present in it (fig. 21), and, being probably morphologically 
 distinct, it has been here termed the " pseudo-velum." 
 
 Unlike the hydra-forms, the medusa-forms of Hydrozoa 
 possess in addition to the tentacles highly-developed sense- 
 organs and gangliouic nerve-centres and nerves. The sense- 
 organs appear to be either eye-spots, or else otocysts, or 
 to combine the functions of both. In addition to these 
 are olfactory tracts or pits connected with the preceding. 
 The sense-organs are placed along the margin of the disc 
 (hence called marginal bodies), and are of three kinds: 
 (1) ocelli rounded pigment spots, rarely provided with a 
 
 Fig. 10. Fig. 11. 
 
 Fio.lO. Ocellus of a medusa (Liaia Koellikeri). or, pigmented ectodermal cells; 
 7, lens. (After Hertwig.) 
 
 Fio. 11. Otocyst (formed entirely by ectoderm) of Phialidtum, one of the 
 vcsiculate medusae, d 1 , superficial layer of ectoderm ; <P, deep layer of ecto- 
 derm ; A, auditory cells of ectoderm ; Ui, auditory liairs ; tip, nerve body ; 
 nr', upper nerve-ring ; r, endoderm cells of the circular canal. The otolith 
 cavity i.s seen above A. 
 
 lens (Lizzia) (fig. 10), always placed at the base of a tentacle 
 or in the radius of one on the oral surface (Lizzia), entirely 
 ectodermal in origin ; (2) vesiculi or otocysts formed (as 
 discovered by the Hertwigs, 1878) by an invagination of the 
 ectoderm (fig. 11) containing concretions and hair cells; 
 either open or entirely closed, generally numerous, and 
 placed between tentacles, sometimes at the bases of tentacles 
 (Obelia) ; (3) teutaculocysts which are reduced and modi- 
 fied tentacles; into them alone of the three kinds of mar- 
 
 /-- hk 
 
 Fie. 12. Simple tentaculocyst of one of the Traekomedusae (Rhopalonema 
 velatum). The process carrying the otolith or concretion M*, formed by 
 endoderm cells, is enclosed by an upgrowth forming the "vesicle," which is 
 not yet quite closed in at the top. (After Hertwig ) 
 
 ginal bodies do the endoderm and, in the more complex, 
 the enteric canal system enter (figs. 12, 13, and 30). The 
 endodermal sac forms the axis of the tentaculocyst, its cells 
 secrete crystalline concretions, and it functions as an otocyst; 
 pigment spots, which may have cornea, lens, and retina 
 well developed, are formed sometimes to the number of 
 six (Charybdcea) on the ectoderm of the tentaculocyst (fig. 
 13). The olfactory sense-epithelium (fig. 14) is either dis- 
 tributed in a continuous band on the margin of the disc 
 (U ' ydromedusce, discovered here by the Hertwigs), or it is 
 
62 
 
 HYDROZOA 
 
 confined to deep pits (fovese nervosse) from each of which 
 a tentaculocyst arises (discovered in the Scyphomedusce in- 
 dependently by Schiifer and Claus). With some exceptions, 
 medusae provided with ocelli are destitute of vesiculi, which 
 alone occur in the vesiculate Leptomedusce. Tentaculocysts 
 
 B 
 
 Fig. 13. Fig. 14. 
 
 Fio. 13. Tentacnloeysts of medusa; (A, of Pelagia ; B, of CharyMcfa). 
 a, the free tentacle hanging in the notch of the disc; b, stalk; c, enteric 
 canal continued into it; d, enlarged portion of the canal; , concretions 
 on endodermal cells ; /, pigmented ectoderm ; g, lens. (From Gegenbaur.) 
 
 Fio. 14. Cells from the olfactory pits (fovese nervosa?) of Aurelia. (After Schafer.) 
 
 characterize to the exclusion of the ocelli and vesiculi the 
 Trachomedusce and Narcomedusce among Hydromedusce and 
 all the Scyphomedusce, except Lucernaria, where they are 
 replaced by '' colleto-cystophors." 
 
 The nervous system has only recently been correctly 
 recognized in medusae, though seen by Agassiz as long ago 
 us 1849, and described both by Fritz Miiller and Haeckel 
 in certain forms (Geryonidce) more recently (1860). It 
 differs remarkably in the two great groups into which the 
 Hydrozoa are divisible. In the Scyphomedusce there is 
 no continuous nerve-centre, but around and about each 
 tentaculocyst nerve-fibres and cells are grouped in such a 
 way as to divide the disc into zones of nerve supply corre- 
 sponding to the number of tentaculocysts (usually eight). 
 
 FIG. 15, Scattered nerve ganglion cells, c, from the sub-umbrella of Aurelia 
 aurita. (After Schafer.) 
 
 Both the Hertwigs (Nerven-System der Medusen, 1878) and 
 Eimer (Die Medusen, 1879) entirely missed in their re- 
 searches the large nerve-fibres and prominent ganglion cells 
 (fig. 15) which were discovered by Professor Schafer of 
 University College, London (Phil. Trans., 1879), in the 
 Scyphomedusce. The writer can confirm Schiifer's observa- 
 tion of the existence of such fibres and ganglion cells in 
 the region of the circular muscular zone on the oral face 
 of the disc of Aurelia, immediately beneath the flattened 
 epithelium of the ectoderm. Professor Claus of Vienna 
 has independently described (" Polypen und Quallen der 
 Adria," 1878) similar nerve-cells and fibres in Chry- 
 saora and Gharybdcea. Professor Schafer failed to ascer- 
 tain satisfactorily the origin and termination of the fibres, 
 which appear, however, to originate in superficial ecto- 
 
 dermal cells ("sense-epithelium") in the neighbourhood 
 of the tentaculocysts and in the cells of those organs, 
 and to terminate without any plexiform connexion with 
 one another in the muscular fibres. Eimer has described 
 very abundant and excessively fine fibres, often moniliform, 
 which extend from epithelial cells in the neighbourhood of 
 tentaculocysts and form a network traversing the gelatinous 
 substance of the disc in every direction. This observation, 
 though supported by the fact that such fibres ars indi- 
 cated by the extended experimental investigation of Eimer 
 and of Romanes (Eimer, Die Medusen; Komanes, Phil. 
 Trans., 1876, et seq.}, is not confirmed by other observers, 
 and the fibres described are regarded as skeletal tissue. If 
 Elmer's fibres do not exist, the muscular tissue of the 
 medusas must be regarded as acting to a large extent inde- 
 pendently of nerve-control ; and this is borne out by Claus's 
 observation of the absence of sense-organs and nerve-fibres 
 from the swimming-bells of the Siphonophora (compound 
 medusa). In the Hydromedusoe the nerve ganglion cells 
 are grouped in a continuous ring around the margin of the 
 disc, separated horizontally into an inferior and superior 
 portion by the insertion of the velum. The difference in 
 the form of the nervous system has led Eimer to propose 
 the names Cycloneura for the Hydromedusce and Toponeura 
 for the Scyphomedusce. Amongst the latter, however, 
 Charybdcea, having a continuous velum like Hydromedusce, 
 has also a continuous nerve-ring. 
 
 Comparison and Relations of Hydriform and Medusiform 
 Persons. A simple shortening of the vertical axis, and a 
 widening of the hypostome, with obliteration of the lumen 
 (but not of the cells) of the endoderm over a considerable 
 region of the disc thus produced, suffice to convert the hydra- 
 form into the medusa-form. 1 This change of proportion 
 made (fig. 16), the sense-organs of the medusiform person 
 have to be added, and the change is complete. Thus it be- 
 comes clear that we have to deal with one fundamental form, 
 appearing in a lower, fixed, nutritive phase and a higher, 
 locomotor, generative phase in the two cases respectively. 
 
 The phylogeny of the Hydrozoa and the historical relation- 
 ship of the two phases (hydriforrn and medusiform) appears 
 to be as follows. 
 
 A two-cell-layered sac-like form, with mouth and with or 
 without tentacles, was the common ancestor of Hydrozoa, 
 Anthozoa, and Sponges. The particular form which the 
 proximate ancestor of the Hydrozoa took (1 in fig. 16) is 
 most nearly exhibited at the present day in Lucernaria 
 and in the scyphistoma larva (hydra-tuba) of Discomedusce. 
 It was a hemispherical cup-like polyp with tentacles in 
 multiples of four, with four lobes to the wide enteric 
 chamber. This polyp, after passing a portion of its life fixed 
 by the aboral pole, loosened itself and swam freely by the 
 contractions of the circular muscular fibres of its hypostome 
 (sub-umbrella), and developed its ovaria and spermaria on 
 the inner walls of the enteric chamber. This ancestor 
 possessed, like its descendants, a very marked power of 
 multiplication, either by buds or by detached fragments of 
 its body. Accordingly it acquired definitely the character 
 of multiplying by bud-formation during the earlier period 
 of its life ; each of the buds so formed completed in the 
 course of time its growth into a free swimming person. 
 We must suppose that the peculiarities of the two phases 
 of development became more and more distinctly developed, 
 the earlier budding phase exhibiting a more elongated form 
 and simple enteric cavity (hydra-form), which subsequently 
 
 1 This relationship, demonstrated by the Hertwigs' discovery of the 
 endoderm layer of the medusa's disc, differs from that supposed to 
 obtain by Professor AHman. He supposed the medusa's disc to 
 represent the coalesced tentacles of a hydra-form, and cited the webbed 
 tentacles of Laomedea flexuosa in support of the identification, which 
 had at the time very much to commend it. 
 
HYDROZOA 
 
 63 
 
 became changed in the course of the ontogeny (develop- 
 ment of the individual) into the umbrella or disc-like 
 form, with coalesced enteric walls and radial and circular 
 surviving spaces (medusa-form). And now the ancestry 
 took two distinct lines, which have given rise respectively 
 to the two great groups into which the Hydrozoa are divi- 
 sible the Scyphomedusce and the Hydromedusce. In the 
 one set the hydriform persons of a colony, instead of each 
 becoming metamorphosed into a medusiform person, pro- 
 ceeded each to break up into a series of transverse divisions ; 
 each division became a medusiform person, and was 
 liberated in its turn as a free swimming organism (figs. 
 26 and 27). We must suppose that this process began 
 historically by the outgrowth of new tentacles around the 
 point where the disc of a person fully transformed from the" 
 
 Fio. IS. Diagrams to exhibit the plan of structure of hydriform and medusiform 
 persons (all eicept 5 are vertical sections). A, base of tentacles, margin of the 
 dice; B, oral margin; Ma, mannbrium; Te, tentacle; CV, circular vessel; 
 EnL, endodenn lamella ; ot, otocyst ; <x, ocellus olf, olfactory pit ; H, hood of 
 tentaculocyst ; mg. genitalia developing in manubrinm; dg, genitalia develop- 
 ing in the disc (wall of a radiating canal) ; GP, sub-genital pits of the sub- 
 umbrella; Gf, gastrai filaments; re. velum. 1, Form Intermediate between 
 medusa-form and hydra-form. 2, Hydra-form with wide disc, manubrinm, 
 and solid tentacles (Tubularian). 3. Hydra-form with narrower disc, and 
 hollow tentacles (Hydra). 4, Medusa-form with endoderm lamella on the 
 left, the section passing through a radiating canal on the right; a velum, two 
 possible positions of the genitalia, and two kinds of sense-organs are shown 
 (Hydromedtace). 5, A similar medusa-form seen from the surface. 6, Section 
 of Aurtlia attrita, to show especially the nature of the sub-genital pits, GP. 
 outside the genital frills, and the position of the gastrai filaments GF, as well 
 as the flattened form of the disc. 
 
 hydriform to the medusiform phase was loosened in its 
 attachment and about to separate from the colony. The 
 " hastening of events," a well-known feature of organic 
 growth-sequences, would complete the development of the 
 newly sprouting person before the loosened medusa had 
 got well away, and so on with a third, fourth, and even 
 with twenty such successive buds. The separation of the 
 adult form from its fixed larva by fission has been justly 
 compared by Louis Agassiz to the separation of the 
 Comatula from its pentacrinoid larval stalk. If the stalk 
 could only produce new Comatulce, the analogy would be 
 complete, Lucernaria is in the same way comparable with 
 the stalked crinoids, being an adult form which retains the 
 characters exhibited by the immature phases of its congeners. 
 The Scyphomedusce do not, however, all exhibit a 
 hydriform phase, and a production of medusae by the 
 
 " strobilation " or " metamerizing " of a scyphistoma. 
 Some of them (Pelagia) " hasten events " so far that the 
 diblastula never fixes itself, but becomes at once a single 
 medusa, the hydriform phase of the ontogeny being alto- 
 gether omitted. Certain peculiarities of the medusa's struc- 
 ture, above all the possession of gastrai filaments (solid 
 filaments like tentacles projecting in four interradial groups 
 near the genitalia into the enteric cavity), serve to unite 
 Pelagia, which has no larval stage, and Lucernaria (which 
 is always of intermediate character between hydra-form 
 and medusa-form) with the numerous species which develop 
 by the strobilation of hydriform larva?. 
 
 The second line of descent which has given rise to those 
 Hydrozoa, known as Hydromedusce not only acquired at 
 the start a different mode of producing medusiform persons, 
 but the medusiform persons acquired characters differing 
 from those of the Scyptomediuce in important (but not 
 fundamental) features. The larval stage in this series 
 developed the property of budding to a very great degree, 
 so as often to form fixed tree-like colonies of considerable 
 size. Then the transformation of the identical colony- 
 forming persons into free-swimming persons was finally and 
 definitively abandoned, and only a late-appearing set of buds 
 proceeded to complete the typical changes and to become 
 medusae. The earlier-produced buds were thus arrested 
 in development, and became specially modified for the 
 purposes of a fixed life as members of a colony. Thus 
 they acquired the elongate form and the sporadic position 
 of the tentacles which we see in some hydriform persons of 
 the Hydromedufce group (figs. 38 and 40), and were adapted 
 to nutrition solely (hence the term trophosome applied by 
 Allman to such colonies). The characters of the mature 
 generative person, with its power of detachment and free 
 locomotion, being confined to the later buds borne on the 
 sides of the hydriform persons or on special portions of the 
 colony, we find that the former became more and more 
 specialized as sexual medusiform persons in proportion as 
 the latter became specialized as asexual hydriform persons, 
 and thus it is that we have the remarkable phenomenon of 
 hydriform colonies, developed from the eggs of medusae, 
 producing as it were crops of medusae (figs. 34 and 37) 
 which detach themselves and swim away to deposit their 
 eggs (alternation of generations). The Hydromedusce never 
 produce medusae by strobilation or transverse division of a 
 hydriform person, although in rare cases the cicatrix left 
 by a detached medusa-bud has been observed to sprout 
 and produce a hydriform person. Neither medusiform 
 nor hydriform persons of the Hydromtdusce series ever 
 have gastrai filaments (unless they are represented by the 
 "villi" of the Siphonophora described by Huxley, Oceanic 
 Hydrozoa), whilst the medusa-forms always possess a velum 
 and a comparatively simple set (four, six, or eight) of radi- 
 ating canals in the disc, the remains of the enteric lumen. 
 
 The complete differentiation of hydriform and medusi- 
 form persons existing on one and the same colony having 
 been attained in the Hydromedusce, further changes of a 
 most remarkable character were brought about in some of 
 the descendants of these forms. The condition which we 
 have so far noted is perpetuated at the present day in 
 Bougainvillia (Eudendrium), Campanvlaria, and a vast 
 number of the so-called hydroid polyps; others have 
 undergone further adaptational changes. We have to 
 notice at least four important additional modifications 
 independent of one another. 
 
 (1.) The hydriform stage was suppressed altogether, 
 and, as in some Scyphomedusce, so here too the diblastula 
 developed directly into a medusa (Trachomedusce, Narco- 
 mfduste, and probably some Leptomedusce like Thaumanliat 
 and JZyuorea, and some Anthomedusce like Oceania, and 
 Turritopsis). 
 
64 
 
 HYDROZOA 
 
 (2.) The medusiform persons being early produced did not 
 separate themselves from the colony, but the whole colony 
 became free (if it ever were fixed), the medusiform persons 
 carrying the hydriform persons away with them. Thus the 
 highly differentiated swimming and floating colonies of the 
 Siphonophora originated. 
 
 (3.) The medusiform persons ceased to detach themselves 
 from the fixed hydriform persons or colonies, and developed 
 the ova and sperm within themselves, whilst still small in 
 size and attached to the hydriform stock. Having once 
 abandoned the detached, free-swimming life, the medusae 
 underwent in different genera a varying amount of degene- 
 ration and atrophy, of which we have in existence all 
 
 Fio. 17. Diagrams illustrating the gradual degeneration of the medusa bud 
 into the form of a sporosac. The black represents the enteric cavity and its con- 
 tinuations; the lighter shading represents the genital products (ova or sperm). 
 A, medusiform person still attached by a stalk at the aboral pole to a colony 
 (phanerocodonic gonophor of Allman) ; B, modified medusiform pel-son, with 
 margin of the disc (umbrella) united above and imperforate (mouthless) manu- 
 brium (adelocodonic gonophor of Allman); C, sporosac, with incomplete 
 extension of the enteric cavity into the umbrella, rudimentary invagination 
 above to form the sub-umbrella cavity; D, sporosac with manubrial portion 
 only of the enteric cavity ; E, sporosac without any trace of manubrium. 
 
 possible degrees, leading from the fixed " phanerocodonic 
 gonophors" (Allman, bell-like genital buds) of many 
 Siphonophora through the " adelocodonic gonophors " 
 (genital buds with the bell no longer open but closed by the 
 union of the margins of the disc) of Cordylophora to the 
 sporosacs of Hydractinia, and even to the simple genital 
 warts of the little degenerate Hydra viridis of fresh waters 
 (see fig. 17, and explanation). By this process a large num- 
 
 Fio. 18. Two female sporosacs (degenerate meilusse) of Hydractinia echinata. 
 (From Gegenbaur, after Van lleneden.) a, ectoderm; 6, endoderm; o, egg- 
 cells; <7, enteric cavity. In A an invagination of the ectoderm, which is 
 more complete in B, represents the rudiment of the sub-umbrella space. 
 
 ber of Hydromedusce (figs. 35, 38, 39, 40, and 42) have lost 
 all evidence of the real characters of their medusa-forms, just 
 as others have suppressed the evidence of their hydra-forms 
 by direct development from the egg ; and inasmuch as both 
 these processes take place in genera having the closest affinity 
 with genera in which both hydra-form and medusa-form are 
 fully preserved, it is not possible to erect groups similar to 
 the Haplomorpha of Cams or the Monopsea of Allman for 
 their reception. The difficulty of classification is, however, 
 rendered very great, fora double system becomes necessary, 
 which shall deal with the characters of hydriform and 
 medusiform persons in parallel equivalent series. The ; 
 difficulty is considerably enhanced when we find that iden- 
 tical medusa-forms may spring from unlike hydra-forms, 
 and, conversely, that closely allied hydra-forms may give 
 rise to very different medusa-forms. The character first 
 noticed by Rapp as distinguishing the hydroid polyps from 
 the coral-polyps, namely, that of developing their geuitalia 
 as external bodies (Exoarii) instead of internally (Endoarii), 
 
 is seen by the considerations just adduced to be fallacious. 
 The Hydromedusce, it is true, often (not always) develop 
 their generative products from the ectoderm, and the geni- 
 talia frequently project as ridges and discharge themselves 
 directly to the exterior in this division. The Hydromedusce 
 contrast in this respect with the Scyphomedusce and An- 
 thozoa, which develop their genitalia from the endoderm, 
 and are (to use Rapp's terms) Endoarii whilst the former 
 are Exoarii. But the bodies mistaken for external generative 
 organs by Rapp and other early observers in many hydroids, 
 and in Hydra itself, are aborted degenerate medusae. 
 
 (4.) A further set of changes, which have affected the 
 original hydriform colonies and their medusa-buds so as to 
 produce new complications of structure among the Hydro- 
 medusce, are summed up under the head of " polymorphism." 
 The differentiation of hydriform and medusiform persons is 
 a case of dimorphism; a further distribution of functions, 
 with corresponding modification of form, gives us "polymor- 
 phism." Polymorphism is unknown in the Scyphomedusce, 
 and it is chiefly confined to two groups of Hydromedusce (the 
 Hydrocorallince and the Siphonophora'). In the hydriform 
 colonies of Hydractinia (one of the GymnoUastea-Anthome- 
 dusce) the outer hydriform persons of the colony (fig. 39) 
 differ in form from the rest, and have wart-like tentacles. In 
 the same genus, and also in many CalyptoUastea, the hydri- 
 form persons which are destined especially to give origin 
 to medusa-buds are devoid of tentacles and mouth, and 
 are known as blastostyles (Allman), (fig. 43). In Hydro- 
 corallines (fig. 53) elongated hydriform persons (dacty- 
 lozooids) with no mouth and sporadic tentacles are set in 
 series around a central short mouth-bearing person (gastro- 
 zooids) forming the " cyclo-systems " of Mr Moseley (figs. 
 52 and 55). In the Siphonophora, in addition to nutritive 
 (hydriform) persons and generative (medusiform) persons, 
 there may be rows of swimming-bells (medusae devoid of 
 mouth and of genitalia), covering-pieces (flattened medusse), 
 and tentacle-bearers (hydriform persons with one long highly- 
 developed tentacle), (see figs. 56 and 57). 
 
 Hypothesis of the Individuation of Organs. The building 
 up of complex individualities, such as a hydrozoon colony, 
 a flowering plant, or a segmented worm or arthropod in 
 any one of which a number of common units are repeated, 
 but with varied form and function in each part of the com- 
 pound body is generally admitted to be explicable in two 
 ways, and which of the two explanations may be adopted 
 in any one case must depend on the ultimate inference 
 from a wide series of observations. The first hypothesis, 
 which undoubtedly applies to the ordinary hydriform 
 colonies of Hydrozoa, to the segments of Tcenia, and to 
 plants formed by the repetition of phyllomes, is that an 
 original unit like those which constitute the composite 
 organism has freely budded, and repeated its own structure 
 in the well-marked units which remain conjoined to form an 
 aborescent or linear aggregate. This is " eumerogenesis," 
 and such aggregates may be termed eumeristic. By a 
 division of labour and consequent modification of form 
 among the units of a eumeristic aggregate, such an aggregate 
 may (in the course of phylogeny) acquire varied shape and 
 definite grouping of its constituent units, and a high speci- 
 alization as an individual. The high degree of individua- 
 tion which may be thus attained is due to the more or 
 less complete synthesis of a eumeristic colony. The more 
 highly individuated Chsetopods and Arthropods are syn- 
 thesized linear colonies. The cyclo-systems of the Hydro- 
 corallince are undoubted examples of synthesized colonies. 
 The second hypothesis is one which is applicable to cases 
 which, in the absence of special evidence to the contrary, 
 might be regarded as highly synthesized colonies. Accord- 
 ing to this second hypothesis, such highly individuated 
 composite organisms have not (in their phylogeny) passed 
 
HYDROZOA 
 
 65 
 
 through a eumeristic phase in which the units were well 
 developed and alike, but the tendency to bud-formation 
 (whether lateral, linear, or radial) has all along acted con- 
 currently with a powerful synthetic tendency, so that new 
 units have from the first made but a gradual and disguised 
 appearance. This is " dysmerogenesis," and such aggregates 
 as exhibit it may be called dysmeristic. In dysmeristic 
 forms the individuality of the primary unit dominates from 
 the first, and the merogenesis (segmentation or bud-forma- 
 tion) cau only show itself by partially here and more com- 
 pletely there compelling (as it were) the organs or regions 
 of ths body of the primary unit to assume the form of new 
 units. The arms of star-fishes are, when we consider them 
 as derived from the antimera of a Holotliurian, explained 
 as examples of dysmerogenesis. So, too, the series of 
 segments constituting a leech, and probably also the 
 segments of a vertebrate. Eumerogenesis and dysmero- 
 genesis are only variations of one process, merogenesis, and 
 no sharp line can be drawn between them. Individuation 
 may appear at any period in the phylogeny of a eumeristic 
 aggregate and synthesize its units. On the other hand, in- 
 dividuation is more or less completely dominant throughout 
 the history of a dysmeristic aggregate, and is gradually 
 broken down as a more and more complete analysis of the 
 primary unit into new units is effected. It will be observed, 
 however, that in dysmerogenesis, the/ori which individua- 
 tion tends to preserve is that of the primary nnit (notably 
 the case in leeches as compared with the ameristic flukes), 
 whereas when we have eumerogenesis followed by synthesis 
 the resulting form-individuality is something absolutely 
 new. Thus, using the terms eumeromorph and dysmero- 
 morph, we have (1) synthesized eumeromorph simulates 
 normal dysmeromorph ; (2) analysized dysmeromorph 
 simulates normal eumeromorph. 
 
 \Vhether the fixed hydriform colonies of the Hydrozoa, 
 with their more or less complete medusiform buds, and 
 further, the floating colonies of Siphonophora, with their 
 polymorphous units, are to be regarded as synthesized 
 eumeromorplis or as dysmeromorphs, more or less analysed, 
 is perhaps still open to discussion. The former view (that 
 adopted here) is that held by Allman (Monograph of the 
 Tubtilarian Hydroids, 1874), by Leuckart (1851), by 
 Gegeubaur (Grundn'ss, 1874), by Claus (Grundzuge der 
 Zoologie, 1876), and by the Hertwigs (Organismu* der 
 Mednsen, 1873). On the other hand, Huxley (Oceanic 
 IfyJrozoa, 1856), formerly Gegeubaur (Zur Lehre der Gene- 
 ralions-Wechsfl, 1854), and, more recently, Ed. Van Beneden 
 (" De la distinction originelle du testicule et de 1'ovaire," 
 Bull. Acad. Boy. elg., 1874) have held that the medusi- 
 form person is a generative wart which has gradually 
 assumed the characters of a bud, and that the various 
 phases presented by it in different genera are so many more 
 or less successful strivings after complete assumption of the 
 hydra-form (from which the medusa-form is thus secondarily 
 derived). Similarly the variously modified units of the 
 siphonophorous colony have been regarded as the organs of 
 a parent unit which have each more or less completely 
 acquired the form of that parent unit, or, in other words, 
 the colonies in question have been held to be dysmero- 
 morphs. Recently ascertained facts as to the polymorphism 
 of Hydrofora'liii<e, but more especially the demonstration 
 of the identity of structure of the medusae of the Scypho- 
 medusan and Hydromedusan groups, and, further, the mode 
 of development of the Scyphomedusce from the scyphistoma 
 and the relations of the generative products to the enteric 
 cavity, combine to render the view that the polymorphous 
 and dimorphous colonies of Hydrozoa are synthesized 
 eumeromorphs more probable, in the judgment of the 
 present writer, than that which would explain them as 
 dysmeromorphs. 
 
 The term "merogenesis," and its subordinate terms, 
 "eumerogenesis, dysmerogenesis," &.C., are applicable to 
 units of the first order, namely, cells, as well as to the 
 " persons " which are built up by them. Ordinary cell- 
 division is an example of eumerogenesis; free-formation of 
 nuclei, as in the fertilized ovum of Arthropods, is dysmero- 
 genesis. A syncytium is usually a synthesized eumero- 
 morph, but may be a dysmeromorph. 
 
 Definition of the Hydrozoa. The Hydrozoa are Caelentera 
 nematophora, distinguished from the fellow-group Anttiozoa 
 (the name applied to Actinozoa when the Ctenopkora are 
 removed from them) by not possessing the latter's constant 
 and sharp differentiation of the arch-enteric cavity into 
 axial digestive and periaxial septate portions, usually by a 
 simpler form of nematocyst, and generally by lower histo- 
 logical differentiation. - 
 
 The following is a brief summary of the chief characters 
 of the larger divisions of the Hydrozoa: 
 
 Sub-class I. SCYPHOMEDUS.E. These are Hydrozoa which 
 in the adult condition al- 
 ways have four or eight 
 interradial groups of 
 gastral filaments (" pha- 
 cell"ofHaeckel)(figs.l6 
 (6), 23, and 26). Thegeni- 
 talia (ovaria and sper- 
 maria) are developed from 
 endoderm, and are always 
 interradial (in the four 
 
 radii formed after the first FJG . 19. Diagrammatic vertical section of a 
 
 four). The hydra-form 
 is not a " hydroid," but a 
 short polyp with broad 
 hypostome the "scyphi- 
 stoma, "which gives rise to 
 medusa-forms by trans- 
 verse fission (strobilatiou), 
 or itself develops genitalia 
 (Liicernarice). Combined visual and auditory organs in 
 the form of modified tentacles (tentaculocysts) to the 
 number of four, eight, or more occur on the edge of the 
 disc (except in Lucernarve, where they are represented 
 by the "colleto-cystophors"). The medusa-form in some 
 cases develops from the egg without the intermediate 
 scyphistoma-stage (Pslagia, Cliarybdcea 1). The edge of 
 its disc is provided with lappets, which cover the sensorial 
 tentaculocysts (hence Steganophthalmia of Forbes), and is 
 not provided with a velum (hence "Acraspeda" of Gegen- 
 baur), excepting the rudimentary velum of Aurelia (fig. 31) 
 and the well-developed vascular velum (pseudo-velum) of 
 Charybdcea (fig. 21). There is no continuous marginal 
 nerve-ring (except in Charybda>a), but several separate 
 marginal nerve centres (hence Toponeura of Eimer). The 
 
 1 Quite recently the Hertwigs (Jtnaische Zeitsdir., bd. vi., new 
 series, 1879) have insisted that in the Hydromeduxe the genitalia 
 (both ova and testes) are developed from the ectoderm, whilst in the 
 Scyphomedusce and in the A nihozoa they develop from the endoderm. 
 On this account they propose to abandon the grouping into Hydrozoa 
 and Anthozoa of Cceleniera nematophora, and suggest two groups, the 
 Eclocarpece and the Endocarpeas the former equivalent to Hydro- 
 meduscr, the latter embracing Scyphomfdusce and Anthozoa. The 
 Anthozoa exhibit a further predominance of the endoderm in its ex- 
 tensive origination in them of muscular fibre, which but rarely and in 
 small quantity develops from endoderm in the Hydromeditsos or in the 
 Scyphmnedusce. The Hertwigs base their generalization on their own 
 studies of medusse, but they have ignored the observations of Van 
 Beneden on Hydractinia and of Ciamician on various Tubularians, in 
 which the origin of either sperm or ova from endoderm is established. 
 Recently Fraipont has repeated an observation of Van Beneden's on 
 Campan itiaria, and shown conclusively that the ova in that form arise 
 from endoderm. Weismann (Zoologischer A nzeige r, May 1880) shows 
 the same for PlumuJaridce and Scrtvlaridce; the reader is referred to 
 his, paper. l 
 
 Luetrnaria in tbe plane of an interradina. 
 a. one of the intenadial angles of tbe 
 disc, giving rise at a' to two groups of 
 tentacles adradul in position ; 6, axial en- 
 teric cavity; r, endoderm; d. band-like 
 genital gland (ovary or testis). adradial in 
 position, and attached to the inierradial 
 septnm which runs along the angular pro- 
 cess of tbe disc, to which tbe letters c, d 
 point ; p, aboral rejnon or " foot " ; r, tbe 
 interradial gastral filaments or phacella?. 
 (After Allman.) 
 
66 
 
 HYDROZOA 
 
 diblastula in all cases, as yet observed, is formed by in- 
 vagination, the blastopore closing up (Balfour). 
 
 Fig. 23. 
 
 FIG. 20. Charybdtea marsupialis (natural size, after Clans). The four annulated 
 tentacles are seen defending from the four lappets placed at the four corners of 
 the quadrangular umbrella. These are interradial. Two of the four perradial 
 enteric pouches of the umbrella, representing radiating canals, are seen of a pale 
 tint. Fg, gastral filaments (interradial); R, the modified perradial tentacles 
 forming tentaculocysts ; 0, corner ridge facing the observer and dividing 
 adjacent pouches of the umbrella; OF, position of one of the genital bands. 
 
 Fir,. 21. View of the margin of the umbrella of Charjibdcea marsupialis (natural 
 size, after Claus). At the four comers are seen the lappets which support the 
 long tentacles, and in the middle of each of the four sides is seen a tentaculo- 
 cyst. Vel, the vascular velum or pseudo-velum, with its branched vessels. 
 
 FIG. 22. Horizontal section through the umbrella and manubrium of Charybdcea 
 marsupialis (modified from Claus). Ma, manubrium; SR, side ridge (perradial); 
 CR, comer ridges, separated by CO, the interradial corner groove ; Ge, the 
 genital lamellae in section, projecting from the interradial angles on each side 
 into UE, the enteric pouches of the umbrella; SU, the sub- umbrella space. 
 
 FIG. 23. Vertical sections of Charybdcea marsupialis, to the left in the plane of 
 an interradius, to the right in the plane of a perradius. Ma, manubrium ; 
 EAz, axial enteron; Gh, gastral filaments (phacellce); CO, corner groove; 
 SR, side ridge ; EnL, endoderm lamella (line of concrescence of the walls of 
 the enteric cavity of the umbrella, whereby its single chamber is broken up into 
 four pouches) ; Ge, line of attachment of a genital band; EU, enterir \iouch of 
 the umbrella, in the left-hand figure, points to the cavity uniting neighbouring 
 pouches near the margin of the umbrella and giving origin to TCa, the tentacular 
 canal; Ve, velum; Fr, frenum of the velum; Tc, tcntacnlocyst. 
 
 The binary division of the Hydrozoa was established by Esch- 
 scholtz (System, der Acalephen, 1829) whose Discophorce phanero- 
 carpce correspond to the Scyphomedusce, whilst his Discophorce 
 eryptoearpce represent the Hydromedusce. The terms point to dis- 
 tinctions which are not valid. In 1853 Kblliker used the term Dis- 
 
 eophora for the Scyphomcdusce alone, an illegitimate limitation of 
 the term which was followed by Louis Agassiz in 1860. Nichol- 
 son has used the term in the reverse sense for a heterogeneous 
 assemblage of those medusae not classified by Huxley as Lucemaridce, 
 nor as yet recognized as derived from hydroid trophosomes. This 
 use of the term adds to the existing confusion, and renders its 
 abandonment necessary. The term LHscomedusce was used for the 
 Scyphomcdusce by Haeckel in his Generelle Morphologic (exclud- 
 ing Charybdcea) whilst Cams (Handbuch, 1867) confines the term 
 " McduscK " to them alone, which is objectionable, since it belongs 
 as justly to the Hydromedusa'. Forties's term for them, Steganoph- 
 thalmia, indicates a true characteristic, failing only in the Luccr- 
 narice, but its complementary term Gymnophthalinia is inaccurate. 
 Similarly the terms Acraspeda and its complement Craspcdota are 
 inacceptable. Eimer Iris proposed to use the terms Toponcura and 
 Cydoneum for the two divisions but Charybdcea appears to break 
 down this division as so many others. The old term Acalephce, 
 which is retained by Gegenbaur in its proper sense for all the 
 Cop.lenlcra nematophora, is used as the designation of the Scypho- 
 medusa: alone by Claus (Grundz&ge der Zool., 1878), which cannot 
 fail to produce confusion. The term Lucemaridce, proposed so long 
 ago as 1856 by Huxley (Med. Times and Gazette), most truly indi- 
 cates the relationships of these organisms which he was the first to 
 recognize, but it seems desirable to restrict this term to the limited 
 order in which Luccmaria is placed, and to employ for the larger 
 group Scyphomeduscc a term which is the true complement of 
 the convenient name assigned to the other division ot Hydrozoa, 
 viz., Hydromedusce. 1 
 
 Order 1. Lucernarice, Scyphomedusai devoid of tenta- 
 culocysts, with the aboral pole of the body produced into 
 an adhesive disc by which the organism (which possesses 
 the power of swimming by contraction of the circular 
 muscular zone of the hypostome) usually affixes itself. The 
 enteric cavity is divided into four perradial chambers by 
 four delicate interradial 2 septa. The genitalia are developed 
 as four-paired ridges at the sides of the interradial septa 
 on the oral wall of the chambers (fig. 19). No reproduc- 
 tion by fission nor "alternation of generations" is known 
 in the group. At the edges of the disc capitate tentacles 
 are developed in eight adradial 2 groups ; between these are 
 modified tentacles in some genera, the marginal anchors 
 or colleto-cystophors. The canal system whicli has sometimes 
 been described in them is a product of erroneous observation. 
 A very few genera and species of this order are known. 
 They may be justly called the coenotype of the medusa; 
 (Jamc-s Clark), and their relationship to the free swimming 
 forms may be compared, as was done by L. Agassiz, to the 
 relationship of the stalked Crinoids to such forms as Coma- 
 tula. Three species are not uncommon on the British coasts. 
 
 By Milne Edwards the animals forming this group were termed 
 Podactinaria and associated with the Anthozoa. Bv Leuekart they 
 were termed Calycozoa ; it is only of late that the closeness of their 
 relationship to the Scyphomcdusce has been fully recognized, though 
 long since insisted on by Huxley and by James Clark. Haeckel in 
 his new system of the medusae (Sitzungsber. der Jenaische GcscJlschaft 
 fur Medicin und Naturwiss. , July 26, 1878) adopts for them the 
 term Scyphomccliis/x in allusion to their permanently maintaining the 
 distinctive features of the scyphistoma larval form of the .-icraspcdce, 
 the term which he adopts from Gegenbaur for our Scyphomcdusce. 
 
 Order 2. Discomedtisce. These are Scyphomedttsce de- 
 veloping as sexual medusiform persons by transverse fission 
 from a scyphistoma, or else directly from the egg. They 
 have eight tentaculocysts, four perradial, four interradial, 
 and sometimes accessory ones (adradial). Four or eight 
 genital lobes (ovaria or spermaria or hermaphrodite) are 
 developed from the endoderm forming the oral floor of the 
 central region of the enteric cavity, whicli is produced into 
 a corresponding number of pouches. The mouth is either 
 a simple opening at the termination of a rudimentary 
 manubrium (sub-order Cubostomai), or it is provided with 
 four or eight arm-like processes (sub-orders Semostomce and 
 Rhizostomce). In the sub-order Rhizostomrv (fig. 24, ), the 
 
 1 Scyphomedusce (<rKv<t>os, a cup) are medusse which are related by 
 strobilation to Scyphistoma, a wide-mouthed polyp with four gastral 
 ridges. Hydromedusa; are medusae related to a Hydra, a narrower 
 polyp, devoid of gastral ridges, by lateral gemmation. 
 
 2 For use of these terms see paragraphs on Amelia below. 
 
HYDROZOA 
 
 67 
 
 edges of the oral opening fuse together at an early age 
 and leave several sucker-like secondary mouths, which were 
 formerly mistaken for independent persons. The central 
 enteric chamber is continued through the disc by a com- 
 plicated often reticulate system of radiating canals, which 
 excavate the endoderm lamella. 
 
 development have recently formed the subject of investiga- 
 tion by Glaus, Eimer, and others. As the current accounts 
 
 FIG. 24. Seyphomtduscr. a, Rfiiiottoma pulmo; b, Chrfsaora hfotceua 
 
 In the Semostomce and Rhizostomce (not in the Cubostomte) 
 four remarkable (respiratory) sub-genital pits (fig. 28) are 
 hollowed out in the gelatinous substance of the sub-umbrella 
 (oral face of the umbrella). These do not communicate, as 
 
 FJG. 25 Fmir stages in the development of Ckiytaora. A, Diblasrnla stage; 
 B, stage after closure of blastopore ; C. fixed larva with commencing stomodajum 
 ororal ingrowth ; D, filed larva with mouth, short tentacles. Ac. ; tp. ectoderm ; 
 Ay. endodenn ; it. stomodxum ; m. mouth ; 6.', blastopore. (From Balfour. after 
 Clans.) 
 
 has been erroneously supposed, with the genital organs, the 
 products of which normally are evacuated by the mouth. 
 In the Tetragamelian Rhizostoirue these pits remain distinct 
 from one another as in Semostomce, but in the Monogamelian 
 RhizostomcE they unite to form one continuous sub-genital 
 cavity placed between the wall of the enteric cavity and 
 the polystomous oral disc. The common English forms, 
 Aurelia, Chrysaora, and Cyancea, are types of the Semo- 
 ftomue, the somewhat less common Rhizostoma of the 
 Monogamelian Rhizostomae, whilst Xausithoe and Disco- 
 medusa represent the simple Cubostomce. 
 
 The writer has adopted the term used by Haeckel for this order, 
 and is indebted to his preliminary notices of a large work on the 
 Medusa, now in the press, for outlines of the classification and de- 
 finitions which hare been introduced with modifications in relation 
 to these and the other Medusae. The term Discophora is used by 
 Claus (GrundzUge) for the Disconieduscc. It is quite clear from the 
 varied and inconsistent use by different authors of that term, and 
 also of the terms Acalephoe and Medusa:, that they must be ejected 
 altogether from use in systematic treatises. 
 
 The structure of the commoo Aurelia aurita and its 
 
 FIG. 26. Later development of Chrvsaora and Aurelia (after Clans). A, Scyphi- 
 stoma of CArytaora. with four perradial tentacles and horny basal perisarc. 
 B. Oral surface of later stage of scyphistoma of Am elia, with commencement 
 of four interradial tentacles. The quadrangular mouth is seen in the centre; 
 the outline of the stomach wall, seen by transparency around it, is nipped in 
 four places intemdially to form the four gastric ridges. C, Oral surface of 
 a sixteen-tentacled scyphistoma of Aurelia. The four gastric interradial 
 ridges are seen through the mouth. D. First constriction of the Aurelia 
 scyphistoma to form the pile of ephyrse or young medusae (see fig 27). The 
 single epbyra carries the sixteen scyphistoma tentacles, which will atrophy 
 and disappear. The four longitudinal gastric ridges are seen by tiansparency. 
 E, Young epliyra just liberated, showing the eight bifurcate arms of tlie disc 
 and the interradial single gastral filaments. F. Ephyra developing into a 
 medusa by the growth of the adradial regions. Ihe gastral filaments have 
 increased to three in each of the four sets. A, margin of the month : Ad. 
 adradial radius: F, gastral filament; In. interradial radius; JG. adradrial 
 gastral canal ; JR=R*. adradial lobe of the disc ; K, lappet of a perradial arm ; 
 JC stomach wall; Hit, muscle of the gastral ridge: Xtr. gastral ridge; 
 J/i, mesoderm; O, tentaculocyst ; P, perradial radius; K 1 , interradial radius; 
 R*. adradial radius ; SG, commencement of lateral vessel. 
 
 in text-books are very inadequate, a short sketch of tho 
 morphology of that form is appended here. 
 
 From the egg, according 
 to the researches of Claus 
 (whose figures, here repro- 
 duced, refer more especially 
 to the closely allied genus 
 Chrysaora, up to the comple- 
 tion of the scyphistoma), a 
 single-cell-layered blastula de- 
 velops which forms a diblastula 
 by invagination (fig. 25, A, B, 
 C). The orifice of invagination 
 closes up, and the ciliated 
 " planula " (as thb stage used 
 to be termed in all Ccelentera), 
 after swimming around for a 
 time, fixes itself, probably by FlG 
 the blastopo^al pole. The true 
 mouth then forms by inruption 
 at the opposite pole. Two ten- 
 tacles now grow out near the 
 mouth opposite to one another 
 (fig. 25, D), and are followed 
 by two more (fig. 26), these 
 indicating the four primary 
 radii of the body which pass 
 through the angles of the four- 
 sided mouth, and are termed pfrradial. Meanwhile 
 the aboral pole narrows and forms a distinct stalk, 
 which in Chrysaora secretes a horny perisarc (fig. 25, 
 
 Above to left, young scyphistoma 
 with four peira'dial tentacles. Be- 
 low to left, scyphistoma with six- 
 teen tentacles and first constriction. 
 To the light, stiobila condition of 
 the scyphistoma, consisting of thir- 
 teen metameric segments ; the up- 
 permost still possesses the sixteen 
 tentacles of the scyphistoma; the 
 remainder have no tentacles, but 
 are ephyrae. each with eight bifid 
 arms (processes of the disc). Each 
 segment when detached becomes 
 an ephyra, such as that drawn in 
 fig. 26. E, F. (From Gegenbanr ) 
 
68 
 
 HYDROZOA 
 
 D). Four new tentacles, those of the intermediate or 
 secondary radii, now appear between the first four, and 
 are termed interradial. At the same time four longi- 
 tudinal ridges grow forward on the wall of the enteric 
 cavity (fig. 26). These interradial ridges have sometimes 
 
 TO: 
 
 C.P. 
 
 It is in con- 
 
 FIG. 28. Surface view of the sub-umbrella or oral aspect of Aurelia aurita, to 
 show the position of the openings of the sub-genital pits, OP. In the centre 
 is the mouth, with four perradial arms corresponding to its angles (compare 
 fig. 26). The four sub-genital pits are seen to be interradial. x indicates the 
 outline of the roof (aboral limit) of a sub-genital pit; y, the outline of Its floor 
 or oral limit, in which is the opening (compare 6 of fig. 16). 
 
 been erroneously described as containing each a longitudinal 
 canal connected with a circular canal at the base of the 
 tentacles. They are in reality solid, as is the margin of the 
 hypostome from which the tentacles spring, 
 nexion with these four 
 ridges that the gastral 
 filaments will subse- 
 quently appear, as also 
 the genital organs either 
 along their middle line 
 or adradially to them. 
 The ridges correspond 
 to the mesenteries of 
 the Anthozoa. Eight 
 additional tentacles 
 placed one on each side 
 of the perradial ten- 
 tacles (or of the inter- 
 radial, according as we 
 may choose to regard 
 the matter) next appear, 
 and are distinguished as 
 adradial. All the ten- 
 tacles reaching an equal 
 size, we obtain the ap- 
 pearance seen in fig. 26, 
 when the young scyphi- 
 stoma is looked at from 
 above. Looked at from 
 the side, with its wide 
 hypostome and short 
 vertical axis, the scy- 
 phistoma differs widely from an ordinary hydra-form, and 
 approaches the medusa-form, to which its four longitudinal 
 gastral ridges further assimilate it. The little creature is 
 now about an eighth of an inch in height ; in other genera, 
 but not in Chrysaora, it may now multiply by the produc- 
 tion of a few buds from its fixed basal disc. After nourish- 
 ing itself for a period, and increasing to four or five times 
 the size just noted, the vertical axis elongates and a series 
 of transverse constrictions appear on the surface, marking 
 off the body of the scyphistoma into a series of discs 
 (figs. 26 and 27), each of which by the development 
 
 FIG. 29. Half of the lower surface of Aurelia 
 aurita. The transparent tissues allow the 
 enteric cavities anil canals to be seen through 
 them, a, marginal lappets hiding tentaculo- 
 cysts; 6, oral arms; r, axial or gastric portion 
 of the enteric cavity; ge, radiating and ana- 
 stomosing canals of the enteric system; on, 
 ovaries. The gastral filaments near to these 
 are not drawn. (From Gegenbaur.) 
 
 of tentacles and completion of the constriction will become 
 a separate medusa (in its young state called " ephyra "). 
 The tentacles of the Aurelia and the structure of the 
 margin of its hypostome are very different from those of 
 the scyphistoma. They are exhibited in their earliest 
 condition (when the Aurelia-medusa, is first liberated from 
 its attachment and is an ephyra) in fig. 26, E, F. The 
 margin of the hypostome is drawn out into eight arms 
 (which are not to be confused with tentacles) ; the end of 
 each arm is bifid, carrying a pair of lappets the marginal 
 lappets which persist in the adult (see figs. 30 and 31). Be- 
 tween the lappets is placed a short and peculiar tentacle, the 
 tentaculocyst or sense-organ. The eight arms of the disc 
 and their tentaculocysts are perradial and interradial. As 
 the organism grows, a set of eight adradial tentacles appear 
 in the notches between the eight arms, but never attain any 
 relatively large size in Aurelia. The asteroid arm-bearing 
 
 FIG. 30. Tentaculocyst and marginal lappets at Aurelia aurita. In the left- 
 hand figure ML, marginal lappets; T, tentaculocyst; A, superior or aboral 
 olfactory pit; MT, marginal tentacles of the disc. The view is from the aboral 
 surface, magnified about 50 diameters. In the right-hand figure A, superior 
 or aboral olfactory pit; B, inferior or adoral olfactory pit; //, bridge between 
 the two marginal lappets forming the hood; T, tentaculocyst ; End, cndodcrm; 
 Ent, canal of the enteric system continued into the tentaculocyst; t'yn, endo- 
 dermal concretion (auditor)'); c, ectodermal pigment (ocellus). The drawing 
 represents a section, taken in a radial vertical plane so as to pass through the 
 long axis of tlie tentaculocyst. (After Elmer.) 
 
 character of the margin of the disc is soon obliterated by 
 
 the relative growth of the intermediate adradial areas, which 
 
 become quite filled up, so that in the adult the tentaculocyst 
 
 is carried in a notch instead of on a prominence, and is 
 
 concealed by the two lappets 
 
 (figs. 28 and 30). The margin 
 
 of the disc between adjacent 
 
 pairs of lappets gives rise to 
 
 a fold which grows inwards 
 
 (toward the mouth) during 
 
 an early stage (fig. 31), and 
 
 numerous small tentacles (the 
 
 fringe) appear along the 
 
 margin of the disc, which 
 
 soon equal in size the first 
 
 adradial tentacle. The in- 
 
 . , FIG. 31. Part of the margin of the disc 
 
 growing fold IS the velum or ofayoungXiuv/fa, to show the rudi- 
 " naonrln vpliim " anH npvpr mentary velum, Vel, extending from 
 
 pseuao-veium, ana never the ^.g^] lapp ets, ML, on either 
 
 increases in size, SO that in 
 
 the adult it is not observ- 
 able. The tentacles also remain very small and fine in 
 Aurelia, forming a continuous fringe along the edge of 
 the disc, interrupted only by the eight notches for the 
 tentaculocysts (fig. 29), 
 
 The sixteen tentacles of the scyphistoma are necessarily 
 attached to the most anterior of the pile of medusa? ; they 
 atrophy, but to what extent they may be metamorphosed 
 to form the parts of the ephyra or young medusa has not 
 been determined. The scyphistoma, having given rise to 
 its pile of ephyrae, may (in some genera, Aureliaf) 
 redevelop its own kind of tentacles below the constriction 
 marking off the last ephyra. Hence scyphistoma tentacles 
 appear sometimes at the top and sometimes at the bottom 
 
 side; T, the small tentacles fringing 
 
HYDROZOA 
 
 69 
 
 of the pile, which has led to diverse accounts of the mode 
 of development of the ephyrae. 
 
 Whilst changes are going on in the configuration of the 
 margin of the disc of an ephyra on its way to the perfect 
 form of the adult Aurdia, the enteric cavity has also under- 
 gone most important changes. Foremost in importance is 
 the development of a single gastral filament on each of the 
 four gastral ridges which necessarily are present in the 
 transverse slice (so to call it) of a scyphistoma, which 
 becomes an ephyra (fig. 26). These rapidly increase in 
 number as the ephyra grows. Further, the enteric cavity 
 at first follows the outline of the ephyra, sending a process 
 into each arm, but then by adhesion of its walls is converted 
 into a four-lobed central chamber, a marginal canal, and an 
 endoderm lamella. A system of canals, the arrangement of 
 which is seen in figs. 29 and 31, subsequently opens out again 
 certain lines and tracts of the conjoined endoderm walls. 
 
 In the adult Aurdia we find the mouth surrounded by 
 four large arm-like perradial processes (figs. 25 and 29) 
 (not tentacles), and leading through a short manubrium 
 into a flattened four-lobed chamber, the lobes being inter- 
 radial, and having on their oral floor numerous gastral 
 filaments (rich in thread cells) (6 in fig. 16). Each pouch 
 or lobe gives off a canal, which runs towards the circular 
 canal at the margin of the disc, but breaks up into three or 
 four secondary canals on its way. Between the pouches 
 come off eight other "radiating" canals (adradial), which 
 do not branch, but go straight to the circular canal 
 
 The oral floor of the concavity of each lobe of the enteric 
 cavity is occupied by a horse-shoe-shaped frill (fig. 29, ov), 
 either testis or ovary (the sexes being in separate indi- 
 viduals). The open arms of the horse-shoe are turned 
 towards the centre of the disc, and the folds of the genital 
 frill are so deep as to show themselves on the outer ecto- 
 dermal wall of the disc. Here, however, there is a very 
 remarkable arrangement, which has rarely, if ever, been 
 correctly described and figured in our common Aurdia. 
 The gelatinous substance of the disc is hollowed out on 
 that part of the oral face corresponding to the position of 
 the genital frills, so as to form four separate extensive pits 
 or chambers. Each of these sub-genital pits has in A urdia 
 a small round opening on the oral face of the disc (fig. 28, 
 GP), but is otherwise entirely closed, having no com- 
 munication with the genital tissues, from which it is 
 separated by a delicate layer of ectoderm (6 in fig. 16). 
 The pits probably serve to admit water for respiratory pur- 
 pases into close proximity with the genital tissues. 
 
 The whole enteric surface, including canals, is ciliated, 
 whilst the ectoderm is not ciliated, but provided with 
 groups of nematocysts. 
 
 The teutaculocyst in the adult Aurdia is relatively an 
 extremely minute body, completely hidden by the two large 
 marginal lappets (fig. 30, T). Above it (that is, on the 
 aboral surface, as the Aurdia swims) is a deep pit (A), 
 Schafer's fovea nervosa superior, sunk in a sort of bridge 
 which connects the two lappets and overhangs the tenta- 
 cubcyst. A similar pit (the fovea inferior) exists on the 
 oral surface. These have been recognized by Claus, Eimer, 
 and the Hertwigs as olfactory organs. The tentaculocyst 
 is seen in section in fig. 30 (right-hand figure), which ex- 
 hibits its central cavity continuous with the enteric cavity, 
 its ectodermal pigment spot (eye), and its endodermal mass 
 of concretions (auditory organ). 
 
 The chief muscular mass of Aurelia, except that of the 
 oral arms, is a circular zone on the oral face of the disc. 
 The muscular fibres are not distinct cells, but transversely- 
 striated processes of the epidermic cells (epidermo-muscular 
 cells) (fig. 9). In the " arms " of other medusae, and pre- 
 sumably of Aurdia, the muscular fibre is formed by inde- 
 pendent nucleated cells (fig. 9). 
 
 The nerve-epithelium from the olfactory pits of Aurdia is 
 drawn in tig. 1 4. Starting from this and from the cells of 
 the tentaculocysts are nerve-fibres, which spread themselves 
 on the surface of the circular muscular zone in the neigh- 
 bourhood of the tentaculocysts, and these are connected each 
 and separately with large isolated nerve-ganglion cells (fig. 
 15). The nerve-fibre is continued beyond the cell, and 
 in some instances has been traced into a broadened ex- 
 pansion lying on a muscular fibre (Schafer). The nerve- 
 ganglion cells lie very superficially immediately below the 
 flat epithelium of the body surface and between it and its 
 muscular processes. 
 
 The ova and spermatozoa of Aurdia develop in the 
 genital frills from endoderm cells in separate individuals. 
 They pass to the exterior through the mouth. 
 
 Order 3. Conomfdusae, Scyphomedusoe with only four 
 tentaculocysts, and these perradiaL A broad velum (so- 
 called pseudo-velum) of complete circular form is present, 
 differing from that of the Hydromedusce in the fact that it 
 is penetrated by canals of the enteric system (Charybdcea). 
 The whole umbrella is bell-shaped. The genital organs are 
 four pairs of lamelliform ridges (fig. 22) which are attached 
 to the four narrow interradial septa that divide the large 
 enteric cavity of the umbrella into four perradial gastro- 
 caual pouches. The lamelliform genital glands hang freely 
 in these pouches. At the edge of the umbrella are four 
 interradial lappet-like prolongations of the gelatinous sub- 
 stance of the disc, which support each a long tentacle (fig. 
 20). The nerve-ring is complete, like that of the Hydro- 
 medusae. 
 
 There is now no doubt that Charybdaa, which has been placed in 
 each of the two large divisions of the Hydrozoa, must be classed 
 with the Scyphomtdusce. The recent investigations of Claus 
 (Arbeiten. atis dem Zool. Institut zu Wien, Bd. i. Hft. ii., 1878), as 
 well as those of Haeckel and Fritz Miiller, lead to this conclusion. 
 The term CoTtomedusec is adopted from Haeckel, who places here, 
 besides Charyldoca and Tamoya, other forms, a fuller description of 
 which may be expected in his forthcoming System der Medusen. In 
 many respects its quadrangular form, its marginal lappets, its 
 broad enteric pouches in place of fine canals, its vascular velum, and 
 its highly complicated tentaculocysts (fig. 13, B) CharybdaM 
 is peculiar. The simplicity of the enteric system and the arrange- 
 ment of the genital glands bring it near to Lueeniaria. The ex- 
 istence of four interradial groups of gastral filaments, and the dis- 
 position of the paired genital glands at the sides of the iuterradial 
 septa, determine its position to be among the Scypkomcduscc. Its 
 development is not known. Figs. 20 to 23 illustrate the structure 
 of Charybdcca. 
 
 Order 4. Peronudusce, Scyphomedusce with four inter- 
 radial tentaculocysts. The enteric system consists of three 
 divisions, an aboral main stomach with four interradial 
 gastral ridges and filament groups ; a mid-stomach, which 
 communicates by means of four perradial slits with a very 
 large ring-sinus (occupying two-thirds of the umbrella) ; and 
 thirdly, an oral portion or pharynx, with four wide per- 
 radial pouches. The genital organs are four pairs of 
 sausage-shaped interradial ridges lying on the oral floor of 
 the ring-sinus. 
 
 This is a new group founded by Haeckel, of which we have at 
 present no further details. 
 
 Sub-class II. HYDROMEDUS.E. These&reffydrozoadevoid 
 of gastral filaments ; the sexual persons are always medusi- 
 form, the genital glands are developed sometimes from ecto- 
 dermal cells, sometimes from endoderm, and are always per- 
 radial (in the radii of the first order). The medusiform per- 
 sons always possess a muscular non-vascular velum (hence 
 Crasjxdota) and a complete nerve-ring (hence Cydoneura of 
 Eimer). The marginal sense-organs are either ocelli or oto- 
 cysts or tentaculocysts. The diblastula, in all cases as yet 
 observed, is formed by delamination (Balfour). The sexual 
 medusiform persons may develop directly from the egg, but 
 more usuallv the egg gives rise to a hydriform person the 
 hydroid which differs from a scyphistoma in its elongate 
 
70 
 
 HYDROZOA 
 
 vertical axis, the indefinite number (often also position) of 
 its tentacles, and its frequent formation of a colony of large 
 size by lateral budding. By lateral budding (not by 
 
 PIG. 32. Diagram showing possible modifications of persons of a gymnoblastic 
 Hydromeduaa. a, hydrocaulus (stem); b, hydrorhiza (root); c, enteric cavity; 
 <J, endoderm; , ectoderm; /, perlsarc (horny case); g, hydranth (hydriform 
 person) expanded; g', hydranth (hydriform person) contracted; h, hypostomc, 
 bearing mouth at its extremity; *, sacciform gonophor (sporosac) springing 
 from the hydrocaulus ; k' t sporosac springing from m, a modified hydriform 
 person (blastostyle): the genitalia are seen surrounding the spadix or manu- 
 brium ; /, medusiform person or medusa ; m, blastostyle. (Alter Allman.) 
 
 metameric fission) medusiform persons wbich alone develop 
 sexual glands are produced on the hydriform colonies; 
 
 FIG. 33. Diagram showing possible modifications of the persons of a Calypto- 
 blastic Hydromedusa. Letters a to h same as in fig. 32. t, the horny cup or 
 hydrotheca of the hydriform persons; /, medusiform person springing from m, 
 a modified hydriform person (blastostyle); n, the horny case or gonangium 
 enclosing the blastostyle and its buds. This and the hydrotheca i give origin 
 to the name Ca/yptoblastea. (After Allman.) 
 
 these may separate from the colony, or may be retained in 
 a more or less degenerate form adherent to it, as generative 
 buds or warts. 
 
 The medusiform persons of this group are the Discopharce crypto- 
 carpce of Eschscholtz, the Craspedota of Gegenbaur (1854), and the 
 Hydromedusida of Kblliker (1853) the last two authors at that 
 time separating the hydriform persons as Hydroidca. Louis 
 Agassiz (1860) includes both sets of persons under the term 
 
 Fig. 34. 
 
 Fig. 35. 
 
 FIG. 34. Diagram of Corymorpha. A, a hydriform person giving rise to 
 medusifomi persons by budding from the margin of the disc; B, free swim- 
 ming medusa (Steenstrupia of Forbes) detached from the same, with manu- 
 brial genitalia (Anthomedusg) and only one tentacle. (After Allman.) 
 
 FIG. 35. Diagram of Tubulana indivisa. A single hydriform person a bearing a 
 stalk carrying numerous degenerate medusiform persons or sporosacs b. (After 
 Allman.) 
 
 Hydroida (together with Lucernaria), which also is the term adopted 
 by Allman in his beautiful monograph(1871-74). J.V. Cams, amend- 
 ing the limitations given by Carl Vogt, was the first to use the term 
 Hydromedusce in the sense here adopted (Handbvch der Zoologie, 
 1863), and it is now employed in the same sense by Gegenbaur 
 (Elements of Comparative Anatomy, London, 1878), namely, to em- 
 brace both the cryptocarpous medusae of Eschscholtz and the 
 
 FIG. 36. Colony of Bougaimillea fruticosa, natural size, attached to the 
 underside of a piece of floating timber. (After Allman.) 
 
 hydroids related to them. The term Hydromedusa is used unwisely 
 by Claus (Grundzilgc d. Z. ) for the whole group of Hydrozoa. It 
 has been the practice of some authors to give a double classification 
 of the group one based on the characters of the medusiform per- 
 sons, the other on that of the hydriform persons. In the present 
 article a double name will in some cases be assigned to a group- 
 but the attempt is made to bring both sets of persons under one 
 system. 
 
 Order 1. Gymnoblastea-Antlwrnedusce. These are Hydro- 
 medusa; which all, as far as is known, pass through a 
 hydriform phase, but in which the medusiform persons 
 may either reach full development or exhibit the extremest 
 degeneration (Hydra). The ectoderm of the hydriform 
 
HYDROZOA 
 
 71 
 
 persons may secrete a horny tubular protective case 
 (perisarc), but this does not form cups for the reception of 
 the tentacular crown nor cases enclosing groups of medusi- 
 form buds (gonangia). The fully-developed rnedusiform 
 
 FIG. 37. Portion of colony of BaugainrUlta (Endedrivm)fr*tiaua {A nthomeduur- 
 calyptMaitea) more magnified. (From Lubbock, after Allman.) 
 
 persons never possess otocysts nor tentaculocysts, but always 
 ocelli at the base of the tentacles. The latter are usually 
 four or six, corresponding to the same number of simple 
 radial enteric canals, but may be more numerous or reduced 
 to one or to two ; rarely they are branched (Cladonema). 
 
 Fig. 38. 
 
 Fig. 39. 
 
 Fig. 40. 
 
 FIG. 38. Diagram of Clara, showing a hydriform person surronnded by a 
 verticil of degenerate mcdusiform persons (sporosacs). (After Allman.) 
 
 FIG. 39. Diagram of a colony of Hydraftinia,' showing four forms of persons, a, 
 hydfiform person; 6, modified hydriform person, or blastustyle, bearing c, 
 degenerate mednsiform persons or sporosacs; d, modified hydriform person 
 situated at the margin of the colony (dactylozooid). (After Airman.) 
 
 FIG. 40. Diagram of a colony of Diearne, showing three forms of persons a, 
 normal hydriform person : 6, modified bud-bearing hydriform person (blasto- 
 style); f, degenerate mednsiform persons (sporosacs).' (After Allman.) 
 
 The sexual glands are placed in the wall of the mauubrium, 
 either equaUy distributed all round it or in four separate 
 perradial groups, which are often divided into eight ad- 
 radial groups by the perradial longitudinal muscles. 
 
 This is a very well defined group, since the Gymnoblastea of 
 Allman, based on the characters of the hydrifonn persons also 
 known as Tubnlaruz and Gymnatokor correspond exactly with the 
 AnUtomeduste of Haeckel's new system. Hydra is included here, 
 though placed in a separate order by Allman. Some of the leading 
 forms of hydriform and medusifonn persons are given in the cuts 
 (figs. 34 to 42). The greatest rauge in the amount of degenera- 
 tion of the mednsiform persons is seen even in genera of the same 
 family e.g., Turns and data the former producing free 
 medusae, the latter sesrile sporosacs. The Occanidte of Gegenbaur 
 (excluding the Williada:, which Haeckel assigns to the next group) 
 correspond on the whole to the medusa-forms of this order. 
 
 Fig. 41. Fig. 42. 
 
 FIG. 41. Hydrifonn person of SfKorfttt, with medosif orm persons budding from 
 
 it. and shown in various stages of development, o, b, c, d, e. (From Gegenbaur, 
 
 after Desor.) 
 FIG. 42. Hvdra riridis. or, ovary; te. testis. 
 
 Order 2. Calyptoblastea-Leptomedusa;. These are Hydro- 
 medusae of which the hydriform phase is known in a 
 large number of cases, whilst of others only the medusa- 
 forms are known ; none are known to develop directly from 
 the egg to the medusa-form. As in the preceding group, 
 the medusiform persons may reach full development or 
 
 Fig. 43. Fig. 44. 
 
 FIG. 43. Diagram of a colony of Campantilaria. showing four forms of per- 
 sons. A, portion of a fixed colony; a. hydriform person; 6, bud-bearing 
 hydriform person (blastostyle) ; H, free - swimming colony, being a sexless 
 mednsiform person (blastocheme of Allman). with modified medusiform persons 
 budding from iis radiating canals, as sporosacs. (After Allman.) 
 
 FIG. 44. Medusiform person (Liiria). one of the Anthomedanx, detached from a 
 hydroid colony of the family EndtndH&r. Ocelli are seen at the base of the 
 tentacles, and two mednsiform buds on the sides of the manubrium. (After 
 Allman.) 
 
 exhibit themselves as degenerate sexual sacs on the hydri- 
 form colonies. The ectoderm of the hydra-forms always 
 secretes a perisarc which forms a cup-like protection (hydro- 
 theca) to the tentacle-crown, and which also encloses the 
 group of medusa-buds in peculiar horny cases (gonangia). 
 The fully-deveJoped medusifonn persons (fig. 47) either 
 
72 
 
 HYDROZOA 
 
 have uo otooysts, but only ocelli (Ocellata<) t or they have 
 otoeysts (tig. 11) (eetodornml sues), four, eight, or over a 
 Immlred, not homologous with tentacles, and sometimes in 
 addition ocelli ( rttifiilahr). The radial ontoric canals uro 
 usually four or eight in number, but may bo more numerous, 
 whilst the marginal tentacles of the disc uro either few or 
 
 FIR. 4R. FiR. 40. 
 
 Km. . MediisWirm pewon (.tirtfoV one of the .4 K/AiiinnfMir. dotachd from 
 hydroM eolonj of the family <\uyin,f,r. b, the long mamibriwn, bearing ( 
 H txcvpiton) m*4\ulfonn I>U,N ; n, mouth. 
 
 Ki. 4ti. Mctlualform |H<raon, one of tho .4MiuMi<ihu<r, detached from n hvdro... 
 colony of .SJOKSII-JW^. Ocelli are awn HI the base of tho tentacles, nml also (as 
 mi MtOtpttUU gnwps of niodusllorm build. 
 
 very numerous. Tho genital glands always arc placed in 
 the course of the radial canals of the disc (not in tho utanu- 
 brium), and stand out as groups of wart-like processes on 
 the sub umbrellar surface (tig. 43). Their mode of dis- 
 charge is uncertain. 
 
 Ku>. 4T. View of (hi' oral surface of on<> of the l.rp(amnlt<r (/raw prihlrida, 
 HaecMk, to show the numerous tentacles iul tho otocysts. p, gvnltal glands; 
 .V. ntauubrlnm; o.', olocysts; tv, the four fnilUtlni; oniU.i; V, . the \t'lum. 
 
 The ( WyutoMii.tfVii of Allnuni, SbfMtohi of Cams, and Ctui>i>tinu- 
 lariir of authors form a wcll-tnarkoil group of hydivids which, when 
 they )t' Vl> ri* 1 * l' vt> iiuxlusio, give rise to those termed Lfptometiu&f 
 by Havkel, oorivsjiotidinj! to the ThiimnaHtiaiitr and Eufopiiltv of 
 r's system. The oalyptoWastie hydroid /.<-^(-i//iAii, t \vhi 1 -h, 
 to Allmnn. gives rise to * /.term-like medusa (AttMo- 
 is the only recorded exception to tliis correspondence. 
 The sKym>riiit and other meihisiv of similar stvuetuiv hv* not been 
 tracwl into wnnexion with any hydritorm tmphosome, but we are 
 not justified theivfoiv incoiioUuliiij; tluit they develop directly from 
 fhe egsj without hydriform phs<>. The chief jx>int distingxiishing 
 the /.<^*>irtfN.w its A lot from til* jtntiit>mt<lus<r is the drvolopniout 
 nf the gonorativt> Kxlies in the radial canals. This pi^sition is simi- 
 lar to that occupied, by the same organs in Tmi-tiMiitiiuMt *&A 
 iSci/j>At>WfWi(.s<r. Allman, howewr, considei's the gvnitl glands of 
 the IsptonuilHsiT, not as n>en> glands like those of Aurtlia or 
 ("Atirw/i^ni, but as a series of buds a generation of aborted 
 medusso or sporosaes. In consequence he terms the medusa of tlie 
 LtptOMmiHMi*. blastocheme ^or bud-pnxluco^. as distinguished l'n>m 
 ftonoeheme (or geniUl-pnxluccr). In sup)K>it of this view, 
 
 Allnmn (Aftmograuh, 1874) adduces tho various remarkable case* 
 of production <>!' buds by mnlus.r \\hi,-h Intve been reeorded (tig. 
 H, -l.'i, -lii), mid, furtlu'f, the M-I\ >tukiiif; Niiiiilnrity between the 
 Htructure of a lobe of the genital gland of Obelin and a spui-osae 
 MII-II us we liml in Jlt/i/nictiiiiii. It seems necesaury to n.v. ].i 
 Alliuan's view on this miillir, unless we are iire|uired tn uliatiilnn 
 the homology of sporosacii with medusa* in the case of hydriform 
 |iersons. 
 
 The eolonies of hydriform persons of the present group dilfer inter 
 e acconling to the arrangement of the cups or hydiotheeie. In 
 Pluiniilnnait they aro sessile, and all on one side of a branch ; in 
 they are sessile, and alternately placed on either side; in 
 iiiir each cup is raised on a pedicel or stalk. The 
 mcduMii'orm persons sometimes remain abortive and sessile in their 
 gomuigia. 
 
 F1.48. fWiwm'i'<i(Wi'r^oi')Ain(nM,oneof thc7>'oc*<*iwir. (Aflei Haul,, 1 t 
 .1. none nut. ', rmlUI imTi' ; A, trntnouloryit ; f, flrcukr cnl; t, vmlmtiiin 
 etmul ; if", ovwry ; A. ini-viilH or ejutlluniiuuis imH'fM aDeoiultiiK hoin the cintt. 
 luminous nmrjjin of tho disc centvipetHllv In the outer Mivfnce of the jcll\ hke 
 dlde; six of those HIV |>enmlial, *1\ interradiiil, coirfspotulinu to the twelve soldi 
 lrvnl leutticled, iTseniblltiK tho^o of CNNI'IHI; Jt, tlllatatlon (sieintu'h) of the 
 nmimbrlum; /, ji'lly of tli* dltr: I', iiMiiubrluni : /, tentacle (hollow and 
 tertiary, *.*., tutiH-ili'it by lx i>errillal ami six Intemullnl solid larval tentacle*) ; 
 n, rarlbngluoud innrcln of the disc covered by thread-cells ; i\ velum. 
 
 Order 3. Trtifhomtdustr, JIy<iroinrdi<f<r which haT6 aa 
 
 sense-organs tcntttcnlocysts. The nd'liilis { \\. r.M ;i n> 
 
 FlG. 49. Plajrr.im of a vertlcul section of OiMiirt<i AtuMfn. (lassjnp on thft 
 ri^lu thronch the \\hoK' length ef a r;uli:itini; cannl, and on the left thn>Uh 
 the outprad lobe of an ovary. /, gelatinous snbstamt' of the disc and siistric 
 talk (muiubriumV. r, radial Ini; canal; n, its outer, rt, its Inner wall; 17, 
 ovaries; *, atomach (illlatation of the manubrlum): 7, tongue-like process of 
 the gelatinous substance; *, cartilaKinous puvess ascending tioiu the tnavginal 
 ring at the site of a tentaculoevst : c. ciivnlar can.-il; A, tentaculocyst ; r, 
 velum; *, cartilaginous marginal ring. (From Gegvnbanr.) 
 
 formed by eudodermic cells as in Scitpfwntfihisa?, and 
 ocelli may or may not be present on the tentaculocyst. 
 
HYDROZOA 
 
 73 
 
 The genital glands have the form of wide outgrowths or 
 lamelliforrn enlargements in the course of the radial canals 
 (figs. 48, 49). No hydriforin phase is kuown in any 
 member of this group, and one at least (Geryonia) has been 
 observed to develop from the egg directly into the medusa- 
 form. 
 
 Order 4. Narcomedusve. These have the same characters 
 as the Trachomedwce, excepting that the genital glands are 
 in the wall of the mauubrium or in pocket-like radial out- 
 growths thereof (figs. 50 and 51). Further, the marginal 
 tentacles of the disc possess peculiar " roots," which can be 
 traced upwards into the gelatinous substance of the body. 
 No hydriform phase has been observed iu this group, 
 whilst ^Egina and jEyinopsis have been shown to develop 
 directly from the egg to the medusa-form. 
 
 FIG. 50. Cunina rhododactyla, one of the Narcomtdtate. c, circular canal; h, 
 ' otoporpae " (ear-rivets) or centripetal process of the marginal cartilaginous 
 ring connected with tentaculocyst; t, stomach; /, jelly of the disc; r, radiat- 
 ing canal (pouch of stomach) ; /f, tentacles ; (IP, tentacle root. (After HaeckeL) 
 The lappets of the margin of the disc, separated by deep notches, above 
 which (nearer the aboral pole) the tentacles project from the disc (not mar- 
 ginal therefore), are characteristic of many 2?artoniedus<e and Trachomeduste. 
 Cartilaginous strands (the mantle rivets or peronias) connect the tentacle root 
 with the solid marginal ring. 
 
 The two orders Trachomedtisce and Narcomedusce are established 
 by Haeckel in his new " system " for the peculiar forms classed by 
 Carus as Haplomoiyha, and by Allmau as Monopsea. These latter 
 names have reference to the fact that no hydriform phase is knouti 
 to occur in the life-history of these organisms, a fact which is not 
 peculiar to them, and, if it should prove to be not universal amongst 
 them, would by no means invalidate their claim to a distinct posi- 
 tion on the grounds afforded by the characters above given. They 
 are remarkable for a certain hardness and stiffness of the gelatinous 
 substance of the disc, or at any rate of the cellular axis of the 
 tentacles, on accout of which the orders are contrasted by Haeckel 
 as Trachylince with Anthomcdusce and Leptoineduscc, which are 
 
 Fio. 51. Diagram of a vertical section through a young Cunina rhododactyla, 
 passing on the right side thruugh a radiating pouch, b, tentaculocyst ; r, 
 circular canal ; g, ovary ; h, marginal cartilage and connecting process 
 springing from a tentaculocyst (otoporpa) ; t. stomach; /, jelly of the disc; 
 r, radiating canal or pouch ; tt. tentacle (solid, cartilaginous) ; lir, tentacle 
 root ; p, velum. (From Gegenbaur.) 
 
 termed Leptolince ; a curious parallelism as to the position of the 
 genitalia exists between Anthomedusas and Nareoinedusoe on the 
 one hand and LeptonuduscE and Trachomedusee, on the other. 
 The orders present a very high degree of development, both in 
 coarser and histological differentiation. At one time it was sup- 
 posed, in accordance with Haeckel's observations, that Geryonia 
 (Carmarina, fig. 48), one of the Trachonudusce, gave rise by buds 
 from its enteric walls to young Cuninoe (Narcomedusoe, tig. 50), 
 but this has been explained by the observations of Franz Sehulze 
 and of Uljanin as due to parasitism, young Cunince in the condition 
 of ciliated Planulos entering the mouth and enteric chamber of the 
 Carmarina. The same explanation probably applies (Claus) to the 
 supposed internal buds of Cunina observed by Gegenbaur, Fritz 
 Miiller, and iletschnikow. The process is sufficiently remarkable 
 according to the last observer, for the first generation of buds pro- 
 duce a second generation by external gemmation, before attaining 
 the characters of the parent Cunina. The anatomy of these forms 
 
 is fully given in Haeckel's memoirs in the Jcnaische Zeitachrifl, vols. 
 i. and ii. , 1864-66 ; also further details as to Carmarina, are given in 
 Elmer's Jtledusen, 1878. 
 
 Order 5. Hydrocorallinve. These are llydromedwae in 
 which the hydriform phase forms large colonies, presenting 
 a copious calcareous deposit 
 in the ectodermal tissue (cce- 
 nosteum of Moseley), leav- 
 ing only the hydranths or ten- 
 tacular region free from such 
 hardening. The inedusiform 
 persons are, at present, only 
 known in the degenerate 
 form of sporosacs, which 
 occupy cavities (ampullae 
 of Moseley) in the har- 
 dened base of the colony 
 (Stylasteridtf). No such 
 
 p.ivitif> Jiavp Kopn rlpfpprjvl 
 in Others (Mllleparidce), which 
 
 may, therefore, give rise to 
 complete medusiform persons. 
 In all a marked polymorphism has been observed (fig. 53), 
 consisting in the differentiation of longer tentacle-like 
 persons (dactylozooids) and shorter mouth-bearing persons 
 (gastrozooids). The persons of both kinds are either 
 scattered irregularly or the dactylozooids are arranged 
 around the gastrozooids in cyclosystems of greater or less 
 definiteness, or in distinct rows (fig. 55). The position m 
 these two kinds of hydriform persons is marked by definite 
 groups of pits (cyclosystems) in the dried calcareous skeleton 
 of the colonies, which simulate the calycles of the stony 
 coral.-; (Anthozoa). 
 
 'G- 52. Portion of the calcareous 
 corallum of Millepora nodota, show- 
 ing the cyclual arrangement of the 
 
 < F n 
 
 FIG. 53. Enlarged view of the surface of a living Millepora. showing five 
 dactylozooids surrounding a central gastrozooid. (From Moseley.) 
 
 Louis Agassiz was the first to recognize the true nature of the 
 MilleporidcE, and his imperfect observations have been fully con- 
 firmed and greatly extended by Mr Moseley (Phil. Trans. , 1878) who 
 added the Stylasteridas previously regarded as Anthozoa to the 
 category of calcigenons hydroids, and founded the order of 
 Hydrocorallinae. The Stylasteridas differ from the Milleporidce in 
 possessing a calcified axial style at the base of the dilated portion of 
 each gastrozooid, and further in the ascertained development of 
 sporosacs, and in the greater complication of their cyclosystems v 
 These forms are abundant in tropical seas, and contribute with the 
 Anthozoa and Corallines to the formation of coral reefs. Allopora 
 and Stylaster occur off the N orwegian coast. The woodcuts illus- 
 trating the structure of this group are borrowed from Mr Moseley 's 
 Notes of a Naturalist on the " Challenger." 
 
 The nearest allies of the Hydrocorallince are such polymorphic 
 Gymnoblastea as Hydraclinia (fig. 39) ; the definite division of labour 
 and the polymorphism in the former, together with their calci- 
 genous peculiarity, entitle them to rank as a distinct ord^er. 
 
74 
 
 HYDROZOA 
 
 Order 6. Siphonophora. These are Hydromedusee in 
 which hydriform persons alone ( Velella) or hydriform 
 persons and sterile medusiform persons are united, under 
 many special modifications of form, to constitute Floating 
 colonies of very definite shape and constitution. In 
 addition to these are developed medusiform sexual persons 
 which usually are spurosacs and 
 only exceptionally attain full de- 
 velopment so as to be liberated 
 from the colony as free-swimming 
 medusaj (Velella, as Chrysomitra; 
 Physalia, only liberating female 
 medusas). The medusiform persons, 
 where sufficiently developed, exhibit 
 the velum characteristic of Hydro- 
 mediisce; the larger mouth-bearing 
 nydriform persons, which are some- 
 times the only representatives of 
 their kind, care remarkable for 
 differentiation into four regions, 
 a proboscis, a stomach, a basal ring, 
 and a short stalk on which the 
 tingle tentacle of great length is 
 situated (fig. 56, f). In the sub- 
 order Physophoridce (fig. 57, C) the 
 persons are united by a short or 
 long and spiral stem, terminated 
 at one end by a flask-like air-sac 
 (pneumatocyst); below the air-sac a Flo " 4 _ Portlon of thecoral . 
 biserial or multiserial range of swim- ium of Astyius 
 ming-bells (nectocalyces = medusae 
 with suppression of manubrium, 
 tentacles, and sense-organs) are 
 placed. Covering pieces (hydro- 
 phyllia, reduced medusas) and dactylozooids are affixed 
 to the succeeding region of the stem, and alternate in 
 definite order with the mouth-bearing hydriform persons 
 (polyps or nutritive persons) and generative medusiform 
 persons. In the suVorder Physalidte the stem is con- 
 verted into an air-sac, enormously enlarged, and the necto- 
 
 (onc of the StylasleriJa?), 
 showing cyclosystems placed 
 at intervals on the branches, 
 each with a central gastro- 
 pdre and zone of slit-like dac- 
 tylopores. (After Moseley.) 
 
 FIG. 55. Diagrams illustrating the successive stages in the development of the 
 cyclosystems of the Stylasteridtt. 1, Sporadopora dichotoma. 2, 3, Allopora 
 nobitis.-.4,Alloporaprofunda, 5, Allopora miniacea. G, Astyius subviridis. 
 7, Distichopora coccinea. s, style ; dp, dactylopore ; ffp, gastropore ; 6, in fig. 6. 
 inner horseshoe-shaped mouth of gastropore. (After Moseley.) 
 
 calyces and hydrophyllia are absent. In the sub-order 
 Calycoplwridce the air-sac is not developed, the nectocalyces 
 are in a biserial group, or reduced to two or to one. 
 Dactylozooids are wanting. The modified persons (append- 
 ages, Huxley) arise from the stem in groups, and can be 
 withdrawn into the cavity of a swimming-bell (fig. 57, B). 
 
 Each group consists of a nutritive person, with long ten- 
 tacle, of generative medusoids, and usually also an umbrella- 
 shaped or funnel-like covering piece. The latter separate 
 in some Diphyida; and lead an independent life as 
 Eudoxicc. 
 
 In the suborder Discoidce the stem is converted into a 
 flattened disi; with a system of canalicular cavities. Above 
 this lies the air sac, a flattened reservoir of cartilaginous 
 consistence. The hydriform persons depend from the disc, 
 centrally a large nutritive person surrounded by smaller 
 similar persons carrying at their bases the generative 
 medusoids ; near the edge of the disc are dactylozooids. 
 The medusoids develop into complete medusiform persons, 
 and develop the genital products after liberation from the 
 colony, when they are known as Chrysomitra. 
 
 FIG. 56. Diagram showing possible modifications of medusiform and hydri- 
 form persons of a colony of Siphonophora. n, pneumatocyst; 1-, necto. 
 calyces (swimming bells); /. hydiophyllium (covering-piece); f, generative, 
 medusifonn person ; y, dactylozooid with attached tentacle, h ; e, nutritive 
 hydriform person, with branched grappling tentacle, /; m, stem. The thick 
 black line represents endoderm, the thinner line ectoderm. (After Allman.) 
 
 The Siphonophora alone, amongst the colonies formed by Hydrozoa, 
 exhibit a high degree of division of labour and consequent individua- 
 tion. The mode of origin of such colonies lias been discussed above. 
 The locomotive habit, as contrasted with the sessile habit of other 
 colonies, is no doubt correlated with the sharply defined individuality 
 which they attain (compare Cristatdla among Polyzoa). Velella 
 and Physalia are occasionally seen on the southern and western 
 shores of England, but as a rule the Siphonophora are met with only 
 in the open ocean and in the Mediterranean. By some authorities 
 the SiphonopJtora are assigned a distinct position among the Hydro- 
 zoa, side by side with the Hydromcdusce nnd Scyphomcdusce ; their 
 interpretation as floating colonies of Hydromcdusa:, an interpre- 
 tation necessitated by the structure of their medusiform persons, 
 forbids their separation from that group. 
 
 FOSSIL HYDROZOA. The researches of Moseley have neces- 
 sitated a redistribution of the group of Anthozoa known as 
 the Tabulata. Among these appear to be a few Hydro- 
 coralliitie, which occur in the fossil state. The Palaeozoic 
 forms known as graptolites are by some authors assigned 
 to the Hydrozoa, but the grounds for placing them in this 
 position are very slight, owing to the imperfect nature of 
 the remains. A discussion of the small amount of structure 
 which they present would be out of place here. 
 
 Remarkable Scyphomediisui have been obtained from the 
 Soleuhofen slates (Jurassic); excepting these, no noteworthy 
 extinct Jfydrozoa are known (see Haeckel in Zeitsch. wiss. 
 
HYDROZOA 
 
 75 
 
 Zool., vols. xv., xix., and Jenaische Zeitsch,, vol. viii., 
 1874). 
 
 Relationship of the Ctenophora to the Hydrozoa. The 
 remarkable medusa-form recently described by Haeckel 
 (Sitzungtber. Jenaifche Gesellseh., 1878) as Ctenaria Cteno- 
 phora, and classed by him amongst the Anthomedusce, seems 
 to furnish a very direct transition from the structure of a 
 medusa to that of such a ctenophor as Cydippe (Pletiro- 
 
 FIG 57 Floating colonies of Siphoxophom. A, Diph 
 
 B, A 
 
 group of appendages from the tm of the same Diphfa.' C, Pkftapliora 
 hydrottatica. D, Separate nectocalyi of the same. , Cluster of female 
 iporosacs (aborted medusae 1 of Agalma tartii. a, stem or axis of the colony; 
 tf, pnenmatocyst (air-bladder); m, nectocalyx; c. sub-urn brellar cavity of 
 nectocalyx ; r, radiating canals of the umbrella of the nectocalyi : o. orifice 
 formed by the margin of the umbrella; /, hydrophyllhi in B. dactylozooids in 
 C; n. stomach; t, tentacles; g, sporosacs, (From Geeenbanr.) 
 
 brachia). The woodcut and appended explanation (fig. 58) 
 copied from Haeckel's memoir will render the relations 
 of the two forms clear. Ctenaria has the margin of its 
 
 disc narrowed so as to give the organism a spherical form. 
 The approximated margins bound an orifice leading to the 
 sub-umbrella space. This orifice corresponds to the so- 
 called mouth of a Cydippe. Further, Ctenaria has two, 
 and only two, long-fringed tentacles, like those of Cydippe, 
 and each springing from a pocket as in that genus, and 
 on the surface of its spheroidal umbrella eight rows of 
 differentiated ectodermal cells, which though not ciliated 
 
 Fio. 58 Ctemaria Cte*ophora (Haeckel), one of the Anlhomfduia-. connecting 
 that croup with the Ctnophora. A. lateral view of the entire medusa ; B, two 
 horizontal views, that to the left representing the surface of the aboral hemi- 
 sphere, that to the right a section passing nearly equatorial!}-, a. the eight 
 (ciliated?) rows of thread-cells, adradial in position, and corresponding to the 
 eight ctenophoral zones of PltvrobraeMa; 6, jelly of the umbrella; c, circular 
 muscle of the sub-umbrella ; <f. longitudinal muscles of the sub-nmbrella; t, 
 stomachal dilatation of the enteric cavity ; /, the sixteen oral tentacles ; g, the 
 four perradial generative glands in the stomach wall (mannbrium); h. the 
 four perradial primary radiating canals; '. the eight adradial bifurcations of 
 the preceding : t. ring canal in the margin of the umbrella ; /. velum ; m, the 
 two Literal tentacle pooches ; n. the two lateral unilaterally fringed tentacles ; 
 o, the apical cavity (infnndibulum) above the stomach. The canal aialiM. 
 with its four primary and eight secondary rami agrees in Clnaria and Ptnro- 
 Irarhia. The mouth of the latter is homologous with the margin of the 
 umbrella of the former. The mouth of Ctt*a:-ia is homologous with the 
 junction of the so-called funnel of Pltttretrathia with its so-called digestive 
 cavity. This last is the homologue of the sub-umbrellar cavity of Ctenaria. 
 The apical opening or openings of the funnel of Ctexophora is paralleled .by 
 the stalk canal of medusa 1 , whilst the agreement between the tentacles anil 
 their pouches in Cttnaria and Ptewbraclria it complete. 
 
 correspond closely in position with the eight ctenophoral 
 ambulacra of Cydippe. The disposition of the enteric canal- 
 system of Ctenaria is, as shown in the cut, also transitional 
 in the direction of Cydippe. Apart from the existence of 
 Ctenaria, the homologies suggested by Haeckel between 
 Hydromedusce and Ctenophora are such as to commend 
 themselves very stronglv to acceptance (E. E. L.) 
 
PL AN ARI AN S 
 
 (By Prof. Ludwig von Graf, University of Graz, Austria.) 
 
 name Planaria was first applied by O. F. Miiller 
 J_ in his Prodromus Zoologize Daniex (1776) to a 
 group of worms, inhabitants of fresh and salt water, 
 characterized, so far as was then known, by a flattened 
 leaf-like form. Ehrenberg in 1831 changed this name to 
 Tvrbellaria on account of the cilia with which the body is 
 furnished, by means of which the worms create a whiry 
 pool in the surrounding water. The extent of this 
 group was subsequently more restricted, and at present 
 the name Tvrbellaria is applied to all those (mainly 
 free-swimming) Platyhelminths whose body is clothed exter- 
 nally with a ciliated epidermis (fig. 9), and which possess 
 a mouth and (with the exception of one division) an 
 alimentary canal, but are without an anus. The Tur- 
 bellarians, excluding the NEMEETINES (q.v.), which until 
 recently were classed with them, form an order of the 
 class Platyhelminthts, and the old name Planaria is now 
 confined to a group of the freshwater representatives of 
 this order. 
 
 Size and External Characters. Many forms of the 
 Turbellarians are so minute as to be hardly visible with 
 the naked eye, while others attain to a length of several 
 inches, and a land Planarian of no less than 9 inches in 
 length has been described by Moseley. The freshwater 
 forms are generally small, the largest representatives 
 of the order being marine or terrestrial. The smaller 
 species are mostly cylindrical, or convex dorsally and 
 flat ventrally ; the anterior extremity is commonly trun- 
 cated and the posterior extremity pointed (fig. 1, o, I). 
 The larger aquatic forms are thinner in proportion to 
 the increasing surface of the body, so that they come to 
 resemble thin leaf-like lamellae (d), while the large land 
 I'lanarians instead of increasing in superficies grow in 
 length (e and /), so that they may be best compared to 
 leeches. The larger aquatic forms are frequently provided 
 with tentacles in the shape of paired finger-like processes 
 or ear-like folds of the anterior part of the body (d and 
 g) ; sometimes the tentacles are papillary outgrowths of 
 the dorsal surface ; the land Planarians are often to be 
 distinguished by a crescent-shaped area at the fore end 
 of the body, which is separated off from the rest (/). 
 In many cases the whole dorsal surface is beset with 
 papillae (d). The aperture of the mouth varies greatly 
 in its position ; sometimes it is situated at the anterior 
 extremity, sometimes in the middle of the ventral surface 
 of the body, occasionally quite close to the posterior ex- 
 tremity ; the single common or distinct male and female 
 generative apertures are also situated upon the ventral 
 surface of the body, and the former in rare cases open in 
 common with the mouth ; the genital apertures always lie 
 behind the mouth. Many Turbellarians have a sucker 
 which serves to attach the animal to surrounding objects, 
 or to another individual during copulation. 
 
 InUgument. The integument is composed of a single 
 layer of ciliated epithelium ; between the cilia there are 
 often long flagella and stiff tactile hairs and even (in a 
 single instance) chitinous spines; these latter must be 
 regarded as local thicken- 
 ings of the firm cuticle 
 which covers the epi- 
 dermic cells. The epi- 
 dermic cells are flat or 
 columnar, and are united 
 to each other by smooth 
 opposed margins or by 
 denticulate processes 
 which fit into similar 
 processes in the adjacent 
 cells (fig. 2). Sometimes 
 the epidermic cells are 
 separated by an inter- 
 stitial nucleated tissue. 
 The structure and func- 
 tions of the cells of the 
 epidermis differ, and four 
 varieties are to be found : 
 (a) indifferent ciliated 
 cells ; (b) cells containing 
 certain definite structures 
 (rhabdites, nematocysts) ; 
 (c) gland cells ; and (d) 
 glutinous cells (Kleb- 
 zellen). The rhabdites 
 are refracting homogene- 
 ous rod-like bodies, of a 
 firm consistency, which 
 are met with in most 
 Turbellaria, and often 
 fill all the cells of the epidermis ; they are not always found 
 entirely within the cells, but the extremity often projects 
 freely on to the exterior of the body. They are readily 
 extruded from the cells by pressure, and are often found in 
 great abundance in the mucus secreted by the glandular 
 cells (many Turbellarians, like snails, deposit threads of 
 mucus along their track) ; in this case the epidermic cells 
 become perforated like a sieve. In many Turbellarians the 
 rhabdites are chiefly massed in the anterior part of the 
 body ; frequently there are several varieties of rhabdites 
 in one and the same species, some being pointed at 
 both ends, others cylindrical with truncated extremities. 
 These structures are either formed directly in the ordinary 
 epidermis cells as a kind of secreted product of the cell, or 
 in special formative cells which lie beneath the integument 
 and are connected with the epidermis cells by protoplasmic 
 filaments, by means of which the rhabdites reach the surface 
 of the body. These cells must be regarded as epidermic 
 
 Fie. 1 
 
 . . , Canrolula paradoia, Oe.; b. Vor- 
 tex riritlit. M. Sch.; c, tloncttu fiaaa, 
 Gff. ; d, Thytemozoon brothii, Gr., with 
 elevated anterior extremity (after Job. 
 Schmidt); e, Rhynchodenxu ttrrettrit, O. 
 F. Multer (after Kennel); /, Bi folium 
 caret, Mos. (after Moseley); g, Poiyeetit 
 eomuta, O. Sch., attached by the pharynx 
 (pA)to a dead worm (after Johnson). All 
 the figure* of natural size, and viewed 
 from the dorsal surface. 
 
78 
 
 PLANARIANS 
 
 cells which have become disconnected with the epidermis 
 itself, and wandered into the subjacent parenchyma. The 
 function of the rhabdites seems to be to support the 
 tactile sense. In rare instances nematocysts are present 
 which in structure and development entirely resemble 
 those of the Ccelentera (see vol. xii. p. 550). Very com- 
 monly structures known as pseudo-rhabdites are present; 
 these have a rod-like form, but instead of being homo- 
 geneous are finely granular ; they are an intermediate step 
 between the rhabdites proper and a granulated secretion 
 occasionally thrown off by the gland cells. The unicellu- 
 lar glands are either situated among the epidermic cells or 
 in the parenchyma, in which case they are connected with 
 the exterior only by the excretory duct. A peculiar modi- 
 fication of the epidermic cells are the so-called " glutinous 
 cells," which occur on the ventral surface or at the hinder 
 end of the body of many Turbellarians, and compensate 
 for the suckers; the surface of these cells is furnished 
 with numerous minute processes by means of which and a 
 sticky secretion the animals can attach themselves to sur- 
 rounding objects. Sometimes the epidermic cells contain 
 calcareous concretions, and very commonly pigment is 
 found either in the cells themselves or within the inter- 
 stitial tissue. The colours of Turbellarians are, however, 
 not always due to the pigment of the epidermis but to 
 pigment contained in the parenchyma. Beneath the 
 epidermis is a basement membrane (fig. 2, bm) which is in 
 
 bm. 
 
 Im 
 
 FIG. 2. Integument of Maoitoma lingua, O. Sch. On the right hand is the 
 epidermis (z) with perforations (0 through which the rhabdites (st) project. 
 Beneath this the basement membrane (6m), and beneath this again the muscular 
 layers consisting of circular (rm), diagonal (sm), and longitudinal (/m) fibres. 
 
 some cases very delicate and structureless, and in other 
 cases much thicker and enclosing branched cells ; this 
 membrane is attached more firmly to the subjacent tissue 
 than to the epidermis. Since this tissue is the strongest in 
 the body, and serves as a surface of attachment for the 
 muscles, it has been termed by Lang a skeletal membrane. 
 The third section of the integument is formed by the 
 muscular layers. These form a continuous covering to 
 the rest of the body, but their arrangement and thickness 
 are very different in different forms. In the smaller species 
 (Bhabdocoelida) there are two layers, an outer circular and 
 an inner longitudinal, only in a few cases the circular layer 
 is external to the longitudinal ; sometimes there are three 
 distinct layers, as in fig 2, where a diagonal layer is inter- 
 posed. The larger forms (Dendrocoslida) have a much 
 more complicated muscular system : in the most differen- 
 tiated forms there are six separate layers (two circular, 
 two diagonal, and two longitudinal), which are, however, 
 always less developed upon the dorsal than upon the 
 ventral surface in that the thickest layer of the ventral 
 surface (the innermost longitudinal) is absent or very 
 feebly developed upon the dorsal side. Besides the 
 
 integumentary muscular system, there are also found dorso- 
 ventral muscular bands which traverse the whole body 
 from the dorsal to the ventral basement membrane, being 
 branched at both extremities, and the special muscles of 
 the pharynx, genital organs, and suckers. 
 
 The perivisceral cavity, bounded by the integument and 
 traversed by the dorso-ventral muscles, contains the 
 organs of the body alimentary canal, excretory system, 
 nervous system, and genital glands. The space left 
 between these organs is filled with parenchyma ; the latter 
 varies much in appearance and is very difficult to study. 
 Generally it consists of a network of fibres and trabeculae, 
 which contain nuclei, and between which is a system of 
 cavities filled during life with the perivisceral fluid. These 
 cavities are generally but few in number and vary with 
 the stronger or feebler development of the reticulum ; 
 they occasionally contain free cells. 
 
 Alimentary Canal. All Turbellarians are furnished 
 with a mouth, which, as there is no anus, serves both 
 to take in nutriment and expel the undigested remains 
 of food. The alimentary canal consists of a muscular 
 pharynx and an intestine. The pharynx (figs 3, 5 to 8, pK) 
 is cylindrical in form, rather complicated in structure, and 
 surrounded by a muscular sheath, which opens on to the 
 exterior by the mouth (m). Often the pharynx consists 
 merely of a circular fold lying within the pharyngeal 
 pouch (fig. 8) ; it can be protruded through the mouth 
 and acts like a sucker, so that the animal can fasten itself 
 upon its prey and draw it into the intestine by suction. 
 At the junction of the pharynx with the intestine open the 
 salivary glands, which are frequently large and well- 
 developed (fig. 5, *). The intestine (i) has a very 
 characteristic form in the different sections, and has long 
 served to divide the Turlellaria into two groups : (1) 
 Rhabdocoelida, with a straight unbranched intestine (figs. 5, 
 6), and (2) Dendroccelida, with a branched intestine (figs. 7, 
 8). In the latter group Lang has recently called attention 
 to further differences that exist in the form of the intestine : 
 in the Tricladida (fig. 7) there is no central " stomach," 
 but three equally-sized intestinal branches (which have 
 secondary ramifications) unite together to open into the 
 pharynx ; in the second group, the Polycladida (fig. 8), 
 there is a median stomach (st), from which numerous 
 intestinal branches arise ; this stomach communicates 
 directly with the pharynx ; the branches of the intestine 
 are much ramified and often form an anastomosing net- 
 work. The epithelium of the intestine is a single layer of 
 cells generally not ciliated, capable of protruding amreboid 
 processes by which the food is absorbed ; the digestion of 
 these animals is intracellular. Sometimes a muscular 
 coat surrounds the intestine, the lumen of which is thus 
 capable of being totally or partially contracted. To the 
 above-mentioned divisions of the group, distinguished 
 from each other by the varying form of the alimentary 
 tract, another has been added, viz., the Accela (Ulianin), 
 which are characterized by the entire absence of any 
 intestine. In these forms (fig. 4) the mouth leads directly 
 into the parenchyma of the body by a short tube which is 
 merely an invagination of the integument ; the paren- 
 chyma is a syncytium, consisting of a soft protoplasmic 
 mass with scattered nuclei, which represents the elements 
 of the intestine and the body parenchyma (ento- and 
 mesoderm) completely fused and without any traces of 
 differentiation. This fact, as well as the disappearance of 
 a nervous and excretory system, reduces the Accela to the 
 lowest position not only among the Tiirbellaria, but among 
 the whole group of the Verities. 
 
 Excretory System.- The excretory system of the Turbel- 
 larians is quite similar to that of the Trematodes and 
 Cestoids ; it consists of (1) the main trunks with their 
 
PLANARIANS 
 
 79 
 
 external aperture, (2) the secondary branches of these, and 
 (3) the excretory cells with the fine tubules leading from 
 them. Karely is there but a single main excretory trunk 
 present opening at the hinder end of the body (Steno- 
 stoma); generally there are a pair of such trunks which 
 open in common at the hinder end of the body, or 
 separately (most Jtkabdoccela), or by the mouth (fig. 3). 
 In the Tridadida there are two or 
 four lateral trunks present which 
 open by a number of pores arranged 
 in pairs upon the dorsal surface of 
 the body ; the same appears to be 
 the case in the Polycladida. The 
 main trunks of the excretory sys- 
 tem are generally much twisted in 
 their course, and anastomose with 
 each other ; they receive the fine 
 tubules either directly or, as in the 
 Rhabdocnla, there is a network of 
 secondary tubules interposed. The 
 excretory cells are pear-shaped ; 
 they are branched and furnished 
 with a nucleus and a large vacuole 
 which is directly continuous with 
 the lumen of the tubule ; from the 
 boundary wall of the vacuole springs 
 a single flagellum, which depends 
 into the lumen of the tubule and is 
 capable of active movement. Lang 
 discovered in a marine form of the 
 Tridadida (Gunda) similar vacuo- FIG. s.-siain trunks of the ex- 
 
 ... ., v . , '. . cretory system of MesotUma 
 
 lated cells With a Single flagellum ehrenberyii, 0. Sch. Open 
 
 among the epithelial cells of the 
 intestine, and came to the conclu- 
 sion that the excretory cells were on that account derived 
 from the epithelium of the intestine. The movements of 
 the excretory fluid towards the external pore are directed 
 by this flagellum as well as by cilia developed upon the 
 walls of the fine tubules ; the motion of all these cilia is 
 such as to drive the contents of the tubules towards the 
 excretory pore. The main trunks of the excretory system 
 are either sparsely (Tridadida according to Jijima) or com- 
 pletely (Polycladida according to Lang) lined with cilia. 
 
 Nervous System. The central organ of the nervous 
 system, the brain (en), is a double ganglion at the anterior 
 end of the body, and has been noticed in all the known 
 forms with the exception of the Acosla. It is situated in 
 front of or above the pharynx ; in those species in which 
 a process of the intestine extends beyond the region of the 
 brain (cf. figs. 7 and 8 viewed from the ventral surface) it is 
 placed below this. In such cases there is sometimes a com- 
 missure encircling the prolongations of the intestine. Each 
 of the two ganglia gives ofi a strong longitudinal nerve 
 cord (figs. 5-8, In) from which arise branches going to the 
 various organs of the body. The structure of the nervous 
 system is somewhat different in the Rhabdoccela, Trida- 
 dida, and Polycladida. In the first group (figs. 5, 6) the 
 two longitudinal cords and their branches are the most 
 feebly developed, and there is but rarely (Mesostoma, 
 Monotus) a transverse commissure uniting the longitudinal 
 cords. These cords are very large in the Tridadida, 
 where the brain is to be regarded as a simple thickening of 
 them ; in this group there are numerous transverse com- 
 missures between the longitudinal nerve cords (fig. 7), and 
 the nerves arising from them and passing to the periphery 
 form a subcutaneous nerve plexus within the muscular 
 coat. Lang has observed a similar nerve plexus in the 
 Polycladida, the central nervous system of which differs 
 from that of the Tridadida in that a number of stout 
 nerve cords radiate outwards from the brain as well as the 
 
 two longitudinal cords; they are all united together by 
 m 
 
 .y- 
 
 Fig. 4. Fig. 5. 
 
 Fro. 4. Plan of an Acoelous Turbellarlan. , eye; m, mouth; of, otolith; or, 
 ovary ; p, digesting parenchyma ; (, testicular follicles ; n, vesicula seminalis 
 $ , male organ of copulation ; j 9 .common sexual aperture. 
 
 FIG. 5. Plan of a Rhabdocoelous Turbellarian. frc, Imrsa copulatrix ; en, brain ; 
 e, eye ; g, germarium ; t, intestine ; In, longitudinal nerve trunk ; m, mouth ; 
 ph, pharynx ; rt, receptacnlum seminis ; s, salivary gland ; I, testis ; a, uterus 
 (containing an egg); r, yelk gland t , vesicula seminalis; g, chitinous 
 copulatory organ; $ 9 > common sexual aperture; be, bursa copulatrtx. 
 
 numerous commissures, and a network is thus formed 
 which extends throughout the body. 
 
 fef 
 
 Fig. 6. Fig. 7. 
 
 FIG. 6. Plan of an Alloiocoelous Tnrbcllarian. Lettering as in fig. 5. 
 FIG. 7. Plan of a Tricladid. f,, anterior, and t'j, i,, paired posterior branches of 
 
 intestine ; od, oviduct; te, tentacle ; r<J, vas deferens ; j. male, and $ , female 
 
 copulatory organ. Other letters as in fig. 5. 
 
 Sense Organs. These are represented by tactile organs, 
 
80 ' 
 
 PLANARIANS 
 
 auditory organs (otoliths), and eyes. The whole surface of 
 the body is very sensitive and (e.g., in the Polydadidd) con- 
 tains cells which end in tufts of fine hairs, so that certain 
 regions thus become specially sensitive and serve as tactile 
 organs. The anterior pointed extremity of the body in 
 the Rhabdocmla is characterized by an abundant develop- 
 ment of rhabdites and tactile hairs, and thus becomes a 
 special tactile organ ; in other cases this region of the body 
 is transformed into a conical tactile proboscis which can be 
 retracted into a sheath (Proboscida). In the freshwater 
 Tricladida the anterior margin of the head is richly inner- 
 vated, and is beset with a special row of tactile cells which 
 contain no rhabdites ; in the terrestrial forms of the same 
 family (Bipalium) Moseley has described a row of papillae 
 along the crescent-shaped anterior extremity which can be 
 
 ov- 
 
 FIG. 8. Plan of a Polycladid. en, brain ; i, intestinal branches ; IL anterior 
 unpaired intestinal branch; In, longitudinal nerve cord; m, mouth; od, 
 oviduct; or, ovarian follicle ; ph, pharynx ; ph t , pharyngeal pout-h ; f, stomach; 
 (, testicular follicle; w, uterus; t?</, vas deferens; $, male copulatory organ, 
 with the male aperture behind; 9 female copulatory organ, with the female 
 aperture before it. The eyes are omitted. 
 
 extended and form tactile organs ; between the papillas are 
 peculiar ciliated grooves connected with nerves. In the 
 Polycladida there are tactile cells with stiff hair-like pro- 
 cesses on the summit of the dorsal papillae and the various 
 tentacular structures ; the tentacles in this family also 
 serve to support the eyes. 
 
 The majority of the Turbellarians possess eyes ; the 
 R/utbdoccelida commonly have two or four, as also have the 
 Tricladida ; the latter, however, are in some instances 
 furnished with a greater number arranged in a continuous 
 row round the anterior end of the body ; in the Poly- 
 cladida there are from fourteen to several hundred eyes 
 arranged in two symmetrical groups round the brain or 
 
 scattered over the whole of the anterior margin of the 
 body and upon the tentacles. The eyes are always situ 
 ated beneath the integument within the parenchyma, 
 sometimes directly upon the brain or connected with it by 
 special optic nerves. In its simplest form the eye is a 
 pigmented spot with or without a refractory lens-like 
 body ; the more complicated eyes consist of a pigmented 
 sheath containing a number of refracting rods which are 
 connected at their outer extremity with a series of retinal 
 cells, one to each rod ; the retinal cells are prolonged into 
 a nerve thread running to the brain ; the arrangement of 
 the visual elements is therefore precisely the same as in the 
 vertebrate eye. Of great interest is the fact that in the 
 Polycladida the number of eyes increases with the growth 
 of the animal, and Lang has shown that the eyes increase 
 in number by actual division. On the other hand Carriere 
 has discovered by experimenting with certain freshwater 
 Tricladida that the compound eyes (those containing a 
 number of rods) are formed by the coalescence of several 
 simple eyes. Only a single eye is found in the Monotida, 
 which has the form of a simple pigment spot in front of 
 the otolith. 
 
 Auditory organs are found in the shape of vesicles filled 
 with fluid and containing circular lenticular or spindle- 
 shaped otoliths formed of carbonate of lime. Otolithic 
 vesicles of this kind are found in many Rhabdoccelida 
 (Accela, Monotida, fig. 4, ot) embedded in a depression on 
 the anterior surface of the brain. In the Dendrocoelida 
 these organs are but rarely present. 
 
 As a sensory organ of unknown function must be men- 
 tioned the paired lateral ciliated grooves which are met 
 with on either side of the brain in many Rhabdoccela (fig. 
 9, c) ; they are also found commonly in NEMERTINES (q. v.), 
 but are here more complicated in structure. 
 
 Reproductive Organs. With a few exceptions all the 
 Turbellarians are hermaphrodite, and reproduce themselves 
 sexually. Only among the Microstomida is there an 
 asexual as well as a sexual reproduction. The male and 
 female organs open to the exterior, either through a com- 
 mon cloaca (atrium yenitale) on the ventral surface (most 
 R/iabdoccelida and all Tricladida, figs. 4-7), or there are 
 separate male and female apertures. In this case the male 
 aperture is generally placed in front of the female aperture 
 (some Rhabdoccelida and all Polycladida, fig. 8), but 
 occasionally the positions are reversed (certain Rhabdo- 
 ccelida). The genital glands display a primitive condition 
 in being paired, though frequently the germarium (fig. 5, 
 y) of the Rhabdoccela, and occasionally also the testis, is 
 developed only upon one side of the body. 
 
 The structure of the female organs varies. In some 
 cases there are simple ovaries (ov in figs. 4, 8) in which 
 the ova originate and become fully mature without being 
 furnished witli the secretion of a second gland ; in other 
 cases there is a division into germarium (fig. 5-7, y) and 
 yelk gland (v ) ; the primordial ova or germs originate in 
 the former, and absorb the products of the yelk gland in 
 the atrium, where they become ready for fertilization. 
 An intermediate condition is seen in those forms where 
 there is but a simple gland present which produces germs 
 in its upper portion and yelk in the lower portion. The 
 ovaries are generally compact round or tubular glands 
 (fig. 4) ; sometimes they are formed of a number of pear- 
 shaped follicles (fig. 8) ; there is usually a simple or paired 
 uterus (u) which retains the ova for some time before 
 they are deposited ; sometimes, however, the ova undergo 
 their development within the uterus and are completely 
 developed before expulsion ; in some cases the egg-shell is 
 detached within the uterus so that the young are produced 
 alive. 
 
 In Turbellarians without a yelk gland the uterus is a 
 
PLANARIANS 
 
 81 
 
 simple widening of the oviduct (fig. 8) ; in those forms 
 which possess additional yelk glands the uterus is a simple 
 or paired diverticulum of the atrium genitale (figs. 5, 7). 
 The ova are either surrounded by a more or less hard 
 chitinous shell, or one shell contains a number of ova 
 ("cocoon" of Tricladida and many Polydadida). The 
 Polydadida deposit an egg-string which like that of the 
 Gastropoda consists of a number of eggs bound together 
 by a transparent albumen-like mass. Many Rhabdoccel 
 Turbellarians (e.g., Mesostoma ehrenbergii) produce two 
 sorts of ova, thin-shelled summer ova and thick-shelled 
 winter ova; the latter are capable of withstanding a 
 considerable amount of dessication, and are deposited in 
 the autumn. The accessory female organs of reproduction 
 are represented by bursae seminales, which receive the 
 semen during copulation and retain it until fertilization 
 is accomplished. A further division of labour is brought 
 about by the presence of two diverticula of the atrium 
 genitale, one of which serves as a bursa copulatrix (fig. 5, 
 be) and the other as a receptaculum seminis (rs) in the 
 same sense as the equivalent organs of insect?. In the 
 place of a special receptaculum seminis the efferent duct 
 of the ovary is often (Mesostomida) metamorphosed into a 
 chamber to contain the semen. In the Tricladida and 
 Polydadida the female efferent duct is often differentiated 
 into a muscular vagina which closely resembles the penis 
 (figs. 7, 8, ? ). 
 
 Finally, the female generative apparatus is furnished 
 with a number of glands which have been termed cement 
 glands, albuminiparous glands, and shell glands. 
 
 The male sexual glands (figs. 4-8, t) resemble the ovaries 
 in being either compact tubular (fig. 5) or follicular 
 (tigs. 4, 6, 7, 8) structures. The vasa deferentia (vd) are 
 often widened out into vesiculae seminales (tigs. 4, 6, vs) ; 
 or there are special vesiculae seminales present, formed by 
 a portion of the penis (fig. 5, vs). In the male organ of 
 copulation there is frequently found in addition to the 
 spermatozoa an accessory granulated secretion produced 
 by special glands, but of unknown function. 
 
 The muscular penis, especially in the Rhahdocoda, has 
 a number of chitinous spines and hooks which serve to 
 assist the animal in maintaining a firm hold during 
 copulation, but also in capturing and retaining its prey. 
 In Macrorhynchus helgolandicus, Gff., there is a peculiar 
 poison dart connected with the male copulatory organ 
 which only serves the latter purpose. Very remarkable 
 is the opening of the penis into the mouth cavity in 
 Stylostomum (Polydadida) and Prorhynchus (Rhabdocoela), 
 and also the existence of several (2-15) pairs of male 
 copulatory organs and genital apertures in certain Poly- 
 dadida. 
 
 The spermatozoa vary much in form, especially ia the 
 Rhabdocoslida, where frequently the species of one and the 
 same genus are distinguished by the different form of the 
 spermatozoa. Copulation in the Turbellarians is generally 
 reciprocal ; only in those cases where both summer and 
 winter ova (see above) are formed do the former arise from 
 self-fertilization ; the latter are the result of the copulation 
 of two individuals. The fertilization of the ova always 
 takes place in the atrium genitale. Many Turbellarians, 
 especially the Accela, display the phenomenon known as 
 " successive hermaphroditism," the male organs of an 
 individual attain to maturity first, and the female organs 
 become ripe subsequently. During copulation, therefore, 
 one individual is physiologically a male and the other a 
 female. 
 
 Asexual generation is met with only in the Microsto- 
 mida ; it takes the form of transverse division accompanied 
 by budding. The posterior third of the body becomes 
 separated off by a septum running from the gut to the 
 
 integument and an external furrow corresponding to this ; 
 this part of the body grows in length until it equals the 
 anterior portion. By further repetition of this double pro- 
 cedure of separation and equalization there, chains of 4, 
 then 8, 16, and 32 buds are formed, which remain attached 
 (tig. 9), and, although fresh mouth apertures (m, m", TO'") 
 have been formed, are still in communication by the 
 intestinal lumen ; this becomes closed before or after the 
 several buds break off from their connexion with each 
 other. Throughout the whole summer chains of zooids 
 are met with ; in autumn this asexual division probably 
 ceases to occur ; the several individuals become sexually 
 mature, separate from each other, and lay eggs which 
 
 Fig. 10. 
 
 Fig. 9. 
 
 FIG. 9. Uicrottoma lintare, Oe., undergoing division. There are 16 individuals, 
 8 with month apertures, snowing the Duds of the first (IB), second ('), third 
 (m"), and fourth (m'") generation. The fifth generation has not yet acquired 
 a month aperture, r, ciliated grooves ; e, eye spots ; i, intestine. 
 
 FIG. 10. Larva of Yungia aurantira, L. (Polycladida). with provisional ciliated 
 processes (after A. Lang). 
 
 remain quiescent during the winter and in the spring 
 develop into fresh individuals reproducing asexually. 
 
 Development. The study of the development of the 
 : Turbellarians is unfortunately not very far advanced, 
 ! particularly among the small Rhabdoccelida, which are 
 extremely difficult to investigate, and about which hardly 
 any developmental facts are known. The larger fresh- 
 water Tricladida and the Polydadida on the contrary have 
 been recently very fully investigated. The Rhabdocoela and 
 the Tricladida appear to develop directly without any meta- 
 morphosis, while a great part of the Polydadida undergo 
 a metamorphosis and pass through a larval condition, 
 during which they are furnished with provisional ciliated 
 processes (fig. 10) ; the Accela have also a free larval form ; 
 pelagic larvae with a coat of long cilia apparently belonging 
 to this group have been observed by Ulianin. The seg- 
 I mentation of the ovum is total, but unequal ; an epibolic 
 gastrula is formed and the aperture of invagination 
 becomes the permanent mouth of the adult. 
 
 Systematic Arrangement and Mode of Life. Order 
 Turbellaria. Platyhelminths with a ciliated integument, 
 a mouth and pharynx, but no anus ; with paired cerebral 
 ganglia and two lateral nerve cords ; sexual organs her- 
 maphrodite ; chiefly free-swimming. 
 
 Li 
 
82 
 
 PLANARIANS 
 
 Sub- order A. Rhabdocoelida. Of small size ; body cylin- 
 drical or depressed ; without an intestine, or with a simple 
 unbranched intestine ; the female genital glands always 
 compact, not follicular ; genital apertures single or distinct. 
 
 Tribe I. Accela (fig. 1, a). With a digestive paren- 
 chyma not differentiated into intestine and parenchyma 
 proper ; with no nervous system or excretory organs ; 
 sexual organs hermaphrodite, with follicular testes and 
 paired ovaries ; generally without a pharynx, but having 
 otoliths ; all the forms marine. Many quite flat, with 
 the lateral margins bent down towards the ventral surface 
 (Convoluta), frequently with brown or green parasitic algse 
 in the parenchyma. 
 
 Tribe II. Rhabdocoela (fig. 1, b). -Intestinal tract and 
 parenchyma separate ; nervous system and excretory 
 organs present ; with compact testes and female genera- 
 tive glands (ovaries or separated germarium and yelk 
 glands) ; with a complicated pharynx, but generally without 
 otolitha. Numerous forms, freshwater and marine ; the 
 genus Prorhynchus (two species) also in damp earth. The 
 Microstomida (fig. 9) propagate asexually. Freshwater 
 forms mostly belong to the families Mesostomida and 
 Vorticida, some of which contain green parasitic algae. 
 Marine forms include representatives of these two families 
 and of the Proboscida (with a tactile proboscis). Of the 
 family Vorticida, the genera Graffilla and Anoplodium are 
 parasitic, the former in Gastropods the latter in Echino- 
 derms (Holothurians). 
 
 Tribe III. Alloiocosla (fig. 1, c). Intestinal tract and 
 parenchyma separate ; nervous system and excretory 
 organs present ; with follicular testes and compact female 
 glands (as in the Rhabdocoela) ; pharynx similarly 
 developed as a shorter or longer sac. One family (Mono- 
 tida), with otoliths. All the species marine, with one 
 exception, Plagiostoma lemani, which lives in the deep 
 water of the Alpine lakes. 
 
 Sub-order B. Dendrocoslida. Large forms, with a 
 flattened body, branched intestine, follicular testes and 
 follicular yelk glands or ovaries ; without otoliths. 
 
 Tribe I. Tricladida. Body elongate ; intestine with 
 three main branches uniting to open into a cylindrical 
 retractile pharynx ; with follicular testes, two round 
 germariums, and numerous yelk follicles, with a single 
 sexual aperture. Planaria, Dendrocodum, Polycelis (fig. 
 1, g) are inhabitants of fresh water (with great power of 
 reproduction). Terrestrial forms (fig. 1, e, f) of leech-like 
 shape, especially met with in the tropics (only two 
 European species Rhynchodemus terrestris and Geodesmus 
 bilineat'tts) ; marine forms Gunda (characterized by a 
 metameric structure), Bdellottra (external parasite of 
 Limulus). 
 
 Tribe II. Polycladida (fig. 1, d). Body leaf-like, thin, 
 and broad, with numerous branched or retiform intestinal 
 coaca which unite to form a central tube (stomach) ; with 
 follicular testes and follicular ovaries, with two separated 
 genital apertures, the male in front of the female ; without 
 (Acotylea) or with (Cotylea) a sucker situated behind the 
 female generative opening. All marine. 
 
 Literature. The most recent works, which also contain a full 
 account of what has gone before, are the following : Rhabdocoela. 
 L. v. Graff, Monographic der Turbellarien : 1. Rhabdocozlida, Leip- 
 sic, 1882, with 20 plates. Marine and Freshwater Tricladida. 
 A. Lang, " Der Bau von Gunda segmcntata und die Verwandtschaft 
 der Platyhelminthen mit Coslenteraten und Hirudineen," in Mitth. 
 Zool. Stat. Ncapel, vol. iii., 1881; El. Metschnikoff, "DieEm- 
 bryologie von P lanaria polychroa," in Zeitsehr. f. iviss. Zool., vol. 
 xxxviii., 1883; Isao Jijima, " Untersuchungen u'ber den Ban und 
 die Entwickelungsgeschichte der Susswasser-Dendrocoelen," in 
 Zeitsehr. f. wiss. Zool., vol. xl., 1884. .Land Planariaiis. H. N. 
 Moseley, "On the Anatomy and Histology of the Land Planariaiis 
 of Ceylon, with some Account of their Habits, and with a Descrip- 
 tion of Two New Species, and with Notes on the Anatomy of some 
 European Aquatic Species," in Phil. Trans. (London, 1874), and 
 "Notes on the Structure of several Forms of Land Planarians, 
 with a Description of Two New Genera and Several New Species, 
 and a List of all Species at present known," in Quart. Jour. Micr. 
 Sci., vol. xlvii., 1877 ; J. v. Kennel, "Die in Deutschland gefun- 
 denen Landplanarien Rhynchodemus terrestris und Geodesmus 
 bilineatus," in Arbeit. Zool.-Zootom. Instil. Wurzburg, v., 1879. 
 Polycladida. A. Lang, "Die Polycladen," in Fauna und Flora 
 des Golfes von Neapel, No. 11, 39 plates, Leipsic, 1884-85. 
 
 (L. v. G.) 
 
NEMEKTINES 
 
 (By A. A. W. Hvbreckt, Ph.D., LL.D., Professor of Zooloyy, University of Utrecht.) 
 
 "VTEMERTINES, or NEMEBTEANS (Nemertea), is the 
 _Ll name of a subdivision of worms, 1 characterized by 
 the ciliation of the skin, by the presence of a retractile 
 proboscis, by the simple arrangement of the generative 
 apparatus, and in certain cases by a peculiar pelagic larval 
 stage to which the name " pilidium " has been given. Many 
 of them are long thread-shaped or ribbon-shaped animals, 
 more or less cylindrical in transverse section. Even the 
 comparatively shortest species and genera can always be 
 termed elongate, the broadest and shortest of all being 
 the parasitic Malacobdella and the pelagic Pelagonemertes. 
 There are no exterior appendages of any kind. The colours 
 are often very bright and varied. They live in the sea, 
 some being common amongst the corals and algae, others 
 hiding in the muddy or sandy bottom, and secreting gelatin- 
 ous tubes which ensheath the body along its whole length. 
 Formerly, they were generally arranged amongst the 
 Platyelminthes as a suborder in the order of the Turbel- 
 lariaus, to which the name of Rhynchocaela was applied, the 
 other suborders being the Dendroccela and the Ehabdoccela. 
 With the advance of our knowledge of these lower 
 worms it has been found desirable to separate them 
 from the Turbellarians and to look upon the Nemertea as 
 a seperate phylum of Platyelminthes. Lately the interest 
 in their morphology has increased since it has been 
 advanced (6, 8) 2 that certain points in their organization 
 appear to indicate a remote degree of relationship to the 
 ancestral forms which must have preceded the Chordata (to 
 which the vertebrate animals also belong), and that this 
 relationship is closer than that which exists betweeu those 
 Protochordata and any other group of invertebrate animals. 
 CLASSIFICATION. The Nemertines are subdivided into 
 three suborders : Hoplonemertea, Schizonemertea, and 
 Palawnemertea (5). The (1) Hoplonemertea embrace all 
 the species with a stylet in the proboscis, and also 
 Malacobdella, which has an unarmed proboscis, but 
 which, by the details of its organization and its develop- 
 ment, must certainly be placed here (parasitism may 
 be the cause of its incipient degeneration). The special 
 characters of this suborder may be gathered from the 
 anatomical descriptions hereafter to be given. In those 
 species of which the embryology has been investigated 
 the development was direct The more common or more 
 important genera are Amphiporus (A. pulcher, British 
 coasts, Mediterranean ; A . gplendidvs, Atlantic), which is 
 comparatively short, Nemertes (X. gracilis, Atlantic and 
 Mediterranean; N. antonina, Mediterranean; N. echino- 
 dertna, Mediterranean), which is long and thread-like, 
 Tftrastemma, Drepo.nophorus (with more complicate arma- 
 ture in the proboscis), Akrostomum, Malacobdella. (2) In 
 the Schizonmertea all those genera and species are united 
 which have deep, longitudinal, lateral cephalic fissures. 
 The development of some (Linens) is characterized by the 
 
 1 Xemertes was a sea nymph, daughter of Xereus and Doris, 
 of the genera was named Semerles by Cuvier. 
 
 * These figures refer to the bibliography at page 88. 
 
 One 
 
 so-ualled larva of Desor, of others (Cerebratvlus) by the 
 curious and characteristic pilidium-larva. The principal 
 genera are Lineus (L. longissimus, Atlantic ; L. obscurus), 
 Cerebratvlus (C. marginatus, C. bilineatus, both Atlantic 
 and Mediterranean ; C. urticans, Mediterrauean ; C.fascio- 
 latus and auraiitiacus, C. hepaticus and dohrnii, Medi- 
 terranean ; C. macintoshii, Madeira), Langia (L. form>jsa), 
 Borlasia (B. elizabetkx). (3) Of the Palxonemertea the 
 most typical and most characteristic genera are Cari- 
 nella and Cephalothrix. They differ considerably both 
 
 jta. 
 
 Fig. 1. 
 
 Fig. 2. 
 
 FIG-. 1, 2. DUgiamsof the organs of a Xemertine, fig. 1 from below, fig. 2 from 
 above, m. mouth : Jir, intestinal diverticula ; a. anus ; or, ovaries ; n, neph- 
 ridia; Br, brain-lobes: tn, longitudinal nerve stems; j/r, proboscis; ps t pro- 
 boscidian sheath ; p.o.. opening for proboscis. 
 
 from Hoplo- and from Schizonemertines, and evidently 
 belong to a lower stage of differentiation. The genera 
 Polia (P. delineata and P. curia, Mediterranean) and Valen- 
 cinia are provisionally arranged in this order because, 
 though less primitive, they are not typical representatives 
 of the other two suborders. The development of these 
 species is not at all, or only very superficially, known. For 
 the further characters of the last two suborders see the 
 anatomical description below. 
 
 Another subdivision generally current is that into the 
 Enopla and the Anopla (14). This does not, however, 
 take into sufficient account the primitive and diverging 
 
84 
 
 NEMERTINES 
 
 P.O. 
 
 brain-lobes; JV, lateral nerves; 
 PS, proboscidian sheatli ; /';, 
 proboscis; P.O., exterior open- 
 ing through which the probos- 
 cis is everted. (Esopliapus iiml 
 mouth shown by dotted lines. 
 
 characters disclosed by the very important less highly 
 organized genera. 
 
 A.na- ANATOMY. (a) Proboscis and Proboscidian Sheath. The 
 
 tomy organ most characteristic of a Nemertine is without doubt 
 the proboscis. With very few exceptions (Malacobdella, 
 Akrostomum, where it has fused with the mouth to a single 
 exterior opening), there is a terminal opening (subterminal 
 in Valencinia) at the foremost tip of the body, out of which 
 the proboscis is seen shooting backwards and forwards, 
 sometimes with so much force that both its interior 
 attachments are severed and it is entirely expelled from 
 the body. It then often retains its vitality for a long 
 time, apparently crawling about as if it were itself a worm, 
 a phenomenon which is at least 
 partially explained by the extra- 
 ordinary development of nervous 
 tissue, equally distributed all 
 through the walls of the proboscis, 
 and either united (10) into nu- 
 merous longitudinal nerve-stems 
 (Drepanophorus, Amphiporus) or 
 spread out into a uniform and 
 comparatively thick layer (Cere- 
 bratulus, sp.). This very effective 
 and elaborate innervation, which 
 has been directly traced (6) to the 
 brain, whence strong nerves (gene- 
 rally two) enter the proboscis, 
 
 % ,. , l'io. 3. Anterior portion of the 
 
 renders it exceedingly probable body of a Nemertine. 
 that the most important functions 
 of the proboscis are of a sensi- 
 ferous, tactile nature, a supposi- 
 tion which is again strengthened 
 by the fact that amongst the Rhabdoccel Turbellarians an 
 organ which may be called the forerunner of the Nemertean 
 proboscis has been proved (3) to be the morphological 
 equivalent of the foremost tip of the body, which, as an 
 organ of delicate touch, has acquired the property of 
 folding inwards. In Nemertines the everted proboscis is 
 retracted in the same way 
 as the tip of a glove finger 
 would be if it were pulled 
 backwards by a thread 
 situated in the axis and 
 attached to the tip. The 
 comparison may be car- 
 ried still further. The 
 central thread just alluded 
 to is represented in the 
 Nemertean proboscis by 
 that portion which is 
 never everted, and the 
 tip of the glove by the 
 boundary between the 
 evertible and non-evert- FlGS 
 ible portion of the pro- 
 boscis a boundary which 
 in the Hoplonemertini is marked by the presence of a 
 pointed or serrated stylet. This stylet is thus situated 
 terminally when the proboscis has reached its maximum 
 eversion. It adds a decisively aggressive character to an 
 organ the original significance of which, as we have seen, 
 was tactile. This aggressive character has a different 
 aspect in several genera which are destitute of a central 
 stylet, but in which the surface that is turned outwards 
 upon eversion of the proboscis is largely provided with 
 nematocysts, sending the urticating rods of different sizes 
 in all directions. In others this surface is beset with 
 thick, glandular, adhesive papillae. 
 
 The comparison with the glove-finger is in so far 
 
 Fig. 4. 
 
 Fig. 5. 
 
 5. Proboscis with stylet, ''reserve" 
 sacs, and muscular bulb of a Iioplonemer- 
 tine. i'ig. 4 retracted ; fig. 5 everted. 
 
 insufficient as the greater portion of the non-evertible half 
 of the proboscis is also hollow and clothed by glandular 
 walls. Only at the very hindermost end does it pass into 
 the so-called retractor-muscle (fig. 2), which is attached to 
 the wall of the space (proboscidian sheath) in which the 
 proboscis moves about. This retractor-muscle, indeed, 
 serves to pull back with great rapidity the extruded 
 proboscis, and is aided in its action by the musculature of 
 the head. The extrusion itself depends entirely upon 
 contraction of the muscular walls of the space just 
 mentioned (proboscidian sheath). As it is (1) closed on 
 all sides, and (2) filled with a corpuscular fluid, the 
 contractions alluded to send this fluid to impinge against 
 the anterior portion, where the proboscis, floating in its 
 sheath, is attached with it to the muscular tissue of the 
 head (fig. 3). Partial extrusion lessening the resistance 
 in this region inevitably follows, and when further con- 
 tractions of the walls of the sheath ensue total extrusion 
 is the consequence. It is worthy of notice that in those 
 Nemertines which make a very free use of their proboscis, 
 and in which it is seen to be continually protruded and 
 retracted, the walls of the proboscidian sheath are enor- 
 mously muscular. On the other hand, they are much less 
 considerably or even insignificantly so in the genera that 
 are known to make a rather sparing use of their proboscis. 
 
 The proboscis, which is thus an eminently muscular 
 organ, is composed of two or three, sometimes powerful, 
 layers of muscles one of longitudinal and one or two of 
 circular fibres. In the posterior retractor the longitudinal 
 fibres become united into one bundle, which, as noticed 
 above, is inserted in the wall of the sheath. At the 
 circular insertion of the proboscis in front of the brain the 
 muscular fibres belonging to the anterior extremity of the 
 body and those connected with the proboscis are very 
 intimately interwoven, forming a strong attachment. 
 
 The proboscis broken off and expelled is generally 
 reproduced, the posterior ribbon-like end of this reproduced 
 portion again fusing with the walls of the 
 sheath (11). There is reason to suppose that, 
 when a wound is inflicted by the central 
 stylet, it is envenomed by the fluid secreted 
 in the posterior proboscidian region being at 
 the same time expelled. A reservoir, a duct, 
 and a muscular bulb in the region (fig. 4) FlG 6 _ Tlle ar . 
 where the stylet is attached serve for this pur- mature from 
 
 m.' *e r / ">e proboscis 
 
 pose. The significance of two or more (in O f Drepano- 
 Drepanophorus very numerous) small sacs con- P hor >">- 
 taining so-called " reserve " stylets resembling in shape that 
 of the central dart is insufficiently known. 
 
 The proboscidian sheath, which by its transverse con- 
 tractions serves to bring about eversion of the proboscis in 
 the way above traced, and the muscular walls of which 
 were similarly noticed, is attached to the musculature of 
 the head just in front of the ganglionic commissures 
 (fig. 3). In nearly all Nemertines it extends backwards 
 as far as the posterior extremity, just above the anus; in 
 Carinella it is limited to the anterior body-region. The 
 corpuscles floating in the fluid it contains are of definite 
 shape, and in Cerebratulus urticans they are deep red from 
 the presence of haemoglobin. Internally the muscular 
 layers are lined by an epithelium. In the posterior 
 portion this epithelium in certain Schizonemertea has a 
 more glandular appearance, and sometimes the interior 
 cavity is obliterated by cell-proliferation in this region. 
 Superiorly the sheath either closely adheres to the muscular 
 body-wall, with which it may even be partly interwoven, 
 or it hangs freely in the connective tissue which fills the 
 space between the intestine and the muscular body-wall. 
 
 (b) Cutaneous System. Externally in all species a layer 
 of ciliated cells forms the outer investment. In it are, 
 
NEMERTINES 
 
 85 
 
 circ.2. 
 
 FIGS. 7 
 
 Fig. 7. Fig. 8. Fig. 9. 
 
 7-9. The layers of the body-wall In Cariaella (fig. 7), the Hoplonemertea 
 (fig. 8), and the Schizonemertea (fig. 9). e, cellular tissue of the integument ; 
 Bm, basement membrane ; circ. 1, outer circular, and long, longitudinal layer 
 of muscular tissue ; tin. 2, long. 1, additional circular and longitudinal layers 
 of the same ; nl, nervous layer. 
 
 (fig. 7). The second is common to all the Schizonemer- 
 tines as well as to Polio, and Valencinia, and also compre- 
 hends three layers, of which, however, two are longitudinal, 
 viz., the external and the internal one, there being a strong 
 circular layer between tnem (fig. 9). To the third type 
 all the Hoplonemertea correspond ; their muscular layers 
 are only two, an external circular and an internal longi- 
 tudinal one (fig. 8). 
 
 The Schizonemertea thus appear to have developed an 
 extra layer of longitudinal fibres internally to those which 
 they inherited from more primitive ancestors, whereas the 
 Hoptonemertea are no longer in possession of the internal 
 circular layer, but have on the contrary largely developed 
 the external circular one, which has dwindled away in 
 the Schizonemertea. In only one instance has the present 
 writer met with a thin exterior circular layer in a very 
 large specimen of Cerebratulus ; younger specimens of the 
 same species did not show it. It is noticeable that Kefer- 
 
 moreover, enclosed unicellular glands pouring their highly 
 refracting contents, of a more or less rod-like shape, directly 
 to the exterior. They appear to be the principal source 
 of the mucus these animals secrete. In Schizonemertines 
 these elements are separated by a thin homogeneous base- 
 ment membrane (fig. 8) from the following, that is, from 
 a layer in which longitudinal muscular fibres are largely 
 intermixed with tortuous glands, which by reason of their 
 deeper situation communicate with the exterior by a much 
 longer and generally very narrow duct. The pigment is 
 also principally localized in this layer, although sometimes 
 it is present even deeper down within the musculature. 
 The passage from this tegumentary layer to the subjacent : 
 longitudinal muscular one is gradual, no membrane ' 
 separating them. In Carinella, Cephalothrix, Polia, and 
 the Hoplonemertines the two tegumentary layers with 
 their different glandular elements are fused into one ; a 
 thick layer of connective tissue is situated beneath them 
 (instead of between them) and keeps the entire cutaneous 
 system more definitely separate from the muscular (figs. 
 7,8). 
 
 (c) Musculature and Connective Tissue. The muscular 
 layers by which the body-wall is constituted have been 
 very differently and to some extent confusingly described 
 by the successive authors on Nemertean anatomy. There 
 is sufficient reason for this confusion. The fact is that not 
 only have the larger subdivisions a different arrangement 
 and even number of the muscular layers, but even within 
 the same genus, nay, in the same species, well-marked 
 differences occur. Increase in size appears sometimes to 
 be accompanied by the development of a new layer of 
 fibres, whereas a difference in the method of preparation 
 may give to a layer which appeared homogeneous in one 
 specimen a decidedly fibrous aspect in another. Never- 
 theless there are three principal types under which the 
 different modifications can be arranged. One of them is 
 found in the two most primitively organized genera, 
 Carinella and Cephalothrix, i.e., an outer circular, a longi- 
 tudinal, and an inner circular layer of muscular fibres 
 
 stein (9) also observed four layers similarly arranged in 
 one of the specimens of Cerebratidui which he investi- 
 gated. The situation of the lateral nerve-stems in the 
 different genera with respect to the muscular layers lends 
 definite support to the interpretation of their homologies 
 here given. 
 
 In Carinella, Cephalothrix, and Polia, as well as in all 
 Hoplonemertines, the basement membrane of the skin 
 already above alluded to is particularly strong and immedi- 
 ately applied upon the muscular layers. In the Schizo- 
 nemertines there is a layer in which the cutaneous elements 
 are largely represented below the thin basement membrane 
 (fig. 8), between it and the bulk of the outer longitudinal 
 muscles. The difference in the appearance of the base- 
 ment membrane- sometimes wholly homogeneous, some- 
 times eminently fibrillar can more especially be observed 
 in differently preserved specimens of the genus Polia. 
 
 The connective tissue of the integument and basement 
 membrane imperceptibly merges into that which surrounds 
 the muscular bundles as they are united into denser and 
 definite layers, and this is especially marked in those forms 
 (Akrostomum) where the density of the muscular body- 
 wall has considerably diminished, and the connective tissue 
 has thus become much more prominent. It can then at 
 the same time be observed, too, that the compact mass of 
 connective tissue ("reticulum," Barrois) which lies between 
 the muscular body-wall and the intestine (1) is directly 
 continuous with that in which the muscular layers are 
 imbedded. Nuclei are everywhere present. The omni- 
 presence of this connective tissue excludes the idea of any 
 true body cavity in Nemertines. 
 
 In Polia the connective tissue enclosed in the external 
 muscular layer is eminently vacuolar, all the interme- 
 diate stages between such cells in which the vacuole pre- 
 dominates and the nucleus is peripheral and those in which 
 the granular protoplasm still entirely fills them being 
 moreover present. 
 
 In addition to the musculature of the proboscis and 
 proboscidian sheath, longitudinal muscular fibres are 
 found in the walls of the oesophagus, whilst transverse 
 ones are numerous and united into vertical dissepiments 
 between the successive intestinal caeca, thus bringing about 
 a very regular internal metamerization (4). The genital 
 products develop in intermediate spaces similarly limited 
 by these dissepiments and alternating with the digestive 
 caeca. 
 
 (d) tfervous System and Sense Organs. The nervous system of 
 Nemertines presents several interesting peculiarities. As central 
 organs we have to note the brain-lobes and the longitudinal lateral 
 cords which form one continuous unsegmented mass of fibrous and 
 cellular nerve-tissue. The fibrous nerve-tissue is more dense in the 
 higher differentiated, more loose and spongy in the lower organized 
 forms ; the cellular nerve-tissue is similarly less compact in the 
 forms that are at the base of the 
 scale. No ganglionic swellings 
 whatever occur in the course of 
 the longitudinal cords. The 
 brain must be looked upon as 
 the anterior thickening of these 
 cords, and at the same time as 
 the spot where the two halves 
 of the central nerve system 
 
 inrprmmmnnirarp Trii i FlGS - 10 > 1L Brain and lateral organ of 
 
 18 a Schizonemertine (fig. 10) and a Hoplo- 
 
 brougnt about by a double com- nemertine(fig.ll). eo, exterior opening; 
 missure, of which the ventral n.L, superior brain-lobe ; p./ M posterior 
 portion is considerably thicker brain-lobe. 
 
 than the dorsal, and which, together with the brain-lobes, consti- 
 tutes a ring through which both proboscis and proboscidian sheath 
 pass. The brain-lobes are generally four in number, a ventral and 
 a dorsal pair, respectively united together by the above-mentioned 
 commissures, and moreover anteriorly interfusing with each other, 
 right and left. In Carinella this separation into lobes of the 
 anterior thickenings of the cords has not yet commenced, the ven- 
 tral commissure at the same time being extremely bulky. There 
 is great probability that the central stems, together with the brain, 
 
86 
 
 NEMERTINES 
 
 must be looked upon as local longitudinal accumulations of nervous 
 tissue in what was in more primitive ancestors a less highly dif- 
 ferentiated nervous plexus, situated in the body-wall in a similar 
 way to that which still is found in the less highly organized 
 Ccelenterates. Such a nervous plexus indeed occurs in the body- 
 wall of all Schizonemertines (7), sometimes even as a compara- 
 tively thick layer, situated, as are the nerve stems, between the 
 external longitudinal and the circular muscles (rig. 9). In Cari- 
 nella, where the longitudinal nerve-stems are situated exteriorly to 
 the muscular layers, 
 this plexus, although 
 present, is much less 
 dense, and can more 
 fitly be compared to 
 a network with wide 
 meshes. In both 
 cases it can be shown 
 to be in immediate 
 continuity with the 
 coating of nerve-cells 
 forming part of the 
 longitudinal cords. 
 It stretches forward 
 as far as the brain, 
 and in Carinella is FIG. 12. The brain of a Nemertine, with its lobes and 
 a<min continued in commissures. S.lf., nerves to sensory apparatus; 
 
 fmnt nf it- -urliovOQC P.N., I1C.TVCS for proboscis ; VO<J, nei'TCS for 0380- 
 
 I , wne phagus; L.N., lateral nerve stems. 
 
 in the Scliizoneiner- 
 
 tines the innervation of the anterior extremity of the head, in 
 front of the brain, takes the form of more definite and less numer- 
 ous branching stems. The presence of this plexus in connexion 
 with the central stems, sending out nervous filaments amongst 
 the muscles, explains the absence, both in Palaeo- and Schizo- 
 nemertines, of separate and distinct peripheral nerve stems spring- 
 ing from the central stems innervating the different organs and 
 body-regions, the only exceptions being the nerves for the pro- 
 boscis, those for the sense organs in the head, and the strong 
 nerve pair (n. vagiis) for the oesophagus. At the same time it 
 renders more intelligible the extreme sensitiveness of the body- 
 wall of the Nemertines, a local and instantaneous irritation 
 often resulting in spasmodic rupture of the animal at the point 
 touched. 
 
 In the Hoplonemertea, where the longitudinal stems lie inside 
 the muscular body-wall, definite and metamerically placed nerve 
 branches spring from them and divide dichotomously in the 
 different tissues they innervate. A definite plexus can here no 
 longer be traced. In certain Hoplonemertiues the lateral stems 
 have been noticed to unite posteriorly by a terminal commissure, 
 situated above the anus, the whole of the central nervous system 
 being in this way virtually situated above the intestine. In others 
 there is an approximation of the lateral stems towards the median 
 ventral line (Drepanophorus) ; in a genus of Schizonemertines 
 (Langia), on the other hand, an arrangement occurs by which the 
 longitudinal steins are no longer lateral, but have more or less 
 approached each other dorsally (6). 
 
 In addition to the nerves starting from the brain-lobes just now 
 especially mentioned, there is a double apparatus which can hardly 
 be treated of in conjunction with the sense organs, because its 
 sensory functions have not been sufficiently made out, and which 
 will therefore rather be considered along with the brain and central 
 nervous system. This apparatus is usually known under the name 
 of the lateral organs. To it belong (a) superficial grooves or deeper 
 slits situated on the integument near the tip of the head, (b) nerve 
 lobes in immediate connexion with the nervous tissue of the brain, 
 and (c) ciliated ducts penetrating into the latter and communicating 
 with the former. Embryology shows that originally these different 
 parts are separately started, and only ultimately become united 
 into one. Two lateral outgrowths of the foremost portion of the 
 cesophagus, afterwards becoming constricted off, as well as two 
 ingrowths from the epiblast, contribute towards its formation, at 
 least as far as both Hoplo- and Schizonemertines are concerned. 
 As to the Palseonemtrtea, their embryology has not yet been studied, 
 and in the most primitive genus, Carinella, we do not find any 
 lateral organs answering to the description above given. What we 
 do find is a slight transverse furrow on each side of the head, close 
 to the tip, but the most careful examination of sections made 
 through the tissues of the head and brain shows the absence of any 
 further apparatus comparable to that described above. Only in one 
 species, Carinella iiiexpectata, a step in advance has been made, in 
 so far as in connexion with the furrow just mentioned, which is 
 here also somewhat more complicated in its arrangement, a ciliated 
 tube leads into the brain, there to end blindly amidst the nerve- 
 cells. No other intermediate stages have as yet been noticed 
 between this arrangement and that of the Schizonemertea, in which 
 a separate posterior brain-lobe receives a similar ciliated canal, and 
 in which the cesophageal outgrowths have made their appearance 
 and are coalesced with the nerve-tissue in the organ of the adult 
 
 animal. The histological elements of this portion remain distinct 
 both by transmitted light and in actual sections. 
 
 These posterior brain-lobes, which in all Schizonemertines are in 
 direct continuity of tissue with the upper pair of principal lobes, 
 cease to have this intimate connexion in the Hoplonemertea ; and, 
 although still constituted of (1) a ciliated duct, opening out exter 
 nally, (2) nervous tissue surrounding it, and (3) histological ele 
 meuts distinctly different from the nervous, and most probably 
 directly derived from the cesophageal outgrowths, they are never- 
 theless here no longer constantly situated behind the upper brain 
 lobes and directly connected with them, but are found sometimes 
 behind, sometimes beside, and sometimes before the brain-lobes. 
 Furthermore, they are here severed from the principal lobes and 
 connected with them by one or more rather thick strings of nerve- 
 fibres. In some cases, especially when the lobes lie before the brain, 
 their distance from it, as well as the length of these nervous con- 
 nexions, has considerably increased. 
 
 With the significance of these parts we are still insufficiently 
 acquainted. There appear to be analogous organs amongst 
 Platyelminthes, but a careful comparative study is wanted. A 
 partial comparison has been hazarded (8) with the anterior 
 oesophageal outgrowths in Balanoylossus and Amphioxus, and for 
 the Schizonemertines arguments have been adduced (6) to prove 
 that here they have the physiological significance of a special 
 respiratory apparatus for the central nervous tissue, which in all 
 these forms is strongly charged with haemoglobin. The hemoglobin 
 would, by its pre-eminent properties of fixing oxygen, serve to fur- 
 nish the nerve system, which more than any other -requires a 
 constant supply, with the necessary oxygen. Such could hardly 
 be obtained in any other way by those worms that have no special 
 respiratory apparatus or delicately ramifying blood-vessels, and that 
 live in mud and under stones, where the natural supply of freshly 
 oxygenated sea-water is practically limited. Whether in the Hoplo- 
 nemertines, where the blood fluid is often provided with lioiino- 
 globiniferous disks, the chief functions of the side organs may not 
 rather be a sensory one must be further investigated. 
 
 The exterior opening of the duct has been several times alluded 
 to. In the Hoplonemertiues it is generally situated towards the 
 middle of a lateral transverse groove on either side of the head, as 
 was noticed for Carinella, and as is also 
 present in Polia. Generally a row of 
 shorter grooves perpendicular to the first, 
 and similarly provided with strong cilia, 
 enlarges the surface of these furrows (fig. 
 14). In Valcncinia there is nothing but 
 a circular opening without furrow. In all 
 Schizonemertines there is on each side of 
 the head a longitudinal slit of varying 
 length but generally considerable depth, 
 
 in the bottom of which the dark red brain FlG ?. 13, 14.-Lateral^vie-s 
 is very plainly visible by transparency. 
 These slits are continued into the ciliated 
 duct, being at the same time themselves 
 very strongly ciliated. In life they are 
 commonly rhythmically opened and shut 
 by a wavy movement. They are the head slits (cephalic fissures, 
 " Kopfspalten ") so characteristic of this subdivision (figs. 10 
 and 13). 
 
 With respect to the sense organs of the Nemertines, we find that 
 eyes are of rather constant occurrence, although many Schizonemer- 
 tines living in the mud appear to be blind. The more highly 
 organized species have often very numerous eyes (Ampldporus, 
 Drepaiwphorus), which are provided with a spherical refracting 
 anterior portion, with a cellular "vitreous body," with a layer of 
 delicate radially arranged rods, with an outer sheath of dark 
 pigment, and with a separate nerve-twig each, springing from a 
 common or double pair of branches which leave the brain as 
 n. optici, for the innervation of the eyes. Besides these more 
 highly differentiated organs of vision, more primitive eyes are 
 present in others down to simple stellate pigment specks without 
 any refracting apparatus. 
 
 Organs of hearing in the form of capsules containing otoliths 
 have only been very rarely observed, apparently only in 
 Hoplonemertea. 
 
 As to the organ of touch, the great sensitiveness of the body has 
 already been noticed, as well as the probable primary significance 
 of the proboscis. Small tufts of tactile hairs or papilla; are some- 
 times observed in small number at the tip of the head (11) ; some- 
 times longer hairs, apparently rather stiff, are seen on the surface, 
 very sparingly distributed between the cilia, and hitherto only in 
 a very limited number of small specimens. They may perhaps be 
 considered as sensory. 
 
 (e) Digestive System. The anterior opening, the mouth, is 
 situated ventrally, close to the tip of the head and in front of the 
 brain in the ffoplonemerte^ somewhat more backward and behind 
 the brain in the other Nemertines. In most Schizonemertines it is 
 found to be an elongated slit with corrugated borders ; in the 
 
 14 
 
 of head of a Schizone- 
 mertine (fig. 13) with 
 longitudinal slit, and of 
 a Hoplonemertine (fig. 14) 
 with transverse groove 
 and furrows. 
 
NEMERTINES 
 
 87 
 
 CT 
 
 Hoplonemertines it is smaller and rounded ; in Malacobdella and 
 Akroslomum. it, moreover, serves for the extrusion of the proboscis, 
 which emerges by a separate dorsal opening just inside the mouth. 
 The oesophagus is the anterior portion of the digestive canal; its 
 walls are folded longitudinally, comparatively thick, and provided 
 with longitudinal muscular fibres. Two layers are specially obvious 
 in its walls, the inner 
 layer bordering the lu- 
 men being composed of 
 smaller ciliated cells, 
 the outer thicker one 
 containing numerous 
 granular cells and hav- 
 ing a more glandular 
 character. Outside the 
 wall of the oesophagus a 
 vascular space has been 
 detected (11) which is 
 in direct continuity 
 with the longitudinal 
 blood-vessels. In cer- 
 tain cases, however, the 
 walls of the ossophagus 
 appear to be very closely 
 applied to the muscular 
 body-wall, and this vas- 
 cular space thereby con- 
 siderably reduced. 
 
 The posterior portion 
 of the intestine is speci- 
 ally characterized by the 
 appearance of the intes- 
 tinal diverticula hori- 
 zontally and symmetric- 
 ally placed right and 
 left and opposite to each 
 other. Sometimes this re- 
 gion, into which the oeso- 
 phagus leads, stretches 
 forwards under the 
 oesophagus (Hoplone- 
 mertines) for a certain 
 distance, anteriorly ter- 
 minating by a cul-de- 
 sac. Cases of asym- 
 metry or irregularity in 
 the arrangement of the 
 caeca, though sometimes 
 occurring, are not nor- 
 mal. At the tip of the 
 
 CT 
 
 IM 
 
 *' A 
 
 LBu 
 
 Fig. 17. 
 
 tail, where the growth FIGS. 15-17. Diagrammatic sections to show dis- 
 of the animal takes position of internal organs in Cariiulla (Palxo- 
 nlapp HIP PIPPA arp al aemerlea), fig. 15, Sehizmemertea, fig. 16, and 
 ice, tne caeca are al- H( , plmeT ^. rUa ; flg . 17. c , cellular portion of 
 integument; B, basement membrane; A, circu- 
 lar muscular layer; A', longitudinal do.; A", 
 second circular (in Carinella) ; A'", second longi- 
 tudinal (in Schizonemertea) ; If, nerrous layer ; 
 i.V, lateral nerves ; PS, cavity of proboscidian 
 sheath (the sheath itself of varying thickness); 
 P, proboscis ; /, intestine ; LBc, lateral blood- 
 vessel ; DBc, dorsal do. ; CT, connective I issue. 
 
 ways eminently regular. 
 So they are throughout 
 the whole body in most 
 of the Hoplonemertines. 
 In Carinella, they are 
 generally deficient and 
 the intestine straight ; 
 in young specimens of this species, however, they occur, though less 
 regular and more in the form of incipient foldings by which the 
 digestive surface is increased. The inner surface of the intestinal 
 cseca is ciliated, the caeca themselves are sometimes especially in 
 the hindermost portion of the body of a considerably smaller lumen 
 than the intermediate genital spaces ; sometimes, however, the 
 reverse is the case, and in both cases it is the smaller lumen that 
 appears enclosed between and suspended by the transverse fibres 
 constituting the muscular dissepiments above mentioned. 
 
 The anus is situated terminally, the muscular body-wall through 
 which the intestine must find its way outwards probably acting in 
 this region the part of a sphincter. The lateral nerve stems mostly 
 terminate on both sides in closest proximity to the anus ; in certain 
 species, however, they interfuse by a transverse connexion above 
 the anus. The longitudinal blood-vessels do the same. The 
 question has been raised whether the regular intestinal caeca of 
 Nemertines might not be compared with those intestinal diverticula 
 of the embryo Amphioxus which ultimately become the mesoblastic 
 somites of the adult (8). This view would be a further extension 
 of the views concerning the coelom first propounded by Huxley. 
 
 (/) Circulatory Apparatus. This consists of three longitudinal 
 trunks, a median and two lateral ones. They are in direct con- 
 nexion with each other both at the posterior and at the anterior 
 end of the body. At the posterior end they communicate together 
 by a T-shaped connexion in a simple and uniform way. Anteriorly 
 there is a certain amount of difference in the arrangement. Whereas 
 in the Hoplonemertines an arrangement prevails as represented in 
 
 fig. 18, the lateral stems in the Schizonemertines, while entirely 
 
 uniform all through the posterior portion of the body, no longer 
 
 individually exist in the oesophageal region, but 
 
 here dissolve themselves into a network of vascu- 
 
 lar spaces surrounding this portion of the di- 
 
 gestive tract (11). The median dorsal vessel, 
 
 however, remains distinct, but instead of con- 
 
 tinuing its course beneath the proboscidian 
 
 sheath it is first enclosed by the ventral muscu- 
 
 lature of this organ, and still farther forwards 
 
 it even bulges out longitudinally into the cavity 
 
 of the sheath. Anteriorly it finally communi- 
 
 cates with the lacunae just mentioned, which 
 
 paratus in the ant 
 rior body-region of 
 a Hoplonemertine. 
 
 , 
 
 lobes of the brain, pass through the nerve nng 
 together with the proboscidian sheath, and are 
 generally continued in front of the brain as a 
 lacunar space in the muscular tissue, one on each side. 
 
 Special mention must be made of the delicate transverse vessels 
 regularly connecting the longitudinal and the lateral ones. They 
 are metamerically placed, and belong to the same metamer as the 
 digestive cceca, thus alternating with the generative sacs. The 
 blood fluid does not flow in any definite direction ; its movements 
 are largely influenced by those of the muscular body-wall. It is 
 colourless, and contains definite corpuscles, which are round or 
 elliptical, and in many Hoplonemertines are coloured red by haemo- 
 globin, being colourless in other species. The circulatory system 
 of Carinella is considerably different, being more lacnnar and less 
 restricted to definite vascular channels. Two lateral longitudinal 
 lacunae form, so to say, the forerunners of the lateral vessels. A 
 median longitudinal vessel and transverse connecting trunks have 
 not as yet been detected. There are large lacunae in the head in 
 front of the ganglia. 
 
 (g) Nephridia. Although these organs were already very well 
 known to Max Schultze (14), their presence in Nemertines was 
 repeatedly and seriously disputed until Von Kennel (10) definitely 
 proved their existence and gave details concerning their histology. 
 With the exception of a few genera where they have not as yet been 
 discovered (Carinella), one pair of nephridia appears to be very 
 generally present. They essentially consist of a complex coiled 
 tube, one on each side of the oesophagus (fig. 1), communicating 
 with the exterior by a duct piercing the body-wall. The two 
 openings of the nephridia are situated sometimes more towards the 
 ventral, at other times more towards the dorsal side. Even in the 
 larger Schizonemertines these pores are only a few millimetres 
 behind the mouth region. Internal funnel-shaped openings, 
 although sought for, have as yet not been detected. The coiled 
 tubes extend both forwards and backwards of the external opening, 
 by far the greater portion being situated backwards. The anterior 
 coils reach forwards till in the immediate vicinity of the posterior 
 brain-lobes. The coils are tubiform, with an internal lumen, only 
 one layer of rather large cells constituting the walls. These cells 
 are ciliated ; in some transparent species the internal ciliary move- 
 ment can be observed during life. In transverse sections the 
 nephridia can be shown to be generally situated in the region 
 limited by (1) the proboscidian sheath, (2) the upper wall of the 
 intestine, (3) the muscular body-wall. No trace of nephridia is 
 found posterior to the oesophagus. 
 
 (h) Generative System. In the Nemertines the sexes are separate, 
 with only very few exceptions (12) (Tctrastemma hermaphroditica, 
 Marion). The generative products are contained in separate 
 pouches placed metamerically in the way noticed above in treating 
 of the digestive system. They are conveyed outwards along narrow 
 canals, one pair for each metamer piercing the muscular body-wall, 
 and visible on the outside in mature individuals as minute light- 
 coloured specks. The ova and spermatozoa, when mature, present 
 no peculiarities. As the ova are in many species deposited in a 
 gelatinous tube secreted by the body-walls, in which they are 
 arranged (three or more together) in flask-shaped cavities, impreg- 
 nation must probably take place either before or at the very moment 
 of their being deposited. The exact mode has not yet been noticed. 
 Another point not yet sufficiently settled is the oogenesis in 
 Nemertines. In several cases the ova appear to originate directly 
 as the lining of the generative pouches, but the exact part which 
 the mesoblastic connective tissue plays, both with regard to these 
 pouches and to the generative products themselves, remains yet to 
 be settled. 
 
 Prosyrhochmus claparedii is a viviparous form. 
 
 DEVELOPMENT. The embryology of the Nemertines offers Develop. 
 some very remarkable peculiarities. Our knowledge of ment 
 the development of the most primitive forms is very scanty. 
 Of that of Carinella absolutely nothing is known. On 
 Cephalothrix we have observations, in certain respects con- 
 tradictory. Both Schizo- and Hoplonemertea, have been 
 more exhaustively studied, the first, as was noticed above, 
 
88 
 
 NEMERTINES 
 
 being characterized by peculiar larval forms, the second 
 developing without metamorphosis. 
 
 The larva of Cerebratulus is called the pilidiuin. In 
 exterior shape it resembles a helmet with spike and ear- 
 lobes, the spike being a strong and long flagellum or a tuft 
 of long cilia, the ear-lobes lateral ciliated appendages 
 (fig. 19). It encloses the primitive alimentary tract. 
 Two pairs of invaginations of the skin, which originally 
 
 FIG. 19. Pllidium larva. B, bunch of cilia or flagellum; , oesophagus ; st, 
 stomach; cs, cesophageal outgrowth for lateral organ; am, amnion; pr.d., 
 prostomial disk; po.d., metastomial disk. 
 
 are called the prostomial and metastomial disks, grow 
 round the intestine, finally fuse together, and form the 
 skin and muscular body-wall of the future Nemertine, 
 which afterwards becomes ciliated, frees itself from the 
 pilidium investment, and developes into the adult worm 
 without further metamorphosis (2, 13). 
 
 The eggs of these species are not enveloped by such 
 massive gelatinous strings as are those of the genus Linens. 
 In the latter we find the young Nemertines crawling about 
 after a period of from six to eight weeks, and probably 
 feeding upon a portion of this gelatinous substance, which 
 is found to diminish in bulk. In accordance with these 
 more sedentary habits during the first phases of life, the 
 characteristic pilidium larva, which is so eminently adapted 
 for a pelagic existence, appears to have been reduced to a 
 close-fitting exterior layer of cells, which is striped off 
 after the definite body-wall of the Nemertiue has similarly 
 
 originated out of four ingrowths from the primary epiblast. 
 To this reduced and sedentary pilidium the name of " larva 
 of Desor " has been given (1). 
 
 In the Hoplonemeriea, as far as they have been investi- 
 gated, a direct development without metamorphosis has 
 been observed. It appears probable that this is only a 
 further simplification of the more complicated metamor- 
 phosis described above. 
 
 As to the development of the different organs, there is 
 still much that remains doubtful. The hypoblast in some 
 forms originates by invagination, in others by delamina- 
 tion. The proboscis is an invagination from the epiblast ; 
 the proboscidian sheath appears in the mesoblast, but is 
 perhaps originally derived from the hypoblast. The origin 
 of the lateral organs has already been noticed ; that of the 
 nerve system is essentially epiblastic. 
 
 Literature. 
 
 (1) J. Barrois, " Reeherches sur 1'embryologie des Nemertes," 
 Annales cles Sc. Naturelles, vi., 1877. 
 
 (2) O. Biitschli, " Einige Bsmerkungen zur Metamorphose des 
 Pilidium," Archivfiir Naturgeschichlc, 1873. 
 
 (3) L. von Graff, Monographic der Turbellarien, 1882. 
 
 (4) A. A. W. Hubreeht, " Untersuchungen iiber Nemertinen 
 a. d. Golf von Neapel," Niederl. Archivfur Zoologie, ii. 
 
 (5) Id. , " The Genera of European Nemerteans critically revised," 
 Notes from the Leyden Museum, 1879. 
 
 (6) Id., "Zur Anatomic u. Physiologic d. Nervensystems d. 
 Nemertinen," Verh. Kon. Akad. v. Wetensch., Amsterdam, 1880, 
 vol. xx. 
 
 (7) Id., "Tlie Peripheral Nervous System of the Palseo- and 
 Sehizonemertini, one of the layers of the body-wall," Quart. 
 Journal of Micr. Science, vol. xx. 
 
 (8) Id., "On the Ancestral Forms of the Chordata," Ib., July 
 1883. 
 
 (9) \V. Kefersteiu, " Untersuchungen iiber niedere Seethiere," 
 Zeitschr. f. wissensch. Zool.,^ vol. xii., 1863. 
 
 (10) J. von Kennel, " Beitritge zur Kenntniss der Nemertinen," 
 ArbeUen a. d. zool.-zoot. Instit., ii., Wiirzburg, 1878. 
 
 (11) W. C. Macintosh, A Monograph of British Annellida : I. 
 Nemcrteaus, Ray Society, 1873-74. 
 
 (12) A. F. Marion, " Reeherches sur les animaux inferieurs du 
 Golfe de Marseille," Ann. des Sc. Nat., 1873. 
 
 (13) E. Metschnikoff, ' ' Studien iiber die Entwiekclung der 
 Eehinodermen und Nemertinen," Mem. de VAcad. Imp. de St. 
 Petcrsb., xiv., 1869. 
 
 (14) Max Sclmltze, Seitriigc zur Naturgeschichte der Turbellarien, 
 Greifswald, 1851, and Zeitschr. fur wissensch. Zoo!., iv., 1852. 
 
 (A. A. W. H.) 
 
ROTIFERA 
 
 (By Prof. A. G. Bourne, Presidency College, Madras.) 
 
 THE Rotifera or Rotatoria form a small, in many 
 respects well-defined, but somewhat isolated class of 
 the animal kingdom. They are here treated of separately, 
 partly on account of the difficulty of placing them in one 
 of the large phyla, partly on account of their special 
 interest to microscopists. 
 
 Now familiarly known as " wheel animalcules " from 
 the wheel-like motion produced by the rings of cilia which 
 generally occur in the head region, the so-called rotatory 
 organs, they were first discovered by Leeuwenhoek (I), 1 to 
 whom we also owe the discovery of Bacteria and ciliate 
 Infusoria. Leeuwenhoek described the Rotifer vulgaris in 
 1702, and he subsequently described Melicerta ringens and 
 other species. A great variety of forms were described 
 by other observers, but they were not separated as a class 
 from the unicellular organisms (Protozoa) with which 
 they usually occur until the appearance of Ehrenberg's 
 great monograph (2), which contained a mass of detail 
 regarding their structure. The classification there put 
 forward by Ehrenberg is still widely adopted, but numer- 
 ous observers have since added to our knowledge of the 
 anatomy of the group (3). At the present day few groups 
 of the animal kingdom are so well known to the micro- 
 scopist, few groups present more interesting affinities to 
 the morphologist, and few multicellular animals such a 
 low physiological condition. 
 
 Genei-al Anatomy. The Rotifera are multicellular 
 animals of microscopic size which present a coelom. They 
 are bilaterally symmetrical and present no true metameric 
 segmentation. A head region is generally well marked, 
 and most forms present a definite tail region. This tail 
 region has been termed the "pseudopodium." It varies 
 very much in the extent to which it is developed. It 
 attains its highest development in forms like Philodina, 
 which affect a leech-like method of progression and use it 
 as a means of attachment. We may pass from this through 
 a series of forms where it becomes less and less highly 
 developed. In such forms as Brachiomis it serves as a 
 directive organ in swimming, while in a large number of 
 other forms it is only represented by a pair of terminal 
 styles or flaps. In the sessile forms it becomes a con- 
 tractile pedicle with a suctorial extremity. A pseudo- 
 podium is entirely absent in Asplanchna, Triarthra, 
 Polyarthra, and a few other genera. The pseudopodium, 
 when well developed, is a very muscular organ, and it may 
 contain a pair of glands (fig. 2, A, gl) which secrete an 
 adhesive material. 
 
 The surface of the body is covered by a firm homogeneous 
 structureless cuticle. This cuticle may become hardened 
 by a further development of chitin, but no calcareous 
 deposits ever take place in it. The cuticle remains softest 
 in those forms which live in tubes. Among the free-living 
 forms the degree of hardening varies considerably. In 
 some cases contraction of the body merely throws the 
 cuticle into wrinkles (Notommata, Asplanchna) ; in others 
 definite ring-like joints are produced which telescope into 
 one another during contraction ; while in others again it 
 becomes quite firm and rigid and resembles the carapace 
 of one of the Entomostraca ; it is then termed a " lorica." 
 The lorica may be prolonged at various points into spines, 
 which may attain a considerable length. The surface may 
 be variously modified, being in some cases smooth, in others 
 marked, dotted, ridged, or sculptured in various ways (fig. 
 1, K). The curved spines of Philodina aculeata (fig. 1, G) 
 and the long rigid spines of Triarthra are further develop- 
 
 1 These numbers refer to the bibliography at p. 93. 
 
 ments in this direction. The so-called setae of Polyarthra on 
 the other hand are more complex in nature, and are moved 
 by muscles, and thus approach the " limbs " of Pedalion. 
 
 Fio. 1. A, Ftoitularia eampanulata, an adnlt male, drawn from a dead specimen 
 (after Hudson): t, testis; oc, eye-spots. B, Floscularia appendicvlata, an 
 adult female (after Gegenbaur): a, the ciliated flexible proboscis. C, Stephana- 
 ceros eichhornii : a, the urceolus. D, Microcodon darns, ventral view (after 
 Grenadier) : m, mouth ; a, bristles ; x, architroch ; , lateral sense-organs. E, 
 Polyarthra platyptera : oc, eye-spot ; x*, Isolated tufts representing a cephalo- 
 troch ; x, branchiotroch ; a, 6, and c, three pairs of appendages which are 
 moved by the muscles m. F, another figure of Polyarthra, to show the position 
 which the appendages may take up. G, Philodina aculeata : oc, eye-spots ; *, 
 calcar. H, Actinurus neptunius: oc, eye-spots ; *, calcar. I, Asplanchna sie~ 
 boldii, male, viewed from the abdominal surface: a, anterior short arms; b, 
 posterior longer arms; m, mouth; 3?, cephalotrochic tufts; x, branchiotroch. 
 J, Asplanchna siebotdii, female ; letters as before. K, Noteus quadricornis, 
 to show the extent to which the lorica may become sculptured. (All, except 
 where otherwise stated, from Pritchard.) 
 
 Several genera present an external casing or sheath or 
 tube which is termed an " urceolus." In Floscularia and 
 Stephanoceros the urceolus is gelatinous and perfectly 
 hyaline ; in Conochilm numerous individuals live in such a 
 hyaline urceolus arranged in a radiating manner. The 
 urceolus, which is secreted by the animal itself, may 
 become covered with foreign particles, and in one species, 
 the well-known Melicerta rinyens, the animal builds up its 
 urceolus with pellets which it manufactures from foreign 
 
 M 
 
90 
 
 R O T I F E R A 
 
 particles, and deposits in a regular oblique or spiral series, 
 and which are cemented together by a special secretion. 
 The urceolus serves as a defence, as the animal can by con- 
 tracting its stalk withdraw itself entirely within the tube. 
 Locomotor Organs. While, as mentioned above, several 
 genera or individual species present long spines, these 
 become movable, and may be spoken of as appendages, in 
 two genera only. In Polyarthra (fig. 1, E, F) there are 
 four groups of processes or plumes placed at the sides of 
 
 Fio. 2.FloscutaHa appendiculata. A and B represent the same animal, some of 
 the organs being shown in one figure and some in the other, oc, eye-spots ; g, 
 nerve ganglion ; p, pharynx (the mouth should be shown opening opposite the 
 letter); ma, the mastax; e, oesophagus; st, stomach; a, anus, opening the 
 cloaca; gl, mucous glands in the pseudopodium ; n, nephridia; /. flame-cells; 
 bl, contractile vesicle ; m, m, muscles. 
 
 the body, each of which groups can be separately moved 
 up and down by means of muscular fibres attached to their 
 bases, which project into the body. The processes them- 
 selves are unjointed and rigid. In Pedalion (fig. 3), a 
 remarkable form discovered by Dr C. J. Hudson in 1871 
 (12, 13, 14, and 15), and found in numbers several times 
 since, these appendages have acquired a new and quite 
 special development. They are six in number. The largest 
 is placed ventrally at some distance below the mouth. Its 
 free extremity is a plumose fan-like expansion (fig. 3, 
 A, a, and H). It is (in common with the others) a hollow 
 process into which run two pairs of broad, coarsely trans- 
 versely striated muscles. Each pair has a single insertion 
 on the inner wall the one pair near the free extremity of 
 the limb, the other near its attachment ; the bands run 
 up, one of each pair on each side and run right round 
 the body forming an incomplete muscular girdle, the ends 
 approximating in the median dorsal line. Below this 
 point springs the large median dorsal limb, which termin- 
 ates in groups of long setrc. It presents a single pair of 
 muscles attached along its inner wall which run up and 
 form a muscular girdle. round the body in its posterior 
 third. On each side is attached a superior dorso-lateral 
 and an inferior ventro-lateral appendage, each with a fan- 
 like plumose termination consisting of compound hairs, 
 found elsewhere only among the Crustacea each of these 
 
 is moved by muscles running upwards towards the neck 
 and arising immediately under the trochal disk, the inferior 
 ventro-lateral pair also presenting muscles which form a 
 girdle in the hind region of the body. Various other 
 muscles are present : there are two complete girdles in the 
 neck region immediately behind the mouth; there are also 
 muscles which move the hinder region of the body. In 
 addition to these the body presents various processes 
 which are perhaps some of them unrepresented in other 
 Rotifers. In the median dorsal line immediately below 
 the trochal disk there is a short conical process presenting 
 a pair of muscles which render it capable of slight move- 
 ment. From a recess at the extremity of this process 
 spring a group of long setose hairs the bases of which are 
 connected with a filament probably nervous in nature. 
 This doubtless represents a structure found in many 
 Rotifers, and variously known as the "calcar," "siphon," 
 " tentaculum," or "antenna." This calcar is double in 
 Tubicolaria and Melicerta. It is very well developed in 
 the genera Rotifer, Philodina, and others, and is, when so 
 developed, slightly retractile. It appears to be repre- 
 sented in many forms by a pit or depression set with hairs. 
 The calcar has been considered both as an intromittent 
 organ and a respiratory tube for the admission of water. 
 It is now, however, universally considered to be sensory 
 in nature. Various forms present processes in other parts 
 
 FIG. 3. Pedalion mira. A, Lateral surface view of an adult female : a, median 
 ventral appendage; b, median dorsal appendage; c, inferior ventro-lateral 
 appendage : d, superior dorso-lateral appendage ; /, dorsal sense-organ (calcar) ; 
 j7, "chin;" x, cephalotroch. B, lateral view, showing the viscera: oc, eye- 
 spots; ?i, nephridia; e, ciliated processes, probably serving for attachment; 
 other letters as above. C, ventral view: x', cephalotroch; r, brancliiotroch; 
 other letters as above. D, ventral view, showing the musculature (</. text). 
 E, dorsal view of a male : a, lateral appendages ; 6, dorsal appendage. F, 
 lateral view of a male. G, enlarged view of the sense-organ marked/. H, 
 enlarged view of the median ventral appendage. (All after Hudson.) 
 
 of the body which have doubtless a similar function, e.g., 
 Microcodon (fig. 1, D, *) with its pair of lateral organs. 
 Pedalion presents a pair of ciliated processes in the 
 posterior region of the body (fig. 3, B, c, and D, e), which 
 it can apparently use as a means of attachment ; Dr 
 Hudson states that he has seen it anchored by these and 
 swimming round and round in a circle. They possibly re- 
 
ROTIFERA 
 
 91 
 
 present the flaps found on the tail of other forms. Pedalion 
 also has a small ciliated muscular process (fig. 3, A, g) placed 
 immediately below the mouth, and termed a " chin," which 
 appears to be merely a greater development of a sort of 
 lower lip which occurs in many Rotifers. 
 
 Muscular System. All the Botifera present a muscular system 
 which is generally very well developed. Transverse striation occurs 
 among the fibres to a varying extent, being well marked in cases 
 where the muscle is much used. The muscles which move the 
 body as a whole are arranged as circular and longitudinal series, 
 but they are arranged in special groups and do not form a com- 
 plete layer of the body- wall as in -the various worms. Some of the 
 longitudinal muscles are specially developed in connexion with the 
 tail or pedicle. Other muscles are developed in connexion with 
 special systems of organs, the trochal disks, the jaw apparatus, 
 and the reproductive system. The muscles in connexion with the 
 trochal disk serve to protrude or withdraw it, and to move it about, 
 when extruded, in various directions. The protrusion is probably, 
 however, generally effected by the elasticity of the integument 
 coming into play during the relaxation of the retractor muscles, and 
 by a general contraction of the body wall. The tentaculiferous 
 apparatus of Polyzoa and Gephyrea is protruded in the same manner. 
 
 Trochal Dish. This structure is the peculiar characteristic of 
 the class. It is homologous with the ciliated bands of the larvse 
 of Ecbinoderms, Chsetopods, Molluscs, tc., and with the tenta- 
 culiferous apparatus of Polyzoa and Gephyrea, and has been termed 
 in common with these a " velum." This velum presents itself in 
 various stages of complexity. It is found as a single circnm-oral 
 ring (pilidium), as a single prse-oral ring (Chaetopod larvae), or as 
 a single prae-oral ring coexisting with one or more post-oral rings 
 (Chsetopod larvse, Holothurian larvae). We may here assume that 
 the ancestral condition was a single cireum-oral ring associated 
 with a terminal mouth and the absence of an anus, and that the exist- 
 ence of other rings posterior to this is an expression of metameric 
 segmentation, i.e., a repetition of similar parts. With the develop- 
 ment of a prostomiate condition a certain change necessarily takes 
 place in the position of this band: a portion of it comes to lie 
 longitudinally; but it may still remain a single band, as in the 
 larva of many Echinoderuis. How have the other above-mentioned 
 conditions of the velum come about ? How has the prae-oral band 
 been developed ? Two views have been held with regard to this 
 question. According to the one view, the fact whether the single 
 band is a pne-oral or a post-oral one depends upon the position in 
 which the anus is about to develop. If the anus develops in such 
 a position that month and anus lie on one and the same side of the 
 band, the latter becomes prse-oral ; if, however, the anus develops 
 so that the mouth and anus lie upon opposite sides of the band, 
 the band becomes post-oraL If we hold this view we must consider 
 any second band, whether pr*- or post-oral, to arise as a new 
 development The other view premises that the anus always forms 
 so as to leave the primitive ring or "architroch" post-oral, i.e., 
 between mouth and anus. Concurrently with the development of 
 a prostomium this architroeh somewhat changes its position and 
 the two lateral portions come to lie longitudinally ; these may be 
 supposed to have met in the median dorsal line and to have 
 coalesced so as to leave two rings the one prse-oral (a " cephalo- 
 troch"), the other post-oral (a " branehiotroch ") ; this latter may 
 atrophy, leaving the single prse-oral ring, or it may become further 
 developed and thrown into more or less elaborate folds. The exist- 
 ing condition of the trochal disk or velum in the Botifera seems to 
 the writer of this article to bear out the latter view as to the way 
 in which modifications of the velum may have come about 
 
 In its simplest condition it forms a single eircum-oral ring, as in 
 Microcodon (fig. 1, D). The structures at the sides of the mouth 
 in this form are stated to be bristles, and have therefore nothing 
 to do with the velum (fig. 4, A, p). This simple ring may become 
 thrown into folds, so forming a series of processes standing up 
 around the month ; this is the condition in Stephanoceros (fig. 4, B,p). 
 There are, however, but few forms presenting this simple condi- 
 tion ; and it must be remembered that the evidence for the assump- 
 tion here made, that this is a persistent architroch and not a bran- 
 chiotroch persisting where a cephalotroch has vanished, is not at 
 present conclusive. This band, may, while remaining single and 
 perfectly continuous, become prolonged around a lobe overhanging 
 the mouth a prostomium. This condition occurs in Philodina 
 (fig. 4, E, r, p); the two sides of the post-oral ring; do not meet 
 dorsally, but are carried up and are continuous with the row of 
 cilia lining the " wheels. " There is thus one continuous ciliated 
 band, a portion of which runs up in front of the mouth. This 
 condition corresponds to that of the Auricularian larva. The fold- 
 ing of the band has become already somewhat complicated ; a 
 hypothetical intermediate condition is shown in fig. 4, c, D. The 
 next stage in the advancing complexity is that the prostomial por- 
 tion of the band (fig. 4, G, H, p') becomes separated as a distinct 
 ring, a cephalotroch ; we find such a stage in Lacinularia (fig. 4, 
 
 G, H), where both cephalotroch and branchiotroch remain fairly 
 simple in shape. In Melicerta (fig. 4, I, j) both cephalotroch and 
 branchiotroch are thrown into folds. Lastly, we find that in such 
 forms as Broxhionus the cephalotroch becomes first convoluted and 
 
 in, 
 
 FIG. 4. Diagrams of the Trochal Disk. A, ilifrofodon. B, Stephanoceros ; the 
 month lies in the centre of a group of tentacles. C, hypothetical intermediate 
 form between Mierocodon and Philodina, showing the development of a pro- 
 stomial portion of the velum. D, dorsal view of the same. E, Philodina. F, 
 dorsal view of the same. G, Lacinutaria: the dotted line represents the por- 
 tion of the velnm which has become separated as a special ring a cephalotroch. 
 U, dorsal view of the same. I, tfelieerla ; the dotted line represents the 
 cephalotroch; both this and the branchiotroch have become thrown into folds. 
 J, dorsal view of the same. K,' Brachionus ; there is a large prae-oral lobe 
 with three ciliated regions, shown by the dotted lines r, c, a discontinuous 
 cephalotroch. L, dorsal view of the same. 
 
 m, mouth ; p, p', velum ; p. architroch ; p', portion of the architroch which 
 becomes carried forward to line the proston.i&l region, but does not become 
 separated ; c, cephalotroch. (Original.) 
 
 then discontinuous (fig. 4, K, L, c), and further it may become so 
 reduced as to be represented only by a few isolated tufts, as in 
 Asplanchna (fig. 1, I, x and af); in such a form as Lindia (fig. 6, c) 
 the branchiotroch has vanished and the cephalotroch has become 
 reduced to the two small patches at the sides of the head. 
 
 The trochal apparatus serves the Rotifera as a locomotive organ 
 and to bring the food particles to the month ; the cilia work so as 
 to produce currents towards the mouth. 
 
 Digestive System. This consists of the following regions: (1) 
 the oral cavity ; (2) the pharynx ; (3) the ctsophagus ; (4) the 
 stomach ; (5) the intestine, which terminates in an anus. The 
 anus is absent in one group. 
 
 The pharynx contains the mastax with its teeth ; these are 
 calcareous structures, and are known as the trophi. In a typical 
 mastax (8, 9) (Bra- j j 
 
 chionus, fig. 5, A) * ~^=~ =^=~- 
 there are a median 
 anvil or incus and 
 two hammer-like 
 portions, mallei. 
 The incus consists 
 of two rami (e) 
 resting upon a cen- 
 tral fulcrum (/) ; 
 each malleus con- 
 sists of a handle or 
 manubrium (c) and 
 a head or uncus 
 (<f), which often 
 presents a comb- 
 like structure. Fig. 
 5 shows some of 
 
 the most important FIG 5 _ Trophi , variong fonrg . A , BnKhionul . t B , 
 modifications Which Digltna /orcipata ; C, Atplanfhaa ; D, Philodixa. /, 
 the apparatus may fulcrum, and e,e, rami, forming the incus; r, manubrium, 
 exhibit The parts ""d d > onca*- forming the malleus. (After Hudson.) 
 may become very slender, as in LHglena fordpata (fig. 5, B) ; the 
 mallei may be absent, as in Asplanchna (fig. 5, c), the rami being 
 highly developed into curved forceps and movable one on the other ; 
 or, the manubria being absent and the fulcrum rudimentary, the 
 rami may become massive and subquadratic, as in Philodina (fig. 
 5, D). All the true Rotifers jossess a mastax. Ehrenberg's group 
 of the Agomphia consisted of a heterogeneous collection of forms, 
 Ichthydium and Chxtonotus being Gaslro'richa, and Cyphonautcf 
 
92 
 
 ROTIFERA 
 
 a Polyzoan larva, while Enteroplea is probably a male Rotifer, and, 
 like the other males, iu a reduced condition. There is no reason for 
 considering this mastax as the homologue of either the gastric mill of 
 Crustaceans on the one hand or the teeth in the Chaetopods' pharynx 
 on the other ; it is merely homoplastic with these structures, but has 
 attained a specialized degree of development. Both the pharynx 
 and the oesophagus which follows it are lined with chitin. The 
 resophagus varies in length and in some genera is absent (Philo- 
 dinadie), the stomach following immediately upon the pharynx. 
 The stomach is generally large ; its wall consists of a layer of very 
 large ciliated cells, which often contain fat globules and yellowish- 
 green or brown particles, and outside these a connective tissue 
 membrane ; muscular fibrillae have also been described. Very 
 constantly a pair of glands open into the stomach, and probably 
 represent the hepato-pancreatic glands of other Invertebrates. 
 
 Following upon the stomach there is a longer or shorter intestine, 
 which ends in the cloaca. The intestine is lined by ciliated cells. 
 In forms living in an urceolus the intestine turns round and runs 
 forward, the cloaca being placed so as to debouch over the margin 
 of the urceolus. The cloaca is often very large ; the nephridia and 
 oviducts may open into it, and the eggs lodge there on their way 
 outwards ; they are thrown out, as are the faecal masses, by an 
 eversion of the cloaca. Asplanchna, Notommata sieboldii, and cer- 
 tain species of Ascomorpha are said to be devoid of intestine or 
 anus, excrementitious matters being ejected through the mouth (11). 
 
 Nephridia. The ccelom contains a fluid in which very minute 
 corpuscles have been detected. There is no trace of a true vascular 
 system. The nephridia (fig. 2, B, n) present a very interesting 
 stage of development. They consist of a pair of tubules with an 
 iutracellular lumen running up the sides of the body, at times 
 merely sinuous, at others considerably convoluted. From these 
 are given off at irregular intervals short lateral branches, each of 
 which terminates in a flame-cell precisely similar in structure to 
 the flame-cells found in Planarians, Trematodes, and Cestotles ; 
 here as there the question whether they are open to the ccelom or 
 not must remain at present undecided. At the base these tubes 
 open either into a permanent bladder which communicates with the 
 cloaca or into a structure presenting apparently no advance in its 
 development upon the contractile vacuole of a ciliate Infusorian. 
 
 Nervous System and Sense- Organs. Various structures have been 
 spoken of as nervous which are now acknowledged to have been 
 erroneously so described (18). There is a supra-cesophageal gang- 
 lion which often attains considerable dimensions, and presents a 
 lobed appearance (fig. 2, A and B, g). Connected with this are the 
 eye-spots, which are seldom absent. Where these are most highly 
 developed a lens-like structure is present, produced by a thicken- 
 ing of the cuticle. In the genus Katifer and other forms these are 
 placed upon'the protrusible portion of the head, and so appear to 
 have different positions at different moments. The number of eye- 
 spots varies from one to twelve or more. They are usually red, red- 
 dish-brown, violet, or black in colour. Other structures are found 
 which doubtless act as sense-organs. The calcar above-mentioned 
 generally bears at its extremity stiff hairs which have been demon- 
 strated to be in connexion with a nerve fibril. On the ventral sur- 
 face of the body just below the mouth a somewhat similar structure 
 is often developed the chin. There are besides at times special 
 organs, like the two lateral organs in Microcodon (fig. 1, D, s), which 
 no doubt in common with the calcar and chin have a tactile function. 
 
 Reproductive Organs and Development. The Rotifera were 
 formerly considered to be hermaphrodite, but, while the ovary was 
 always clear and distinct, there was always some difficulty about 
 the testis, and various structures were put forward as representing 
 that organ. One by one, however, small organisms have been dis- 
 covered and described as the males of certain species of Rotifers, 
 until at the present time degenerated males are known to occur in 
 all the families except that of the Philodinadx. The male Rotifers 
 are provided with a single circlet of cilia (a peritroch), a nerve 
 ganglion, eye-spots, muscles, and nephridial tubules all in a some- 
 what reduced condition, but there is usually no trace of mouth or 
 stomach, the main portion of the body being occupied by the testi- 
 cular sac. There is an aperture corresponding with the cloaca of the 
 female, where the testis opens into the base of an eversible penis. 
 The males of Floscularia are shown in fig. 1. The male of Pedalion 
 mira possesses rudimentary appendages. The ovary is usually a 
 large gland lying beside the stomach connected with a short oviduct 
 which opens into the cloaca. The ova often present a reddish hue 
 (Philodina roseola, Brachionus rubens), due doubtless, like the red 
 colour of many Crustacean ova, to the presence of tetronerythrin. 
 
 Up to the present our embryological knowledge of the group is 
 very incomplete. Many Rotifers are known to lay winter and 
 summer eggs of different character. The winter eggs are provided 
 with a thick shell and probably require fertilization. Two or three 
 of them are often carried about attached to the parent (Brachionus, 
 Notommata), but they are usually laid and fall into the mud, there 
 to remain till the following spring. The summer eggs are of two 
 kinds, the so-called male and female ova, both of which are stated 
 to develop parthenogenetically. They may be carried about in 
 
 large numbers in the cloaca or oviduct or attached to the body of 
 the parent. The female ova give rise to female and the male ova 
 to male individuals. Male individuals are only formed in the 
 autumn in time to fertilize the winter ova. 
 
 Habitat and Mode of Life. The Rotifera are distri- 
 buted all over the earth's surface, inhabiting both fresh 
 and salt water. The greater number of species inhabit 
 fresh water, occurring in pools, ditches, and streams. A 
 few species will appear in countless numbers in infusions 
 of leaves, <fec., but their appearance is generally delayed 
 until the putrefaction is nearly over. Species of Rotifer 
 and Philodina appear in this way. A few marine forms 
 only have been described Brachionus mulleri, B. hepta- 
 (onus, Synchxta laltica, and others. 
 
 A few forms are parasitic. Albertia lives in the intestine 
 of the earthworm ; a form has been described as occurring 
 in the body-cavity of Synapta; a small form was also 
 observed to constantly occur in the velar and radial canals 
 of the freshwater jelly-fish, Limnocodium. Notommata 
 parasitica leads a parasitic existence within the hollow 
 spheres of Volvox globator, sufficient oxygen being given 
 off by the Volvox for its respiration. 
 
 Many Rotifers exhibit an extraordinary power of resist 
 ing drought. Various observers have dried certain species 
 upon the slide, kept them dry for a certain length of time, 
 and then watched them come to life very shortly after the 
 addition of a drop of water. The animal draws itself to- 
 gether, so that the cuticle completely protects all the softer 
 parts and prevents the animal itself from being thoroughly 
 dried. This process is not without parallel in higher 
 groups ; e.g., many land snails will draw themselves far into 
 the shell, and secrete a complete operculum, and can remain 
 in this condition for an almost indefinite amount of time. 
 The eggs are also able to withstand drying, and are pro- 
 bably blown about from place to place. The Rotifera can 
 bear great variations of temperature without injury. 
 
 Since their removal from among the Protozoa various 
 attempts have been made to associate the Rotifera with 
 one or other large phylum of the animal kingdom. 
 Huxley, insisting upon the importance of the trochal disk, 
 put forward the view that they were " permanent Echino- 
 derm larvae," and formed the connecting link between 
 the Nemei-tidse. and the Nematoid worms. Ray Lankester 
 proposed to associate them with the Cheetopoda, and 
 Arthropoda in a group Appendiculata, the peculiarities in 
 the structure of Pedalion forming the chief reason for 
 such a classification. There is, however, no proof that we 
 thus express any genetic relationship. The well-developed 
 coelom, absence of metameric segmentation, persistence of 
 the trochal disk in varying stages of development, and the 
 structure of the nephridia are all characters which point to 
 the Rotifera as very near representatives of the common 
 ancestors of at any rate the Mollusca, Arthropoda, and 
 C/txtopoda. But the high development of the mastax, 
 the specialized character of the lorica in many forms, the 
 movable spines of Polyarthra, the limbs of Pedalion, and 
 the lateral appendages of Asplanchna, the existence of a 
 diminutive male, the formation of two varieties of ova, all 
 point to a specialization in the direction of one or other of 
 the above mentioned groups. Such specialization is at 
 most a slight one, and does not justify the definite associa- 
 tion of the Rotifera in a single phylum with any of them. 
 
 Classification. The following classification has been 
 recently put forward by Dr C. T. Hudson (19). 
 
 CLASS ROTIFERA. 
 Order I. Rhizota. 
 
 Fixed forms ; foot attached, transversely wrinkled, noil-retractile 
 truncate. 
 
 Fam. 1. FLOSCULARIAD.S. Floscularia, Stephanoceros. 
 Fam. 2. MELICERTADJE. Melicerta, Cephalosiphon, Megalo- 
 trocha, Limnias, ^Ecistes, Lacinularia, Conochilus. 
 
ROTIFERA 
 
 93 
 
 Order II. Bdelloida. 
 
 Forms which swim and creep like a leech ; foot retractile 
 jointed, telescopic, termination furcate. 
 
 Fam. 3. PHILODIXAD.S. PhUodina, Rotifer, Callidina. 
 
 Order III. Ploima. 
 Forms which swim only. 
 
 Grade A. ILLORICATA. 
 
 Fam. 4. HYDATIXAD.&. Hydatina, Ehinops. 
 Fam. 5. SYXCHJJTAD.E. Synchsta, PolyarAra. 
 Fam. 6. NOTOJIMATAD.S. Notonimata, Diglena, Furcularia, 
 
 Scandium, Pleurotrocha, Distemma. 
 Fam. 7. TRIARTHUAD.K. -friarthra. 
 Fam. 8. ASPLAXCHXAD.S. Asplanchn 
 
 Grade B. LOEICATA. 
 Fam. 9. BRACHIONID.E. Brachionus, Noteus, Anursa, Sac- 
 
 culus. 
 
 Fam. 10. PrERODlNADjE. Ptcrodina, Pompholyx. 
 Fam. 11. EUCHLAXID.E. Euchlanis, Salpina, Diplax, Mono- 
 
 stylo,, Colurus, Monura, lletopodia, Stephanops, Monocerca, 
 
 Mastigocerca, Dinocharis. 
 
 Order IV. Scirtopoda. 
 
 Forms which swim with their ciliary wreath, and skip by means 
 of hollow limbs with internal locomotor muscles. 
 
 Fani. 12. PEDALIONIDJB. Pedalion. 
 
 The above list includes only the principal genera. There are, 
 however, a number of forms which could not be placed in any of 
 the above families. 
 
 ABERRANT FORMS. 
 
 Trochosphxra sequatorialis (fig. 6, G), found by Semper 
 in the Philippine Islands, closely resembles a monotrochal 
 
 FIG. 6. Various aberrant forms. A, Balatro talrta (after Claparede) : a, mastax. 
 B. Seison ndsalite (after Clans) : m, month ; rd, position of the aperture of the 
 vas dcfcrcns. C. Lindia torulcta: a, ciliated processes at the sides of the heat 
 representing cephalotroch ; of, eye-spots. D, E, and F, Apsilta lentiformit 
 (after Mecznikow). D, adult female with expanded proboscis: m, position of 
 the mouth : *, lateral sense-organs. E. yonng free-swimming female. F. adult 
 male. G, Trochotphxra Kquatorialit (after Semper) : m, mouth ; y, ganglion : 
 n, anus; 6, velum; oc, eye-spot; c, muscles. 
 
 polychsetons larva while possessing undoubtedly Rotiferal 
 characters. Mecznikow has described a remarkable form, 
 Apsilus lentiformis (fig. 6, D, E, and F), the adult female 
 of which is entirely devoid of cilia but possesses a sort of 
 retractile hood ; the young female and the males are not 
 thus modified. Claparede discovered fixed to the bodies 
 of small Oligochaetes a curious non-ciliated form, Balatro 
 calvus (fig. 6, A), which has a worm-like very contractile 
 body and a well-developed mastax. As mentioned above, 
 the ciliatiou is reduced to a minimum in the curious worm- 
 like form Lindia (fig. 6, c). Seison nebalix (fig. 6, B), 
 living on the surface of Ifebalix, which was described 
 originally by Grube, is the same form as the Satcobdella 
 nebalise, which was supposed by Van Beneden and Hesse 
 to be a leech. It has been shown by Glaus to be merely 
 an aberrant Rotifer. 
 
 Of the curious aquatic forms Idhydium, Chsetonotus, 
 Turbanella, Dasyditis, Cephalidium, Chxtura, and Hemi- 
 dasys, which Mecznikow and Claparede included under 
 the name Gastrotricha, no further account can be given 
 here. They are possibly allied to the Rotifera, but are 
 devoid of mastax and trochal disk. 
 
 Bibliography, 
 
 The following are some of the more important memoirs, &c., on 
 the Rotifera. 
 
 (1) Leenwenhoek, Phil. Trans., 1701-1704. 
 
 (2) TZhieu\XTg,DieInfusiimst}iierche>ialsvollkommeneOrganisme>i, 
 
 1838. 
 
 (3) II. F. Dujanlin, Hist. Nat. des Zoophytes: Infusoires, ]841. 
 
 (4) W. C. Williamson, " On Mclicerta ringens," Quart. Jour. 
 
 Micr. Sci., 1853. 
 
 (5) Ph. H. Gosse, "On Mtlieerta ringens," Quart. Jour. Micr. 
 
 Sci., 1853. 
 
 (6) T. H. Huxley, "OnLaeinulariasocialis," Trans. Micr. Soc., 
 
 1853. 
 
 (7) FT. Leydig, " Ueber den Ban nnd die systematische Stellung 
 
 der Raderthiere," Zeit. /. w. Zool., vi., 1854. 
 
 (8) Ph. H. Gosse, Phil. Trans., 1856. 
 
 (9) F. Cohn, Zeit.f. w. Zoo!., vii., ix., and xiL 
 
 (10) Ph. H. Gosse, Phil. Trans., 1858. 
 
 (11) Pritchard, Infusoria, 1861. 
 
 (12, 13, 14) C. T. Hudson, " On Pedalion," Quart. Jour. Micr. 
 Sci., 1872, and Monthly Micr. Jour., 1871 and 1872. 
 
 (15) E. Ray Lankester, "On Pedalion," Quart. Jour. Sci., 1872. 
 
 (16) El. Mecznikow, "On Apsilus Itntiformis," Zeit^f. w. Zool., 
 
 1872. 
 
 (17) C. Semper, "On Trochosphasra," Zeit. f. w. Zool., xxiL. 
 
 1872. 
 
 (18) K. Eckstein, "Die Rotatorien der Umgegend von Giessen," 
 
 Zcit.f. w. Zool., 1883. 
 
 (19) C. T. Hudson, "On an Attempt to reclassify Rotifers," 
 
 Quart. Jour. Micr. Sci., 1884. 
 
 (A. G. B.) 
 
M L L U S C A 
 
 THE Mollusca form one of the great " phyla," or sub- 
 kingdoms of the Animal Pedigree or Kingdom. 
 
 Literary History of the Group. The shell-bearing forms 
 belonging to this group which were known to Linnaeus were 
 placed by him (in 1748) in the third order of his class 
 Vermes under the name "Testacea," whilst the Echino- 
 derms, Hydroids, and Annelids, with the naked Molluscs, 
 formed his second order, termed " Zoophyta." Ten years 
 later he replaced the name "Zoophyte" by "Mollusca," 
 which was thus in the first instance applied, not to the 
 Mollusca at present so termed, but to a group consisting 
 chiefly of other organisms. Gradually, however, the term 
 Mollusca became used to include those Mollusca formerly 
 placed among the "Testacea," as well as the naked Mollusca. 
 
 It is important to observe that the term /mAaicia, of which 
 Mollusca is merely a Latinized form, was used by Aristotle 
 to indicate a group consisting of the Cuttle-fishes only. 
 
 The definite erection of the Mollusca into the position 
 of one of the great primary groups of the animal kingdom 
 is due to George Cuvier (1788-1800), who largely occupied 
 himself with the dissection of representatives of this type (I). 1 
 An independent anatomical investigation of the Mollusca 
 had been carried on by the remarkable Neapolitan natur- 
 alist Poli (1791), whose researches (2) were not published 
 until after his death (1817), and were followed by the 
 beautiful works of another Neapolitan zoologist, the illus- 
 trious Delle Chiaje (3). 
 
 The " embranchement " or sub-kingdom Mollusca, as de- 
 fined by Cuvier, included the folio wing classes of shell-fish : 
 1, the cuttles or poulps, under the name CEPHALOPODA; 2, 
 the snails, whelks, and slugs, both terrestrial and marine, 
 under the name GASTEOPODA; 3, the sea-butterflies or 
 winged-snails, under the name PTEEOPODA ; 4, the clams, 
 mussels, and oysters, under the name ACEPHALA; 5, the 
 lamp-shells, under the name BEACHIOPODA ; 6, the sea- 
 squirts or ascidians, under the name NUDA ; and 7, the 
 barnacles and sea-acorns, under the name CIBRHOPODA. 
 
 The main limitations of the sub-kingdom or phylum 
 Mollusca, as laid down by Cuvier, and the chief divisions 
 thus recognized within its limits by him, held good to the 
 present day. At the same time, three of the classes con- 
 sidered by him as Mollusca have been one by one removed 
 from that association in consequence of improved know- 
 ledge, and one additional class, incorporated since his day 
 with the Mollusca with general approval, has, after more 
 than forty years, been again detached and assigned an 
 independent position owing to newly-acquired knowledge. 
 
 The first of Cuvier's classes to be removed from the Mol- 
 lusca was that of the Cirrhopoda. Their affinities with the 
 lower Crustacea were recognized by Cuvier and his contem- 
 poraries, but it was one of the brilliant discoveries of that 
 remarkable and too-little-honoured naturalist, J. Vaughan 
 Thompson of Cork, which decided their position as Crus- 
 tacea. The metamorphoses of the Cirrhopoda were described 
 and figured by him in 1830 in a very complete manner, 
 and the legitimate conclusion as to their affinities was for- 
 mulated by him (4). Thus it is to Thompson (1830), and 
 not to Burmeister (1834), as erroneously stated by Kefer- 
 stein, that the merit of this discovery belongs. The next 
 class to be removed from Cuvier's Mollusca was that of the 
 Nuda, better known as Tunicata. In 1 866 the Russian 
 embryologist Kowalewsky startled the zoological world with 
 a minute account of the developmental changes of Ascidia, 
 one of the Tunicata (5), and it became evident that the 
 
 1 These figures refer to the bibliography at the end of the article, 
 p. 695. 
 
 affinities of that class were with the Yertebrata, whilst their 
 structural agreements with Mollusca were only superficial. 
 The last class which has been removed from the Cuvierian 
 Mollusca is that of the Lamp-shells or Brachiopoda. The 
 history of its dissociation is connected with that of the 
 class, viz., the Polyzoa or Bryozoa, which has been both 
 added to and again removed from the Mollusca between 
 Cuvier's date and the present day. The name of J. 
 Vaughan Thompson is again that which is primarily con- 
 nected with the history of a Molluscan class. In 1830 
 he pointed out that among the numerous kinds of " polyps" 
 at that time associated by naturalists with the Hydroids, 
 there were many which had a peculiar and more elaborate 
 type of organization, and for these he proposed the name 
 Polyzoa (6). Subsequently (7) they were termed Bryozoa 
 by Ehrenberg (1831). 
 
 Henri Milne-Edwards in 1844 demonstrated (8) the affi- 
 nities of the Polyzoa with the Molluscan class Brachiopoda, 
 and proposed to associate the three classes Brachiopoda, 
 Polyzoa, and Tunicata in a large group " Molluscoidea," 
 coordinate with the remaining classes of Cuvier's Mollusca, 
 which formed a group retaining the name Mollusca. By 
 subsequent writers the Polyzoa have in some cases been kept 
 apart from the Mollusca and classed with the " Yermes ; " 
 whilst by others (including the present writer) they have, 
 together with the Brachiopoda, been regarded as true Mol- 
 lusca. The recent investigation by Mr. Caldwell (1882) 
 of the developmental history of Phoronis (9), together 
 with other increase of knowledge, has now, however, estab- 
 lished the conclusion that the agreement of structure 
 supposed to obtain between Polyzoa and true Mollusca is 
 delusive ; and accordingly they, together with the Brachi- 
 opoda, have to be removed from the Molluscan phylum. 
 Further details in regard to this, the last revolution in Mol- 
 luscan classification, will be found in the article POLYZOA. 
 As thus finally purified by successive advances of em- 
 bryological research, the Mollusca are reduced to the 
 Cuvierian classes of Cephalopoda, Pteropoda, Gastropoda, 
 and Acephala. Certain modifications in the disposition of 
 these classes are naturally enough rendered necessary by 
 the vast accumulation of knowledge as to the anatomy and 
 embryology of the forms comprised in them during fifty 
 years. Foremost amongst those who have within that 
 period laboured in this group are the French zoologists 
 ; Henri Milne-Edwards (20) and Lacaze Duthiers (21), to 
 the latter of whom we owe the most accurate dissections 
 and beautiful illustrations of a number of different types. 
 ' To Kolliker (22), Gegenbaur (23), and more recently Spengel 
 (24), amongst German anatomists, we are indebted for 
 I epoch-making researches of the same kind. In England, 
 j Owen's anatomy of the Pearly Nautilus (10), Huxley's dis- 
 cussion of the general morphology of the Mollusca (11), 
 1 and Lankester's embryological investigations (12), have 
 aided in advancing our knowledge of the group. Two 
 ; remarkable works of a systematic character dealing with 
 ' the Mollusca deserve mention here the Manual of the 
 Mollusca, by the late Dr. S. P. Woodward, a model of clear 
 : systematic exposition, and the exhaustive treatise on the 
 : Malacozoa or Weichthiere by the late Professor Keferstein 
 of Gottingen, published as part of Bronn's Classen und 
 Ordnvngen des Thier-Reichs. The latter work is the most 
 completely illustrated and most exhaustive survey of exist- 
 ing knowledge of a large division of the animal kingdom 
 which has ever been produced, and, whilst forming a monu- 
 ment to its lamented author, places the student of Mol- 
 luscan morphology in a peculiarly favourable position. 
 
96 
 
 MOLLUSCA 
 
 Classes of the Mollusca. The classes of the Mollusca 
 which we recognize are as follows : 
 Phylum MOLLUSCA. 
 
 BRANCH A. Glossophora. BRANCH B. Lipocephala 
 
 ( = Acephala, Cuvier). 
 
 Class 1. GASTROPODA. Class 1. LAMELLIBRANCHIA 
 
 Br. a. Isopleura. (Syn. Conchifera). 
 
 Examples Chiton, Neo- Examples Oyster, Mussel, 
 
 menia. Clam, Cockle. 
 Br. b. Anisopleura. 
 Examples Limpet, Whelk, 
 Snail, Slug. 
 
 Class 2. SCAPHOPODA. 
 Example Tooth-shell . 
 
 Class 3. CEPHALOPODA. 
 
 Br. a. Pteropoda. 
 Examples Hyalsea, Pneu- 
 modermon. 
 
 Br. Jt. Siphonopoda. 
 Examples Nautilus, Cut- 
 tles, Poulp. 
 
 General Characters of the Mollusca. The forms com- 
 prised in the above groups, whilst exhibiting an extreme 
 range of variety in shape, as may be seen on comparing 
 an Oyster, a Cuttle-fish, and a Sea-slug such as Doris; 
 whilst adapted, some to life on dry land, others to the 
 depths of the sea, others to rushing streams ; whilst capable, 
 some of swimming, others of burrowing, crawling, or jump- 
 ing, some, on the other hand, fixed and immobile; some 
 amongst the most formidable of carnivores, others feed- 
 ing on vegetable mud, or on the minutest of microscopic 
 organisms yet all agree in possessing in common a very 
 considerable number of structural details which are not 
 possessed in common by any other animals. 
 
 The structural features which the Mollusca do possess 
 in common with other animals belonging to other great 
 phyla of the animal kingdom are those characteristic of 
 the Ccelomata, one of the two great grades (the other and 
 lower being that of the Ccelentera) into which the higher 
 animals, or Enterozoa as distinguished from the Protozoa, 
 are divided (13). The Enterozoa all commence their indivi- 
 dual existence as a single cell or plastid, which multiplies 
 itself by transverse division. Unlike the cells of the Proto- 
 zoa, these embryonic cells of the Enterozoa do not remain each 
 like its neighbour and capable of independent life, but pro- 
 ceed to arrange themselves in two layers, taking the form 
 of a sac. The cavity of the two-cell-layered sac or Diblas- 
 tula thus formed is the primitive gut or ARCH-ENTERON. 
 In the Ccelentera, whatever subsequent changes of shape 
 the little sac may undergo as it grows up to be Polyp or 
 Jelly-fish, the original arch-enteron remains as the one 
 cavity pervading all regions of the body. In the Ccelomata 
 the arch-enteron becomes in the course of development 
 divided into two totally distinct cavities shut off from one 
 another an axial cavity, the MET-ENTERON, which retains 
 the function of a digestive gut ; and a peri-axial cavity, 
 the CCELOM or body-cavity, which is essentially the blood- 
 space, and receives the nutritive products of digestion and 
 the waste products of tissue-change by osmosis. The 
 Mollusca agree in being Ccelomate with the phyla Verte- 
 brata, Platyhelmia (Flat- worms), Echinoderma, Appendicu- 
 lata (Insects, Ringed-worms, &c.), and others, in fact, 
 with all the Enterozoa except the Sponges, Corals, Polyps, 
 and Medusae. 
 
 In common with all other Ccelomata, the Mollusca 
 are at one period of life possessed of a PROSTOMIUM 
 or region in front of the mouth, which is the essential 
 portion of the " head," and is connected with the property 
 of forward locomotion in a definite direction and the steady 
 carriage of the body (as opposed to rotation of the body 
 on its long axis). As a result, the Ccelomata, and with 
 them the Mollusca, present (in the first instance) the general 
 
 condition of body known as BILATERAL SYMMETRY; the 
 dorsal is differentiated from the ventral surface, whilst a 
 right and a left side similar to, or rather the complements 
 of, one another are permanently established. In common 
 with all other Ccelomata, the Mollusca have the mouth and 
 first part of the alimentary canal which leads into the 
 met-enteron formed by a special invagination of the outer 
 layer of the primitive body- wall, not to be confounded with 
 that which often, but not always, accompanies the ante- 
 cedent formation of the arch-enteron ; this invagination 
 is termed the STOMOD^EUM. Similarly, an anal aperture is 
 formed in connexion with a special invagination which 
 meets the hinder part of the met-enteron, and is termed 
 the PROCTOD.EUM. 
 
 In common with many (if not all) Coelomata, the Mol- 
 lusca are provided with at least one pair of tube-like organs, 
 which open each by one end into the ccelom or body cavity, 
 and by the other end to the exterior, usually in the neigh- 
 bourhood of the anus. These are the NEPHRIDIA. 
 
 Like all other Ccelomata, the Mollusca are also provided 
 with special groups of cells forming usually paired or median 
 growths upon the walls of the ccelomic cavity, the cells 
 being specially possessed of reproductive power, and dif- 
 ferentiated as egg-cells and sperm-cells. These are the 
 GONADS. As in other Ccelomata, the cells of the gonads 
 may escape to the exterior in one of two ways either 
 through the nephridia, or, on the other hand, by special 
 apertures. 
 
 As in all other Ccelomata, the cells, which build up 
 respectively the primary outer layer of the body, the 
 lining layer of the met-enteron, and the lining layer of the 
 ccelom, are multiplied and differentiated in a variety of 
 ways in the course of growth from the early embryonic 
 condition. TISSUES are formed by the adhesion of a num- 
 ber of similarly modified cells in definite tracts. As in all 
 Ccelomata, there is a considerable variety of tissues char- 
 acterized by, and differentiated in relation to, particular 
 physiological activities of the organism. Not only the 
 Ccelomata but also many Ccelentera show, in addition to 
 the EPITHELIA (the name given to tissue which bounds a 
 free surface, whether it be that of the outer body-wall, of 
 the gut, or of a blood-space), also deeper lying tissues, 
 of which the first to appear is MUSCULAR tissue, and the 
 second NEEVOUS tissue. 
 
 The epithelia are active in throwing off their constituent 
 cells (blood-corpuscles from the wall of the ccelom), or in 
 producing secretions (glands of body-wall and of gut), or 
 in forming horny or calcareous plates, spines, and pro- 
 cesses, known as CUTICULAR PRODUCTS (shells and bristles 
 of the body- wall, teeth of the tongue, gizzard, &c.). 
 
 In the Mollusca, as in all other Ccelomata, in correspond- 
 ence with the primary bilateral symmetry and in relation 
 to the special mechanical conditions of the prostomium, 
 the nervous tissue which is in Ccelentera, and even in Flat- 
 worms, diffused over the whole body in networks, tends 
 to concentrate in paired lateral tracts, having a special 
 enlargement in the prostomium. The earlier plexiform 
 arrangement is retained in the nervous tissue of the walls 
 of the alimentary canal of many Ccelomata, whilst a con- 
 centration to form large nerve-masses (GANGLIA), to which 
 numerous afferent and efferent fibres are attached, affects 
 the nervous tissue of the body- wall. 
 
 In all Ccelomata, including Mollusca, muscular tissue is 
 developed in two chief layers, one subjacent to the deric or 
 outer epithelium (SOMATIC MUSCULATURE), and a second sur- 
 rounding the alimentary canal (SPLANCHNIC MUSCULATURE). 
 Thus, primarily, in Ccelomata the body has the character of 
 two muscular sacs or tubes, placed one within the other 
 and separated from one another by the ccelomic space. 
 The somatic musculature is the more copious and develops 
 
MOLLUSCA 
 
 97 
 
 very generally an outer circular layer (i.e., a layer in which 
 the muscular fibres run in a direction transverse to the 
 long axis of the body) and a deeper longitudinal layer ; 
 to these oblique and radiating fibres may be added. The 
 splanchnic musculature, though more delicate, exhibits a 
 circular layer nearer the enteric epithelium, and a longi- 
 tudinal layer nearer the coelomic surface. 
 
 In Ccelomata and in many Coelentera there are found 
 distributed between the tracts of muscular tissue, bounding 
 them and giving strength and consistency also to the walls 
 of the body, of the alimentary canal, of the ccelom, and of 
 the various organs and tissue-masses (such as nerve-centres, 
 gonads, fcc.) connected with these, tracts of tissue the 
 function of which is skeletal. The SKELETAL TISSUE of 
 Mollusca, in common with that of other Ccelomata, exhibits 
 a wide range of minute structure, and is of differing density 
 in various parts ; it may be fibrous, membranous, or carti- 
 laginous. The Mollusca, in common with the other Ccelo- 
 mata, exhibit a remarkable kind of association between the 
 various forms of skeletal tissue and the epithelium which 
 lines the ccelomic cavity. The coelomic cavity contains a 
 liquid which is albuminous in chemical composition (BLOOD- 
 LYMPH or H.EMOLYMPH), and into this liquid cells are shed 
 from the eoelomic epithelium. They float therein and are 
 known as BLOOD CORPUSCLES or LYMPH CORPUSCLES. The 
 ccelomic space with its contained haemolymph is not usually 
 in Ccelomata, and is not in Mollusca, a simple even-wailed 
 cavity, but is broken up into numerous passages and re- 
 cesses by the outgrowths, both of the alimentary canal and 
 of its own walls. By the adhesion of its opposite walls, 
 and by an irregularity in the process of increase of its area 
 during growth, the ccelom becomes to a very large extent a 
 spongy system of intercommunicating LACUSJ; or irregular 
 spaces, filled with the ccelomic fluid. At the same time, the 
 ccelomic space has a tendency to push its way in the form of 
 narrow canals and sinuses between the layers of skeletal tissue, 
 and thus to permeate together with the skeletal tissue in 
 the form of a spongy, or it may be a tubular, network all 
 the apparently solid portions of the animal body. This 
 association of the nutritive and skeletal functions is accom- 
 panied by a complete identity of the tissues concerned in 
 these functions. Not only is there complete gradation 
 from one variety of skeletal tissue to another (e.g., from 
 membranous to fibrous, and from fibrous to cartilaginous) 
 even in respect of the form of the cells and their intercellular 
 substance, but the ccelomic epithelium, and consequently 
 the hsemolymph with its floating corpuscles derived from 
 that epithelium, is brought into the same continuity. The 
 skeletal and blood-containing and -producing tissues in fact 
 form one widely- varying but continuous whole, which may 
 be called the SKELETO-TEOPHIC SYSTEM OF TISSUES. 
 
 In many Coelomata not only do the skeletal tissues 
 allow the coelomic space with its fluid and corpuscles to 
 penetrate between their layers, but a special mode of 
 extension of that space is found, which consists in the 
 hollowing out of the solid substance of elongated cells 
 having the form of fibres, which thus become tubular, 
 and, admitting the nutritive fluid, serve as channels for 
 its distribution. These are " capillary vessels," and it has 
 yet to be shown that such are formed in the Mollusca. 
 Larger vessels, however, concerned in guiding the move- 
 ment of the coelomic fluid in special directions are very 
 usually developed in the Mollusca, as in other Ccelomata, 
 by the growth of skeletal tissue around what are at first 
 ill-defined extensions of the ccelomic space. In this way 
 a portion of the ccelomic space becomes converted into 
 vessels, whilst a large part remains with irregular walls 
 extending in every direction between the skeletal tissues 
 and freely communicating with the system of vessels. As 
 in many other Ccelomata, muscular tissue grows around 
 
 the largest vessel formed from the primitive ccelom, which 
 thus becomes a contractile organ for propelling the blood- 
 lymph fluid. This " HEART " has in Mollusca, as in most 
 other Ccelomata in which it is developed, a dorsal position. 
 A communication of the blood-lymph space with the 
 exterior by means of a pore situated in the foot or else- 
 where has been very generally asserted to be characteristic 
 of Mollusca. It has been maintained that water is intro- 
 duced by such a pore into the blood, or admitted into a 
 special series of water-vessels. It has also been asserted 
 that the blood-fluid is expelled by the Mollusca from these 
 same pores. Recent investigation (14) has, however, made 
 it probable that the pores are the pores of secreting glands, 
 and do not lead into the vascular system. There is, it there- 
 fore appears, no admission or expulsion of water through 
 such pores in connexion with the blood, although in some 
 other Ccelomata it is established that water is taken into 
 the ccelomic space through a pore (Echinoderms), whilst in 
 some others there is no doubt that the ccelomic haemolymph 
 is occasionally discharged in quantity through pores of defi- 
 nite size and character (Earthworm, ic.). 
 
 We have thus seen that the Mollusca possess, in common 
 with the other Ccelomata 1, a body composed of a vast 
 number of cells or plastids, arranged so as to form a sac- 
 like body-wall, and within that a second sac, the met-enteron, 
 the wall of which is separated from the first by a coflom or 
 blood-lymph space ; 2, a stomodxum and a proctodxum ; 
 3, a prostomium, together with a differentiated dorsal and 
 ventral surface, and consequently right and left sides, i.e., 
 bilateral symmetry ; 4, a pair of nephridia ; 5, gonads 
 developed on the wall of the ccelom ; 6, deric epithelium 
 (producing horny and calcareous deposits on its surface), 
 enteric epithelium, and ccelomic epithelium; 7, laterally 
 paired masses of nerve-tissue, especially large in the pro- 
 stomial region (nerve-centres or ganglia) ; 8, muscular 
 tissue, forming a somatic tunic and a splanchnic tunic ; 9, 
 skeleto-trophic tissues, consisting of membranous, fibrous, and 
 cartilaginous supporting tissues, and of blood-vessels and the 
 walls of blood-spaces, the coelomic epithelium, and the liquid 
 tissue known as kxmolymph (commonly blood). 
 
 Schematic Mollusc. Starting from this basis of structural 
 features common to them and the rest of the Ccelomata, 
 we may now point out what are the peculiar developments 
 of structure which characterize the Mollusca and lead to 
 the inference that they are members of one peculiar branch 
 or phylum of the animal pedigree. In attempting thus to 
 set forth the dominating structural attributes of a great 
 group of organisms it is not possible to make use of arbi- 
 trary definitions. Of Mollusca, as of other great phyla, it 
 is not possible categorically to enunciate a series of struc- 
 tural peculiarities which will be found to be true in refer- 
 ence to every member of the group. We have to remember 
 that the process of adaptation in the course of long ages 
 of development has removed in some cases one, in other 
 cases another, of the original features characteristic of the 
 ancestors from which the whole group may be supposed to 
 have taken origin, and that it is possible (and actually is 
 realized in fact) that some organisms may have lost all the 
 primary characteristics of Molluscan organization, and yet 
 be beyond all doubt definitely stamped as Mollusca by 
 the retention of some secondary characteristic which is so 
 peculiar as to prove their relationship with other Mollusca. 
 An example in point is found in the curious fish-like form 
 Phyllirhoe (fig. 58), which has none of the primary char- 
 acteristics of a Mollusc, and yet is indisputably proved to 
 belong to the Molluscan phylum by possessing the peculiar 
 and elaborate lingual apparatus present in one branch of 
 the phylum, the Glossophora. 
 
 In order to exhibit concisely the peculiarities of organi- 
 zation which characterize the Mollusca, we find it most 
 
08 
 
 MOLLUSCA 
 
 convenient to construct a schematic Mollusc, which shall 
 possess in an unexaggerated form the various structural 
 arrangements which are more or less specialized, exagger- 
 ated, or even suppressed in particular members of the group. 
 Such a schematic Mollusc is not to be regarded as an arche- 
 
 d 
 
 m. 
 
 9P l 
 
 \ l> I I I If 7 
 
 * (IV 1 
 
 y.pe z.l a y.ab 
 
 Fio. 1. Schematic Mollusc. A. Dorsal aspect. B. Ventral aspect. C. The 
 heart, pericardium, gonacls, and nephridia shown in position. D. The nervous 
 system ; the reader is requested to note that the cord passing backwards 
 from g.pe lies beneath, and does not in any way unite with the cord which 
 passes from g.ab to g.pl. E. Diagram in which the body-wall is represented 
 as cut in the median antero-posterior plane, so as to show organs in position, 
 the shell-sac is seen in section, but the shell is omitted. 
 Letters in all the figures as follows : a, cephalic tentacle ; i>, head ; c, edge 
 
 of the mantle-skirt or limbus pallialis ; d, dotted line indicating the line of 
 origin of the free mantle-skirt from the sides of the visceral hump ; e, outline 
 of the foot seen through the mantle-skirt in A, which is supposed to be trans- 
 
 parent, allowing the position of this and of the various parts h, i, k, I, m, to 
 be seen through its substance ; /, edge of the shell-follicle ; g, the shell ; h, 
 the osphradium, paired (Spengel's olfactory organ) ; i, the ctenidimn, paired 
 (gill-plume) ; k, aperture of the gonad, paired ; I, aperture of one of the two 
 nephridia ; m, anus ; n, posterior region of the foot reaching farther back 
 than the mass of viscera (dorsal hump) which it carries ; o, mouth ; p, plantar 
 surface of the foot ; q, cut edge of the body-wall of the dorsal region ; r, 
 coelomic space (blood-lymph space or body-cavity), mostly occupied by liver, 
 but to some extent retained as blood-channels and lacunee ; s, pericardia! 
 cavity ; t, gonad (ovary or spermary), paired ; 1(, nephridium, paired ; v, ven- 
 tricle of the heart receiving the right and the left auricles at its sides, and 
 sending off anteriorly a large vessel, posteriorly a small one ; to, the cephalic 
 eye, paired ; x, dotted ring to show the position occupied by the oesophagus 
 in relation to the nerve ganglia and cords ; y, the otocyst, paired ; z.l, the 
 digestive gland (so-called "liver") of the left side ; z.g, duct of the digestive 
 gland of the right side ; g.c, cerebral ganglion united by the cerebral com- 
 missure to its fellow ; g.pl, pleural ganglion xinited by the cerebro-pleural 
 connective to the cerebral ganglion, and by the pleuro-pedal connective to 
 the pedal ganglion ; g.pe, the pedal ganglion united to its fellow by the pedal 
 commissure the two sending off posteriorly the long ladder-like pair of pedal 
 nerves ; g.v, the visceral ganglion (of the left side) united by the visceral 
 loop or commissure to the similar ganglion on the right side, and by the 
 viscero-pleural connective to the pleural ganglion ; g.ati, abdominal ganglion 
 developed on the course of the visceral loop ; g.olf, olfactory ganglion placed 
 near the osphradium on a nerve taking its origin from the visceral ganglion. 
 
 type, in the sense which has been attributed to that word, 
 nor as the embodiment of an idea present to a creating mind, 
 nor even as an epitome of developmental laws. Were know- 
 ledge sufficient, we should wish to make this schematic 
 
 Mollusc the representation of the actual Molluscan ancestor 
 from which the various living forms have sprung. To defi- 
 nitely claim for our schematic form any such significance 
 in the present state of knowledge would be premature, 
 but it may be taken as more or less coinciding with what 
 we are justified, under present conditions, in picturing to 
 ourselves as the original Mollusc or archi-Mollusc (more 
 correctly Archimalakion). After describing this schematic 
 form, we shall proceed to show how far it is realized or 
 justified in each class and order of Mollusca successively. 
 
 The schematic Mollusc (fig. 1, A to E) is oblong in 
 shape, bilaterally symmetrical, with strongly differentiated 
 dorsal and ventral surface, and has a well-marked HEAD, 
 consisting of the prostomium (6) and the region imme- 
 diately behind the mouth. Upon the head we place a 
 pair of short CEPHALIC TENTACLES (a). The mouth is 
 placed in the median line anteriorly, and is overhung by 
 the prostomium (B, o) ; the anus is placed in the median 
 line posteriorly, well raised on the dorsal surface (A, m). 
 The apertures of a pair of NEPHRIDIA are seen in the 
 neighbourhood of the anus right and left (A, I). Near 
 the nephridial apertures, and in front of them, right and 
 left, are the pair of apertures (k) appropriate to the ducts 
 of the GONADS (generative pores). 
 
 The most permanent and distinctive Molluscan organ 
 is the FOOT (Podium). This is formed by an excessive 
 development of the somatic musculature along the ventral 
 surface, distinctly ceasing at the region of the head, below 
 which it suddenly projects as a powerful muscular mass 
 (B, p ; E, p). It may be compared, and is probably genetic- 
 ally identical, with the muscular ventral surface of the 
 Planarians and with the suckers of Trematoda, but is more 
 extensively developed than are those corresponding struc- 
 tures. The muscular tissue of the foot, and of all other 
 parts of the body of all Mollusca, is cellular and unstriated, 
 as distinguished from the composite muscular fibre (con- 
 sisting of cell-fusions instead of separable cells) which 
 occurs in Arthropoda and in Vertebrata, and which has 
 the further distinction of being composed of alternating 
 bands of substance of differing refractive power (hence 
 " striated "). The appearance of cross striation seen in 
 the muscular cells of some Molluscs (odontophore of 
 Haliotis, Patella, <fec.) requires further investigation. It 
 is by no means altogether the same thing as the mark- 
 ing characteristic of striated muscular fibre. 
 
 Contrasting with the ventral foot is the thin -walled 
 dorsal region of the body, which may be termed the anti- 
 podial region. This thin-walled region is formed by soft 
 viscera covered in by the comparatively delicate and non- 
 muscular body- wall (fig. 1, E). As the ventral foot is 
 clearly separate from the projecting head, so is this dorsal 
 region, and it is conveniently spoken of as the VISCERAL 
 HUMP or "dome" (cupola). Protecting the visceral dome 
 is a SHELL (conchylium) consisting of a horny basis impreg- 
 nated with carbonate of lime, 1 and secreted by the deric 
 epithelium of this region of the body (g). The shell 
 in our schematic Mollusc is single, cap-shaped, and sym- 
 metrical. It does not lie entirely naked upon the surface 
 of the visceral dome, but is embedded all round its margin, 
 to a large extent in the body-wall. In fact, the integu- 
 ment of the visceral dome forms an open flattened sac 
 in which the shell lies. This is the PRIMARY SHELL- 
 SAC, or FOLLICLE (A and E, /). The wall of the body pro- 
 jects all round the visceral dome in the form of a flap or 
 skirt, so as to overhang and conceal to some extent the 
 head and the sides of the foot. This skirt, really an out- 
 
 1 As to the minute structure of the shell in various classes, see 
 Carpenter's article " Shell " in the Cyclop. o/Anat. and Physiol. The 
 limits of our space do not permit us to deal with this or other histo- 
 logical topics. 
 
MOLLUSCA 
 
 99 
 
 growth of the dorsal body-wall, is called the MANTLE-FLAP 
 (limbus pallialis), or more shortly the MANTLE or PALLIUM 
 (c). The space between the overhanging mantle-flap and the 
 sides and neck of the animal which it overhangs is called 
 the SUB-PALLIAL SPACE or CHAMBER. Posteriorly in this 
 space are placed the anus and the pair of nephridial aper- 
 tures (see fig. 1, E). 
 
 The development of the mantle-skirt and its sub-pallial 
 space appears to have a causal relation, in the way of pro- 
 tection, to a pair of processes of the body-wall which 
 spring, one on the right and- one on the left, from the sides 
 of the body, nearer the anus than the mouth, and are 
 concealed by the mantle-flap to some extent (A, B, t). 
 These processes have an axis in which are two blood-vessels, 
 and are beset with two rows of flattened filaments, like the 
 teeth of a comb in double series. These are the CTENIDIA 
 or gill-combs. Usually, as will be seen in the sequel, they 
 play the part of gills, but since in many Molluscs (Lamelli- 
 branchs) their function is not mainly respiratory, and since 
 also other completely-formed gills are developed as special 
 organs in some Molluscs to the exclusion of these pro- 
 cesses, it is well not to speak of them simply as " gills " or 
 " branchiae," but to give them a non-physiological name 
 such as that here proposed. Near the base of the stem of 
 each ctenidium is a patch of the epithelium of the body- 
 wall, peculiarly modified and supplied with a special nerve 
 and ganglion. This is Spengel's olfactory organ, which 
 tests the respiratory fluid, and is persistent in its position 
 and nerve -supply throughout the group Mollusca. We 
 propose to call it the OSPHBADIUM. 
 
 Passing now to the internal organs, our schematic 
 Mollusc is found to possess an ALIMENTARY CANAL, which 
 passes from mouth to anus in the middle line, leaving 
 between it and the muscular body-wall a more or less 
 spongy, in parts a spacious, CCELOM. The stomodaeum is 
 large and muscular, the proctodxum short ; the bulk of 
 the alimentary canal is therefore developed from the met- 
 enteron or remnant of the arch-enteron after the coalom 
 has been pinched off from it. A paired outgrowth of the 
 met-enteron forms the glandular diverticulum known as 
 the digestive gland or (commonly) liver (E, zg, zl). 
 
 Dorsally to the alimentary tract the ccelom is spacious. 
 The space (C, E, *) is termed the PERICARDIUM, since it is 
 traversed by a vessel running fore and aft in the median 
 line, which has contractile muscular walls and serves as a 
 heart to propel the coelomic blood-fluid. This pericardial 
 space, although apparently derived from the original ccelom, 
 is not in communication with the other spaces and blood- 
 vessels derived from the ccelom ; it never (or perhaps in a 
 very few instances) contains in the adult the Molluscan blood 
 or hsemolymph, and is always in free communication with 
 the exterior through the tubes called nephridia (renal 
 organs). The HEART receives symmetrically on each side, 
 right and left, a dilated vessel bringing aerated blood from 
 the ctenidia. These dilated vessels are termed the auricles 
 of the heart, whilst the median portion itself, at the point 
 where these vessels join it, is termed the ventricle of the 
 heart (C, v). The vessel passing fore and aft from the 
 ventricle gives off a few trunks which open into spaces 
 of the ccelom, the so-called lacunae ; these are excavated in 
 every direction between the viscera and the various bundles 
 of fibrous and muscular tissue, and may assume more or 
 less the character of tube-like vessels with definite walls. 
 Right and left opening into the pericardial coslom is a 
 coiled tube, the farther extremity of which opens to the 
 exterior by the side of the anus. These two tubes (C, u) 
 are the symmetrically disposed NEPHRIDIA (renal organs). 
 
 The GONADS (ovaries or spermaries) are placed in the 
 mid-dorsal region of the ccelom (C, t), and have their own 
 apertures in the immediate neighbourhood of those of the 
 
 nephridia. The apertures are paired right and left, and so 
 are the ducts into which they lead ; but at present we have 
 no ground for determining whether the gonad itself was 
 primarily in Molluscs a paired organ or a median organ, 
 nor have we any well-founded conception as to the nature 
 of the ducts when present, and their original relationship 
 
 ft- 
 
 6r 
 
 Fro. 2. Ctenidia of various Molluscs (original). A. Of Chiton ; /.., fibrons 
 tissue ; a.b.v., afferent blood-vessel ; e.b.v., efferent blood-vessel ; g.l., later- 
 ally paired lameUse. B. Of Sepia ; letters as in A. C. Of Fissnrella ; letters 
 as 'in A. D. Of Nncula ; <J, position of axis with blood-vessels ; a, inner ; 
 6 and c, outer row of lamellae. E. Of Paludina ; t, intestine running parallel 
 with the axis of the ctenidium and ending in the anus a ; br, rows of elongate 
 processes corresponding to the two series of lamellae of the upper figures. 
 
 to the gonads. The genital ducts of some organisms are 
 modified nephridia, but the nature of those of Mollusca, 
 of Arthropoda, of Echinoderma, of Nematoidea, and of 
 some Vertebrata has yet to be elucidated. 
 
 The disposition of the nerve-centres is highly character- 
 istic. There are four long cords composed of both nerve- 
 fibres and nerve-cells which are disposed in pairs, two right 
 and left of the pedal area or foot, two more dorsally and 
 tending to a deeper position than that occupied by the 
 pedal cords, so as to lie freely within the ccelomic space 
 unattached to the body- wall. These are respectively the 
 
 PEDAL NERVE-CORDS and the VISCERAL NERVE-CORDS. The 
 
 latter meet and join one another posteriorly. A right and 
 left (D, g.v), and a median abdominal (g.ab) ganglion are 
 placed on these cords, and from them are given off the 
 osphradial nerves which have special ganglia (g.olf). In the 
 region of the prostomium the pedal nerve-cords are enlarged 
 behind the mouth, forming fo&ptdal ganglia (ff.pe), and 
 are united by nerve-fibres to one another. From this spot 
 they are continued forward into the prostomium, where 
 they enlarge to form the right and left cerebral ganglia (ff.c), 
 which are united to one another by nerve-fibres in front of 
 
100 
 
 MOLLUSCA 
 
 the mouth, just as the pedal ganglia are behind it. The 
 right and left pedal ganglia are joined by transverse cords 
 to the right and left visceral cords respectively, the point 
 of union being marked on either side by a swelling (ff.pl) 
 known as the pleural ganglion. The visceral nerve-cord 
 can also be traced up on each side beyond the pleural 
 ganglion to the cerebral ganglion. Thus we have a 
 nearly complete double nerve-ring formed around the oeso- 
 phagus by the two pairs of nerve-cords which are in this 
 region drawn, as it were, towards each other and away 
 from their lateral position both behind and before the 
 stomodaeal invagination. Whilst the swollen parts of the 
 nerve-tracts are termed ganglia, the connecting cords 
 are conveniently distinguished either as connectives or as 
 commissures. Commissures connect two ganglia of the 
 same pair We have a cerebral commissure, a pedal com- 
 missure and a visceral commissure. Connectives connect 
 ganglia of dissimilar pairs, and we speak accordingly of 
 the cerebro- pedal connective, the cerebro- pleural con- 
 nective, the pleuro- pedal connective, and the viscero- 
 pleural connective. 
 
 An ENTERIC NERVOUS SYSTEM forming a plexus on the 
 walls of the alimentary canal exists, but does not exhibit 
 cords and ganglia visible to the naked eye except in the 
 large Dibranchiate Cephalopods. 
 
 Our schematic Mollusc is provided with certain ORGANS 
 OF SPECIAL SENSE. Tactile organs occur on the head in the 
 form of short CEPHALIC TENTACLES (a). Deeply placed are 
 
 Fio. 3. Development of the Pond-Snail, Limnteus stagnalis (after Lankester, 
 15). dc, directive corpuscles (prseseminal outcast cells); ch, egg-envelope 
 or chorion ; or, oral end of the blastopore ; r, anal end of the blastopore. 
 A. Formation of the Diblastula by the invagination of larger cells into the 
 area of smaller cells (optical section). B. View of the same specimen from 
 the surface of invagination ; the smaller cells are seen at the periphery ; by 
 division they will multiply and extend themselves over the four larger cells. 
 C. Fully-formed Diblastula, surface view to show the elongated form of the 
 orifice of invagination or blastopore ; its middle portion closes up and coin- 
 cides with the region of the foot ; the extremity, or, coincides with the mouth 
 and stomodseum, the opposite extremity, r, with the anus. D. Optical section 
 of an embryo a little older than A. E. Surface view of the same embryo. 
 
 a pair of closed vesicles containing each a calcareous con- 
 cretion and acting as auditory organs ; these are known as 
 OCTOCYSTS (D, y). They are situated behind the mouth 
 in the foremost portion of the foot. At the base of each 
 cephalic tentacle is a pigmented eye-spot the CEPHALIC 
 EYE (D, w). The OSPHRADIUM (/t), or peculiar patch of 
 olfactory epithelium at the base of the ctenidium, has 
 already been mentioned. 
 
 To the scheme thus exhibited of the possible organization 
 of the ancestral Mollusc we shall now add a sketch of 
 the mode in which this form of body and series of internal 
 organs are developed from the egg. 
 
 The egg-cell of Mollusca is either free from food material 
 a simple protoplasmic corpuscle or charged with food 
 
 material to a greater or less extent. Those cases which 
 appear to be most typical that is to say, which adhere to a 
 
 Fio. 4. Development of the Pond-Snail, Llmnseus stagnalis (after Lankester, 
 15). r, directive corpuscle ; bl, blastopore ; en, endoderm or enteric cell 
 layer ; ec, ectoderm or deric cell-layer ; t), velum ; m, mouth ; /, foot ; (, ten- 
 tacles ; /j), pore in the foot (belonging to the pedal gland ?) ; m/, the mantle- 
 flap or limbus pallialis ; sh, the shell ; I, the sub-pallial space, here destined 
 to become the lung. A. First four cells resulting from the cleavage of the 
 original egg-cell. B. Side view of the same. C. Diblastula stage (see fig. 3), 
 showing the two cell-layers and the blastopore. D, E, F. Trochosphere 
 stage, D older than E or F. G. Three-quarter view of a Diblastula, to show 
 the orifice of invagination of the endoderm or blastopore, U. H, I. Veliger 
 stage later than D. (Compare fig. 70 and fig. 72***). 
 
 procedure which was probably common at one time to all 
 then existing Mollusca, and which has been departed from 
 A B ^^ G 
 
 Fie. 5. Early stages of division of the fertilized egg-cell in Nassa mutabilis 
 (from Balfuur, after Bobretzky). A. The egg-cell has divided into two 
 spheres, of which the lower contains more food-material, whilst the upper is 
 again incompletely divided into two smaller spheres. Resting on the divid- 
 ing upper sphere are the eight-shaped "directive corpuscles," better called 
 " prseseminal outcast cells or apoblasts," since they are the result of a cell- 
 division which affects the egg-cell before it is impregnated, and are mere 
 refuse, destined to disappear. B. One of the two smaller spheres is reunited 
 to the larger sphere. C. The single small sphere has divided into two, and 
 the reunited mass has divided into two, of which one is oblong and practi- 
 cally double, as in B. D. Each of the four segment-cells gives rise by divi- 
 sion to a small pellucid cell. E. The cap of small cells has increased in 
 number by repeated formation of pellucid cells in the same way, and by 
 division of those first formed. The cap will spread over and enclose the four 
 segment-cells, as in fig. 3, A, B. 
 
 only in later and special lines of descent show approxi- 
 
MOLLUSCA 
 
 mately the following history. By division of the egg-cell 
 (fig. 3, A, B ; fig. 4, A, B ; and fig. 5) a mulberry-mass of 
 embryonic-cells is formed (Morula), which dilates, forming 
 a one-cell-layered sac (Blastula). By invagination one 
 
 Fio. 6. Development of the Oyster, Ostrea edvlis (modified from Horst, 16). 
 A. Blastula stage (one-cell-layered sac), with commencing invagination of 
 the wall of the sac at bl, the blastopore. B. Optical section of a somewhat 
 later stage, in which a second invagination has commenced namely, that 
 of the shell-gland st ; U, blastopore ; en, invaginated endodenn (wall of the 
 future arch-enteron) ; ec, ectoderm. C. Similar optical section at a little 
 later stage. The invagination connected with the blastopore is now more 
 contracted, d; and cells, , forming the mesoblast from which the ccelom 
 and muscular and skeleto-trophic tissues develop, are separated. D. Similar 
 section of a later stage. The blastopore, U, has closed ; the anus will sub- 
 sequently perforate the corresponding area. A new aperture, m, the month, 
 has eaten its way into the invaginated endodennal sac, and the cells pushed 
 in with it constitute the stoniodseum. The shell-gland, sir, is flattened out, 
 and a delicate shell, s, appears on its surface. The ciliated velar ring is cut 
 in the section, as shown by the two projecting cilia on the upper part of the 
 figure. The embryo is now a Trochosphere. B. Surface view of an embryo 
 at a period almost identical with that of D. F. Later embryo seen as a 
 transparent object, m, mouth ; ft, foot ; o, anas ; e, intestine ; st, stomach ; 
 tp, velar area of the prostominm. The ertent of the shell and commencing 
 upgrowth of the mantle-skirt is indicated by a line forming a curve from o 
 toF. 
 
 X.B. In this development, as in that of Pisidium (figs. 150, 151X no part of 
 the blastopore persists either as mouth or as anus, but the aperture closes, 
 the pedicle of invagination, or narrow neck of the invaginated arch-enteron, 
 becoming the intestine. The month and the anus are formed as independent 
 in-pushiugs, the mouth with storaodteum first, and the short anal proctodaeum 
 much later. This interpretation of the appearances is contrary to that of 
 Horst (16), from whom our drawings of the oyster's development are taken. 
 The account given by the American naturalist Brooks (19) differs greatly as 
 to matter of fact from that of Horst, and appears to be erroneous in some 
 respects. 
 
 portion of this sphere becomes tucked into the other as 
 in the preparation of a woven nightnp for the head (fig. 
 6, B ; fig. 7, A). The orifice of invagination (blastopore) 
 narrows, and we now have a two-cell-layered sac, the 
 Diblastula. The invaginated layer is the enteric cell-layer 
 or endoderm ; the outer cell-layer is the deric cell-layer or 
 ectoderm. The cavity communicating with the blastopore 
 and lined by the endoderm is the arch-enteron. The blas- 
 topore, together with the whole embryo, now elongates. 
 The blastopore then closes along the middle portion of its 
 extent, which corresponds with the later developed foot. 
 At the same time the stomodaeum or oral invagination 
 forms around the anterior remnant of the blastopore, and 
 the proctodaeum or anal invagination forms around the 
 posterior remnant of the blastopore. There are, however, 
 
 variations in regard to the relation of the" bllstcbors id tV& 
 mouth and to the anus which are probably mdfjificatitins ' 
 the original process described above. An examination of 
 figs. 3, 4, 5, 6, 7, and of others illustrative of the embryo- 
 logy of particular forms which occur later in this article, 
 is now recommended to the reader. The explanation of 
 the figures has been made very full so as to avoid the 
 
 E 
 
 FIG. 7. Development of the River-Snail, Paludina riripara (after Lankester, 
 17). de, directive corpuscle (outcast cell) ; <K, arch-enteron or cavity lined 
 by the enteric cell-layer or endodenn ; 6?, blastopore ; rr, velum or circlet 
 of ciliated cells ; rfr, velar area or cephalic dome ; sm, site of the as yet un- 
 formed mouth ; /, foot ; met, rudiments of the skeleto-trophic tissue*; pi, 
 the pedicle of invagination, the future rectum ; sAjJ, the primitive shell-sac 
 or shell-gland ; m, mouth ; an, anus. A. Diblastula phase (optical section). 
 B. The Diblastula has become a Trochosphere by the development of the 
 ciliated ring rr (optical section). C. Side view of the Trochosphere with 
 commencing formation of the foot. D. Further advanced Trochosphere 
 (optical section). E. The Trochosphere passing to the Veliger stage, dorsal 
 view showing the formation of the primitive shell-sac. F. Side view of the 
 same, showing foot, shell-sac (dig!), velum (rr), month, and anus. 
 
 K.B. In this development the blastopore is not elongated ; it persists as 
 the anas. The month and stomodseom form independently of the blastopore. 
 
 necessity of special descriptions in the text. Internally, by 
 the nipping off of a pair of lateral outgrowths (forming 
 part of the indefinable " mesoblast ") from the enteric cell- 
 layer the foundations of the coelomic cavity are laid. In 
 some Coelomata these outgrowths are hollow and of large 
 size. In Mollusca they are not hollow and large, which is 
 probably the archaic condition, but they consist at first of 
 a few cells only, adherent to one another ; these cells then 
 diverge, applying themselves to the body-wall and to the 
 gut-wall so as to form the lining layer of the coelomic 
 cavity. Muscular tissue develops from deep-lying cells, and 
 the rudiments of the paired nerve-tracts from thickenings 
 of the deric-cell layer or ectoderm. 
 
 The external form meanwhile passes through highly char- 
 acteristic changes, which are on the whole fairly constant 
 throughout the Mollusca. A circlet of cilia forms when the 
 embryo is still nearly spherical (fig. 4, F ; fig. 6, E ; fig. 7, 
 
102: 
 
 MOLLUSCA 
 
 B-},* in Tan ^eqx'atoBial position. As growth proceeds, one 
 Ihttnisphere FerijaujS relatively small, the other elongates and 
 enlarges. Both mouth and anus are placed in the larger 
 area ; the smaller area is the prostomium simply ; the cili- 
 ated band is therefore in front of the mouth. The larval 
 form thus produced is known as the Trochosphere. It 
 exactly agrees with the larval form of many Chaetopod 
 worms and other Ccelomata. Most remarkable is its 
 agreement with the adult form of the Wheel animalcules 
 or Rotifera, which retain the prae-oral ciliated band as their 
 chief organ of locomotion and prehension throughout life. 
 So far the young Mollusc has not reached a definitely 
 Molluscan stage of development, being only in a condition 
 common to it and other Ccelomata. It now passes to the 
 veliger phase, a definitely Molluscan form, in which the 
 disproportion between the area in front of the ciliated 
 circlet and that behind it is very greatly increased, so 
 that the former is now simply an emarginated region of 
 the head fringed with cilia (fig. 8 ; fig. 6, F ; fig. 7, F ; 
 and fig. 60, A). It is termed the " velum," and is fre- 
 quently drawn out into lobes and processes. As in the 
 Rotifera, it serves the veliger larva as an organ of loco- 
 
 Fio. 8. "Veliger" embryonic form of Mollusea (from Gegenbaur). v, velum; 
 c, visceral dome with dependent mantle-skirt ; p, foot ; (, cephalic tentacles ; 
 op, opcrculum. A. Earlier, and B, later, Veliger of a Gastropod. C. Veli- 
 ger of a Pteropod showing lobe-like processes of the velum and the great 
 paired outgrowths of the foot. 
 
 motion. In a very few Molluscs, but notably in the Com- 
 mon Pond-Snail, the emarginated bilobed velum is re- 
 tained in full proportions in adult life (fig. 70), having 
 lost its marginal fringe of specially long cilia and its 
 locomotor function. The body of the Veliger is char- 
 acterized by the development of the visceral hump on 
 one surface, and by that of the foot on the other. Growth 
 is greater in the vertical dorso-ventral axis than in the 
 longitudinal oro-anal axis ; consequently the foot is rela- 
 tively small and projects as a blunt process between mouth 
 and anus, which are not widely distant from one another, 
 whilst the antipodal area projects in the form of a great 
 hump or dome. In the centre of this antipedal area there 
 has appeared (often at a very early period) a gland-like 
 depression or follicle of the integument (fig. 6, C, sk ; fig. 7, 
 E, F, shgl ; fig. 60, B ; fig. 68, shs fig. 72***, ss). Thia is 
 the primitive shell-sac discovered by Lankester (18) in 1871, 
 and shown by him to precede the development of the perma- 
 nent shell in a variety of Molluscan types. The cavity of 
 this small sac becomes filled by a horny substance, and then 
 it very usually disappears, whilst a delicate shell, commenc- 
 
 ing from this spot as a centre, forms and spreads upon the 
 surface of the visceral dome. 
 
 The embryonic shell-sac or shell-gland represents in a 
 transient form, in the individual development of most 
 Mollusea, that condition of the shell-forming area which 
 we have sketched above in the schematic Mollusc. In 
 very few instances (in Chiton, and probably in Limax), as 
 we shall see below, the primitive shell-sac is retained and 
 enlarged as the permanent shell-forming area. It is sup- 
 planted in other Molluscs by a secondary shell-forming 
 area, namely, that afforded by the free surface of the 
 visceral hump, the shell-forming activity of which extends 
 even to the surface of the depending mantle-skirt. Accord- 
 ingly, in most Mollusea the primitive shell is represented 
 only by the horny plug of the primitive shell-sac. The 
 permanent shell is a new formation on a new area, and 
 should be distinguished as a secondary shell. 
 
 The ctenidia, it will be observed, have not yet been 
 mentioned, and they are indeed the last of the charac- 
 teristic Molluscan organs to make their appearance. Their 
 possible relation to the prae-oral and post-oral ciliated bands 
 of embryos similar to the Trochosphere are discussed by 
 the writer in the Quart. Jour. Micr. Sci., vol. xvii. p. 
 423. The Veliger, as soon as its shell begins to assume 
 definite shape, is no longer of a form common to various 
 classes of Mollusea, but acquires characters peculiar to its 
 class. At this point, therefore, we shall for the present 
 leave it. 
 
 SYSTEMATIC REVIEW OF THE CLASSES AND OKDEES OF 
 MOLLUSCA. 
 
 We are now in a position to pass systematically in 
 review the various groups of Mollusea, showing in what 
 way they conform to the organization of our schematic 
 Mollusc, and in what special ways they have modified or 
 even suppressed parts present in it, or phases in the repre- 
 sentative embryonic history which has just been sketched. 
 It will be found that the foot, the shell, the mantle-skirt, 
 and the ctenidia, undergo the most remarkable changes of 
 form and proportionate development in the various classes 
 changes which are correlated with extreme changes and 
 elaboration in the respective functions of those parts. 
 
 Division of the Phylum into two Branches. The Mollusea 
 are sharply divided into two great lines of descent or 
 branches, according as the prostomial region is atrophied 
 on the one hand, or largely developed on the other. 
 
 The probabilities are in favour of any ancestral form 
 the hypothetical archi-Mollusc which connected the Mol- 
 lusea with their non-Molluscan forefathers having pos- 
 sessed, as do all the more primitive forms of Coslomata, a 
 well-marked prostomium, and consequently a head. The 
 one series of Mollusea descended from the primitive head- 
 bearing Molluscs have acquired an organization in which 
 the Molluscan characteristics have become modified in 
 definite relation to a sessile inactive life. As the most 
 prominent result of the adaptation to such sessile life they 
 exhibit an atrophy of the cephalic region. They form the 
 branch LIPOCEPHALA the mussels, oysters, cockles, and 
 clams. The other series have retained an active, in many 
 cases a highly aggressive, mode of life ; they have, corre- 
 spondingly, not only retained a well-developed head, but 
 have developed a special aggressive organ in connexion 
 with the mouth, which, on account of its remarkable nature 
 and the peculiarities of the details of its mechanism, serves 
 to indicate a very close genetic connexion between all such 
 animals as possess it. This remarkable organ is the odon- 
 tophore, consisting of a lingual ribbon, rasp, or radula, 
 with its cushion and muscles. On account of the pos- 
 session of this organ this great branch of the Molluscan 
 phylum may be best designated GLOSSOPHOEA. Any term 
 
MOLLUSCA 
 
 103 
 
 which merely points to the possession of a head is objec- 
 tionable, since this is common to them and the hypotheti- 
 cal archi-Mollusca from which they descend. The term 
 Odontophora, which has been applied to them, is also un- 
 suitable, since the organ which characterizes them is not a 
 tooth, but a tongue. 
 
 r 
 
 B 
 
 t / / / 
 
 i y I 
 
 9 ' * 
 
 Fio. 9. Odontophore of Glossophorous Molluscs. 
 
 A. Diagram showing mouth, oesophagus, and lingual apparatus of a Gastro- 
 
 pod in section. o, upper lip ; al, lower lip ; i>, calcareo-corneous jaw of 
 left side ; c, outer surface of the snout ; d, esophagus ; e, fold in the 
 Tall of the oesophagus behind the radnlar sac (n) ; /, anterior termina- 
 tion of the radula and its bed, the point at which it wears away ; g 1 
 base of the radnlar sac or recess of the pharynx ; A, cartilaginous piece 
 developed in the floor of the pharynx beneath the radula, and serving 
 for the attachment of numerous muscles, and for the support of the 
 radula ; t, anterior muscles ; I, posterior muscles attached to the carti- 
 lage : f, muscle acting as a retractor of the buccal mass ; m, muscle 
 attached to the lower lip ; n, posterior extremity of the radular sac ; 
 o, the bed of the radula or layer of cells by which its lower surface is 
 formed ; p, the homy radula or lingual ribbon ; q, opening of the radular 
 sac into the pharynx or buccal cavity ; r, cells at the extreme end of 
 the inner surface of the radular sac which produce as a "cuticular 
 secretion " the rows of teeth of the npper surface of the radula. 
 
 B. Radula or lingual ribbon of Paludina n'rrpara, stripped from its bed, a 
 
 horny, cuticular product. 
 
 C. A single row of teeth from the radula of Trodau cinerarius. Rhipido 
 
 glossate ; formula, x.5.1.5.x. 
 
 D. A single row of teeth from the radnla of Favltimajragilis. Ptenoglossate ; 
 
 formula, x.O.x. 
 
 E. A single row of teeth from the radnla of Chiton cinertus. Too elaborate 
 
 for formulation. 
 
 F. A single row of teeth from the radnla of Patella vulgata. Formula, 3.1.4.1.3. 
 
 G. A single row of teeth from the radula of Cyprxa lulvota. Ta?nioglossate ; 
 
 formula, 3.1.3. 
 
 H. A single row of teeth from the radnla of Nona annvlata. Rachiglossate ; 
 formula, 1.1.1. The Common Whelk is similar to this. 
 
 The general structure of the odontophore ( = tooth- 
 bearer, in allusion to the rasp-like ribbon) of the glosso- 
 phorous Mollusca may be conveniently described at once. 
 Essentially it is a tube-like outgrowth the radular sac (fig. 
 9, A, ff, n) in the median line of the ventral floor of the 
 stomodseum, upon the inner surface of which is formed a 
 chitinous band (the radula) beset with minute teeth like a 
 
 rasp (p). Anteriorly the ventral wall of the diverticulum 
 is converted into cartilage (h), to which protractor and re- 
 tractor muscles are attached (/-, t), so that by the action of 
 the former the cartilage, with the anterior end of the ribbon 
 resting firmly upon it, may be brought forward into the 
 space between the lips of the oral aperture (au, al), and 
 made to exert there a backward and forward rasping action 
 by the alternate contraction of retractor and protractor 
 muscles attached to the cartilage. But in many Glosso- 
 phora (f.g., the Whelk) the apparatus is complicated by the 
 fact that the diverticulum itself, with its contained radula, 
 rests but loosely on the cartilage, and has special muscles 
 attached to each end of it, arising from the body wall ; 
 these muscles pull the whole diverticulum or radular sac 
 alternately backwards and forwards over the surface of the 
 cartilage. This action, which is quite distinct from the 
 movement of the cartilage itself, may be witnessed in a 
 Whelk if the pharynx be opened whilst it is alive. It has 
 also been seen in living transparent Gastropods. The chi- 
 tinous ribbon is continuously growing forward from the 
 tube-like diverticulum as a finger-nail does on its bed, and 
 thus the wearing away of the part which rests on the car- 
 tilage and is brought into active use, is made up for by 
 the advance of the ribbon in the same way as the wearing 
 down of the finger-nail is counterbalanced by its own for- 
 ward growth. And, just as the new substance of the 
 finger-nail is formed in the concealed part, sunk posteriorly 
 below a fold of skin, and yet is continually earned forward 
 with the forward movement of the bed on which it rests, 
 and which forms its undermost layers, so is the new sub- 
 stance of the radula formed in the compressed extremity 
 of the radular sac (;), and carried forward by the forward 
 movement of the bed (p) on which it rests, and by which 
 is formed its undermost layer. This forward-moving bed 
 is not merely the ventral wall of the radular diverticulum, 
 but includes also that portion of the floor of the oral cavity 
 to which the radula adheres (as far forward as the point / 
 in fig. 9, A). At the spot where the radula ceases, the for- 
 ward growth-movement of the floor also ceases, just as in 
 the case of the finger-nail the similar growth-movement 
 ceases at the line where the nail becomes free. 
 
 The radula or cuticular product of the slowly-moving 
 bed can be stripped off, and is then found to consist of a 
 ribbon-like area, upon which are set numerous tooth-like 
 processes of various form in transverse rows, which follow 
 one another closely, and exactly resemble one another in the 
 form of their teeth (fig. 9, B). The tooth-like processes in a 
 single transverse row are of very different shape and num- 
 ber in different members of the Glossophora, and it is pos- 
 sible to use a formula for their description. Thus, when 
 in each row there is a single median tooth with three teeth 
 on each side of it more or less closely resembling one 
 another, as in fig. 9, G, we write the formula 3.1.3. When 
 there are additional lateral pieces of a different shape to 
 those immediately adjoining the central tooth, we indi- 
 cate them by the figure 0, repeated to represent their 
 number, thus 0000.1.1.1.0000 is the formula for the 
 lingual teeth of Chiton Stflleri. A single median tooth, 
 an admedian series, and a lateral series may be thus dis- 
 tinguished. In some Glossophora only median teeth are 
 present, or large median teeth with a single small ad- 
 median tooth on each side of it (fig. 9, H); these are 
 termed Rachiglossa (formula, .1. or 1.1.1). In a large 
 number of Glossophora we have three admedian on each 
 side and one median, no lateral pieces (fig. 9, G) ; these 
 are termed Tsenioglossa (formula, 3.1.3). Those with nume- 
 rous lateral pieces, four to six or more admedian pieces, 
 and a median piece or tooth (fig. 9, C) are termed Ehipi 
 doglossa (formula, x.6. 1.6.x, where x stands for an inde- 
 finite number of lateral pieces). The Toxoglossa have 
 
104 
 
 MOLLUSCA 
 
 1.0.1, the central tooth being absent and the lateral teeth 
 peculiarly long and connected with muscles. The term 
 Ptenoglossa (fig. 9, D) is applied to those Glossophora 
 in which the radula presents no median tooth, but an 
 indefinite and large number of admedian teeth, giving 
 the formula x.O.x. When the admedian teeth are inde- 
 finite (forty to fifty), and a median tooth is present, the 
 term Myriaglossa is applied (formula, x.l.x). It must be 
 understood that the pieces or teeth thus formulated may 
 themselves vary much in form, being either flat plates, or 
 denticulated, hooked, or spine-like bodies. We shall revert 
 to the terms thus explained in the systematic descriptions 
 of the groups of Glossophora. 
 
 The muscular development in connexion with the whole 
 buccal mass, and with each part of the radular apparatus, 
 is exceedingly complicated, as many as twenty distinct 
 muscles having been enumerated in connexion with this 
 organ. In addition to the radula, and correlated with its 
 development, we find almost universally present in the 
 Glossophora a pair of horny jaws (usually calcified) de- 
 veloped as cuticular productions upon the epidermis of the 
 lips (fig. 9, A, 4). The radula and the shelly jaws of the 
 Glossophora enable their possessors not only to voraciously 
 attack vegetable food, but the radula is used in some in- 
 stances for boring the shells of other Mollusca, and the 
 jaws for crushing the shells of Crustacea, and for wound- 
 ing even Vertebrata. 
 
 PHYLUM MOLLUSCA. 
 
 BRANCH A. GLOSSOPHORA. 
 
 Characters. Mollusca with head-region more or less 
 prominently developed ; always provided with a peculiar 
 rasping-tongue the odontophore rising from the floor of 
 the buccal cavity. 
 
 The Glossophora comprise three classes, chiefly distin- 
 guished from one another by the modifications of the foot. 
 
 Class I. GASTROPODA. 
 
 Characters. Glossophora in which (with special excep- 
 tion of swimming forms) the FOOT is simple, median in 
 position, and flattened so as to form a broad sole-like sur- 
 face, by the contractions of which the animal crawls, often 
 divided into three successive regions the pro-, meso-, and 
 meta-podium by lateral constrictions. 
 
 The Gastropoda exhibit two divergent lines of descent 
 indicated by the term sub-class (see p. 649). 
 
 Sub-class 1. GASTROPODA ISOPLEURA. 
 
 Characters. Gastropoda in which not only the head 
 and foot but also the visceral dome with its contents and 
 the mantle retain the primitive BILATERAL SYMMETRY of 
 the archi-Mollusc. The anus retains its position in the 
 median line at the posterior end of the body. The whole 
 visceral mass together with the foot is elongated, so that 
 the axis joining mouth and anus is relatively long, whilst 
 the dorso-pedal axis at right angles to it is short. The 
 
 CTENIDIA, the NEPHRIDIA, GENITAL DUCTS, and CIRCULA- 
 TORY ORGANS are paired and bilaterally symmetrical. The 
 pedal and visceral NERVE-CORDS are straight, parallel with 
 one another, and all extend the whole length of the body ; 
 the ganglionic enlargements are feebly or not at all deve- 
 loped. The Isopleura comprise three orders. 
 
 Order 1. Polyplacophora (the Chitons). 
 
 Characters. Gastropoda Isopleura with a metameric re- 
 petition of the shell to the number of eight. The shells of 
 the primitive type are partially or wholly concealed in shell- 
 sacs comparable to the single embryonic shell-sac of other 
 Mollusca. On the surface of the mantle-flap numerous 
 
 calcified spines and knobs are frequently developed. The 
 ctenidia are of the typical form, small in size and meta- 
 merically repeated along the sides of the body to the 
 
 B C v ' '--" A 
 
 Fio. 10. Three views of Chiton. A. Dorsal view of Chiton Wosnessenksii, 
 MM'l., showing the eight shells. (After Middendorf.) B. View from the 
 pedal surface of a species of Chiton from the Indian Ocean, p, foot ; o, 
 mouth (at the other end of the foot is seen the anus raised on a papilla) ; kr, 
 oral fringe ; br, the numerous ctenidia (branchial plumes) ; spreading beyond 
 these, and all round the animal, is the mantle-skirt. (After Cuvier.) C. The 
 same species of Chiton, with the shells removed and the dorsal integument 
 reflected. 6, buccal mass ; m, retractor muscles of the buccal mass ; m, 
 ovary ; od, oviduct ; i, coils of intestines ; ao, aorta ; c 1 , left auricle ; c, 
 ventricle. 
 
 number of sixteen or more ; an osphradium or area of 
 " olfactory epithelium " (Spengel) is found at the base of 
 each ctenidium. The other organs are not subject to 
 metameric repetition. The odontophore is highly devel- 
 oped ; the teeth of the lingual ribbon are varied in form, 
 several in each transverse row (fig. 9, E). Paired genital 
 ducts distinct from the paired nephridia are present. 
 
 The order Polyplacophora contains but one family, the 
 Chitonidx, with the genera: Chiton, Lin. (figs. 10, 15, &c.); 
 Cryptochiton, Midd., 1847 ; and Cryptoplax ( = Chitonellm), 
 Blainv., 1818. 
 
 Order 2. Neomeniae. 
 
 Characters. Gastropoda Isopleura devoid of a shell, 
 which is replaced by innumerable microscopic calcified 
 plates or spicules set in the dorsal epidermis ; mantle-flap 
 not lateral, but reduced to a small collar surrounding the 
 
 A BCD 
 
 Fio. 11. Neomenia mrinata, Tullberg (after Tullberg). A. Lateral view. B. 
 Ventral view. C. Dorsal view. D. Ventral view of a more extended speci- 
 men, o, anterior ; 6, posterior extremity ; c, furrow, in which the narrow 
 foot is concealed. 
 
 anus ; ctenidia represented by a symmetrical group of bran- 
 chial filaments on either side of the anus ; foot very narrow, 
 sunk in a groove; odontophore feebly developed, but the 
 radula many-toothed ; gonads placed in the pericardium 
 discharging by the nephridia ; no special generative ducts. 
 The order Neomeniae contains the two genera Neomenia, 
 Tullberg (Solenopus, Sars) (fig. 11); and Proneomenia, 
 Hubrecht. 
 
 Order 3. Chsetoderma. 
 
 Characters. Gastropoda Isopleura devoid of a shell, 
 which is replaced by numerous minute calcareous spines 
 
 Fio. 12. Chsetoderma nitidulum, Loven (after Graff). The cephalic enlarge- 
 ment is to the left, the anal chamber (reduced pallial chamber, containing 
 the concealed pair of ctenidia) to the right. 
 
 standing up like hairs on the surface of the body ; body 
 
MOLLUSCA 
 
 105 
 
 much elongated so as to be vermiform ; mantle-flap as in 
 Neomeniae ; ctenidia in the form of a pair of branchial 
 plumes, one on each side of the anus ; foot aborted, its 
 position being indicated by a longitudinal furrow ; odonto- 
 phore greatly reduced, the radula only represented by a 
 single tooth ; gonads and nephridia as in Neomenia. 
 
 The order Chaetoderma contains the single genus Chx- 
 toderma (fig. 12). 
 
 Further remark* on the Isopleurous Gastropods. The 
 union of the Chitons with the remarkable worm-like forms 
 Neomenia and Chaetoderma was rendered necessary by 
 Hubrecht's discovery (25) in 1881 of a definitely consti- 
 tuted radula and odontophore in his new genus Proneo- 
 menia, founded on two specimens brought from the arctic 
 regions by the Barents Dutch expedition. 
 
 By some writers e.g., Keferstein the Chitons have been 
 too intimately associated with the other Gastropoda, whilst, 
 on the other hand, Gegenbaur seems to have gone a great 
 deal too far in separating them altogether from the other 
 Mollusca as a primary subdivision of that phylum, inas- 
 o much as they are Ulti- 
 
 mately bound to the 
 other Glossophora by 
 the possession of a 
 thoroughly typical 
 and well - developed 
 odontophore. They 
 undoubtedly stand 
 nearer to the archi- 
 Mollusca than any 
 other Glossophora in 
 having retained a com- 
 plete bilateral sym- 
 metry and the primi- 
 tive shell-sac, though 
 the metameric repe- 
 tition of this organ 
 and of the ctenidia is 
 a complication of, and 
 departure from, the 
 primitive character. 
 It is not improbable 
 that in the calcareous 
 spines and plates of 
 the dorsal integument 
 of Neomenia and Chae- 
 toderma, which occur 
 
 FIG. 13.-Diagrams of the alimentary canal of also On the part of 
 Isoplenra (from Hubrecht). o, mouth ; a, the dorSUm Uncovered 
 anus; d, alimentary canal; I, liver (digestive , in- /-xi-, 
 gland). A. Neomenia and Proneomenia. B. by Sfiell in Lniton, We 
 ChKtoderma. C. Chiton. h ave t jj e retention Of 
 
 a condition preceding the development of the solid Mol- 
 luscan shell, or a reversion to it. The minute calcareous 
 bodies may have the same relation to a compact shell which 
 the shagreen denticles of the sharks have to a continuous 
 dermal bone. 
 
 The anatomy of the Gastroj>oda Isopleura has been largely 
 elucidated within the past year by the researches of 
 Hubrecht and of Sedgwick, who have been the first to 
 apply the method of sections to the study of this group. 
 
 The leading points in the modifications of mantle-flap, 
 foot, and ctenidia are set forth in the preceding summaries, 
 and in the accompanying references to the figures. With 
 regard to other organs, we have to note the form of 
 the alimentary canal (fig. 13), which is simplest in 
 Chaetoderma, symmetrically sacculated in Neomenia, and 
 wound upon itself, forming a few coils, in Chiton. The 
 latter has a compact liver with arborescent duct, which is 
 represented by the sacculi in Neomenia and by a single 
 
 caecum in Chaetoderma. Salivary glands are present in 
 Chiton and in Proneomenia. The radula is highly devel- 
 oped in Chiton, and, though present in Proneomenia, has 
 not been described in Neomenia. A single tooth in Chae- 
 toderma appears to represent the radula in a reduced state. 
 The circulatory organs of Chiton alone are known with 
 any degree of detail (fig. 10, C). There is a median dorsal 
 blood-vessel the aorta which is enlarged to form a 
 ventricle in the posterior region of the body. On either 
 side the ventricle is connected to a well-developed auricle, 
 which pours into it the aerated blood from the gills 
 (ctenidia). The extent to which vascular trunks are 
 developed has not been determined, but vessels to and 
 from the ctenidia, and in the mid-line of the foot, are 
 known. As in other Mollusca, the vessels do not extend 
 far, but lead into lacunae between the organs and tissues. 
 Dorsal and ventral vessels have been detected in Neomenia 
 and Chaetoderma, but no specialized heart. 
 
 A B 
 
 FIG. 14. Diagrams of the eicretory and reproductive organs of Isoplenra (after 
 Hubrecht)L 0, ovary ; P, pericardium ; A T , nephridinm ; , external apertnre 
 
 of nephridinm ; y. external apertnre of the genital dnct of Chiton ; r, rectum ; 
 CZ, cloacal or pallia! chamber of Neomenise and Chartoderma ; Br, ctenidia 
 (branchial plumes). A. Chietoderma. B. Neomenia. C. Proneomenia. D. 
 Chiton. 
 
 The heart of Chiton lies in a space which is to be 
 regarded as a specialized part of the ccelom, and, as in 
 other Molluscs, is termed the pericardium. In front of 
 this space in Chiton lies the ovary (fig. 14, D). In the 
 other Isopleura the genital bodies (gonads) lie in the peri- 
 cardium, which has a longer form and extends dorsally 
 above the intestine. Opening into the pericardium equally 
 in all the Isopleura (fig. 14) is a pair of bent tubes which 
 lead to the exterior. These are the nephridia, which in 
 Chiton are essentially renal in function. Their disposition 
 has been determined by Sedgwick (26), who has shown that 
 each nephridium is much bent on itself, so that, as in the 
 
 O 
 
106 
 
 MOLLUSCA 
 
 S.O.. 
 
 F- 
 
 nephridia of Conchifera (organ of Bojanus), the internal 
 aperture lies near the external. From the folded stem of 
 the nephridium very numerous secreting cseca are given off, 
 omitted in the dia- 
 gram (fig. 14, D), but 
 accurately drawn in 
 fig. 15. The sexes in 
 Chiton are distinct, 
 and the ovary or testis, 
 as the case may be, 
 though lying in and 
 filling a chamber of 
 the original ccelom, 
 does not discharge into 
 the pericardium, but 
 has its own ducts, 
 which pass to the ex- 
 terior just in front of 
 those of the nephridia 
 (fig. 14, D, g, and fig. ' nt- 
 16). In this respect 
 Chiton is less primi- 
 tive than the other Iso- 
 pleura, and even than 
 some other Gastropods 
 (the Zygobranchia), 
 and some Conchifera 
 (Spondylus, &c.), which 
 have no special genital 
 apertures, but make use 
 of the nephridia for 
 
 this purpose. InChifon F '- 15. Dissection of the renal organs (neph- 
 , . , ndia) of Chiton siculus, after Haller (Arbeiten, 
 
 dtscrepans, in which 
 
 there are sixteen pairs 
 
 of ctenidia, the orifices 
 
 of the nephridia are 
 
 coincident with the six- 
 teenth pair of ctenidia, 
 
 those of the genital 
 
 ducts with a point between the thirteenth and fourteenth 
 
 ctenidia. 
 
 In the Neomenise and Cheetoderma the nephridia are 
 short and wide (N in fig. 14, A, 
 B, C), and function as excretory 
 ducts for the genital products, the 
 gonads being lodged in the long 
 pericardium. Their separate or 
 united apertures open near the anus 
 into the small chamber formed by 
 the restriction of the mantle-skirt 
 to the immediate neighbourhood of 
 the anus. 
 
 The nervous system of the Gas- 
 tropoda Isopleura is represented in 
 the diagram fig. 17. In all it is 
 important to observe that nerve- 
 ganglion cells are by no means 
 _ a limited .to special swellings the 
 ganglia but are abundant along 
 the whole course of the four great 
 longitudinal trunks. This is a pri- 
 mitive character comparable to that 
 
 FIO. 16. Ovary and oviducts presented by the nerve-cords of Ne- 
 i*>r inr dt ^ h fift anterior niertine worms, and oi tne -A.rt.uro- 
 and posterior s'usp'ensor of pod Peripatus. Higher differen- 
 iarged V part of" oviduct) [oj tiation in other Mollusca leads to 
 oviduct predominance if not an exclusive 
 
 presence of nerve-fibres in the cords, and of nerve-ganglion 
 
 cells in the specialized ganglia. The numerous transverse 
 
 connexions of the pedal nerve-cords in Chiton and Neo- 
 
 Zool. Instit., Vienna, 18S2). F, foot ; L, edge of 
 the mantle not removed in the front part of 
 the specimen ; s.o., oesophagus ; a/, anus ; gg, 
 genital duct ; go, external opening of the same ; 
 eg, stem of the nephridium leading to no, its 
 external aperture ; nk, reflected portion of the 
 nephridial stem ; ng, tine cseca of the nephri- 
 dium, which are seen ramifying transversely 
 over the whole inner surface of the pedal mus- 
 cular mass. 
 
 menia (seen also in Fissurella (fig. 36) and some other 
 Gastropods) are comparable to the transverse connexions 
 of the ventral nerve- . 
 
 cords of Chsetopod 
 worms and Arthro- 
 pods. In the abund- 
 ance of the nervous 
 network connected 
 with its longitudinal 
 nerve-tracts, Chiton 
 appears to retain some- 
 thing of the early con- 
 dition of the Coelo- 
 mate nervous system 
 when it had the form 
 of a sub-epidermic net- 
 work or nerve -tunic 
 (seen more clearly in 
 Planarians and some 
 Nemertines), and when 
 the concentration into 
 definitely compacted 
 cords had not set in. 
 
 Ganglia are, how- 
 ever, distinguishable 
 upon the nervous cords 
 of Chiton (fig. 18). The 
 cerebral ganglia are 
 not distinguishable as 
 such, but a pair of 
 buccal ganglia (B in 
 fig. 18) are developed 
 on two connectives 
 which pass forward 
 from the cerebral re- 
 gion to the great mus- 
 cular mass of the 
 mouth. These buccal 
 ganglia are special de- 
 velopments connected 
 
 with the snprial mils- Fl - 17. Diagrams of the nervous system of 
 
 Wlin me sp Isopleura (after Hubrecht, loc. eit.). c, cere- 
 
 CUiarity OI tne lips and bral ganglia ; s, sublingual ganglia ; v, pedal 
 
 nrlnnrrmVinrp flnri nrp (ventral) nerve-cord ; I, visceral (lateral) nerve- 
 
 lontopnore, ana are cord . p ^ pos t-anal junction of the visceral 
 
 found in all G10SSO- nerve-cords. A. Proneomenia. B. Neomeuia. 
 
 . , , i C. Chsetoderma. D. Chiton. 
 
 phora, but not in the 
 
 Lipocephala. Such special ganglia related to special 
 organs (and not introduced in our schematic Mollusc, fig. 
 
 1) we find in connexion with 
 the siphons of the Lipoce- 
 phala, and in various posi- 
 tions upon the visceral nerve- 
 cords of other Mollusca, both 
 Glossophora and Lipocephala. 
 A pair of pedal ganglia but 
 little developed (p in fig. 18), 
 and a special group of sub- 
 lingual ganglia are present in 
 Chiton. On the whole, the 
 nervous system of the Iso- 
 pleura is exceedingly simple 
 and archaic, whilst it does not 
 well serve as a type with 
 
 FIG. Ik-Anterior parf of the nervous which to compare that of 
 system of Chiton citiereus, in more de- other Mollusca On account of 
 tail(fromGejrpnbaur, KlementsofComv- .1 n < 
 
 Anatomy). B, buccal ganglia (con- the small amount of concen- 
 cerned with the odontophore) ; c, tration of its nerve-ganglion 
 
 cerebral nerve-mass; P, pedal gan- . , 
 
 glion and commencement of pedal cells into ganglia, SUCil as We 
 nerve-cord; pi, visceral nerve-cord. fl j 11 Jpvpl-,! ; n nHipr 
 Tlie sublingual ganglia are not let- nnd wel1 developed in OUier 
 tered. forms. 
 
 The development of Neomenia and Chaitoderma from 
 
M O L L U S C A 
 
 107 
 
 the egg is entirely unknown, that of Chiton only par- 
 tially. Impregnation is effected when the eggs have been 
 discharged and are lying beneath the mantle-skirt. A 
 trochosphere larva is developed from the Diblastula of 
 Chiton (Loven). 
 
 The Chitons are found in the littoral zone in all parts of 
 the world, and are exclusively marine. Neomenia, Proneo- 
 menia, and Chaetodenna have hitherto been dredged from 
 considerable depths (100 fathoms and upwards) in the 
 North Sea, Proneomenia also in the Mediterranean (Marion). 
 
 Sub-class 2. GASTROPODA ANISOPLEURA. 
 
 Characters. Gastropoda in which, whilst the head and 
 foot retain the bilateral symmetry of the archi-Mollusca, 
 the visceral dome, including the mantle-flap dependent from 
 it, and the region on which are placed the ctenidia, anus, 
 generative and nephridial apertures, have been subjected 
 to a ROTATION tending to bring the anus from its posterior 
 median position, by a movement along the right side, 
 forwards to a position above the right side of the animal's 
 neck, or even to the middle line above the neck. This 
 torsion is connected mechanically with the excessive vertical 
 growth of the visceral hump and the development upon 
 its surface of a heavy shell. The SHELL is not a plate en- 
 closed in a shell-sac, but the primitive shell-sac appears 
 and disappears in the course of embryonic development, and 
 a relatively large nautiloid shell (with rare exceptions) 
 develops over the whole surface of the visceral hump and 
 mantle-skirt. Whilst such a shell might retain its median 
 position in a swimming animal, it and the visceral hump 
 necessarily fall to one side in a creeping animal which 
 carries them uppermost. 
 
 The shell and visceral hump in the Anisopleura incline 
 
 cer -y-. 
 
 FIG. 19. Sketch of a model designed so as to show the effect of torsion or rotation 
 of the visceral hump in Streptoneurous Gastropoda; A, unrelated ancestral 
 condition . B, quarter-rotation ; C, complete semi-rotation (the limit) ; ax, anna ; 
 to, rn, primarily left nephridium and primarily right nephridium ; Irg, primarily 
 left (subsequently the sub-intestinal) visceral ganglion ; riy, primarily right 
 (subsequently the sub-intestinal) visceral ganglion ; eery, cerebral ganglion ; 
 pig, pleural ganglion ; ptdg, pedal ganglion ; aby, abdominal ganglion ; biux, 
 bnccal mass; W, wooden arc representing the base-line of the wall of the 
 visceral hump ; x, if, pins fastening the elastic cord (representing the visceral 
 nerve loop) to IT. 
 
 normally to the right side of the animal. As mechanical 
 results, there arise a one-sided pressure and a one-sided 
 strain, together with a one-sided development of the 
 muscular masses which are related to the shell and foot. 
 Both the TORSION THROUGH A SEMICIRCLE of the base of the 
 visceral dome and the continued leiotropic spiral growth 
 of the visceral dome itself, which is very usual in the 
 Anisopleura, appear to be traceable to these mechanical 
 conditions. ATROPHY of the representatives on one side 
 of the body of paired organs is very usual. Those placed 
 primitively on the left side of the rectum, which in virtue 
 of the torsion becomes the right side, are the set which suffer 
 (see fig. 19). Some Anisopleura, after having thus acquired 
 a strongly-marked inequilateral character in regard to such 
 organs as the ctenidia, nephridia, genital ducts, heart,, and 
 rectum, appear by further change of conditions of growth to 
 have acquired a superficial bilateral symmetry, the second- 
 
 ary nature of which is revealed by anatomical examination 
 
 (Opisthobranchia, Xatantia). 
 
 In all groups of Anisopleura examples are numerous in 
 
 which the shell is greatly developed, forming a " house " 
 into which the whole animal can be with- 
 drawn, the entrance being often closed 
 by a second shelly piece carried upon 
 the foot (the operculum). The power of 
 rapidly extending and of again contract- 
 ing large regions of the body to an 
 enormous degree is 
 usual, as in the T.i- 
 pocephalous Mol- 
 lusca. In spite of 
 the theories which 
 have been held on 
 this matter, it ap- 
 pears highly prob- 
 able that no fluid 
 from without is in- 
 troduced into the 
 *P /! blood, nor is any ex- 
 
 pelled during these 
 -o changes of form. 
 A large mucous 
 gland with a med- 
 ian pore is usually 
 
 developed On the *" Streptoneurous ~eon- 
 . t dition. B, buccal (sub- 
 
 ventral surface of 
 
 oesophageal) ganglion ; C, 
 cerebral ganglion ; Co, 
 pleural ganglion ; P, pe- 
 dal ganglion with otocyst 
 attached ; j>, pedal nerve; 
 A, abdominal ganglion 
 thelong-looiidEuthy- LipOCephala, and in * *e extremity of the 
 
 nnn ~*n.,~,..*i:*4 n TV~ r r I Mini mlm 'i 1 "lonn" 
 
 the foot, compar- 
 able to the similar 
 
 neurons condition. The 
 
 twisted visceral "loop" 
 
 u*ricuitu K-"K""" >tj i i . i 
 
 pleural ganglion pe, has been mistaken *o,sub-intestinalganglion 
 
 SSfiPSAa for a water-pore. 3SS $&< 
 
 which represents also The leiotropic genbaur, after Jhering.) 
 
 gangiion P f "strep^ torsion of the visceral dome has had 
 neura and gives off the iggg deep -seated effect in one series of 
 
 nerve to the osphra- , . , F "V . 
 
 dium (olfactory organ) Anisopleura than in another. Accord- 
 leSr^s^died^'^ ^g 1 ?' ^ ^ lo P formed by the two 
 
 nital" ganglion. The VISCERAL NERVES (fig. 19) is Or is not 
 buccal nerves and tsan- i * ^i . 
 
 giia are omitted. (After caught, as it were, in the twist, we are 
 
 SpengeL) a y e ^o Distinguish one branch or line of 
 
 descent with straight visceral nerves the EDTHYSEURA 
 
 FIG. 22. Xervous system of the Pond-Snail, Limneeus stagntil\s, as a type of 
 the short-looped Euthynenrons condition. The short visceral "loop" with 
 its three ganglia is lightly-shaded, ce, cerebral ganglion ; pt, pedal ganglion ; 
 ft, pleural ganglion ; ab, abdominal ganglion ; sp, visceral ganglion of the 
 left side ; opposite to it is the visceral ganglion of the right side, which 
 gives off the long nerve to the olfactory ganglion and osphradinm o. In 
 Planorbis and in Auricula (Pnlmonata, allied to Limmeus) the olfactory organ 
 is on the left side and receives its nerve from the left visceral ganglion. 
 (After Bpengel.) 
 
 (fig. 20) from a second branch with the visceral nerves 
 
108 
 
 MOLLUSCA 
 
 twisted into a figure-of-eight the STREPTONEURA. (fig. 
 21). Probably the Euthyneura and the Streptoneura 
 have developed independently from the ancestral bilater- 
 ally symmetrical Gastropods. The escape of the visceral 
 nerve-loop from the torsion depends on its having acquired 
 a somewhat deeper position and shorter extent, previously 
 to the commencement of the phenomenon of torsion, 
 in the ancestors of the Euthyneura than in those of 
 the Streptoneura. In the ancestral Streptoneura the 
 visceral loop was lateral and superficial as in the living 
 Isopleura. 
 
 Branch a. STREPTONEURA (Spengel, 1881). 
 
 Characters. Gastropoda Anisopleura in which the 
 visceral "loop" (the conterminous visceral nerves) was 
 adherent to the body-wall and so shared in the torsion of 
 the visceral hump, the right cord crossing above the left 
 so as to form a figure-of-eight (see fig. 19). 
 
 The Streptoneura comprise two orders the Zygo- 
 branchia and the Azygobranchia. 
 
 Order 1. Zygotoranchia. 
 
 Characters. Streptoneura in which, whilst the visceral 
 torsion is very complete so as to bring the anus into the 
 middle line anteriorly or nearly so, the atrophy of the 
 primitively left-side organs is not carried out. The right 
 and left ctenidia, which have now become left and right 
 respectively, are of equal size, and are placed symmetrically 
 on either side of the neck in the pallial space. Belated 
 to them is a simple pair of osphradial patches. Both right 
 
 FIG. 23.ffaliotis tuberculata. d, foot; i, tentacular processes of the mantle. 
 (From Owen, after Cuvier.) 
 
 and left nephridia are present, the actual right one being 
 much larger than the left. Two auricles may be present 
 right and left of a median ventricle (Haliotis), or only one 
 (Patella). The Zygobranchia are further very definitely 
 characterized by the archaic character of absence of special 
 genital ducts. The generative products escape by the 
 larger nephridium. The sexes are distinct, and there is 
 no copulatory or other accessory generative apparatus. 
 The teeth of the lingual ribbon are highly differentiated 
 (Rhipidoglossate). The visceral dome lies close upon the 
 oval sucker-like foot, and is coextensive with its prolonga- 
 tion in the aboral direction. 
 
 The Zygobranchia comprise three families, arranged in two sub- 
 orders. 
 
 Sub-order 1. Ctcnidiolranchia. 
 
 Character. Large paired ctenidia acting as gills. 
 Family 1. Haliotidss. 
 
 Genera : Haliotis (Ear-Shell, Ormer in Guernsey); mostly tropical ; 
 
 Teinotis. 
 Family 2. Fissurellidai. 
 
 Genera : Fissurella (Key-hole Limpet) (figs. 24, 36), Emarginula, 
 Parmophonis (fig. 25) ; mostly tropical. 
 
 Sub-order 2. Phyllidiobranchia. 
 Characters. Ctenidia reduced to wart-like papilla; special sub- 
 
 pallial lamellae, similar to those of the Opisthobranch Pleuro- 
 phyllidia, perform the function of gills. 
 Family 3. Patcllidas. 
 
 Genera : Patella (Limpet, figs. 26, &c.), A'acella (Bonnet-Limpet), 
 Lottia. 
 
 Further Remarks on Zygobranchia. The Common Limpet 
 is a specially interesting and abundant example of the 
 remarkable order Zygobranchia. A complete and accurate 
 account of its anatomy has yet to be written. Here we 
 have only space for a brief outline. The foot of the 
 Limpet is a nearly circular disc of muscular tissue; in 
 front, projecting from and 
 raised above it, are the head 
 and neck (figs. 26, 30). The 
 visceral hump forms a low 
 conical dome above the sub- 
 circular foot, and standing out 
 all round the base of this dome 
 so as to completely overlap the 
 head and foot, is the circular 
 mantle-skirt. The depth of 
 free mantle-skirt is greatest in 
 front, where the head and neck 
 are covered in by it. Upon 
 the surface of the visceral 
 dome, and extending to the 
 edge of the free mantle-skirt, 
 is the conical shell. When 
 
 ,, Fio. 24. Dorsal aspect of a specimen 
 
 the shell IS taken away (best of Fissurella from which the shell 
 pffpptprl Vvv immprqinn in nor nas been remove <l. whilst the ante- 
 
 enectea oy immersion in no rior area of the mantle . skirt has 
 
 water) the Surface of the vis- teen longitudinally slit and its sides 
 
 t / 1 t, T . , reflected, a. cephalic tentacle ; b. 
 
 ceral dome IS tound to be foot; d, left (archaic right) gill- 
 
 rvwprprl Viv a VilflpV pnlniirprl plume ; e, reflected mantle-flap ; fl, 
 
 the fissure or hole in the mantle-flap 
 epithelium, which may be re- traversed by the longitudinal inci- 
 
 moved, enabling the observer s - i n: . / ' right (archaic left) ^"i" 
 
 dium's aperture ; g, anus ; h, left 
 rchaic right) aperture of nephri- 
 um ; p, snout. (Original.) 
 
 The muscular columns (c) 
 
 to note the position of some 
 
 organs lying below the trans- 
 
 parent integument (fig. 27). 
 
 attaching the foot to the shell form a ring incomplete in 
 front, external to which is the free 
 mantle -skirt. The limits of the 
 large area formed by the flap over 
 the head and neck (ecr) can be traced, 
 _ and we note the anal papilla show- 
 ing through and opening on the right 
 shoulder, so to speak, of the animal 
 into the large anterior region of the 
 sub-pallial space. Close to this the 
 small renal organ (i, mediad) and the 
 larger renal organ (k, to the right 
 and posteriorly) are seen, also the 
 pericardium (I) and a coil of the in- 
 testine (ini) embedded in the com- 
 pact liver. 
 
 On cutting away the anterior part 
 of the mantle-skirt so as to expose 
 the sub-pallial chamber in the region 
 of the neck, we find the right and 
 left renal papillae (discovered by Lan- 
 in 1867) on either side 
 
 mouth ; T, cephalic ten- the anal papilla (fig. 28), but no gills. 
 
 tacle ; br, one of the two T , > v " , , 
 
 symmetrical giiis placed on If a similar examination be made 
 
 the neck. (Original.) of the alHed genug F i ssure l la ( fig _ 
 
 24, d), we find right and left of the two renal apertures 
 a right and left gill-plume or ctenidium, which by their 
 presence here and in Haliotis furnish the distinctive char- 
 acter to which the name Zygobranchia refers. In Patella 
 no such plumes exist, but right and left of the neck are 
 seen a pair of minute oblong yellow bodies (fig. 28, d), 
 which were originally described by Lankester as orifices 
 possibly connected with the evacuation of the generative 
 
MOLLUSCA 
 
 109 
 
 products. On account of their position they were termed 
 by him the "capito-pedal orifices," being placed near the 
 junction of head and foot. Spengel (24) has, however, in 
 a most ingenious way shown that these bodies are the repre- 
 sentatives of the typical pair of ctenidia, here reduced to a 
 mere rudiment. Near to each rudimentary ctenidium Spengel 
 
 FIG. 26. The Common Limpet (PattUa wlgata) in its shell, seen from the pedal 
 surface, z, y, the median an two-posterior axis ; a, cephalic tentacle ; b, 
 plantar surface of the foot ; c, free edge of the shell ; d, the branchial effe- 
 rent vessel carrying aerated blood to the auricle, and here interrupting the 
 circlet of gill lamellae ; e, margin of the mantle-skirt ; / gill lamellae (not 
 ctenidia, but special pallial growths, comparable to those of Plenrophyllidia); 
 g, the branchial efferent vessel ; A, factor of the branchial advehent vessel ; 
 i, interspaces between the muscular bundles of the root of the foot, causing 
 the separate areas seen in fig. 27, c. (Original.) 
 
 has discovered an olfactory patch or osphradium (consisting 
 of modified epithelium) and an olfactory nerve-ganglion 
 (fig. 32). It will be remembered that, according to 
 Spengel, the osphra- 
 dium of Mollusca is 
 definitely and inti- 
 mately related to the 
 gill - plume or cteni- 
 dium, being always 
 placed near the base 
 of that organ ; further, 
 Spengel has shown 
 that the nerve-supply 
 of this olfactory organ 
 is always derived from 
 the visceral loop. Ac- 
 cordingly, the nerve- ' 
 supply affords a means 
 of testing the conclu- 
 sion that we have in 
 Lankester's capito- 
 pedal bodies the rudi- 
 mentary ctenidia. The 
 accompanying dia- 
 grams (figs. 34, 35) of 
 the nervous systems of 
 Patella and of Haliotis, 
 as determined by Spen- 
 gel, show the identity in the origin of the nerves passing 
 from the visceral loop to Spengel's olfactory ganglion of 
 the Limpet, and that of the nerves which pass from the 
 visceral loop of Haliotis to the olfactory patch or osphra- 
 dium, which lies in immediate relation on the right and 
 on the left side to the right and the left gill-plumes 
 (ctenidia) respectively. The same diagrams serve to de- 
 
 I0 - ^- Dors 8 ' surface of the Limpet removed 
 from its shell and deprived of its black pig- 
 mented epithelium; the internal organs are 
 seen through the transparent body-watt, c, 
 muscular bandies forming the root of the foot, 
 and adherent to the shell; , free mantle- 
 skirt ; nn, tentaculiferous margin of the same ; 
 i, smaller (left) nephridium ; t, larger (right) 
 nephridium ; /, pericardium ; Lr, fibrous septum, 
 behind the pericardium; it, liver; int, intes- 
 tine ; KT, anterior area of the mantle-skirt over- 
 hanging the head (cephalic hood). (Original.) 
 
 monstrate the Streptoneurous condition of the visceral loop 
 in Zygobranchia. 
 
 Thus, then, we find that the Limpet possesses a sym- 
 metrically-disposed pair of ctenidia in a rudimentary con- 
 dition, and justifies 
 its position among 
 Zygobranchia. At 
 the same time it pos- 
 sesses a totally dis- 
 tinct series of func- 
 tional gills, which 
 are not derived from 
 the modification of 
 the typical Mollus- 
 can ctenidium. 
 These gills are in 
 
 the form of delicate FIG. 28. Anterior portion of the same Limpet, with 
 
 . . 
 
 the overhanging cephalic hood removed, a, ce- 
 phalic tentacle ; b, foot ; c, muscular substance 
 forming the root of the foot ; d, the capito-pedal 
 organs of Lankester ( = rudimentary ctenidia) ; e , 
 mantle-skirt ; /, papilla of the larger nephridinm ; 
 p, anus ; A, papilla of the smaller nephridium ; i, 
 
 er nephridium ; t, larger nephridium ; I, peri- 
 cardium ; , cnt edge of the mantle-skirt ; *, 
 
 "* (Original.) 
 
 lamellae (fig. 26,/), 
 which form a series 
 extending com- 
 pletely round the 
 inner face of the 
 depending mantle- 
 skirt. This circlet of gill-lamellae led Cuvier to class the 
 Limpets as Cyclobranchiata, and, by erroneous identifica- 
 tion of them with 
 
 the series of meta- 
 merically repeated 
 ctenidia of Chiton, 
 to associate the 
 latter Mollusc 
 with the former. 
 The gill -lamellae 
 of Patella are 
 processes of the 
 mantle compar- 
 able to the plait- 
 e * e like folds often 
 
 FIG. 29. The same specimen viewed from the left observed on the 
 front, so as to show the sub-anal tract (/) of the * f *l, K 
 larger nephridium, by which it communicates with roC 
 the pericardium, o, month; other letters as in fig. 28. gjijal chamber in 
 
 other Gastropoda (e.g., Buccinum and Haliotis). They are 
 termed pallial gills. The only other Molluscs in which 
 they are exactly represented 
 are the curious Opistho- 
 branchs Phyllidia and 
 Pleurophyllidia (fig. 57). 
 In these, as in Patella, the 
 typical ctenidia are aborted, 
 and the branchial function 
 is assumed by close -set 
 lamelliform processes ar- 
 ranged in a series beneath 
 the man tie -skirt on either 
 side of the foot. In fig. 26, 
 d the large branchial vein of 
 Patella bringing blood from 
 the gill-series to the heart 
 is seen ; where it crosses 
 the series of lamellae there 
 is a short interval devoid 
 of lamellae. 
 
 The heart in Patella con- 
 sists of a single auricle (not 
 two as in Haliotis and Fis- 
 surella) and a ventricle ; the 
 former receives the blood 
 from the branchial vein, the 
 latter distributes it through a large aorta which soon leads 
 into irregular blood-lacunae. 
 
 FIG. 30. Diagram of the two renal organs 
 (nephridiaX to show their relation to the 
 rectum and to the pericardium. / pa- 
 pilla of the larger nephridinm ; g, anal 
 papilla with rectum leading from it ; A, 
 papilla of the smaller nephridium, which 
 is only represented by dotted outlines ; 
 J, pericardium indicated by a dotted out- 
 line, at its right side are seen the two 
 reno- pericardia! pores ; /, the sub-anal 
 tract of the large nephridium given off 
 near its papilla and seen through the 
 unshaded smaller nephridinm ; fcs.o, an- 
 terior superior lobe of the large ne- 
 phridium ; fctJ, left lobe of same ; kt.p, 
 posterior lobe of same; te.i, inferior 
 sub- visceral lobe of same. (Original. ) 
 
110 
 
 MOLLUSCA 
 
 The existence of two renal organs in Patella, and their 
 relation to the pericardium (a portion of the coelom), is 
 
 ocim 
 
 Fio. 31. Diagram of a vertical antero-postero median section of a Limpet. 
 Letters as in flgs. 28, 29, with following additions : q, intestine in transverse 
 section ; r, lingual sac (radular sac) ; rd, radula ; s, lamcllated stomach ; t, 
 salivary gland ; u, duct of same ; , buccal cavity ; w, gonad ; br.a, branchial 
 advehent vessel (artery); br.v, branchial efferent vessel (vein); to, blood- 
 vessel ; odm, muscles and cartilage of the odontophore ; cor, heart within the 
 pericardium. (Original.) 
 
 important. Each renal organ is a sac lined with glandular 
 epithelium (ciliated cells with concretions) communicating 
 
 FIG. 32. A. Section in a plane vertical to the surface of the neck of Patella 
 through a, the rudimentary ctenidium (Lankester's organ), and b, the ol- 
 factory epithelium (osphradium) ; c, the olfactory (osphradial) ganglion. 
 (After Spengel.) B. Surface view of a rudimentary ctenidium of Patella, 
 excised and viewed as a transparent object. (Original.) 
 
 with the exterior by its papilla, and by a narrow passage 
 with the pericardium. The connexion with the pericar- 
 
 Fia. S3. Vertical section in a plane running right and left through the 
 anterior part of the visceral hump of Patella, to show the two renal organs 
 and their openings into the pericardium, a, large or external or right renal 
 organ ; 06, narrow process of the same running below the intestine and lead- 
 ing by k into the pericardium ; b, small or median renal organ ; c, peri- 
 cardium ; d, rectum ; e, liver ; f, manyplies ; g, epithelium of the dorsal sur- 
 face ; h, renal epithelium lining the renal sacs ; i, aperture connecting the 
 small sac with the pericardium ; fc, aperture connecting the large sac with 
 the pericardium. (From an original drawing by Mr J. T. Cunningham, Fellow 
 of University College, Oxford.) 
 
 dium of the smaller of the two renal organs was demon- 
 strated by Lankester in 1867, at a time when the fact 
 
 .pi 
 
 that the renal organ of the Mollusca, as a rule, opens into 
 the pericardium, and is therefore a typical nephridium, 
 was not known. Subsequent investigations (27) carried on 
 under the direction of the same , 
 
 naturalist have shown that the 
 larger as well as the smaller renal 
 sac is in communication with the 
 pericardium. The walls of the 
 renal sacs are deeply plaited and 
 thrown into ridges. Below the 
 surface these walls are excavated 
 with blood-vessels, so that the sac 
 is practically a series of blood-ves- 
 sels covered with renal epithelium, t _\ 
 and forming a mesh-work within 
 a space communicating with the 
 exterior. The larger renal sac (re- 
 markably enough, that which is 
 aborted in other Anisopleura) ex- 
 tends between the liver and the 
 
 integument of the visceral dome FIG. 34. Nervous system of Pa- 
 very widely. It also bends round 
 the liver as shown in fig. 30, and 
 
 T 
 
 tella ; the visceral loop 
 lightly shaded ; the buccal 
 ganglia are omitted, ce, cere- 
 
 e i ir j j.i bral ganglia ; c'e,cerebral coin- 
 
 forms a large Sac On half of the missure; ^, pleural ganglion; 
 
 upper surface of the muscular mass ** P 6 ^ 1 ^ngi'on ; /, pedal 
 
 * , . nerve ; s, s', nerves (right and 
 
 Of the foot. Here it lies close left) to the mantle ; o, olfac- 
 
 nnnn flip cfpnital bnrlv /nvarv nr tory ganglion, connected by 
 upon tne geniiai ooay ^ ovary c n <, rve to ^ gtreptoneurous 
 
 testis), and in such intimate rela- visceral loop. (After Spengel.) 
 tionship with it that, when ripe, the gonad bursts into the 
 renal sac, and its products are carried to the exterior by 
 the papilla on the right side of the anus (Eobin, Ball). 
 This fact led Cuvier erroneously to the belief that a duct 
 existed leading from the gonad to this papilla. The 
 position of the gonad, best seen in the diagrammatic 
 
 fC. 
 
 Fio. 85. Nervous system of Haliotis ; the visceral loop is lightly shaded ; 
 the buccal ganglia are omitted, ce, cerebral ganglion ; pl.pe, the fused pleural 
 and pedal ganglia ; pe, the right pedal nerve ; ce.pl, the cerebro-pleural con- 
 nective ; cf.pe, the cerebro-pedal connective ; s, s', right and left mantle 
 nerves ; ab, abdominal ganglion or site of same ; o, o, right and left olfactory 
 ganglia and osphradia receiving nerve from visceral loop. (After Spengel.) 
 
 section (fig. 31), is, as in other Zygobranchia, devoid of 
 a special duct communicating with the exterior. This 
 condition, probably an archaic one, distinguishes the Zygo- 
 branchia among all Glossophorous Mollusca. 
 
 The digestive tract of Patella offers some interesting 
 features. The odontophore is powerfully developed ; the 
 radular sac is extraordinarily long, lying coiled in a space 
 
MOLLUSCA 
 
 111 
 
 between the mass of the liver and the muscular foot. The 
 radula has 160 rows of teeth with twelve teeth in each row. 
 Two pairs of salivary ducts, each leading from a salivary 
 gland, open into the buccal chamber. The oesophagus leads 
 into a remarkable stomach, plaited like the manyplies of a 
 sheep, and after this the intestine takes a very large num- 
 ber of turns embedded in the yellow liver, until at last it 
 passes between the two renal sacs to the anal papilla. A 
 curious ridge (spiral t valve) which secretes a slimy cord is 
 found upon the inner wall of the intestine. The general 
 structure of the Molluscan intestine has not been suffi- 
 ciently investigated to render any comparison of this struc- 
 ture of Patella with that of other MoLlusca possible. The 
 eyes of the Limpet (28) deserve mention as examples of 
 the most primitive kind of eye in the Molluscan series. 
 They are found one on each cephalic tentacle, and are 
 simply minute open pits v 
 
 or depressions of the ff 
 epidermis, the epidermic 
 cells lining them being 
 pigmentedand connected 
 with nerves (compare fig. 
 118). 
 
 The Limpet breeds 
 upon the southern Eng- 
 lish coast in the early 
 part of April, but its de- 
 velopment has not been 
 followed. It has simply 
 been traced as far as the 
 formation of a Diblastula 
 which acquires a ciliated 
 band, and becomes a 
 nearly spherical Trocho- 
 sphere. It is probable 
 that the Limpet takes 
 several years to attain 
 full growth, and during 
 that period it frequents 
 the same spot, which 
 becomes gradually sunk 
 below the surrounding 
 
 Surface, especially if the 
 
 rrw>lr \v farhonatonf limp 
 lime, 
 
 At low tide the Limpet 
 
 /l_ . . i > 
 
 (being a Strictly inter- 
 
 tidal Organism} is ex- 
 j i it. j 
 
 posed to the air, and is 
 to be found upon its spot of fixation ; but when the water 
 again covers it, it (according to trustworthy observers) 
 quits its attachment and walks away in search of food 
 (minute encrusting algae), and then once more as the tide 
 falls returns to the identical spot, not an inch in diameter, 
 which belongs, as it were, to it. Several million Limpets 
 twelve million in Berwickshire alone are annually used 
 on the east coast of Britain as bait. 
 
 Order 2. Azygobranchia. 
 
 Characters. Streptoneura which, as a sequel to the 
 torsion of the visceral hump, have lost by atrophy the 
 originally left ctenidium and the originally left nephridium, 
 retaining the right ctenidium as a comb-like gill-plume to 
 the actual left of the rectum, and the right nephridium 
 (that which is the smaller in the Zygobranchia) also to the 
 actual left of the rectum, between it and the gill-plume. 
 The right olfactory organ only is retained, and may assume 
 the form of a comb-like ridge to the actual left of the 
 ctenidium or branchial plume. It has been erroneously 
 described as the second gill, and is known as the para- 
 branchia. The rectum itself lies on the animal's right 
 
 ^_ Semm svstem of 
 
 pallial nerve ; p, pedal nerve ; A, abdomi- 
 nal E*"? 11 * fa tte Streptonenrous visceral 
 
 glion on each 
 
 supra- and sab-intestine 
 side ; B, buccal ganglia ; 
 
 . 
 
 sre ; o, otocyrtatoched to the cerebro- 
 pedal connectives. (From Gegenbeor, after 
 Jhering.) 
 
 shoulder. The presence of glandular plication of the surface 
 of the mantle-flap (fig. 46, x) and an adrectal gland (purple- 
 gland, fig. 47, gp) are frequently observed. The sexes are 
 always distinct; a special genital duct (oviduct or sperm 
 duct) unpaired is present, opening either by the side of the 
 anus or, in the males, on the right side of the neck in con- 
 nexion with a large penis. The shell is usually large and 
 spiral; often an operculum is developed on the upper sur- 
 face of the hinder part of the foot. The dentition of the 
 lingual ribbon is very varied. In most cases the visceral 
 hump and the foot increase along axes at right angles to 
 one another, so that the foot is extended far behind the 
 visceral hump in the ab-oral direction, whilst the visceral 
 hump is lofty and spirally twisted. 
 
 This is a very large group, and is conveniently divided 
 into two sections, the Reptantia and the Xatantia, The 
 former, containing the immense majority of the group, 
 breaks up into three sub-orders, the Holochlamyda, Pneu- 
 monochlamyda, and Siphonochlamyda, characterized by the 
 presence or absence of a trough-like prolongation of the 
 margin of the mantle-flap, which conducts water to the 
 respiratory chamber (sub-pallial space where the gill, anus, 
 &c., are placed), and notches the mouth of the shell by 
 its presence, or again by adaptation to aerial respira- 
 tion. The sub-orders are divided into groups according to 
 the characters of the lingual dentition. In some Azygo- 
 branchia the mouth is placed at the end of a more or less 
 elongated snout or rostrum which is not capable of intro- 
 version (Rostrifera) ; in the others (Proboscidifera) the 
 rostrum is partly invaginated and is often of great length. 
 It is only everted when the animal is feeding, and is with- 
 drawn (introverted) by the action of special muscles ; the 
 over-worked term " proboscis " is applied to the retractile 
 form of snout. The term " introversible snout," or simply 
 "introvert," would be preferable. The presence or absence 
 of this arrangement does not seem to furnish so natural a 
 division of the Reptant Azygobranchia as that afforded by 
 the characters of the mantle-skirt. 
 
 Section a.REPTAXTIA. 
 
 Characters. Azygobranchia adapted to a creeping life ; foot either 
 wholly or only the mesopodium in the form of a creeping disc. 
 
 Sub-order 1. Holochlamyda. 
 
 Characters. Reptant Azygobranchia with a simple margin to the 
 mantle-skirt, and, accordingly, the lip of the shell unnotehed ; 
 mostly Bostrifera (i.e., with a non-introversible snout), and vege- 
 tarian ; marine, brackish, fresh-water, terrestrial. 
 
 a. Xhipidoglossa (x.4 to 7.1.4 to 7.x). 
 Family 1. Trochidte. 
 Genera : Turbo, Lin. ; Phasiandla, Lam. ; Imperator, Montf. ; 
 
 Trochus, Lin.; RotcUa, Lam.; Euomphalus, Low. 
 Family 2. Xeritidse. 
 Genera : XerUa, L. ; Neritina, Lam. ; PiUolus, Low ; XanceUa, 
 
 Lam. 
 
 Family 3. Pleurotomarwlte. 
 
 Genera : Pleurotomaria, Defr. ; Anaiomus, Montf. ; Stomatia, 
 Helbing. 
 
 ft. Ptenoglossa (x.0.x). 
 Family i.Scalaridx. 
 
 Genus : Scalaria, Lam. 
 Family 5. Janihinidte. 
 Genera : Janthina, Lam. (fig. 44) ; Eefiuzia, Petit 
 
 y. TteniogJossa (3.1.3). 
 Family 6. CcritKidee. 
 
 Genera : Ccrtihium, Brng. ; Potamides, Brong. ; Nerimea, Defr. 
 Family 7. Melanidte. 
 
 Genera : Jfelania, Lam. ; ifelanopsis, Fer. ; Ancylotvs, Lay. 
 Family 8. PyramidtUidse. 
 
 Genera : Pyramidtlla, Lam. ; Stylina, Flem. ; Aclis, Loven. 
 Family 9.furrtidlidte. 
 
 Genera: Turriteila, Lam.; Ctecum, Flem.; Fernutus. Adans. ; 
 
 Siliquaria, Brag. 
 Family 10. Xenophoridse. 
 
 Genus : Phorus, Montf. (fig. 39). 
 
112 
 
 MOLLUSCA 
 
 TABULAR VIEW OF THE SUBDIVISIONS OF THE CLASS GASTROPODA, ARRANGED so AS TO SHOW THEIR SUPPOSED GENETIC 
 
 RELATIONSHIPS. 
 
 Class. GASTROPODA. 
 
 (Archisopleurum. ) 
 
 Sub-class 1. ISOPLEURA. 
 
 Sub-class 2. ANISOPLEURA. 
 (ArchwutJiyneurum. ) 
 
 %. Si 
 
 Branch a. STREPTONEURA. 
 (Archizygobranchium. ) 
 
 Branch b. EUTHYNEURA. 
 (Archiopisthobranchium. ) 
 
 Order 1. ZYGOBRANCHIA. 
 
 Order 2. AZYQOBRANCHIA. 
 (Archiholochlamydum. ) 
 
 Order 1. OPISTHOBRANCHIA. 
 (Archipalliatum. ) 
 
 Order 2. PULMONATA. 
 
 (Archibasommatum . ) 
 
 Sect. a. 
 Heptardia. 
 
 Sect. 6. 
 Natantia. 
 
 Sect. a. 
 Palliata. 
 
 I 
 
 r 
 
 I I 
 
 *xj cj 
 
 i 
 
 1 1 
 
 CO K> 
 I I 
 
 Sect. 6. 
 Non-Palliata. 
 
 <r 
 I 
 
 I 
 I 
 
 \ \ 
 
 Family 11. Naticidse. 
 
 Genera : Natica, Lam. ; Sigaretus, Lam. ; Neritopsis, Gratel. 
 Family 12. Entoconchidie. 
 
 The single genus and species Entoconcha mirabilis, discovered by 
 Joh. Miiller in 1851, parasitic in Synapta digitata. The adult 
 form is not known. 
 Family 13. Marsenidse. 
 
 Genera : Marsenia, Leaeh ; Onchidiopsis, Beck. 
 Family 14. Acirueidee. 
 Genera : Acmsea, Eschsch. ; Lottia, Gr. ; (probably these will be 
 
 found to belong to the Zygobranchia). 
 Family 15. Capwlidas. 
 Genera : Capulus, Montf. ; Calyptram, Lam. (fig. 40) ; Trochita, 
 
 Schum. 
 Family 16. Littorinidse. 
 
 Genera : Littorina (the Periwinkles, fig. 46) ; Modulus, Gray ; 
 Lacuna, Turt. ; Rissoa, Frein. ; Hydrobia, Hartm. ; Assiminia, 
 Leach. 
 
 Family 17. Palvdinidss. 
 Genera : Paludvna (River-Snail) (figs. 7, 21) ; Bithynia, Gray ; 
 
 Tanalia, Gray. 
 Family 18. Valvatidse. 
 
 Genus : Valvata (fig. 45), fresh-water. 
 Family 19. Ampullaridee. 
 
 Genus : Ampidlaria (can breathe air by means of the walls of 
 the pallial chamber as well as water by the gill ; fresh-waters 
 of tropical America, Africa, and East Indies). 
 
 Sub-order 2. Pneumonechlamyda. 
 
 Characters. Pallial chamber a lung-sac; no gill; mouth on a 
 rostrum, not a retractile proboscis ; terrestrial habit. 
 
 Family 20. Cydostcmiidse. 
 
 Genera : Cyclostoma, Lam. ; Cyclophorus, Montf. ; Ferussina, 
 
 Gratel. ; Pupina, Vignard. 
 Family 21. Hclicinides (radula rhipidoglossate rather than tsenio- 
 
 glossate). 
 Genera : Stoastoma, Adams ; Trochatella, Swains. ; Helicina, 
 
 Lam. ; Proserpina, Guild. 
 Family 22. Aciculidse. 
 Genera: Acicula, Hartm.; Oeomelania, Pfr. 
 
 Sub-order 3. Siphonochlamyda. 
 
 Characters. Reptant Azygobranchia with the margin of the 
 mantle drawn out to form a trough-like siphon which notches the 
 lip of the shell ; shell always spiral ; usually an operculum, horny 
 or lamelliform ; either a rostrum or a retractile proboscis ; exclusively 
 marine ; mostly carnivorous. 
 
 * Teenioglossa (3.1.8). 
 Family 1. Slrombidse. 
 
 Genera : Stromlnts, L. ; Ptcroccras, Lam. ; Eostellaria., Lam. 
 
 (fig. 43). 
 Family 2. Aporrhaid/e. 
 
 Genus : Aporrhais, Da Costa. 
 Family 3. Pcdicularidas. 
 
 Genus : Pcdicularia, Swains. 
 Family 4. Dolidse. 
 
 Genera : Cassis, Lam. ; Cassidaria, Lam. ; Dolivm, Lam. ; Ficula, 
 
 Swains. 
 Family 5. Tritonidse. 
 
 Genera : Trttonium, Cuv. (fig. 42) ; Ranclla, Lam. 
 Family 6. Cyprssidas (the Cowries). 
 
 Genera : Cyprsea, L. ; Ovuhim, Brag. (fig. 41) ; Erato, Risso. 
 
 *Toxiglossa (1.0.1). 
 Family 7. Conidee. 
 Genus : Conus, L. 
 Family 8. Terebridai. 
 
 Genus : Tercbra, Adans. 
 Family 9. Pleurotomidee. 
 
 Genus : Pleurotoma, Lam. 
 Family 10. Cancellaridas. 
 Genus : Cancellaria, Lam. 
 
 *Rachiylossa (1.1.1 or .1.). 
 Family 11. Muricidas. 
 
 Genera : Murex, L. ; Trophon, Montf. ; Fusus, Brug. ; Pyrula, 
 
 Lam. (fig. 38) ; Turbinclla, Lam. 
 Family 12. Bucdnidee. 
 
 Genera : Buccinum, L. ; Nassa, Lam. (fig. 5) ; Purpura, Brug. 
 
 (fig. 47) ; Concholepas, Lam. ; Magilus, Montf. 
 Family 13. Mitridse. 
 Genus : Mitra, Lam. 
 
MOLLUSCA 
 
 113 
 
 Family 14. . 
 
 Genera : Olim, Brug. ; Ancilla, Lam.; Harpa, Lam. 
 FamQy 15. Volutidai. 
 
 Genera : Voluta, L. ; Cymbium, Montf. ; Margindla, Lam. ; 
 Volvaria Lam. 
 
 Further Remarks on the Reptant Azygobranchia. The 
 very large assemblage of forms coining under this order 
 comprise the most highly developed predaceous sea-snails, 
 numerous vegetarian species, a considerable number of 
 
 Fio. 37. A. Triton varuyatum, to show the proboscis or bnccal introvert (t) 
 in a state of aversion, a, siphonal notch of the shell occupied by the siphonal 
 fold of the mantle-skirt (Siphonochlamyda) ; 6, edge of the mantle-skirt rest- 
 ing on the shell ; c, cephalic eye ; d, cephalic tentacle ; e, everted buccal 
 introvert (proboscis) ; / foot ; g, operculum ; A, penis ; i, under surface of 
 the inantle-skirt forming the roof of the sub-pallia! chamber. B. Sole of the 
 foot of Pyrula tvha, to show o, the pore usually said to be "aquiferous" 
 but probably the orifice of a gland ; b, median line of foot. 
 
 fresh-water, and some terrestrial forms. The partial dis- 
 section of a male specimen of the Common Periwinkle, 
 Littorina littoralis, drawn in fig. 46, will serve to exhibit 
 the disposition of viscera which prevails in the group. 
 
 retractor muscle of the foot, which clings to the spiral 
 column or columella of the shell (see fig. 42). This col- 
 urnella muscle is the same thing as the muscular surface 
 marked c in the figures of Patella, marked k in fig. 91 of 
 Nautilus, and the posterior adductor of Lamellibranchs 
 (fig. 131). 
 
 The surface of the neck is covered by integument forming 
 the floor of the branchial cavity. It has not been cut into. 
 
 FIG. 38. Animal and shell of Pyrula Irrigate- a, siphon ; 6, head-tentacles ; C, head, the letter 
 eye ; i, the foot, expanded as in crawling ; A, the mantle-skirt reflected over the sides of the 
 
 The branchial chamber formed by the mantle-skirt over- 
 hanging the head has been exposed by cutting along a line 
 extending backward from the letters rd to the base of the 
 columella muscle me, and the whole roof of the chamber 
 thus detached from the right side of the animal's neck has 
 been thrown over to the left, showing the organs which lie 
 upon the roof. No opening into the body-cavity has been 
 made ; the organs which lie in the coiled visceral hump 
 show through its transparent walls. The head is seen in 
 front resting on the foot and carrying a median non-retractile 
 snout or rostrum, and a pair of cephalic tentacles at the 
 base of each of which is an eye. In many Gastropoda the 
 eyes are not thus sessile but raised upon special eye-tentacles 
 (figs. 43, 69). To the right of the head is seen the muscular 
 penis p close to the termination of the vas deferens (sper- 
 matic duct) vd. The testis t occupies a median position in 
 the coiled visceral mass. Behind the penis on the same 
 side is the hooklike columella muscle, a development of the 
 
 FIG. 39. Animal and shell of Pkortii mtus. a, snout (not introversible) ; i. 
 cephalic tentacles ; c, right eye ; d, pro- and meso-podinm, to the right of 
 this is seen the metapodium bearing the sculptured operculum. 
 
 Of the organs lying on the reflected mantle-skirt, that which 
 
 in the natural state lay nearest to the vas deferens on the 
 right side of the median line of 
 the roof of the branchial chamber 
 is the rectum t', ending in the 
 anus a. It can be traced back to 
 the intestine i near the surface of 
 the visceral hump, and it is found 
 that the apex of the coil formed 
 by the hump is occupied by the 
 liver h and the stomach v. Pha- 
 rynx and oesophagus are con- 
 cealed in the head. The enlarged 
 glandular structure of the walls 
 of the rectum is frequent in the 
 Azygobranchia, as is also though 
 not universally the gland marked 
 y, next to the rectum. It is the 
 adrectal gland, and in the genera 
 Murex and Purpura secretes a 
 colourless liquid which turns 
 to the at- 
 used by the 
 
 ancients as a dye. 
 
 Near this, and less 
 
 advanced into the 
 
 branchial chamber, 
 
 is the single renal 
 
 organ or nephri- 
 
 dium r with its 
 
 opening to the ex- 
 terior r. Internally 
 
 this glandular sac 
 
 presents a second 
 
 slit or aperture 
 
 which leads into the 
 
 pericardium (as is 
 
 now found to be , 
 
 the case in 
 
 lusca). The heart 
 
 c lying in the pericardium is seen in close proximity to 
 
 -placed near the right P ur P le u P n exposure 
 
 shell. (From Owen, mosphere, and was us 
 
 \r 1 Fl - 40. Shell of Calyptrsea, sn from below so as 
 JUOl- to show the inner whorl 6, concealed by the cap- 
 like outer whorl o. 
 
114 
 
 MOLLUSCA 
 
 the renal organ, and consists of a single auricle receiving 
 blood from the gill, and of a single ventricle which pumps 
 it through the body by an anterior and posterior aorta 
 (see fig. 105). The 
 surface x of the 
 mantle between the 
 rectum and the gill- 
 plume is thrown 
 into folds which 
 in many sea-snails 
 (Whelks, (fee.) are 
 very strongly deve- 
 
 , j mif ' r. i Fio. 41. Animal and shell of Ovulum. fc, cephalic 
 
 loped. Ihe Whole tentacles ; d, foot ; h, mantle-skirt, which is natu- 
 
 of this Surface ai)- ral 'y carried in a reflected condition so as to 
 
 . * cover in the sides of the shell. 
 
 pears to be active 
 
 in the secretion of a mucous-like substance. The single 
 gill-plume br lies to the left of the median line in natural 
 position. It corresponds to the 
 right of the two primitive cten- 
 idia in the untwisted archaic 
 condition of the Molluscan body, 
 and does not project freely into 
 the branchial cavity, but its 
 axis is attached (by concres- 
 cence) to the mantle-skirt (roof 
 of the branchial chamber). It 
 is rare for the gill-plume of an 
 Anisopleurous Gastropod to 
 stand out freely as a plume, 
 but occasionally this more ar- 
 chaic condition is exhibited, as 
 in Valvata (fig. 45). Next be- 
 yond (to the left of) the gill- 
 plume we find the so-called para- 
 branchia, which is here simple, 
 but sometimes lamellated as in 
 Purpura (fig. 47). This organ 
 has, without reason, been sup- 
 posed to represent the second F[Q 42 
 ctenidium of the typical Mollusc, 
 which it cannot do on account 
 
 of its position. It should be ', <"> whorls of the shell ; s, . 
 
 ,1 . . i> .1 tures. Occupying the axis, and 
 
 to the right OI the anus were exposed by the section, is seen the 
 
 this the case. Recently Spengel "coiumeiia "or spiral pillar. The 
 
 columella " or spir 
 
 upper whor]s of g sldl are seen 
 
 to be divided into separate chain- 
 
 has shown that the parabran- 
 
 chia of Gastropods is the typical 
 
 olfactory organ or osphradium Owen.) 
 
 in a highly-developed condition The minute structure 
 
 of the epithelium which clothes it, as well as the origin of 
 
 Fio. 43. Animal and shell of Rostellaria rectirostris. a, snout or rostrum; 
 6, cephalic tentacle ; c, eye ; d, propodium and mesopodium ; e, metapodium ; 
 /, operculum ; &', prolonged siphonal notch of the shell occupied by the 
 siphon, or trough-like process of the mantle-skirt. (From Owen.) 
 
 the nerve which is distributed to the parabranchia, proves 
 it to be the same organ which is found universally in Mol- 
 
 luscs at the base of each gill-plume, and tests the indrawn 
 current of water by the sense of smell. The nerve to this 
 
 Milll. 
 
 Fio. 44. Female Janthina, with egg-float (a) attached to the foot ; 6, egg- 
 capsules ; c, ctenidium (gill-plume) ; d, cephalic tentacles. 
 
 organ is given off from the superior (original right, see 
 fig. 19) visceral ganglion. 
 
 The figures which are here given of various Azygo- 
 branchia are in most cases suffi- 
 ciently explained by the refer- 
 ences attached to them. As an 
 excellent general type of the 
 nervous system, attention may 
 be directed to that of Paludina 
 drawn in fig. 21. On the whole, 
 the ganglia are strongly indivi- 
 dualized in the Azygobranchia, 
 nerve-cell tissue being concen- 
 trated in the ganglia and absent Flo . 4 5.-ravata 
 from the cords (contrast with Zy- < mouth ; P. operculum ; & 
 
 , , . j T i \ , ctenidium (branchial plume); x, 
 
 gobranchia and Isopleura). At filiform appendage (Trudiment- 
 
 the same time, the junction of ^tfn^cteiild'iumoft ^Tcafform 
 
 the visceral loop above the in- not having its axis fused to the 
 *,.(;,, + n ; !! cu roof of the. branchial chamber is 
 
 testme prevents m all Strepto- the notable character of this 
 neura the shortening of the vis- genus. 
 ceral loop, and it is rare to find a fusion of the visceral 
 ganglia with either pleural, pedal, or cerebral a fusion 
 which can and does 
 take place where the 
 visceral loop is not 
 above but below the 
 intestine, e.g., in the 
 Euthyneura (fig. 67), 
 Cephalopoda(fig.ll2), 
 and Lamellibranchia 
 (fig. 144). As con- 
 trasted with the Zygo- 
 branchia and the Iso- 
 pleura, we find that in 
 the Azygobranchia the 
 pedal nerves are dis- 
 tinctly nerves given off 
 from the pedal ganglia, 
 rather than cord-like 
 nerve- tracts contain- 
 ing both nerve -cells 
 or ganglionic elements 
 and nerve-fibres. Yet 
 in some Azygobran- 
 chia (Paludina) a lad- Fio. 46. Male of Httorlna ^itloraUs, Lin., re- 
 -, .., moved from its shell; the mantle-skirt cut along 
 
 der-llke arrangement its right line of attachment and thrown over 
 to the left side of the animal so as to expose the 
 organs on its inner face, a, anus ; i, intestine ; 
 r, nephridium (kidney); r', aperture of the 
 nephridium ; c, heart ; br, ctenidium (gill- 
 plume); pbr, parabranchia ( = the osphradium 
 tected (30). The his- r olfactory patch) ; x, glandular lamella of 
 , i c ,\_ the inner face of the mantle-skirt ; y, adrectal 
 
 tolOgy OI the nerVOUS (purpuriparous) gland ; t, testis ; t'rf, vas de- 
 c-iratem nf Afnllnspn ferens ; p.penis; mr,columella muscle(muscular 
 bvsl process grasping the shelly v, stomach; h, liver, 
 
 has yet to be Sen- N.B. Note the simple snout or rostrum not in- 
 
 ously inquired into. troverte<1 as a " P roboscis -" 
 
 The alimentary canal of the Azygobranchia presents 
 little diversity of character, except in so far as the buccal 
 region is concerned. Salivary glands are present, and in 
 some carnivorous forms (Dolium) these secrete free sul- 
 
 of the two pedal 
 nerves and their lateral 
 branches has been de- 
 
MOLLUSCA 
 
 115 
 
 left line of attachment and 
 thrown over to the right side 
 of the animal so as to expose 
 the organs on its inner lace, 
 a, anus ; r j, vagina ; gp, adrec- 
 tal pnrporiparons gland ; r 1 , 
 aperture of the nephndium (kid- 
 ney) ; br t ctenidium (branchial 
 plume): ?T', parabranchia(=the 
 comb-like osphradium or olfac- 
 tory organ X 
 
 phuric acid (as much as two per cent is present in the 
 secretion), which assists the animal in boring holes by 
 means of its rasping tongue through the shells of other 
 Molluscs upon which it preys. A crop-like dilatation of 
 the gut and a recurved intestine, embedded in the com- 
 pact yellowish-brown liver, the ducts of which open into it, 
 form the rest of the digestive tract and occupy a large 
 bulk of the visceral hump. The buccal region presents a 
 pair of shelly jaws placed laterally upon the lips, and a 
 wide range of variation in the form of the denticles of the 
 lingual ribbon or radula, the nature of which will be un- 
 derstood by a reference to fig. 9, whilst the systematic list 
 of families given above shows the particular form of den- 
 tition characteristic of each division of the order. 
 
 The modification in the form of the snout upon which 
 the mouth is placed, leading to the 
 distinction of " proboscidif erous " 
 and " rostriferous " Gastropods, re- 
 quires further notice. The condi- 
 tion usually spoken of as a "pro- 
 boscis " appears to be derived from 
 the condition of a simple rostrum 
 (having the mouth at its extrem- 
 ity) by the process of incomplete 
 introversion, of that simple rostrum. 
 There is no reason in the actual _, 
 
 . ,, - , , , , FIG. 47. Female of Purpura la- 
 
 sigmhcance ot the word why the paius removed from its shell ; 
 term "proboscis" should be applied ? "P*""*-. 81 "'* cut along its 
 to an alternately introversible and 
 eversible tube connected with an 
 animal's body, and yet such is a 
 very customary use of the term. 
 The introversible tube may be 
 completely closed, as in the "pro- 
 boscis " of Nemertean worms, or 
 it may have a passage in it leading into a non-eversible 
 oesophagus, as in the present case, and in the case of the 
 eversible pharynx of the predatory Chaetopod worms. The 
 diagrams here introduced (fig. 48) are intended to show 
 certain important distinctions which obtain amongst the 
 various "introverts," or intro- and e-versible tubes so fre- 
 quently met with in animal bodies. Supposing the tube 
 to be completely introverted and to commence its ever- 
 sion, we then find that eversion may take place, either 
 by a forward movement of the side of the tube near its 
 attached base, as in the proboscis of the Nemertine worms, 
 the pharynx of Chaetopods, and the eye-tentacle of Gastro- 
 pods, or, by a forward movement of the inverted apex 
 of the tube, as in the proboscis of the Rhabdocoel Planar- 
 ians, and in that of Gastropods here under consideration. 
 The former case we call " pleurecbolic " (fig. 48, A, B, C, 
 H, I, K), the latter " acrecbolic " tubes or introverts (fig. 
 48, D, E, F, G). It is clear that, if we start from the 
 condition of full eversion of the tube and watch the pro- 
 cess of introversion, we shall find that the pleurecbolic 
 variety is introverted by the apex of the tube sinking in- 
 wards ; it may be called acrembolic, whilst conversely the 
 acrecbolic tubes are pleurembolic. Further, it is obvious 
 enough that the process either of introversion or of eversion 
 of the tube may be arrested at any point, by the develop- 
 ment of fibres connecting the wall of the introverted tube 
 with the wall of the body, or with an axial structure such 
 as the oesophagus ; on the other hand, the range of move- 
 ment of the tubular introvert may be unlimited or complete. 
 The acrembolic proboscis or frontal introvert of the Nemer- 
 tine worms has a complete range. So has the acrembolic 
 pharynx of Chaetopods, if we consider the organ as ter- 
 minating at that point where the jaws are placed and the 
 oesophagus commences. So too the acrembolic eye-tentacle 
 of the snail has a complete range of movement, and also the 
 
 pleurembolic proboscis of the Ehabdoccel prostoma. The 
 introverted rostrum of the Azygobranch Gastropods pre- 
 sents in contrast to these a limited range of movement. 
 The " introvert " in these Gastropods is not the pharynx as 
 in the Chaetopod worms, but a prae-oral structure, its apical 
 limit being formed by the true lips and jaws, whilst the 
 apical limit of the Chaetopod's introvert is formed by the 
 jaws placed at the junction of pharynx and oesophagus, so 
 that the Chaetopod's introvert is part of the stomodaeum 
 or fore-gut, whilst that of the Gastropod is external to the 
 alimentary canal altogether, being in front of the mouth, 
 not behind it, as is the Chaetopod's. Further, the Gastro- 
 pod's introvert is pleurembolic (and therefore acrecbolic), 
 and is limited both in eversion and in introversion ; it can- 
 
 FIG. 48. Diagrams explanatory of the nature of so-called proboscides or "intro- 
 verts." A. Simple introvert completely introverted. B. The same, partially 
 everted by eversion of the sides, as in the Nemertine proboscis and Gastropod 
 eye-tentacle = plenrecbolic. C. The same, fully everted. D, E. A similar 
 simple introvert in course of eversion by the forward movement, not of its 
 sides, bat of its apex, as in the proboscidean Rhabdocoels = acrecbolic. F. 
 Acrecbolic ( = pleurembolic) introvert, formed by the snout of the prpboscidi- 
 ferous Gastropod, al, alimentary canal ; d, the true mouth. The introvert 
 is not a simple one with complete range both in eversion and introversion, 
 but is arrested in introversion by the fibrous bands at c, and similarly in 
 eversion by the fibrous bands at i>. G. The acrecbolic snout of a probos- 
 cidiferous Gastropod, arrested short of complete eversion by the fibrous band 
 i>. H. The acrembolic (= pleurecbolic) pharynx of a Chsetopod fully intro- 
 verted, al, alimentary canal ; at d, the jaws ; at a, the month ; therefore a 
 to d is stomodstum, whereas in the Gastropod (F) a to d is inverted body- 
 surface. L Partial eversion of H. K. Complete eversion of H. (Original.) 
 
 not be completely everted owing to the muscular bands 
 (fig. 48, G), nor can it be fully introverted owing to the bands 
 (fig. 48, F) which tie the axial pharynx to the adjacent 
 wall of the apical part of the introvert. As in all such 
 intro- and e-versible organs, eversion of the Gastropod 
 proboscis is effected by pressure communicated by the 
 muscular body- wall to the liquid contents (blood) of the 
 body-space, accompanied by the relaxation of the muscles 
 which directly pull upon either the sides or the apex of 
 the tubular organ. The inversion of the proboscis is effected 
 directly by the contraction of these muscles. In various 
 members of the Azygobranchia the mouth-bearing cylinder 
 is introversible (i.e., is a proboscis) with rare exceptions 
 these forms have a siphonate mantle-skirt. On the other 
 hand, many which have' a siphonate mantle-skirt are not 
 provided with an introversible mouth-bearing cylinder, but 
 have a simple non-introversible rostrum, as it has been 
 
116 
 
 MOLLUSCA 
 
 termed, which is also the condition presented by the mouth- 
 bearing region in nearly all other Gastropoda. One of the 
 best examples of the introversible mouth-cylinder or pro- 
 boscis which can be found is that of the Common Whelk 
 and its immediate allies. In fig. 37 the proboscis is seen 
 in an everted state ; it is only so carried when feeding, 
 being withdrawn when the animal is at rest. Probably 
 its use is to enable the animal to introduce its rasping 
 and licking apparatus into very narrow apertures for the 
 purpose of feeding, e.g., into a small hole bored in the shell 
 of another Mollusc. 
 
 The foot of the Azygobranchia, unlike the simple mus- 
 cular disc of the Isopleura and Zygobranchia, is very often 
 divided into lobes, a fore, middle, and hind lobe (pro-, 
 meso-, and meta-podium, see figs. 39 and 43). Very usually, 
 but not universally, the meta-podium carries an operculum. 
 The division of the foot into lobes is a simple case of that 
 much greater elaboration or breaking up into processes and 
 regions which it undergoes in the class Cephalopoda. Even 
 among some Gastropoda (viz., the Opisthobranchia), we 
 find the lobation of the foot still further carried out by 
 the development of lateral lobes, the epipodia, whilst there 
 are many Azygobranchia, on the other hand, in which the 
 foot has a simple oblong form without any trace of lobes. 
 
 The development of the Azygobranchia from the egg has 
 been followed in several examples, e.g., Paludina, Purpura, 
 Nassa, Vermetus, Neritina. As in other Molluscan groups, 
 we find a wide variation in the early process of the forma- 
 tion of the first embryonic cells, and their arrangement as 
 a Diblastula dependent on the greater or less amount of 
 food-yelk which is present in the egg-cell when it com- 
 mences its embryonic changes. In fig. 7, the early stages 
 of Paludina vivipara are represented. There is but 
 very little food-material in the egg of this Azygobranch, 
 and consequently the Diblastula forms by invagination ; 
 the blastopore or orifice of invagination coincides with the 
 anus, and never closes entirely. A well-marked Trocho- 
 sphere is formed by the development of an equatorial 
 ciliated band; and subsequently, by the disproportionate 
 growth of the lower hemisphere, the Trochosphere becomes 
 a Veliger. The primitive shell-sac or shell-gland is well 
 marked at this stage, and the pharynx is seen as a new 
 ingrowth (the stomodaeum), about to fuse with and open 
 into the primitively invaginated arch-enteron (fig. 7, F). 
 
 In other Azygobranchs (and such variations are repre- 
 sentative for all Mollusca, and not characteristic only of 
 Azygobranchia), we find that there is a very unequal 
 division of the egg-cell at the commencement of embryonic 
 development, as in Nassa (fig. 5). Consequently there is 
 strictly speaking no invagination (emboly), but an over- 
 growth (epiboly) of the smaller cells to enclose the larger. 
 The general features of this process and of the relation of 
 the blastopore to mouth and anus have been explained 
 above in treating of the development of Mollusca generally. 
 In such cases the blastopore may entirely close, and both 
 mouth and anus develop as new ingrowths (stomodaeum 
 and proctodseum), whilst, according to the observations of 
 Bobretzky, the closed blastopore may coincide in position 
 with the mouth in some instances (Nassa, &c.), instead of 
 with the anus. But in these epibolic forms, just as in the 
 embolic Paludina, the embryo proceeds to develop its cili- 
 ated band and shell-gland, passing through the earlier con- 
 dition of a Trochosphere to that of the Veliger. In the 
 veliger stage many Azygobranchia (Purpura, Nassa, &c.) 
 exhibit, in the dorsal region behind the head, a contractile 
 area of the body-wall. This acts as a larval heart, but 
 ceases to pulsate after a time. Similar rhythmically con- 
 tractile areas are found on the foot of the embryo Pulmo- 
 nate Limax and on the yelk-sac (distended foot-surface) 
 of the Cephalopod Loligo (see fig. 72**). 
 
 The history of the shell in the development of Azygo- 
 branchia (and other Gastropods) is important. Just as 
 the primitive shell-sac aborts and gives place to a cap-like 
 or boat-like shell, so in some cases (Marsenia, Krohn) has 
 this first shell been observed to be shed, and a second shell 
 of different shape is formed beneath it. 
 
 A detailed treatment of what is known of the histo- 
 genesis in relation to the cell-layers in these Mollusca would 
 take us far beyond the limits of this article, which aims at 
 exposing only the well-ascertained characteristic features 
 of the Mollusca and the various subordinate groups. There 
 is still a great deficiency in our knowledge of the develop- 
 ment of the Gastropoda, as indeed of all classes of animals. 
 The development of the gill (ctenidium) as well as of the 
 renal organ, and details as to the process of torsion of the 
 visceral hump, are still quite insufficiently known. 
 
 One further feature of the development of the Azygobran- 
 chia deserves special mention. Many Gastropoda deposit 
 their eggs, after fertilization, enclosed in capsules ; others, as 
 Paludina, are viviparous ; others, again, as the Zygobranchia, 
 agree with the Lamellibranch Conchifera (the Bivalves) in 
 having simple exits for the ova without glandular walls, 
 and therefore discharge their eggs unenclosed in capsules 
 freely into the sea- water ; such unencapsuled eggs are 
 merely enclosed each in its own delicate chorion. When 
 egg-capsules are formed they are often of large size, have 
 tough walls, and in each capsule are several eggs floating 
 in a viscid fluid. In some cases all the eggs in a capsule 
 develop ; in other cases one egg only in a capsule (Neri- 
 tina), or a small proportion (Purpura, Buccinum), advance 
 in development ; the rest are arrested either after the first 
 process of cell-division (cleavage) or before that process. 
 The arrested embryos or eggs are then swallowed and 
 digested by those in the same capsule which have advanced 
 in development. The details of this history require renewed 
 study, our present knowledge of it being derived from the 
 works of Koren and Danielssen, Carpenter and Claparede. 
 In any case it is clearly the same process in essence as that 
 of the formation of a vitellogenous gland from part of the 
 primitive ovary, or of the feeding of an ovarian egg by 
 the absorption of neighbouring potential eggs ; but here 
 the period at which the sacrifice of one egg to another 
 takes place is somewhat late. What it is that determines 
 the arrest of some eggs and the progressive development 
 of others in the same capsule is at present unknown. 
 
 Section b (of the Azygobranchia). NA TANTIA. 
 
 Characters. Azygobranchiate Streptoneura which have the 
 form anil texture of the body adapted to a free-swimming pelagic 
 habit. They appear to be derived from holochlamydic forms of 
 Reptant Azygobranchia. The foot takes the form of a swimming 
 organ. The nervous system and sense-organs (eyes, otocysts, and 
 osphradium) are highly developed. The odontophore also is re- 
 markably developed, its admediau teeth being mobile, and it serves 
 as an efficient organ for attacking other pelagic forms upon which 
 the Natantia prey. The sexes are distinct as in all Streptoneura ; 
 and genital ducts and accessory glands and pouches are present as 
 in all Azygobranchia. The Natantia exhibit a series of modifica- 
 tions of the form and proportions of the visceral mass and foot, 
 leading from a condition readily comparable with that of a typical 
 Azygobranch such as Rostellaria, with the three regions of the foot 
 (pro-, meso-, and meta-podium) strongly marked, and a coiled 
 visceral hump of the usual proportions, up to a condition in which 
 the whole body is of a tapering cylindrical shape, the foot a plate- 
 like vertical fin, and the visceral hump almost completely atrophied. 
 Three steps of this modification may be distinguished as three sub- 
 orders, the Atlantacea, the Carinariacca, and the Ptcrotracheacea. 
 
 Sub-order 1. Atlantacea. 
 
 Characters. Natantia with a large spirally- wound visceral hump, 
 covered by a hyaline spiral shell ; mantle-skirt large, overhanging 
 a well-developed sub-pallial branchial chamber as in Azygobranchia, 
 to the wall of which is attached the branchial ctenidium ; foot 
 well developed, divisible into a mobile propodium, a mesopodium 
 on which is formed a sucker, and a metapodium which, when the 
 animal is expanded, extends backwards beyond the shell and visceral 
 
MOLLUSCA 
 
 117 
 
 hump ; upon the upper surface of the metapodium is developed an 
 operculum. 
 
 Genera : Atlanta, Oxygurus. Probably here belong the Palaeozoic 
 fossils Btllerophon. 
 
 Sub-order 2. Carinariaeea. 
 
 Characters. Visceral hump greatly reduced in relative size; 
 shell small, cap-like, hyaline ; 
 ctenidium (branchial plume) 
 projecting from the small sub- 
 pallial chamber ; body cylin- 
 drical ; of the foot-lobes only 
 the mesopodium is prominent, 
 provided with a sucker, and 
 compressed laterally so as to 
 form a vertical plate -like fin 
 projecting from the ventral 
 surface; the propodium forms 
 simply the ventral surface of 
 the anterior region of the cy- 
 lindrical body whilst the me- 
 tapodium forms its posterior 
 region. 
 
 Genera : Carinaria, Cardio- 
 poda. 
 
 Sub-order 3. Pterotracheacea. 
 
 Characters. Visceral hump 
 still further reduced, forming 
 a mere oval sac embedded in 
 the posterior dorsal region of 
 the cylindrical body ; no shell ; 
 foot as in Carinariaeea, except 
 that the sucker is absent from 
 the mesopodium in the females. 
 
 Genera : Pterotrachea, Firu- 
 
 "". -. 
 
 the visceral loop of the Natantia is Streptoneurous. Special 
 to the Natantia is the high elaboration of the lingual 
 ribbon, and, as an agreement with some of the Opistho- 
 branchiate Euthyneura but as a difference from the Azygo- 
 branchia, we find the otocysts closely attached to the cerebral 
 ganglia. This is, however, less of a difference than it was 
 
 10. 50. Carinaria medittrranea. A. The animaL R The shell removed. C, D. Two views of the shell of Cardiopoda. 
 a, month and odontophore ; b, cephalic tentacles ; c, eye ; tf, the fin-like mesopodium ; d', its sucker ; t, metapodinin ; 
 /, salivary glands ; a, border of the mantle-flap ; i, ctenidium (gill-plume) ; m, stomach ; n, intestine ; o, anus ; p, liver ; 
 t, aorta, springing from the ventricle ; , cerebral ganglion ; u, pleura! and pedal ganglion ; IP, testis ; x, visceral ganglion ; 
 y, vesicula seminalis ; z. penis. (From Owen.) 
 
 Further Remarks on the 
 Natantia Azygobranchia. 
 Logically the Xatantia should stand as we have placed them, 
 viz., as a special branch or section of the Azygobranchia, 
 related to them somewhat as are the Birds to the Reptiles. 
 They are true Azygobranchia which have taken to a pelagic 
 life, and the peculiarities of structure which they exhibit 
 
 FIG. 49. Atlanta (Oxygura) Keraudrenii (magnified 20 diametersX o, mouth 
 and odontophore ; 6, cephalic tentacles ; c, eye ; d, propodium (JB) and meso- 
 podinm ; t, metapodium ; f t operculum ; ft, mantle-chamber ; i, ctenidium 
 (gill-plume) ; k, retractor muscle of foot ; f, optic tentacle ; m, stomach ; n, 
 dorsal surface overhung by the mantle-skirt, the letter is close to the salivary 
 gland ; o, rectum and anus ; p, liver ; g, renal organ (nephridium)J; 5, ven- 
 tricle ; u, the otocyst attached to the cerebral ganglion ; w, testis ; z, auricle 
 of the heart ; y, vesicle on genital duct ; z, penis. (From Owen.) 
 
 are strictly adaptations of the structure common to them 
 and the Azygobranchia consequent upon their changed 
 mode of life. Such adaptations are the transparency and 
 colourlessness of the tissues, and the modifications of the 
 foot, which still shows in Atlanta the form common in 
 Azygobranchia (compare fig. 49 and fig. 39). 
 
 The cylindrical body of Pterotracheacea is paralleled by 
 the slug-like forms Of Euthyneura. Spengel has shown that 
 
 at one time supposed to be, for it has been shown by Lacaze 
 Duthiers, and also by Leydig, that the otocysts of Azygo- 
 branchia even when lying close upon the pedal ganglion 
 (as in fig. 21) yet receive their special nerve (which can 
 sometimes be readily isolated) from the cerebral ganglion (see 
 fig. 36). Accordingly the difference is one of position of the 
 otocyst and not of its nerve-supply. The Natantia are further 
 remarkable for the high development of their cephalic eyes, 
 and for the typical character of their osphradium (Spengel's 
 olfactory organ). This is a groove, the edges of which are 
 raised and ciliated, lying near the branchial plume in 
 the genera which possess that organ, whilst in Firuloides, 
 which has no branchial plume, the osphradium occupies a 
 corresponding position. Beneath the ciliated groove is 
 
 FIG. 51. Pterotmdun vmtim. seen from the right side, a, pouch for reception 
 of the snout when retracted : c, pericardium ; ph, pharynx ; oc, cephalic eye ; 
 g, cerebral ganglion ; ff 7 , pieuro-pedal ganglion ; j>r, foot (mesopodium) ; F, 
 stomach ; i, intestine ; n, so-called nucleus ; frr, branchial plume (ctenidium); 
 IT, osphradium ; *, foot (metapodium) ; i, caudal appendage. (After Kefer- 
 stein.) 
 
 placed an elongated ganglion (olfactory ganglion) connected 
 by a nerve to the supra-intestinal (therefore the primitively 
 dextral) ganglion of the long visceral nerve-loop, the strands 
 of which cross one another, this being characteristic of 
 Streptoneura (Spengel). 
 
 The Xatantia belong to the " pelagic fauna " occurring 
 near the surface in the Mediterranean and great oceans in 
 company with the Pteropoda, the Siphonophorous Hydrozoa, 
 Salpae, Leptocephali, and other specially-modified trans- 
 parent swimming representatives of various groups of the 
 animal kingdom. In development they pass through the 
 typical trochosphere and veliger stages provided with boat- 
 like shell. 
 
118 
 
 MOLLUSCA 
 
 Branch b. EUTHYNEURA (Spengel, 1881). 
 
 Characters. Gastropoda Anisopleura in which the 
 visceral loop (the conterminous visceral nerves) does not 
 share in the torsion of the visceral hump, but, being suuk 
 entirely below the body-wall, remains straight and un- 
 twisted. Although the anus is not brought so far forward 
 
 FIG. 52. Bulla texilhim (Chemnitz), as seen crawling, d, oral hood (compare 
 with Tethys, fig. 62, 13), possibly a continuation of the epipodia ; 6,6', cephalic 
 tentacles. (From Owen.) 
 
 by the visceral torsion as in the Streptoneura, and may even 
 by secondary growth assume a posterior median position, yet, 
 as fully developed, an asymmetry has resulted as in the 
 Azygobranchia, only the original right renal organ, right 
 ctenidiuro (if any), right osphradium, right side of the heart, 
 and right genital ducts being retained. All the Euthy- 
 neura are hermaphrodite. The lingual ribbon has very 
 usually numerous fine denticles 
 undifferentiated into series in 
 each row. The shell is light 
 and little calcified ; often it is 
 rot developed in the adult, 
 
 though present in the embryo. 
 
 An operculum, often found in d 
 
 the embryo, is never present in F>. 53. Tornateiia. K, shell ; 6, 
 the adult (except in Tornateiia, llhood : d ' foot : ' opercul 
 fig. 53). Many Euthyneura show a tendency to, or a 
 complete accomplishment of, the suppression of the mantle- 
 skirt as well as of the shell, also of the ctenidium, and ac- 
 quire at the same time a more or less cylindrical (slug-like) 
 form of body. 
 
 The Euthyneura comprise two orders, the Opistho- 
 branchia and the Pulmonata. 
 
 Order 1. Opisthobranchia. 
 
 Marine Euthyneura the more archaic forms of which 
 have a relatively large foot and a small visceral hump, 
 from the base of which projects on the right side a short 
 mantle-skirt. The anus is placed in such forms far back 
 
 Fio. 54. Umbrella mediterranea. a, mouth ; 6, cephalic tentacle ; h, gill 
 (ctenidium). The free edge of the mantle is seen just below the margin of 
 the shell (compare with Aplysia, fig. 63). (From Owen.) 
 
 beyond the mantle-skirt. In front of the anus, and only 
 partially covered by the mantle-skirt, is the ctenidium with 
 its free end turned backwards. The heart lies in front of, 
 instead of to the side of, the attachment of the ctenidium, 
 hence Opisthobranchia as opposed to " Prosobranchia," 
 
 which correspond to the Streptoneura. A shell is possessed 
 
 in the adult state by but few Opisthobranchia, but all pass 
 
 through a veliger larval stage with a nautiloid shell (fig. 60). 
 
 Many Opisthobranchia have 
 
 by a process of atrophy lost 
 
 the typical ctenidium and the 
 
 mantle-skirt, and have deve- 
 
 loped other organs in their 
 
 place. As in some Azygo- 
 
 branchia, the free margin of 
 
 the mantle-skirt is frequently 
 
 reflected over the shell when 
 
 a shell exists ; and, as in some 
 
 Azygobranchia, broad lateral 
 
 outgrowths of the foot (epi- 
 
 podia) are often developed, 
 
 which, as does not occur in Azy- 
 
 gobranchia, may be thrown 
 
 over the shell or naked dorsal Flo ^_ nmbreUa 
 
 surface of the body. f ro ' above, h, mouth 
 
 rm . . f -11 tentacles ; k. 
 
 The variety of special deve- Kcfcrstein.) 
 
 t, cephalic 
 penis-sheath. (After 
 
 lopments of structure accom- 
 panying the atrophy of typical organs in the Opisthobranchia 
 and general degeneration of organization is very great, and 
 renders their classification difficult. Two sections of the 
 order may be distinguished, according as the typical 
 Molluscan mantle-skirt (limbus pallialis) is or is not atro- 
 phied, and within each section certain sub-orders. 
 
 Section a. PALLIATA (= Tectibranchiata, Woodward) the 
 typical Molluscan mautle-skirt or pallium retained. 
 
 Sub-order 1. Ctenidiobranchia. 
 
 Characters. Palliata in which the ctenidium is retained as the 
 branchial organ ; with rare exceptions a delicate shell, which may 
 be very small or completely enclosed by the reflected margin of the 
 mantle; epi podia (lateral outgrowths of the foot) fi equently present. 
 Family 1. Tornatcllidse. 
 
 Genera : Tornateiia, Lam. (fig. 53) ; Cinulia, Gray, &c. 
 Family 2. Bullidse, 
 
 Genera : Bulla, Lam. (fig. 52) ; Accra, Miiller ; Scaphander, 
 Montf. ; Bullsea, Lam. ; Doridium, Meckel ; Gastropteron, 
 Meckel, &c. 
 Family 3. Aplysiidse. 
 
 Genera: Aplysia, Gmelin (the Sea-Hare) (figs. 20, 56, &c.) ; 
 
 Dolobella, Lam.; Lobiger, Krohn, &c. 
 Family 4. Plcurobraiichidas. 
 
 Genera: Pleurobranchus, Cuvier; Umbrella, Chemnitz (figs. 54, 
 55); Euncina, Forbes, &c. 
 
 Sub-order 2. Phyllidiobranchia. 
 
 Characters. Palliata in which the ctenidia have atrophied ; much 
 as in Patellid;e among the Zygobranchiate Streptoneura their place 
 is taken by laterally-placed lamellae, developed from the inner surface 
 of the bilaterally-disposed mantle-skirt in two lateral rows. 
 Family 5. Phyllidiadas. 
 Genera : Phyllidia, Cuiver ; Pleurophyllidia, Heck. (fig. 57). 
 
 Section &. NON-PALLIA TA. 
 
 Characters. The typical Molluscan mantle-skirt is atrophied in 
 the adult. No shell is present in the adult, though the dorsal 
 integument may be strengthened by calcareous spicules (Doris). The 
 otocysts are not sessile on the pedal ganglia as in other Gastropods, 
 but, as in the Natantia Azygobranchia, lie close to the cerebral ganglia. 
 In one sub-order (Pygobranchia) the typical ctenidium appears to 
 be retained in a modified form ; in the others special developments 
 of the body-wall take its place, or no special respiratory processes 
 exist at all. The general form of the body is slug-like, the foot 
 and visceral hump being coextensive, and a secondary bilateral 
 symmetry is asserted by the usually median (sometimes right-sided) 
 dorsal position of the anus on the hinder part of the body. 
 Sub-order 1. Pygobranchia,. 
 
 Characters. The ctenidium assumes the form of a circlet of pinnate 
 processes surrounding the median dorsal anus ; a strongly-marked 
 epipodial fold may occur all round the foot and simulate a mantle- 
 skirt (see fig. 62, C, Doris) ; papillae or " cerata " of the dorsal integu- 
 ment may occur as well as the true ctenidium (fig. 61). 
 Family 6. Dorididse. 
 
 Genera : Doris, L. ; Goniodoris, Forbes ; TriojM, Johnst. ; dZgirus, 
 Loven ; Thecacera, Fleming ; Polyccra, Cuvier ; Idalia, Leuck- 
 art ; Ancula, Loven ; Ceratosoma, Adams ; Onchidoris, Blaiuv. 
 
MOLLUSCA 
 
 119 
 
 Sub-order Z.Ceratonota. 
 
 Characters. The typical Molluscan ctenidium is not developed 
 upon the dorsal area is developed a more or less numerous series o 
 cylindrical or branched processes (the cerata) into each of which the 
 intestine usually sends a process ; anus dorsal, median, or right-sided. 
 Family 7. Tritoniada. 
 
 Genera: Tritonia, Cuvier; ScyUtea, L.; TeOtys, L. (fig. 62, B); 
 
 Dendronotus, A. and H. ; Doto, Oken. 
 Family 8. Eolidse. 
 
 Genera : Eolis, Cnvier (fig. 62, A) ; Glaucus, Forster ; Fiona, A. 
 and H. (fig. 67) ; Embletmiia, A. and H. ; Prodonotus, A. and 
 H. ; Antiopa, A. and H.; Herman, Loven ; Alderio, Allman. 
 
 Sub-order S. Haplvmorpha. 
 
 Characters. "So ctenidia, cerata, mantle-skirt, or other processes 
 of the body-wall ; degenerate forms of small size. 
 Family 9. PhyllirTwidte. 
 
 Genera : Phyllirhoc, Peron and Lesueur (fig. 58) ; Acura, Adams. 
 Family 10. Elysiadte. 
 
 Genera: Elysia, Eisso (fig. 62, D, E) ; Acteonia, QuatreC ; Cenia, 
 A. and H. ; Limapontia, Johnston ; Rhodopc, KolL 
 
 Further Remarks on the Opisthobranchia. The Opis- 
 thobranchia present the same wide range of superficial 
 appearance as do the Azygobranchiate Streptoneura, forms 
 
 Flo. 56. Three views of Aplysia sp., in various conditions of expansion and 
 retraction, t, anterior cephalic tentacles ; f, posterior cephalic tentacles ; 
 e, eyes ; / meta podium ; rp, epi podium ; g, gill-plume (ctenidium) ; m, mantle- 
 flap reflected over the thin oval shell ; os, *, orifice formed by the unclosed 
 border of the reflected mantle- skirt, allowing the shell to show ; pe, the sper- 
 matic groove. (After Cuvier.) 
 
 carrying well-developed spiral shells and large mantle- 
 skirts being included in the group, together with flattened 
 or cylindrical slug- 
 like forms. But in 
 respect of the substi- 
 tution of other parts 
 for the mantle -skirt 
 and for the gill which 
 the more degenerate 
 Opisthobranchia ex- 
 hibit, this Order 
 stands alone. Some 
 Opisthobranchia are 
 striking examples of 
 degeneration (some 
 Haplomorpha), hav- 
 ing none of those re- 
 gions or processes of FlQ .. _ Dorsa , gnd Tentral Tfaw rf j^^^ 
 
 tne DOdy developed dr'/inata(ptto),oneofthePhyllidiobranch'iate 
 
 which rlisrincni;ili Palliate Opisthobranchs. 6, the mouth : 1, the 
 
 tingUlbQ lamelliform snb-pallial gills, which (as in Patella) 
 
 the archaic MollllSCa replace the typical Molluscan ctenidium. (After 
 
 f in. Keferstein.) 
 
 from such flat- worms 
 
 as the Dendroccel Planarians. Indeed, were it not for their 
 retention of the characteristic odontophore we should have 
 little or no indication that such forms as Phyllirhoe and 
 
 B 
 
 Limapontia really belong to the Mollusca at all. The inter- 
 esting little Rhodope Veranyii, which has no odontophore, 
 has been associated by systematists both with these simpli- 
 fied Opisthobranchs and with Rhabdocoel Planarians (29). 
 
 In many respects 
 
 the Sea-Hare (Aply- , ^szzm&.S^S 
 
 sia) of which several 
 species are known 
 (some occurring on 
 the English coast), 
 serves as a conven- 
 ient example of the 
 fullest development -'s-iieTa te^t~ ?*' ^^o^Z 
 
 of the Organization branch. The internal organs are shown as seen 
 
 fe ? . " by transmitted light, a, month ; b, radular sac ; 
 
 Characteristic Of c, o?sophagus ; <f, stomach ; c', intestine ; f, anus ; 
 
 Oi>i<stVmhranplii'a ?,?'. ?W, the four lobes of the liver ; , the 
 
 " l * heart (auricle and ventricle) ; J, the renal sac (ne- 
 
 The Woodcut (fi> 56) phridinm) ; r. the ciliated communication of the 
 
 t -.ir i ' renal sac with the pericardium ; w, the external 
 
 gives a laitnlUl repre- opening of the renal sac ; R, the cerebral ganglion ; 
 
 sentation of the great " S| e ^P 1 )*^ ^t^jf*: /. the genital pore; 
 
 .... y, the ovo-testes; v, the parasitic hydromedusa 
 
 mobility Of the van- ifnestm, usually found attached in this' position by 
 OUSpartSof thebody. tt "bral pole ofta umbrella. (After Keferstein.-) 
 
 The head is well marked and joined to the body by a some- 
 what constricted neck. It carries two pairs of cephalic 
 tentacles and a pair of sessile eyes. The visceral hump is 
 low and not drawn out into a spire. The foot is long, 
 carrying the oblong visceral mass upon it, and projecting 
 (as metapodium) a little beyond it (/). Laterally the 
 foot gives rise to a pair of mobile fleshy lobes, the epipodia 
 (ep), which can be thrown up so as to cover in the dorsal 
 
 Fir.. 59. ^orni lullata. A single row of teeth of the radula. (Formula, X.LX. ) 
 
 surface of the animal. Such epipodia are common, though 
 by no means universal, among Opisthobranchia. The 
 torsion of the visceral hump is not carried out very fully, 
 
 A "^ss 
 
 Fir,. 60. A. Veliger-larva of an Opisthobranch (Polycera). / foot ; op, oper- 
 culnm ; tnn, anal papilla ; ry, dry, two portions of nnabsorbed nutritive 
 yelk on either side the intestine. The right otocyst is seen at the root of 
 the foot. B. Trochosphere of an Opisthobranch (Plenrobranchidium) show- 
 ing : shgr, the shell-gland or primitive shell-sac ; r, the cilia of the velum ; 
 pft , the commencing stomodseum or oral invagination ; of, the left otocyst ; 
 pg, red-coloured pigment spot, C. Diblastula of an Opisthobranch (Poly, 
 cera) with elongated blastopore oi. (All from Lankester.) 
 
 the consequence being that the anus has a posterior posi- 
 n a little to the right of the median line above the 
 metapodium, whilst the branchial chamber formed by the 
 overhanging mantle-skirt faces the right side of the .body 
 .nstead of lying well to the front as in Streptoneura and 
 as in Pulmonate Euthyneura. The gill-plume which in 
 Aplysia is the typical Molluscan ctenidium is seen in fig. 
 
120 
 
 MOLLUSCA 
 
 63 projecting from the brancliial sub-pallial space. The 
 relation of the delicate shell to the mantle is peculiar, 
 since it occupies an oval area upon the visceral hump, 
 the extent of which is indicated in fig. 
 56, C, but may be better understood 
 by a glance at the figures of the allied 
 genus Umbrella (figs. 54, 55), in which 
 the margin of the mantle-skirt coin- 
 cides, just as it does in the Limpet, 
 with the margin of the shell. But in 
 Aplysia the mantle is reflected over 
 the edge of the shell, and grows over 
 its upper surface so as to completely 
 enclose it, excepting at the small cen- 
 tral area s where the naked shell is 
 exposed. This enclosure of the shell 
 is a permanent development of the 
 arrangement seen in many Strepto- 
 neura (e.g., Pyrula, Ovulum, see figs. 
 38 and 41), where the border of the 
 mantle can be, and usually is, drawn 
 over the shell, though it is withdrawn 
 (as it cannot be in Aplysia) when they F ~ 61 ._ Po ,,, CCT . a 
 are irritated. From the fact that one of the pygobranciu- 
 Aplysia commences its life as a free- 
 swimming Veliger with a nautiloid 
 shell not enclosed in any way by the 
 border of the mantle, it is clear that 
 the enclosure of the shell in the adult 
 is a secondary process. Accordingly, 
 the shell of Aplysia must not be con- 
 founded with a primitive shell in its 
 shell-sac, such as we find realized in 
 the shells of Chiton and in the plugs 
 which form in the remarkable tran- 
 sitory "shell-sac" or "shell-gland" of Molluscan embryos 
 
 ate Opisthobranchs (dor- 
 sal view), a, anus ; 6r, 
 the ctenidium peculiarly 
 modified so as to encircle 
 theanus; t, cephalic ten- 
 tacles. External to the 
 branchial ctenidium are 
 seen ten club-like pro- 
 cesses of the dorsal wall, 
 these are the "cerata" 
 which are characteristic- 
 ally developed in another 
 sub - order of Opistho- 
 branchs, the Ceratonota 
 (see fig. 62, A). (From 
 Gegenbaur, after Alder 
 and Hancock.) 
 
 FIG. 62. 
 
 A. Rolls papillosa (Lin.), dorsal view, a, 1>, posterior and anterior cephalic 
 
 tentacles; c, the dorsal "cerata" (hence Ceratobranchia). 
 
 B. Tethys leporina, dorsal view, a, the cephalic hood ; 6, cephalic tentacles ; 
 
 c, neck ; rf, genital pore ; e t anus ; /, large cerata ; g, smaller cerata ; 
 ft, margin of the foot. 
 
 C. Doris (Actinocydus) tubercitlatus (Cuv.), seen from the pedal surface. TO, 
 
 inouth ; b, margin of the head ; /, sole of the foot ; sp, the mantle-like 
 epipodium. 
 
 D. E. Dorsal and lateral view of Elysia (Actteon) viridis. ep, epipodial out- 
 
 growths. (After Keferstein.) 
 
 (see figs. 7, 68, and 72***). Aplysia, like other Mollusca, 
 
 develops a primitive shell-sac in its trochosphere stage of 
 development (fig. 68), which disappears and is succeeded 
 by a nautiloid shell (fig. 60). This forms the nucleus of 
 the adult shell, 
 and, as the ani- 
 mal grows, be- 
 comes enclosed 
 by a reflexion of 
 the mantle-skirt. 
 In reference to 
 the possible com- 
 parison of the 
 enclosed shell of 
 Aplysia and its 
 allies with those 
 of some Slugs and 
 of Cuttle-fishes, 
 the reader is re- 
 ferred to the para- 
 graphs dealing 
 especially with 
 those Molluscs. 
 When the shell 
 of an Aplysia 
 
 enclosed in its 
 
 mantle is pushed 
 well to the left, 
 the sub-pallial 
 
 io. 63. Aplysia leporina (mmelus, Cuv.), with epipodia 
 and mantle reflected away from the mid-line, a, an- 
 terior cephalic tentacle; 6, posterior do.; between a 
 and 6, the eyes ; c, right epipodium ; d, left epipo- 
 dium ; e, hinder part of visceral hump ; fp, posterior 
 extremity of the foot ; fa, anterior part of the foot 
 
 , ,-, underlying the head ; g, the ctenidium (branchial 
 
 space IS Hilly ex- plume) ; ft, the mantle-skirt tightly spread over the 
 ho: 
 
 posed as in fig. 
 63, and the vari- 
 
 ous apertures OI tory organ of Spe'ngel) ;n, outline of "part of the" renal 
 the body are Seen. sac (nephridium) below the surface ; o, external aper 
 
 rny shell and pushed with it towards the left side ; 
 
 the spermatic groove ; t, the common genital pore 
 (male and female) ; I, orifice of the grape-shaped (sup- 
 posed poisonous) gland ; m, the osphradium (olfac- 
 
 Posteriorly we 
 
 ture of the nephridium ; p, anus. (Original.) 
 
 have the anus, in front of this the lobate gill-plume, be- 
 tween the two (hence corresponding in position to that of 
 the Azygobranchia) we have the aperture of the renal 
 organ. In front, near the anterior attachment of the gill- 
 plume, is the osphradium (olfactory organ) discovered by 
 Spengel, yellowish in colour, in 
 the typical position, and overly- 
 ing an olfactory ganglion with 
 typical nerve-connexion (see fig. 
 20). To the right of Spengel's 
 osphradium is the opening of a 
 peculiar gland which has, when 
 dissected out, the form of a bunch 
 of grapes ; its secretion is said to 
 be poisonous. On the under side 
 of the free edge of the mantle are 
 situated the numerous small cu- 
 taneous glands which, in the large 
 Aplysia camelus (not in other 
 species), form the purple secretion 
 which was known to the ancients. 
 In front of the osphradium is the 
 single genital pore, the aperture 
 
 . -. IT, Fio. 64. Gonad, ami accessory 
 
 ot the common or hermaphrodite glands and ducts of Aplysia. 
 duct From this point there ^Snl^g^ 
 stretches forward to the right 
 
 this point there 
 
 f, vesicula seminalis ; , open- 
 side of the head a groove the into f tiie e heni"irmT di^ 
 
 spermatic groove down which ', hermaphrodite duct (uterine 
 
 the spermatic fluid passes. In 
 
 other Euthyneura this groove may 
 
 close up and form a canal. At 
 
 its termination by the side of the head is the muscular 
 
 introverted penis. In the hinder part of the foot (not 
 
 shown in any of the diagrams) is the opening of a large 
 
 mucous-forming gland very often found in the Molluscan 
 
 foot. 
 
 portion) ; ft, vaginal portion of 
 the uterine duct ; c, spenna- 
 theea; ri, its duct; a, genital 
 pore. (Original.) 
 
MOLLUSCA 
 
 121 
 
 With regard to internal organization we may commence 
 with the disposition of the renal organ (nephridium), the 
 external opening of which has already been noted. The 
 position of this opening and other features of the renal 
 organ have been determined recently by Mr. J. T. Cunning- 
 ham, Fellow of University College, Oxford, who writes as 
 follows from Naples, February 1883 : 
 
 "There is considerable uncertainty with respect to the names of 
 the species of Aplysia, There are two forms which are very common 
 in the Gulf of Naples, and which I have used in studying the ana- 
 tomy of the renal organ in the genus. One is quite black in colour, 
 and measures when outstretched eight or nine inches in length. 
 The other is light brown and somewhat smaller, its length usually 
 not exceeding seven inches. The first is flaccid and sluggish in its 
 movements, and has not much power of contraction ; its epipodial 
 lobes are enormously developed and extend far forward along the 
 body ; it gives out when handled an abundance of purple liquid, 
 which is derive' 1 from cutaneous glands situated on the under side 
 of the free edge of the mantle. In the Zoological Station this form 
 is known as Ap. leporina, ; but according to Blochmann it is iden- 
 tical with A. Cameius of Cuvier. The other species is A. depilans 
 it is firm to the touch, and contracts forcibly when irritated ; the 
 secretion of the mantle-glands is not abundant, and is milky white 
 in appearance. The kidney has similar relations in both genera, 
 and is identical with the organ spoken of by many authors as the 
 triangular gland. Its superficial extent is seen when the folds 
 covering the shell are cut away and the shell removed ; the external 
 surface forms a triangle with its base bordering the pericardium and 
 its apex directed posteriorly and reaching to the left-hand posterior 
 corner of the shell-chamber. The dorsal surface of the kidney 
 extends to the left beyond the shell-chamber beneath the skin in 
 the space between the shell-chamber and the left epipodium. 
 
 When the animal is turned on its left-hand side and the mantle- 
 chamber widely opened, the gill being turned over to the left, a 
 part of the kidney is seen beneath the skin between the attachment 
 of the gill and the right epipodium (fig. 63). On examination 
 this is found to be the under surface of the posterior limb of the 
 gland, the upper surface of which has just been described as lying 
 beneath the shell. In the posterior third of this portion, close to 
 that edge which is adjacent to the base of the gill, is the external 
 opening (fig. 63, o). 
 
 "When the pericardium is cut open from above in an animal 
 otherwise entire, the anterior face of the kidney is seen forming 
 the posterior wall of the pericardial chamber ; on the deep edge of 
 this face, a little to the left of the attachment of the auricle to the 
 floor of the pericardium, is seen a depression ; this depression con- 
 tains the opening from the pericardium into the kidney. 
 
 "To complete the account of the relations of the organ : the right 
 anterior corner can be seen superficially in the wall of the mantle- 
 chamber above the gill. Thus the base of the gill passes in a slant- 
 ing direction across the right-hand side of the kidney, the posterior 
 end being dorsal to the apex of the gland, and the anterior end 
 ventral to the right-hand corner. 
 
 " As so great a part of the whole surface of the kidney lies adjacent 
 to external surfaces of the body, the remaining part which faces 
 the internal organs is small ; it consists of the left part of the under 
 surface ; it is level with the floor of the pericardium, and lies over 
 the globular mass formed by the liver and convoluted intestine. 
 
 " Mi-re dissection does not give sufficient evidence concerning such 
 communications as these of the kidney in Aplysia. I studied the 
 external opening by taking a series of sections through the sur- 
 rounding region of the gland ; to demonstrate the internal aperture 
 injected a solution of Berlin blue into the pericardium ; it did not 
 fill the whole kidney easily, but ran down into the part adjacent to 
 the base of the gill. " 
 
 Thus the renal organ of Aplysia is shown to conform to 
 the Molluscan type. The heart lying within the adjacent 
 pericardium has the usual form, a single auricle and ven- 
 tricle. The vascular system is not extensive, the arteries 
 soon ending in the well-marked spongy tissue which builds 
 up the muscular foot, epipodia, and dorsal body-wall. 
 
 The alimentary canal commences with the usual buccal 
 mass ; the lips are cartilaginous, but not armed with horny 
 jaws, though these are common in other Opisthobranchs ; 
 the lingual ribbon is multidenticulate, and a pair of salivary 
 glands pour in their secretion. The O2sophagus expands 
 into a curious gizzard, which is armed internally with large 
 horny processes, some broad and thick, others spinous, fitted 
 to act as crushing instruments. From this we pass to a 
 stomach and a coil of intestine embedded in the lobes of a 
 voluminous liver ; a caecum of large size is given off near 
 
 the commencement of the intestine. The liver opens by 
 two ducts into the digestive tract. 
 
 The generative organs lie close to the coil of intestine 
 and liver, a little to the left side. When dissected out they 
 appear as represented in fig. 64. The essential reproductive 
 
 A B - 
 
 
 
 FIG. 65. Follicles of the hermaphrodite gonads of Eathynenrona Anisoplenra. 
 A, of Helix ; B, of Eolidia. a, ova ; fe, developing spennatozoids ; c, com- 
 mon efferent duct. 
 
 organ or gonad consists of both ovarian and testicular 
 cells (see fig. 65). It is an ovo-testis. From it passes a 
 common or hermaphrodite duet, which very soon becomes 
 entwined in the spire of a gland the albuminiparous gland. 
 The hitter opens into the common duct at the point r, and 
 here also is a small diverticulum of the duct y. Passing 
 on, we find not far from the genital pore a glandular spherical 
 body (the spermatheca a) opening by means of a longish 
 duct into the common duct, and 
 then we reach the pore (fig. 63, 
 k). Here the female apparatus 
 terminates. But when the male 
 secretion of the ovo-testis is 
 active, the seminal fluid passes 
 from the genital pore along the 
 spermatic groove (fig. 63,) to 
 the penis, and is by the aid of 
 that eversible muscular organ 
 introduced into the genital pore 
 of a second Aplysia, whence it 
 passes into the spermatheca, there 
 to await the activity of the fe- 
 male element of the ovo-testis of 
 this second Aplysia. After an 
 interval of some days possibly 
 weeks the ova of the second 
 Aplysia commence to descend 
 the hermaphrodite duct ; they 
 become enclosed in a viscid secre- FIG. es. Enteric eani of 
 
 tion at the noint whprp flip al papaitaa. pA, pharynx ; m, mid- 
 
 e al ~ gut. with its hepatic appendages 
 
 buminimrous eland opens into *. " of whi h re not figured ; 
 
 .1 j ,. . L j -it -^ O'nd gnt ; an, anus. (From 
 the dUCt intertwined With it ; Gegen uaur, after Alder and Han- 
 
 and on reaching the point where cock -> 
 the spermathecal duct debouches they are impregnated by 
 the spermatozoa which escape now from the spermatheca 
 and meet the ova. 
 
 The development of Aplysia from the egg presents many 
 points of interest from the point of view of comparative 
 embryology, but in relation to the morphology of the 
 Opisthobranchia it is sufficient to point to the occurrence 
 of a trochosphere and a veliger stage (fig. 60), and of a 
 shell-gland or primitive shell-sac (fig. 68, s/t-s), which is suc- 
 ceeded by a nautiloid shell. 
 
 The nervous system of Aplysia will be found on com- 
 parison of fig. 20, which represents it, with our schematic 
 Mollusc (fig. 1, D) to present but little modification. It is 
 in fact a nervous system in which the great ganglion-pairs 
 are well developed and distinct. The Euthyneurous visceral 
 loop is long, and presents only one ganglion (in Ajilysia 
 camelus, but two distinct ganglia joined to one another in 
 
 Q 
 
122 
 
 MOLLUSCA 
 
 Aplysia kybrida of the English coast), placed at its extreme 
 limit, representing both the right and left visceral ganglia 
 and the third or abdominal ganglion, which are so often 
 separately present. The diagram (fig. 20) shows the nerve 
 connecting this abdomino- 
 visceral ganglion with the 
 olfactory ganglion of Spen- 
 gel. It is also seen to be 
 connected with a more re- 
 mote ganglion the genital. 
 Such special irregularities 
 in the development of gan- 
 glia upon the visceral loop, 
 and on one or more of the 
 main nerves connected with 
 
 it, are, as the figures of Fl<J . 67 *_ C entral nervous system of Fiona 
 
 Molluscan nervous systems ( ne f tne Ceratonotous Opistho- 
 
 .,. i- i i_ branchs), showing a tendency to fusion 
 
 given in this article show, of the great ganglia. A, cerebral, pleu- 
 
 very frequent. Our figure !j an ^ Yion e ^c' 8 Tuccai U gar?giLn'- */>" 
 
 of the nervous System of ccsophageal ganglion connected with the 
 
 Ai>lvifl rlnpq not rrivp trip buccal; a, nerve to superior cephalic 
 
 Aplysia Q tentacle ; 6, nerves to inferior cephalic 
 
 small pair of buccal ganglia tentacles; c, nerve to generative organs; 
 
 * . i . ,, 7^. d^ pedal nerve ; e, pedal commissure ; e', 
 
 Which are, as in all (jrlOSSO- visceral loop or commissure (?). (From 
 
 phoTOUS Molluscs, present Gegenbaur, after Bergh.) 
 
 upon the nerves passing from the cerebral region to the 
 odontophore. 
 
 For a comparison of various Opisthobranchs, Aplysia will 
 be found to present a convenient starting-point. It is 
 one of the more typical Opisthobranchs, that is to say, 
 it belongs to the section Palliata, but other members of the 
 Palliata, namely, Bulla and Tornatella (figs. 52 and 53), 
 are less abnormal than Aplysia in regard to their shells and 
 the form of the visceral hump. They have naked spirally- 
 twisted shells which may be concealed from view in the 
 living animal by the expansion and reflexion of the epipodia, 
 
 Fin. 68. Young veliger larva of an Opisthobranch (Pleuro-branchiilium). m, 
 mouth ; v, ciliated band marking otf the velum ; ng, cerebral ganglion de- 
 veloping from epiblast, within the velar area ; at, otocyst also developing 
 from epiblast ; /, foot ; i, intestine ; ry, residual nutritive yelk ; shs, primi- 
 tive shell-sac or shell-gland. (From Lankester.) 
 
 but are not enclosed by the mantle, whilst Tornatella is 
 remarkable amongst all Euthyneura for possessing an oper- 
 culum like that of so many Streptoneura. 
 
 The great development of the epipodia seen in Aplysia 
 is usual in Palliate Opisthobranchs ; it occurs also in Elysia 
 (fig. 62, D) among Non-Palliata ; in Doris it seems prob- 
 able that the mantle-like fold overhanging the foot is to 
 be interpreted as epipodium, the mantle-skirt being alto- 
 gether absent, as shown by the naked position of the gills 
 and anus on the dorsal surface (figs. 61 and 62, C). The 
 whole surface of the body becomes greatly modified in 
 those Non-Palliate forms which have lost, not only the 
 mantle-skirt and the shell, but also the ctenidium. Many 
 of these (Ceratonota) have peculiar processes developed 
 on the dorsal surface (fig. 62, A, B), or retain purely 
 
 negative characters (fig. 62, D). The chief modification of 
 internal organization presented by these forms, as compared 
 with Aplysia, is found in the condition of the alimentary 
 canal. The liver is no longer a compact organ opening 
 by a pair of ducts into the median digestive tract, but we 
 find very numerous hepatic diverticula on a shortened 
 axial tract (fig. 66). These diverticula extend usually one 
 into each of the dorsal papillae or " cerata " when these are 
 present. They are not merely digestive glands, but are 
 sufficiently wide to act as receptacles of food, and in them 
 the digestion of food proceeds just as in the axial portion 
 of the canal. A precisely similar modification of the liver 
 or great digestive gland is found in the Scorpions, where 
 the axial portion of the digestive canal is short and straight, 
 and the lateral ducts sufficiently wide to admit food into 
 the ramifications of the gland there to be digested ; whilst 
 in the Spiders the gland is reduced to a series of simple 
 caeca. 
 
 The typical character is retained by the heart, peri- 
 cardium, and the communicating nephridium or renal organ 
 in all Opisthobranchs. An interesting example of this is 
 furnished by the fish-like transparent Phyllirhoe (fig. 58), 
 in which it is possible most satisfactorily to study in the 
 living animal, by means of the microscope, the course of 
 the blood-stream, and also the reno-pericardial communi- 
 cation. With reference to the existence of pores placing 
 the vascular system in open communication with the 
 surrounding water, see the paragraph as to Mollusca gener- 
 ally. In a form closely allied to Aplysia (Pleurobranchus) 
 such a pore leading outwards from the branchial vein has 
 been precisely described by Lacaze Duthiers. No such pore 
 has been detected in Aplysia. In many of the Non-Palliate 
 Opisthobranchs the nervous system presents a concentra- 
 tion of the ganglia (fig. 67), contrasting greatly with what 
 we have seen in Aplysia. Not only are the pleural ganglia 
 fused to the cerebral, but also the visceral to these (see in 
 further illustration the condition attained by the Pulmonate 
 Limnaeus, fig. 22), and the visceral loop is astonishingly short 
 and insignificant (fig. 67, e). That the parts are rightly thus 
 identified is probable from Spengel's observation of the os- 
 phradium and its nerve-supply in these forms ; the nerve to 
 that organ, which is placed somewhat anteriorly on the dor- 
 sal surface being given off from the hinder part (visceral) of 
 the right compound ganglion the fellow to that marked A in 
 fig. 67. The Ceratonotous Opisthobranchs, amongst other 
 specialities of structure, are stated to possess (in some cases 
 at any rate) apertures at the apices of the " cerata " or 
 dorsal papillje, which lead from the exterior into the hepatic 
 caeca. This requires confirmation. Some amongst them 
 (Tergipes, Eolis) are also remarkable for possessing 
 peculiarly modified epidermic cells placed in sacs at the 
 apices of these same papillae, which resemble the " thread- 
 cells " of the Planarian Flatworms and of the C'nelentera. 
 The existence of these thread-cells is sufficiently remark- 
 able, seeing that the Non-Palliate Opisthobranchs resemble 
 in general form and habit the Planarian w r orms, many of 
 which also possess thread-cells. But it is not conceivable 
 that theirpresence is an indication of genetic affinity between 
 the two groups, rather they are instances of homoplasy. 
 The development of many Opisthobranchia has been 
 examined e.g., Aplysia, Pleurobranchidium, Elysia, Poly- 
 cera, Doris, Tergipes. All pass through trochosphere and 
 veliger stages, and in all a nautiloid or boat-like shell is 
 developed, preceded by a well-marked "shell-gland" (seefigs. 
 60 and 68). The transition from the free-swimming veliger 
 larva with its nautiloid shell (fig. 60) to the adult form has 
 not been properly observed, and many interesting points as 
 to the true nature of folds (whether epipodia or mantle or 
 velum) have yet to be cleared up by a knowledge of such 
 development in forms like Tethys, Doris, Phyllidia, Ac. 
 
MOLLUSCA 
 
 123 
 
 As in other Molluscan groups, we find even in closely- 
 allied genera (for instance, in Aplysia and Pleurobran- 
 chidium, and other genera observed by Lankester) the 
 greatest differences as to the amount of food-material by 
 which the egg-shell is encumbered. Some form their 
 Diblastula by emboly (fig. 7), others by epiboly (fig. 5) ; 
 and in the later history of the further development of the 
 enclosed cells (arch-enteron) very marked variations occur 
 in closely-allied forms, due to the influence of a greater or 
 less abundance of food-material mixed with the protoplasm 
 of the egg. 
 
 Order 2 (of the Euthyneura). Pulmonata. 
 
 Characters. Euthyneurous Anisopleurous Gastropoda, 
 probably derived from ancestral forms similar to the 
 Palliate Opisthobranchia by adaptation to a terrestrial life. 
 The ctenidium is atrophied, and the edge of the mantle-skirt 
 is fused to the dorsal integument by concrescence, except at 
 one point which forms the aperture of the mantle-chamber, 
 thus converted into a nearly closed sac. Air is admitted 
 to this sac for respiratory and hydrostatic purposes, and it 
 thus becomes a lung. An operculum is never present ; a 
 contrast being thus afforded with the operculate Pulmonate 
 Streptoneura (Cyclostoma, <fcc.), which differ in other 
 essential features of structure from the Pulmonata. The 
 Pulmonata are, like the other Euthyneura, hermaphrodite, 
 with elaborately-developed copulatory organs and accessory 
 glands. Like other Euthyneura, they have very numerous 
 small denticles on the lingual ribbon. The ancestral 
 Pulmonata appear to have retained both the right and the 
 left osphradia (Spengel's olfactory organs), since in some 
 (Planorbis, Auricularia) we find the single osphradium to 
 be that of the original left side, whilst in others (Limnaeus) 
 it is that of the original right side. 
 
 In some Pulmonata (Snails) the foot is extended at right 
 angles to the visceral hump, which rises from it in the 
 form of a coil as in Streptoneura ; in others the visceral 
 hump is not elevated, but is extended with the foot, and 
 the shell is small or absent (Slugs). 
 
 The Pulmonata are divided into two sub-orders according to the 
 position of the cephalic eyes. 
 
 Sub-order 1. Basommatophora. 
 
 Characters. Eyes placed mediad of the cephalic tentacles at their 
 base ; the embryonic velar area retained in adult life as a pair of 
 cephalic lobes (fig. 70, r) ; male and female generative apertures 
 separate, placed (as is typical in Anisopleura) on the right side of 
 the neck ; visceral hump well developed, with a well-developed 
 shell ; aquatic in habit 
 Family 1. Limnseidte. 
 
 Genera: Limtueits, Lam. (figs. 3, 4, &c.); Chilinia, Gray; Physa, 
 
 Draparn. ; Ancylus, Geoff. ; Planorbis, MulL, &c. 
 Family 2. Auriculidas. 
 
 Genera: Auricula, Lam. ; Conomilus, Lam.; PitAarella, Wood. 
 &c. 
 
 Sub-order Z. Stylommatophora. 
 
 Characters. Eyes placed on the summit of two hollow tentacles ; 
 visceral hump well or not at all developed ; shell large and coiled, 
 or minute or absent ; almost exclusively terrestrial. 
 Family 1. Helicidas. 
 
 Genera : Helix, L. (figs. 69, A; 72*) ; Vtirina, Draparn. ; Suc- 
 cinea, Draparn. ; Bulimus, Scopoli ; Achatina, Lam. ; Pupa, 
 Lam. ; Clausilia, Draparn., &e. 
 Family 2. Limacidse (Slugs). 
 
 Genera : Umax, L. ; Incilaria, Benson ; Arian, Ferussac (fig. 
 69, D) ; Pannafdla, Cuvier ; Testacella, Cuvier (fig. 69, C), &c. 
 Family 3.Oncidiadie. 
 
 Genera : Oncidium, Buchanan ; Peronia, Blainv. (fig. 72) ; 
 Vaginulus, Ferussac, &c. 
 
 Further Remarks on Pulmonata. The land-snails and 
 slugs forming the group Pulmonata are widely distinguished 
 from a small set of terrestrial Azygobranchia, the Pneumo- 
 nochlamyda (see above), at one time associated with them 
 on account of their mantle-chamber being converted, as in 
 
 Pulmonata, into a lung, and the ctenidium or branchial 
 plume aborted. The Pneumonochlamyda (represented in 
 England by the common genus Cyclostoma) have a twisted 
 
 FIG. 69. A series of Stylommatophorons Pulmonata, showing transitional forms 
 between snail and slug. 
 
 A. Htlii pomalia (tram Keferstein). 
 
 B. Htlicophanta brtvipes (from Keferstein, after Pfeiffer). 
 
 C. Testacetla haliotidta (from Keferstein). 
 
 D. Arion ater, the great Black Slug (from KefersteinX 
 
 a, Shell in A, B, C, shell-sac (closed) in D ; 6, orifice leading into the 
 subpallial chamber (lung). 
 
 visceral nerve-loop, an operculum on the foot, a complex 
 rhipidoglossate or taenioglossate radula, and are of distinct 
 sexes ; they are, in fact, Azygobranchiate Streptoneura. 
 The Pulmonata have a straight visceral nerve-loop, never 
 an operculum (even in the embryo), and a multidenticulate 
 
 FIG. "p. A, B, C. Three views of Li'miuetu ftagnalis, in order to show the 
 persistence of the larval velar area r, as the circum-oral lobes of the adult, 
 m, mouth ; /, foot ; r, velar area, the margin r corresponding with the 
 ciliated band which demarcates the velar area or velum of the embryo Gas- 
 tropod (see fig. 4, D, E, F, H, I, t>X (Original.) 
 
 radula, the teeth being equi-formal ; and they are hermaphro- 
 dite. Some Pulmonata (Limnaeus, ic.) live in fresh-waters 
 although breathing air. The remarkable discovery has 
 been made that in deep lakes such Limnaei do not breathe 
 air, but admit water to the lung-sac and live at the bottom. 
 The lung-sac serves undoubtedly as a hydrostatic apparatus 
 in the aquatic Pulmonata, as well as assisting respiration. 
 It is not improbable that here, and in other air-breathing 
 animals, the hydrostatic function was the primary one, and 
 the respiratory a later development. 
 
124 
 
 MOLLUSCA 
 
 The same general range of body-form is shown in Pul- 
 inonata as in the Natant Azygobranchia and in the Opis- 
 thobranchia ; at one extreme we have Snails with coiled 
 visceral hump, at the other cylindrical or flattened Slugs 
 (see fig. 69). Limpet-like forms are also 
 found (fig. 71, Ancylus). The foot is al- 
 ways simple, with its flat crawling surface 
 extending from end to end, but in the 
 embryo Limnseus (fig. 4, H) it shows a 
 bilobed character, which leads on to the 
 condition characteristic of Pteropoda. 
 
 The adaptation of the Pulmonata to ter- 
 restrial life has entailed little modification 
 of the internal organization. The vascular system appears 
 to be more complete in them than in other Gastropoda, 
 fine vessels and even capillaries being present in place of 
 lacunae, in which arteries and veins find their meeting- 
 point. The subject has not, however, been investigated 
 by the proper methods of recent histology, and our know- 
 
 form aquatic Pui- 
 
 FIG. 72. Peronia Tonga?, a littoral Pulmonate, found on the shores of the Indian 
 and Pacific Oceans (Mauritius, Japan). 
 
 ledge of it, as of the vascular system of Molluscs generally, 
 is most unsatisfactory. In one genus (Planorbis) the 
 plasma of the blood is coloured red by haemoglobin, this 
 being the only instance of the pre- 
 sence of this body in the blood of 
 Glossophorous Mollusca, though it 
 occurs in corpuscles in the blood 
 of the bivalves Area and Solen 
 (Lankester, 31). 
 
 The generative apparatus of the 
 Snail (Helix) may serve as an ex- 
 ample of the hermaphrodite appa- 
 ratus common to the Pulmonata 
 and Opisthobranchia (fig. 72*). 
 From the ovo-testis, which lies 
 near the apex of the visceral coil, 
 a common hermaphrodite duct v.e 
 proceeds, which receives the duct 
 of the compact white albumini- 
 parous gland JE.d., and then be- 
 comes much enlarged, the addi- 
 tional width being due to the 
 development of glandular folds, 
 which are regarded as forming a 
 uterus u. Where these folds cease 
 the common duct splits into two 
 
 portions, a male and a female. Fir..72. Hermaphroditerepro 
 
 The male duct v.d becomes fleshy 
 and muscular near its termination 
 at the genital pore, forming the 
 penis />. Attached to it is a diver- 
 ticulum fl., in which the sperma- 
 tozoa which have descended from 
 the ovo-testis are stored and mo- 
 delled into sperm ropes or sperma- 
 tophores. The female portion of 
 the duct is more complex. Soon 
 after quitting the uterus it is joined by a long duct leading 
 from a glandular sac, the spermatheca (R.f). In this duct 
 and sac the spermatophores received in copulation from 
 another snail are lodged. In Helix hortensis the sperma- 
 
 l>pan 
 (Hell 
 
 den Snail (Helix hortensis). 
 ovo-testis ; v.e, hermaphro 
 dite duct ; E.d. t albuminipar 
 ous gland ; u, uterine dilata 
 tion of the hermaphrodite 
 duct; d, digitate accessory 
 glands on the female duct ; 
 p.s, calciferous gland or dart- 
 sac on the female duct ; R.f, 
 spermatheca or receptacle of 
 the sperm in copulation, open- 
 ing into the female duct ; v.d, 
 male duct (vas deferens); p, 
 penis ; fl., flagellum. 
 
 theca is simple. In other species of Helix a second duct 
 (as large in Helix asjwsa as the chief one) is given off from 
 the spermathecal duct, and in the natural state is closely 
 adherent to the wall of the uterus. This second duct has 
 normally no spermathecal gland at its termination, which 
 is simple and blunt. But in rare cases in Helix aspersa a 
 second spermatheca is found at the end of this second duct. 
 Tracing the widening female duct onwards we now come 
 to the openings of the digitate accessory glands d, d, which 
 probably assist in the formation of the egg-capsule. Close 
 to them is the remarkable dart^sac ps, a thick-walled sac, 
 in the lumen of which a crystalline four-fluted rod or dart 
 consisting of carbonate of lime is found. It is supposed 
 to act in some way as a stimulant in copulation, but pos- 
 sibly has to do with the calcareous covering of the egg- 
 capsule. Other Pulmonata exhibit variations of secondary 
 importance in the details of this hermaphrodite apparatus. 
 
 The nervous system of Helix is not favourable as an 
 example on account of the fusion of the ganglia to form 
 an almost uniform ring of nervous matter around the 
 oesophagus. The Pond-Snail (Limnaeus) furnishes, on the 
 other hand, a very beautiful case of distinct ganglia and 
 connecting cords (fig. 22). The demonstration which it 
 affords of the extreme shortening of the Euthyneurous vis- 
 ceral nerve-loop is most instructive and valuable for com- 
 parison with and explanation of the condition of the nervous 
 centres in Cephalopoda, as also of some Opisthobranchia. 
 The figure (fig. 22) is sufficiently described in the letter- 
 press attached to it ; the pair of buccal ganglia joined by 
 the connectives to the cerebrals are, as in most of our figures, 
 omitted. Here we need only further draw attention to the 
 osphradium, discovered by Lacaze Duthiers (32), and shown 
 by Spengel to agree in its innervation with that organ in all 
 other Gastropoda. On account of the shortness of the 
 visceral loop and the proximity of the right visceral 
 ganglion to the O3sophageal nerve-ring, the nerve to the 
 osphradium and olfactory ganglion is very long. The posi- 
 tion of the osphradium corresponds more or less closely 
 with that of the vanished right ctenidium, with which it is 
 normally associated. In Helix and Limax the osphradium 
 has not been described, and possibly its discovery might 
 clear up the doubts which have been raised as to the nature 
 of the mantle-chamber of those genera. In Planorbis, which 
 is dexiotropic (as are a few other genera or exceptional 
 varieties of various Anisopleurous Gastropods) instead of 
 being leiotropic, the osphradium is on the left side, and 
 receives its nerve from the left visceral ganglion, the whole 
 series of unilateral organs being reversed. This is, as might 
 be expected, what is found to be the case in all " reversed " 
 Gastropods. It is also the case in the Pulmonate Auricula, 
 which is leiotropic. 
 
 The shell of the Pulmonata, though always light and 
 delicate, is in many cases a well-developed spiral "house," 
 into which the creature can withdraw itself ; and, although 
 the foot possesses no operculum, yet in Helix the aperture 
 of the shell is closed in the winter by a complete lid, the 
 "hybernaculmn," more or less calcareous in nature, which 
 is secreted by the foot. In Clausilia a peculiar modifica- 
 tion of this lid exists permanently in the adult, attached 
 by an elastic stalk to the mouth of the shell, and known as 
 the " clausilium." In Limnseus the permanent shell is 
 preceded in the embryo by a well-marked shell-gland or 
 primitive shell-sac (fig. 72***), at one time supposed to be 
 the developing anus, but shown by Lankester to be identical 
 with the " shell-gland " discovered by him in other Mol- 
 lusca (Pisidium, Pleurobranchidium, Neritina, etc.). As in 
 other Gastropoda Anisopleura, this shell-sac may abnorm- 
 ally develop a plug of chitonous matter, but normally it 
 flattens out and disappears, whilst the cap-like rudiment of 
 the permanent shell is shed out from the dome-like surface 
 
MOLLUSCA 
 
 125 
 
 of the visceral hump, in the centre of which the shell-sac 
 existed for a brief period. 
 
 In Clausilia, according to the observations of Gegenbaur, 
 the primitive shell-sac does not flatten out and disappear, 
 but takes the form of a flattened closed sac. Within this 
 closed sac a plate of calcareous matter is developed, and 
 after a time the upper wall of the sac disappears, and the 
 calcareous plate continues to grow as the nucleus of the 
 permanent shell. In the slug Testacella (fig. 69, C) the 
 shell-plate never attains a large size, though naked. In 
 other slugs, namely, Limax and Arion, the shell-sac remains 
 permanently closed over the'shell-plate, which in the latter 
 genus consists of a granular mass of carbonate of lime. 
 The permanence of the primitive shell-sac in these slugs is 
 a point of considerable interest. It is clear enough that 
 the sac is of a different origin from that of Aplysia (described 
 in the section treating of Opisthobranchia), being primitive 
 instead of secondary. It seems probable that it is identical 
 with one of the open sacs in which each shell-plate of a 
 Chiton is formed, and the series of plate-like imbrications 
 which are placed behind the single shell-sac on the dorsum 
 of the curious slug, Plectrophorus, suggest the possibility 
 of the formation of a series of shell-sacs on the back of 
 that animal similar to those which we find in Chiton. 
 Whether the closed primitive shell-sac of the slugs (and 
 with it the transient embryonic shell-gland of all other 
 Mollusca) is precisely the same thing as the closed sac in 
 which the calcareous pen or shell of the Cephalopod Sepia 
 
 FIG. 72**. Comparative diagrams of an embryo Slag, Umax (teftX and an 
 embryo Cattle-fish, Loligo (right), sft, internal shell ; pfc, embryonic renal 
 organ (Stiebel's canal) in Lima.* ; mf, edge of the mantle-flap in Loligo ; op, 
 cephalic eye ; t, cephalic tentacle ; , position of the mouth ; Ft, the foot ; 
 .Fu, the hinder part of the foot drawn out to form the funnel of Loligo ; con, 
 the contractile yelk -sac or hernia-like protrusion of the mid-region of the foot, 
 corresponding to the line of closure of the blastopore in Limnaeus. N.B. 
 The blastopore in the embryo of Loligo, which, like that of a bird, is much 
 distorted by excess of food-yelk, dots close at the extremity of the yelk-sac 
 con. (Original.) 
 
 and its allies is formed, is a further question, which we 
 shall consider when dealing with the Cephalopoda. It 
 is important here to note that Clausilia furnishes us 
 with an exceptional instance of the continuity of the shell 
 or secreted product of the primitive shell-sac with the 
 adult shell. In most other Mollusca (Anisopleurous 
 Gastropods, Pteropods, and Conchifera) there is a want of 
 such continuity; the primitive shell-sac contributes no 
 factor to the permanent shell, or only a very minute knob- 
 like particle (Neritina and Paludina). It flattens out and 
 disappears before the work of forming the permanent shell 
 commences. And just as there is a break at this stage, 
 so (as observed by Krohn in Marsenia = Echinospira) there 
 may be a break at a later stage, the nautiloid shell formed 
 on the larva being cast, and a new shell of a different form 
 being formed afresh on the surface of the visceral hump. 
 It is, then, in this sense that we may speak of primary, 
 secondary, and tertiary shells in Mollusca, recognizing the 
 fact that they may be merely phases fused by continuity 
 of growth so as to form but one shell, or that in other 
 cases they may be presented to us as separate individual 
 things, in virtue of the non-development of the later phases, 
 
 or in virtue of sudden changes in the activity of the mantle- 
 surface causing the shedding or disappearance of one phase 
 of shell-formation before a later one is entered upon. 
 
 The development of the aquatic Pulmonata from the 
 egg offers considerable facilities for study, and that of 
 Lininasus has been elucidated by Lankester, whilst Rabl 
 has with remarkable skill applied the method of sections 
 to the study of the minute embryos of Planorbis. The 
 chief features in the development of Limnaeus are exhibited 
 in the woodcuts (figs. 3, 4, and 72***). There is not a 
 very large amount of food-material present in the egg of 
 this snail, and accordingly the cells resulting from division 
 are not so unequal as in many other cases. The four cells 
 first formed are of equal size, and then four smaller cells 
 are formed by division of these four so as to lie at 
 one end of the first four (the pole corresponding to 
 that at which the " directive corpuscles " dc are extruded 
 and remain). The smaller cells now divide and spread 
 over the four larger cells (fig. 3) ; at the same time a space 
 
 *** 
 
 FIG. 72***. Embryo of Limnaevs ftagnajis, at a stage when the Trochosphere 
 is developing foot and shell-gland and becoming a Veliger, seen as a transparent 
 object under slight pressure, pk, pharynx (stomodaaal invagination) ; r, r, 
 the ciliated band marking out the velum ; ng, cerebral nerve-ganglion ; re, 
 Stiebel's canal (left side), probably an evanescent embryonic nephridium ; sh, 
 the primitive shell-sac or shell-gland ; j*t, the rectal peduncle or pedicle of 
 invagination, its attachment to the ectoderm is coincident with the hindmost 
 extremity of the elongated blastopore of fig. 3, C ; tye, mesoblastic (skeleto- 
 trophic and muscular) cells investing as, the bilobed arch-enteron or lateral 
 vesicles of invagiiiated endoderm, which will develop into liver ; /, the foot. 
 (Original.) 
 
 the cleavage cavity or blastocrel forms in the centre 
 of the mulberry-like mass. Then the large cells recom- 
 mence the process of division and sink into the hollow 
 of the sphere, leaving an elongated groove, the blastopore, 
 on the surface (fig. 3, C, and fig, 4, G). The invaginated 
 cells (derived from the division of the four big cells) form 
 the endoderm or arch-enteron ; the outer cells are the ecto- 
 derm. The blastopore now closes along the middle part of 
 its course, which coincides in position with the future "foot." 
 One end of the blastopore becomes nearly closed, and an 
 ingrowth of ectoderm takes place around it to form the 
 stomodseum or fore-gut and mouth. The other extreme 
 end closes, but the invaginated endoderm cells remain in 
 continuity with this extremity of the blastopore, and form 
 the "rectal peduncle" or "pedicle of invagination" of 
 Lankester (see also the account and figures (fig. 151, A) of 
 the development of the bivalve Pisidium), although the 
 endoderm cells retain no contact with the middle region 
 of the now closed-up blastopore. The anal opening forms 
 at a late period by a very short ingrowth or proctodasum 
 coinciding with the blind termination of the rectal peduncle 
 (fig. 72*** pi). 
 
 The body-cavity and the muscular, fibrous, and vascular 
 tissues are traced partly to two symmetrically-disposed 
 
126 
 
 MOLLUSCA 
 
 "mesoblasts," which bud off from the invaginated arch- 
 enteron, partly to colls derived from the ectoderm, which 
 at a very early stage is connected by long processes with 
 the invaginated endoderm, as shown in fig. 3, D. The ex- 
 ternal form of the embryo goes through the same changes 
 as in other Gastropods, and is not, as was held previously 
 to Lankester's observations, exceptional. When the middle 
 and hinder regions of the blastopore are closing in, an 
 equatorial ridge of ciliated cells is formed, converting the 
 embryo into a typical " Trochosphere " (fig. 4, E, F). 
 
 The foot now protrudes below the mouth (fig. 4), and the 
 post-oral hemisphere of the Trochosphere grows more rapidly 
 than the anterior or velar area. The young foot shows a 
 bilobed form (fig. 4, D, / ). Within the velar area the eyes 
 and the cephalic tentacles commence to rise up (fig. 4, D, <), 
 and on the surface of the post-oral region is formed a cap- 
 like shell and an encircling ridge, which gradually increases 
 in prominence and becomes the freely depending mantle- 
 skirt. The outline of the velar area becomes strongly 
 emarginated and can be traced through the more mature 
 embryos to the cephalic lobes or labial processes of the 
 adult Limnseus (fig. 70). 
 
 This permanence of the distinction of the part known 
 as the velar area through embryonic life to the adult state 
 is exceptional among Mollusca, and is therefore a point of 
 especial interest in Limnasus. None of the figures of 
 adult Limnseus in recent works on Zoology show properly 
 the form of the head and these velar lobes, and accordingly 
 the figures here given have been specially sketched for the 
 present article. The increase of the visceral dome, its 
 spiral twisting, and the gradual closure of the space over- 
 hung by the mantle-skirt so as to convert it into a lung-sac 
 with a small contractile aperture, belong to stages in the 
 development later than any represented in our figures. 
 
 We may now revert briefly to the internal organization 
 at a period when the Trochosphere is beginning to show a 
 prominent foot growing out from the area where the mid- 
 region of the elongated blastopore was situated, and having 
 therefore at one end of it the mouth and at the other the 
 anus. Fig. 72*** represents such an embryo under slight 
 compression as seen by transmitted light. The ciliated 
 band of the left side of the velar area is indicated by a 
 line extending from v to v the foot f is seen between the 
 pharynx ph and the pedicle of invagination pi. The mass 
 of the arch-enteron or invaginated endodermal sac has 
 taken on a bilobed form (compare Pisidium, fig. 151), and 
 its cells are swollen (gs and tge). This bilobed sac becomes 
 entirely the liver in the adult ; the intestine and stomach 
 are formed from the pedicle of invagination, whilst the 
 pharynx, oesophagus, and crop form from the stomodseal 
 invagination ph. To the right (in the figure) of the 
 rectal peduncle is seen the deeply invaginated shell-gland 
 ss, with a secretion sk protruding from it. The shell-gland 
 is destined in Linmseus to become very rapidly stretched 
 out, and to disappear. Farther up, within the velar area, 
 the rudiments of the cerebral nerve-ganglion ng are seen 
 separating from the ectoderm. A remarkable cord of cells 
 having a position just below the integument occurs on each 
 side of the head. In the figure the cord of the left side is 
 seen, marked re. This paired organ consists of a string of 
 cells which are perforated by a duct. The opening of the 
 duct at either end is not known. Such cannulated cells 
 are characteristic of the nephridia of many worms, and it 
 is held that the organs thus formed in the embryo Limnseus 
 are embryonic nephridia. The most important fact about 
 them is that they disappear, and are in no way connected 
 with the typical nephridium of the adult. In reference 
 to their first observer they are conveniently called "Stiebel's 
 canals." Other Pulmonata possess, when embryos, Stiebel's 
 canals in a more fully-developed state, for instance, the 
 
 common slug Limax (fig. 72**, pk). Here too they disap- 
 pear during embryonic life. Further knowledge concern- 
 ing them is greatly needed. It is not clear whether there 
 is anything equivalent to them in the embryos of marine 
 Gastropoda or other Mollusca, the ectodermal cells called 
 " embryonic renal organs" in some Gastropod embryos hav- 
 ing only a remote resemblance to them. The three pairs 
 of transient embryonic nephridia of the medicinal leech, 
 the ciliated cephalic pits of Nemertines, and the anterior 
 nephridia of Gephyrseans, all suggest themselves for com- 
 parison with these enigmatical canals. 
 
 Marine Pulmonata. Whilst the Pulmonata are essen- 
 tially a terrestrial and fresh-water group, there is one 
 genus of slug-like Pulmonates which frequent the sea- 
 coast (Peronia, fig. 72), whilst their immediate congeners 
 (Onchidium) are found in marshes of brackish water. Sem- 
 per (33) has shown that these slugs have, in addition to 
 the usual pair of cephalic eyes, a number of eyes developed 
 upon the dorsal integument. These dorsal eyes are very 
 perfect in elaboration, possessing lens, retinal nerve-end 
 cells, retinal pigment, and optic nerve. Curiously enough, 
 however, they differ from the cephalic Molluscan eye (for 
 an account of which see fig. 118) in the fact that, as in 
 the vertebrate eye, the filaments of the optic nerve pene- 
 trate the retina, and are connected with the surfaces of the 
 nerve-end cells nearer the lens instead of with the opposite 
 end. The significance of this arrangement is not known, 
 but it is important to note, as shown by Hensen, Hickson, 
 and others, that in the bivalves Pecten and Spondylus, 
 which also have eyes upon the mantle quite distinct from 
 typical cephalic eyes, there is the same relationship as in 
 Onchidiadse of the optic nerve to the retinal cells (fig. 145). 
 In both Onchidiadse and Pecten the pallial eyes have prob- 
 ably been developed by the modification of tentacles, such 
 as coexist in an unmodified form with the eyes. The 
 Onchidiadce are, according to Semper, pursued as food 
 by the leaping fish Periophthalmus, and the dorsal eyes 
 are of especial value to them in aiding them to escape 
 from this enemy. 
 
 Class II. SCAPHOPODA. 
 
 Characters. Mollusca Glossophorawith the FOOT adapted 
 to a BURROWING life in sand (figs. 73, 74, /). The body, 
 
 D 
 
 Flo. 73.r>enMium rvlgare, Da C. (after Lacaze Duthiers). A. Ventral view 
 of the animal removed from its shell. B. Dorsal view of the same. C. Late- 
 ral view of the same. D. The shell in section. E. Surface view of the shell 
 with gill-tentacles exserted as in life, a, mantle ; a', longitudinal muscle ; 
 a", fringe surrounding the anterior opening of the mantle-chamber ; a"', the 
 posterior appendix of the mantle ; b, anterior circular muscle of the mantle ; 
 V, posterior do. ; c, c', longitudinal muscle of mantle ; e, liver ; / gonad ; k, 
 bucca] mass (showing through the mantle) ; q, left nephridium ; s\ club-shaped 
 extremity of the foot ; iv, w', longitudinal blood-sinus of the mantle. 
 
 and to a much greater extent the mantle-skirt and the foot, 
 are elongated along the primitive antero-posterior (oro-anal) 
 
MOLLUSCA 
 
 127 
 
 axis, and retain, both externally and in the disposition of 
 internal organs, the archi-Molluscan BILATERAL SYMMETRY. 
 The margins of the mantle-skirt of opposite sides (right 
 and left) meet below the foot and fuse by concrescence ; 
 only a small extent in front and a small extent behind of 
 the mantle-margin is left unfused. Thus a CYLINDRICAL 
 FORM is attained by the mantle, and on its surface a TUBU- 
 LAR shell (incomplete along the ventral line in the youngest 
 stages) is secreted (fig. 73, D). The FOOT is greatly elon- 
 gated, and can be protruded from the anterior mantle- 
 aperture. It has a characteristic clavate form (fig. 74, /). 
 The pair of typical CTENIDIA are symmetrically deve- 
 loped in the form of numerous gill-filaments (fig. 74, A, g) 
 
 Fi<-.. 74. Diagrams of the anatomy of Dentaliam. A. The anterior portion of 
 the tubular mantle is slit open along the median dorsal line, and its cut 
 margins (i) reflected so as to expose the foot-, snoot, and gills. B. Lateral 
 view with organs showing as though by transparency. O. Similar lateral 
 view to show the number and position of the nerve-ganglia and cords, a, 
 the mantle-skirt ; 6, anterior free margin of the same ; e, hinder extension of 
 the mantle-skirt ; d, the appendix of the mantle-skirt separated by a valve 
 from the peri-anal portion of the sub-pallial chamber, * ; i, the snout or oral 
 process ; /, the foot ; g, the ctenidial filaments ; ft, the peri-anal part of the 
 sub-pallial chamber ; , the peri-oral part of the same chamber ; t, the anus ; 
 /, the left nephridium ; w, the mouth surrounded by pinnate tentacles ; , 
 the buceal mass and odontophore ; o, oesophagus ; p. the left lobe of the 
 liver; g.p, pedal ganglion-pair; g.e, cerebral ganglion-pair; g.pl, pleura! 
 ganglion-pair; g.v, visceral ganglion-pair. Possibly further research will 
 show that g.pl is the typical visceral ganglion-pair, and that g. r is a pair of 
 olfactory ganglia placed on the visceral loop as in the Lipocephala according 
 to Spengel. 
 
 placed at the base of the cylindrical cephalic prominence 
 or snout (fig. 74, e). A pair of NEPHRIDIA (fig. 74, /) are 
 present, opening near the anus (fig. 74, t). The right 
 serves as a genital duct, the left is apparently renal in 
 function. The LIVER (p) is large and bilobed, the lobes 
 divided into parallel lobules. The NERVE-GANGLIA are 
 present (fig. 74, C) as well-marked cerebral, pleural, pedal, 
 and visceral pairs, the typical pleural pair being closely 
 joined to the cerebral. The visceral loop or commissure is 
 untwisted, that is to say, the Scaphopoda are EUTHYJTEUE- 
 ous. HEART and distinct VESSELS are not developed ; a 
 colourless blood is contained in the sinuses and networks 
 formed by the body-cavity. The GONADS are either male 
 or female, the sexes being distinct. 
 
 The embyro is remarkable for developing five ciliated 
 rings posterior to the ciliated ring and tuft characteristic 
 of the trochosphere larval condition of Molluscs generally. 
 These rings are comparable to those of the larva of Pneu- 
 modermon (fig. 84), and like them disappear. 
 
 The class Scaphopoda is not divisible into orders or 
 families. It contains only three genera : Dentnlivm, L. (figs. 
 73, 74) ; Siphonodtntalium, Sars. ; and Entalium, Dfr. 
 
 They inhabit exclusively the sand on the sea-coast in 
 depths of from 10 to 100 fathoms. 
 
 It is worthy of remark that the Scaphopoda constitute 
 among the Glossophora a parallel to the sand-boring forms 
 so common among the Ljpocephala (such as Solen and Mya). 
 This parallelism is seen in the special mode of elongation 
 of the body, in the form of the foot, and in the tubular 
 form of the mantle brought about by the concrescence of 
 its ventral margins, as in the Lipocephala mentioned. 
 The cylindrical shell of Dentalium is also comparable to 
 the two semi-cylindrical valves of the shell of Solen ; or, 
 better, to the tubular shell of Aspergillum and Teredo. 
 Nevertheless, it is necessary to consider the Scaphopoda as 
 standing far apart from the Lipocephala, and as having no 
 special genetic but only a homoplastic relationship to them, 
 in consequence of their possessing a well-developed odonto- 
 phore, the characteristic organ of the Glossophora never 
 possessed by any Lipocephala. 
 
 Class ni. CEPHALOPODA. 
 
 Characters. Mollusca Glossophora with the FOOT prim- 
 arily adapted to a FREE-SWIMMING mode of life. The 
 archi-Molluscan BILATERAL SYMMETRY predominates both 
 in the external and internal organs generally, though in 
 many cases (especially the smaller forms) a one-sided dis- 
 placement of primitively median organs and a suppression 
 of one of the primitively paired organs is to be noted. 
 
 An ANTERIOR, MEDIAN, and POSTERIOR region of the 
 FOOT can be distinguished (fig. 75, (4), (5), (6)), corre- 
 sponding to but probably not derived from the pro-, rneso-, 
 
 *p 
 
 (1) 
 
 (2) 
 
 FIG. 75. Diagrams of a series of Molluscs to show the form of the foot and its 
 regions, and the relation of the visceral hump to the antero-posterior and 
 dorso-ventral axes. (1) A Chiton. (2) A Lamellibranch. (3) An Anisoplenr- 
 ons Gastropod. (4) A Thecosomatous Pteropod. (5) A Gymnosomatous 
 Pteropod. (6) A Siphonopod (Cuttle). A, P, antero-posterior horizontal 
 axis ; O, V, dorso-ventral vertical axis at right angles to A, P ; o, mouth ; 
 a, anus ; ins, edge of the mantle-skirt or flap ; sj>, sub-pallial chamber or 
 space ; f, fore-foot ; m/, mid-foot ; */, hind-foot ; t, cephalic eyes ; cd, centro- 
 dorsal point (in 6 only). 
 
 and meta-podium of Gastropoda. The fore-foot invariably 
 has the HEAD MERGED into it, and grows up on each side 
 (right and left) of that part so as to surround the mouth, 
 the two upgrowths of the fore-foot meeting on the dorsal 
 aspect of the snout, whence the name Cephalopoda. In 
 the more typical forms of both branches of the class, the 
 peri-oral portion of the foot is drawn out into paired arm- 
 
128 
 
 MOLLUSCA 
 
 like processes, either very short and conical (Clio, Eurybia), 
 or lengthy (Pneumodermon, Octopus) ; these may be beset 
 with suckers or hooks, or both. The mid-foot (fig. 75, mf) 
 is expanded into a pair of muscular lobes right and left, 
 which either are used for striking the water like the wings 
 of a butterfly (Pteropoda), or are bent round towards one 
 another so that their free margins meet and constitute a 
 short tube, the siphon or funnel (Siphonopoda). The hind 
 foot is either very small or absent. 
 
 A distinctive feature of the Cephalopoda is the ABSENCE 
 of anything like the TORSION of the visceral mass seen in 
 the Anisopleurous Gastropoda, although as an exception 
 this torsion occurs in one family (the Limacinidae). 
 
 The ANUS, although it may be a little displaced from 
 the median line, is (except in Limacinidae) approximately 
 median and posterior. The MANTLE-SKIRT may be aborted 
 (Gymnosomatous Pteropoda) ; when present it is deeply 
 produced posteriorly, forming a large sub-pallial chamber 
 around the anus. As in our schematic Mollusc, by the side 
 of the anus are placed the single or paired apertures of the 
 NEPHRIDIA, the GENITAL APERTURES (paired only in Nau- 
 tilus, in female Octopoda, female Ommastrephes, and male 
 Eledone), and the paired CTENIDIA (absent in all Pteropoda). 
 The VISCERAL HUMP or dome is elevated, and may be very 
 much elongated (see fig. 75, (4), (5), (6)) in a direction 
 almost at right angles to the primary horizontal axis (A, P 
 in fig. 75) of the foot. 
 
 A SHELL is frequently, but not invariably, secreted on 
 the visceral hump and mantle-skirt of Cephalopoda ; but 
 there are both Pteropoda and Siphonopoda devoid of any 
 shell. The shell is usually light in substance or lightened 
 by air-chambers in correlation with the free-swimming 
 habits of the Cephalopoda. It may be external, when it is 
 box-like or boat-like, or internal, when it is plate-like. Very 
 numerous minute pigmented sacs capable of expansion and 
 contraction, and known as CHROMATOI>HORES, are usually 
 present in the integument in both branches of the class. The 
 GONADS of both sexes are developed in one individual in some 
 Cephalopoda (Pteropoda), in others the sexes are separate. 
 
 SENSE-ORGANS, especially the cephalic eyes and the oto- 
 cysts, are very highly developed in the higher Cephalopoda. 
 The osphradia have the typical form and position in the 
 lower forms, but appear to be more or less completely 
 replaced by other olfactory organs in the higher. The 
 normal NERVE-GANGLIA are present, but the connectives are 
 shortened, and the ganglia concentrated and fused in the 
 cephalic region. Large special ganglia (optic, stellate, and 
 supra-buccal) are developed in the higher forms (Siphono- 
 poda). 
 
 The Cephalopoda exhibit a greater range from low to 
 high organization than any other Molluscan class, and hence 
 they are difficult to characterize in regard to several groups 
 of organs ; but they are definitely held together by the 
 existence in all of the encroachment of the fore-foot so as 
 
 Fig. 76. 
 
 Fig. 77. 
 
 Fio. 76. SpiriaZis Tmlimoiiles, Soul., one of the Limaeinida; enlarged (from 
 Owen). C C, pteropodial lobes of the mid-foot ; /, operculum carried on the 
 hind-foot ; g, spiral shell. 
 
 FIG. 11. Operculum of Spirialis enlarged. 
 
 to surround the head, and by the functionally important 
 
 BILOBATION OF THE MID-FOOT. 
 
 Two very distinct branches of the Cephalopoda are to 
 be recognized : the one, the Pteropoda, more archaic in 
 the condition of its bi- 
 lobed mid-foot, including 
 a number of minute, and 
 in all probability degen- 
 erate, oceanic forms of 
 simplified and obscure 
 organization ; the other, 
 the Siphonopoda, con- 
 taining the Pearly Nau- 
 tilus and the Cuttles, 
 which have for ages (as 
 their fossil remains show) 
 dominated among the in- 
 habitants of the sea, be- 
 ing more highly gifted 
 in special sense, more 
 varied in movement, 
 more powerful in pro- 
 portion to size, and more 
 
 i j ..i Flo. 77. Uymtiulia 1'eronil. Cuvier (from 
 
 heavily equipped With Owen). C, C, the expanded pteropodial 
 destructive Weapons of lobes or win g- lik e fins of the mid-foot. 
 
 offence than any other marine organisms. 
 
 Branch a.PTEKOPODA. 
 
 Characters. Cephalopoda in which the mid-region of 
 the foot is (as compared with the Siphonopoda) in its more 
 primitive condition, being 
 relatively largely developed 
 and drawn out into a pair 
 of wing-like muscular lobes 
 (identical with the two halves 
 of the siphon of the Siphon- 
 opoda) which are used as 
 paddles (see figs. 76-86). The 
 hind -region of the foot is 
 often aborted, but may carry 
 an operculum (figs. 76, 77). 
 The fore -region of the foot 
 (that embracing the head) is 
 also often rudimentary, but 
 may be drawn out into one 
 or more pairs of tentacles, 
 simulating cephalic tentacles, 
 and provided with suckers 
 (figs. 84, 85). 
 
 Though the visceral hump 
 is not twisted except in the 
 Limacinida? (fig. 76), there is 
 a very general tendency to 
 one-sided development of the 
 viscera, and of their external 
 apertures (as contrasted with 
 Siphonopoda). The ctenidia 
 are aborted, with the possible 
 exception of the processes (fig. 
 85, c) at the end of the body 
 of Pneumodermon. The vas- 
 cular system resembles that 
 of the Gastropoda. The ne- 
 
 phridium is a single tubular FIG. IS.StyUola acimla, Rang. sp. en 
 
 Vinrlv rnT-rrmnnnrli'nrr tr> trip l ar ged (from OwenX C, C, the wing- 
 
 oav ' like lobes of the mid-foot; d, median 
 
 right nephridium of the typi- fold of same ; e, copulatory organ ; K, 
 
 i j. ,1 !_ ii pointed extremity of the shell ; t, an- 
 
 Cal pair 01 the archl-MolluSC. ferior margin of the shell; n, stomach; 
 
 The anal aperture is usually liver = " hermaphrodite gonad. 
 placed a little to the left of the median line, more rarely 
 to the right. In the Limacinidse it has an exceptional 
 position, owing to the torsion of the visceral mass, as in 
 Anisopleurous Gastropoda. 
 
MOLLUSCA 
 
 129 
 
 Jaws and a lingual ribbon are present as in typica] 
 Glossophora, the dentition of the ribbon and the number of 
 jaw-pieces presenting a certain range of variation. Sense- 
 
 C 
 
 tt 
 
 Fig. 79. 
 
 FIG. 79. Camiinia trideniata, Forsk. torn the Mediterranean, magnified two 
 diameters (from OwenX a, month ; b, pair of cephalic tentacles ; C, C, ptero- 
 podial lobes of the mid-foot ; d, median web connecting these ; e, e, processes 
 of the mantle-skirt reflected over the surface of the shell ; a. the shell en- 
 closing the visceral hump ; *, the median spine of the shell. 
 
 Fio. 80. Shell of Carolinia tridentata, seen from the side. /, postero-dorsal 
 surface ; g, antero-ventral surface ; \, median dorsal spine ; i, month of the 
 shell. 
 
 organs are present in the form of cephalic eyes in very few 
 
 forms (Cavolinia, Clione, and in an undescribed form dis- 
 
 covered by Suhm during the "Challenger" Expedition); oto- 
 
 cysts are universally present. The osphradia are present 
 
 in typical form, although the ctenidia are aborted ; only 
 
 one osphradium (the 
 
 right of the typical 
 
 pair) is present (fig. 
 
 87). The gonads are 
 
 both male and female 
 
 in the same individual. 
 
 The genital aperture is 
 
 single. Copulatory or- 
 
 gans, often of consider- 
 
 able size, are present 
 
 (fig. 86, *). 
 
 The mantle -skirt is 
 present in one divi- 
 sion of the Pteropoda 
 (Thecosomata), and in 
 these an extensive sub- 
 pallial chamber is de- 
 veloped, the walls of 
 which in the absence 
 of ctenidia have a 
 branchial function. In 
 asecond division (Gym- F, O . si.-Embryo 
 
 nOSOmata). which com- 
 
 t i , , , . 
 
 prises forms highly de- , heart ; i, intestine ;Totocyst ; Sl shell -r' 
 
 veloped in regard to ne l*!! dium \*. *ophagus i ; <r, sac containing 
 
 . r , nntnfave yelk; mb, mantle-skirt; *c, nb- 
 
 the processes of the pallial chamber ; K*, contractile sinus. 
 
 fore-foot, the mantle-skirt is aborted. A shell is developed 
 on the surface of the visceral hump and mantle-skirt of the 
 Thecosomata, whilst in the Gymnosomata, which have no 
 mantle-skirt, there is in the adult animal no shell. The 
 embryo passes through a trochosphere and a veliger stage 
 (fig. 81), provided with boat -like shell, except in some 
 Gymnosomata in which the Trochosphere with its single 
 velar ciliated band becomes metamorphosed into a larva 
 which has three additional ciliated bands but no velum 
 (resembling the larva of the Scaphopod Dentalium) ; this 
 banded larva does not form a larval shell (fig. 84). 
 The Pteropoda are divided into two orders. 
 
 Order 1. Thecosomata. 
 Characters. Pteropoda provided with a mantle-skirt, 
 
 . . tndntata (from 
 
 Blfour, after roL). a, anus ; / median portion 
 of the foot ; jm. pteropodial lobe of the foot 
 
 ' 
 
 and with a delicate hyaline shell developed on the surface 
 
 of the visceral hump and mantle-skirt ; visceral hump, and 
 
 consequently the shell, 
 
 spirally twisted in one 
 
 family, the Limacinida? ; 
 
 shell often with con- 
 
 tracted mouth and di- 
 
 lated body, its walls 
 
 sometimes drawn out 
 
 into spine-like processes, 
 
 which are covered by 
 
 reflexions of the free 
 
 margin of the mantle 
 
 (Cavolinia, figs. 79, 80). 
 
 Family 1. Cymbuliidx. 
 Genera : Tiedema.mn.ia, 
 Chj. ; Halopsyche, The- 
 ceuryina (figs. 82, 83), 
 Cymbulia, P. and L. 
 (% 77a). 
 Family 2. Conulariidse 
 
 (fossil). 
 
 Genus : Convlaria, Hill. 
 Family 3. Tentaculitids 
 
 (fossil). FIG. 82. Thfcnn/bia GaiuHdwvdii, SonL, 
 
 Genera : TentaculUes (from Owen). Much enlarged ; the body-wall 
 ranuilitss re ved. a, the mouth ; c, the pteropodial 
 i^ ' lobes of the foot : /. >e centrally -placed 
 Coleopnon, hind-foot; d, L, e, three pairs of tentacle-like 
 processes placed at the sides of the month, 
 ""* developed (in aU probability) from the 
 fore-foot ;o', anus ;, genital pore ;t retractor 
 musc ies ; and p, the liver ; , r, w/genitali*. 
 
 Sohlrh 
 
 ir 
 bcnltn. 
 
 Sandb. 
 
 Family 4. HvaUidse 
 /-!, -,? , , 
 
 Genera : Tnptcra, Q. and 
 
 G. ; Styliola, Les. (fig. 
 78) ; Balantium, Lch. ; 
 
 FIG. 83. Shell 
 
 . Vaginella, Dand. ; Cleodora, P. and 
 
 L. ; Diacria, Gr. ; Plturopws, Esch, ; Cavolinia, Gioni. (figs. 
 / 9, 80 f 81). 
 Family 5.Thecidx. 
 
 Genera : Theca, Low ; Pterotheea, Salt 
 Family 6. Limacinidse. 
 
 Genera : Eccyliomphaluis, Porti ; Eeterofusus, Fig. ; 
 Spirialvi, E. S. (fig. 76) ; Limacina, Cuv. 
 
 Order 2. Gymnosomata. 
 
 Characters. Pteropoda devoid of man tie - 
 skirt and shell ; tentacular processes of the 
 fore-foot well developed and provided with 
 suckers. 
 
 Family 1. Pterocymodoceidx. 
 Genus : Pterocymadoce, Kef. 
 Family 2. Clionida. 
 Genera : Cliodita, Q. and G. ; Clionopsis, Trosch. ; 
 
 Clione, PalL (fig. 86). lower ' figure 
 
 Family 3. Pntumodtrmidx. shows the na- 
 
 Gen'era : Trichoeyelus, Esch.; Spongobranehia, ta * l8ize - 
 d'Orb. ; Pneumodermopsis, Kef. ; Pneumodermon, Cuv. (fig. 85). 
 
 Branch b.SIPHOXOPODA. 
 
 Cephalopoda in which the two primarily divergent right 
 and left lobes of the mid-region of the foot have their free 
 borders recurved towards the middle line, where they are 
 either held in apposition (Tetrabranchiata), or fused with 
 one another to form a complete cylinder open at each end 
 (Dibranchiata). This fissured or completely closed tube is 
 the siphon (fig. 75, (6), mf) characteristic of the Siphono- 
 poda, and is used to guide the stream of water expelled 
 by the contractions of the walls of the branchial chamber. 
 The pallial skirt is accordingly well developed and muscular, 
 subserving by its contractions not only respiration but 
 locomotion. The visceral hump is never twisted, and ac- 
 cordingly the main development of the pallial skirt and 
 chamber is posterior, the excretory apertures, anus, and 
 gills having a posterior position, as in the archi-Mollusc. 
 At the same time the visceral hump is usually much elon- 
 gated in a direction corresponding to an oblique line be- 
 tween the vertical dorso-ventral and the horizontal antero- 
 posterior axes (see fig. 75, (6)). 
 
 R 
 
130 
 
 MOLLUSCA 
 
 The fore-part of the foot which surrounds the mouth, as 
 in all Cephalopoda, is drawn out into four or five pairs of 
 lobes, sometimes short, but usually elongated and even fili- 
 
 Fig. 84. Fig. 85. 
 
 Fia. 84. Larvse of Pneumodermon (from Balfour, after Gegenbaur). The 
 prae-oral ciliated band of the trochosphere stage (velum) has atrophied. In 
 A three post-oral circlets of cilia are present. The otocysts are seen, and 
 the rudiments of a pair of processes growing from the head. In B the fore- 
 most ciliated ring has disappeared ; the cephalic region is greatly developed, 
 and, as compared with the adult (fig. 85), is large and free ; the pair of hook- 
 bearing processes on each side of the mouth are retractile, probably part of 
 the fore-foot. At the base of the cephalic snout are seen the pair of arm- 
 like processes (fore-foot) provided with suckers, and behind these the broad 
 pteropodial lobes or wing-like fins of the mid-foot. 
 
 Fio. 85. Pmiimadermon violaceum, d'Orb. ; magnified five diameters, a, the 
 sucker-bearing arms ; b, the fins of the mid-foot (in the middle line, between 
 these, is seen the sucker-like median portion of the foot, by means of which 
 the animal can crawl as a Gastropod) ; c, the four branchial processes. (After 
 Keferstein.) 
 
 form. These lobes either carry peculiar sheathed tentacles 
 (Nautilus), or, on the other hand, acetabuliform suckers, which 
 may be associated with claw-like hooks (Dibranchiata). 
 The hind-foot is probably represented by the valve which 
 depends from the inner ___ c 
 
 wall of the siphon in 
 many cases. 
 
 A shell (figs. 89, 100) 
 is very generally present, 
 affording protection to 
 the visceral mass and 
 attachment for muscles. 
 It may be external or en- 
 closed in dorsal upgrow- 
 ing folds of the mantle, 
 which (except in Spirula) 
 close up at an early period 
 of development, so as to 
 form a shut sac in which 
 the shell is secreted. The 
 
 a 
 
 - 86. Clione borealis, L. ; magnified two 
 diameters, postero-ventral aspect, a, the 
 cephalic region carrying a' three pairs of 
 cephalic cones provided each with very nu- 
 merous minute sucker-like processes, and 
 surrounded by a hood-like upgrowth, 
 and b, the more elongated tentacles (the 
 retractile eye-tentacles are not seen, being 
 placed dorsally) ; c, the pteropodial fins ; 
 rf, the median portion of the foot ; o, the 
 anus ; y, the vagina ; z, the penis. (From 
 Owen, after Eschricht.) 
 
 CeR 
 
 ctenidia are well deve- 
 loped as paired gill-plumes, serving as the efficient bran- 
 chial organs (figs. 101, 103, 
 and fig. 2, B). 
 
 The vascular system is 
 very highly developed ; the 
 heart consists of a pair of 
 auricles and a ventricle (figs. 
 104, 105). Branchial hearts 
 are formed on the advehent 
 vessels of the branchiae. It 
 is not known to what extent 
 the minute subdivision of 
 the arteries extends, or 
 whether there is a true 
 capillary system. 
 
 The pericardium is ex- 
 
 Fio. 87. Enlarged diagram of the nerve- 
 centres of Pneumodermon (from Spen- 
 gel, after Spuleyet). CeR, right cere- 
 bral ganglion ; Pl.R, right pleural 
 ganglion ; Pe, right pedal ganglion ; 
 Vis.R., right visceral ganglion ; I'is.L., 
 left visceral ganglion ; cpe, right cere- 
 bro-pedal connective ; cpl, right cere- 
 bro-pleural connective ; Osp., osphra- 
 dium connected by a nerve with the 
 right visceral ganglion. 
 
 tended so as to form a very 
 large sac passing among 
 the viscera dorsal wards and 
 sometimes containing the 
 ovary or testis the viscero- 
 pericardial sac which opens to the exterior either directly 
 
 or through the nephridia. It has no connexion with the 
 vascular system. The nephridia are always paired sacs, 
 the walls of which invest the branchial advehent vessels 
 (figs. 104, 108). They open each by a pore into the viscerc- 
 
 . 1 C 
 
 9 
 
 Fia. 88. Male (upper) and female (lower) specimens of Nautilus pompilivs as 
 seen in the expanded condition, the observer looking down on to the buccal 
 cone e; one-third the natural size linear. The drawings have been made 
 from actual specimens by A. G. Bourne, B.Sc., and serve to show the 
 natural disposition of the tentaculiferous lobes and tentacles of the circum- 
 oral portion of the foot in the living state, as well as the great differences 
 between the two sexes, a, the shell ; b, the miter ring-like expansion (annular 
 lobe) of the circum-oral muscular mass of the fore-foot, carrying nineteen 
 tentacles on each side posteriorly this is enlarged to form the "hood" 
 (marked v in fig. 89 and m. in figs. 90 and 91), giving off the pair of tentacles 
 marked g in the present figure ; c, the right and left inner lobes of the fore- 
 foot, each carrying twelve tentacles in the female, in the male subdivided 
 intoj), the "spadix" or hectocotylns on the left side, and q, the "anti-spadix," 
 a group of four tentacles on the right side, it is thus seen that the subdivided 
 right and left inner lobes of the male correspond to the undivided right and 
 left inner lobes of the female ; rf, the inner inferior lobe of the fore-foot, a 
 bilateral structure in the female carrying two groups, each of fourteen tenta- 
 cles, separated from one another by a lamellated organ , supposed to be 
 olfactory in function in the male the inner inferior lobe of the fore-foot is 
 very much reduced, and has the form of a paired group of lamella: (d in the 
 upper figure); e, the buccal cone, rising from the centre of the three inner lobes, 
 and fringing the protruded calcareous beaks or .jaws with a series of minute 
 papilla: ; /, the tentacles of the outer circum-oral lobe or annular lobe of the 
 fore-foot projecting from their sheaths ; g, the two most posterior tentacles 
 of this series belonging to that part of the annular lobe which forms the 
 hood (m. in figs. 90 and 91) ; i, superior ophthalmic tentacle ; k, inferior 
 ophthalmic tentacle ; I, eye ; m, paired laminated organ on each side of the 
 
 of the left inner lobe of the fore-foot representing four modified tentacles, 
 ei"ht being left unmodified ; q, the anti-spadix (in the male), being four of 
 the twelve tentacles of the right inner lobe of the fore-foot isolated from 
 the remaining eight, and representing on the right side the differentiated 
 spadix of the left side. The four tentacles of the anti-spadix are set, three 
 on one base and one on a separate base. 
 
 There are thus in the female, where they are most numerous, ninety-four 
 tentacles, thirty-eight on the outer annular lobe, four ophthalmic (a pair to 
 each eye), twelve on each of the right and left inner lobes, and twenty-eight 
 on the inner inferior lobe. 
 
 pericardial sac except in Nautilus. The anal aperture is 
 median and raised on a papilla. Jaws (fig. 88, e ) and a lin- 
 gual ribbon (fig. 107) are well developed. The jaws have 
 the form of a pair of powerful beaks, either horny or calcified 
 (Nautilus), and are capable of inflicting severe wounds. 
 
MOLLUSCA 
 
 131 
 
 Sense-organs are highly developed ; the eye exhibits a 
 veiy special elaboration of structure in the Dibranchiata, 
 and a remarkable archaic form in the Nautilus. Otocysts 
 are present in alL The typical osphradium is not present, 
 
 term hectocotylization is applied to this modification (see 
 figs. 88, 95, 96). Elaborate spermatophores or sperm-ropes 
 are formed by all Siphonopoda, and very usually the female 
 possesses special capsule-forming and nidamental glands for 
 providing envelopes to the eggs (fig. 101, g.n.). 
 The egg of all Siphonopoda is large, and the 
 development is much modified by the presence 
 of an excessive amount of food-material diffused 
 in the protoplasm of the egg-celL Trochosphere 
 and veliger stages of development are conse- 
 quently not recognizable. 
 
 The Siphonopoda are divisible into two 
 orders, the names of -which (due to Owen) de- 
 scribe the number of gill-plumes present ; but 
 in fact there are several characters of as great 
 importance as those derived from the gills by 
 which the members of these two orders are 
 separated from one another. 
 
 Order 1. Tetrabranchiata ( = Schizosiphona, 
 Tentaculifera). 
 
 Characters. Siphonopodous Cephalopods 
 in which the inrolled lateral margins of the 
 mid-foot are not fused, but form a siphon by 
 apposition (fig. 101). The circum-oral lobes 
 of the fore-foot carry numerous sheathed ten- 
 tacles (not suckers) (fig. 88). There are two 
 pairs of ctenidial gills (hence Tetrabranchiata), 
 and two pairs of nephridia, consequently four 
 nephridial apertures (fig. 101). The viscero- 
 
 FIG. 89. Lateral view of the fenmle Pearly Xantflns, contracted by spirit and lying in its shell, j- i L u j 
 
 the right half of which is cut away (from Gegenbaor, after Owen). ", visceral hump; 6,po7- pericardia! Chamber Opens by two independent 
 tton of the free edge of the mantle-skirt reflected on to the shell, file edge of the mantle-skirt anpi-tiirpi: tn trip p-rtprior anrl not intn trip 
 can be traced downwards and forwards around the base of the- mid-foot or siphon i ; /, I, super- a P 6 ' ^^ TO 6 
 
 shell, of which a small piece (s) i 
 siphuncular pedicle, which is broken 
 
 ficialoriginof the retractor muscle of the mid-foot (siphonX more or less firmly attached to the nephridial SaCS. There are two Oviducts 
 )is seen between the letters!, i; s (farther back) points to the /-j^t an J l p f t \ : n *}. f PTna lp an H two 
 oken off short and not continued, as in the perfect state, through (, n o m; ana - II ) in me lemaie ana IWO 
 
 the whole length of the siphuncle of the shell, also marked* and if; o points to the right eye; ducts in the male, the left duct in both 
 t is placed near the extremities of the contracted tentacles of the outer or annular lobe of the , , 
 
 fore-foot, the join ted tentacles are seen protruding a little from their long cylindrical sheaths ; r, SCXCS being rudimentary. 
 
 the dorsal "hood" formed by an enlargement in this region of the annular lobe of the fore- A ] aro -p p-rtprnal shpll pitripr roilprl or straio-Vit 
 foot (m. in figs. 90, 91) ; V, a swelling of the mantle-skirt; indicating the position on its inner . A Iar 8 e external s ' or Straignt 
 
 face of the nidamental gland (see fig. 101, J.R.). is present, and is not enclosed by reflexions of 
 
 except in Nautilus, but other organs are present in the the mantle-skirt, except such narrow-mouthed shells as 
 
 that of Gomphoceras, which were probably enclosed by the 
 
 FIG. !K>. Spirit specimen of female Pearly Nautilus, removed from its shell, 
 and seen from the antero-dorsal aspect (drawn from nature by A. G. 
 Bourne), m., the dorsal "hood" formed by the enlargement of the outer or 
 annular lobe of the fore-foot, and corresponding to the sheaths of two tenta- 
 cles (g, g in fig. 88) ; n., tentacular sheaths of lateral portion < ~>f the annular 
 lobe ; M., the left eye ; 6., the nuchal plate, continuous at its right and left 
 posterior angles with the root of the mid-foot, and corresponding to the 
 nuchal cartilage of Sepia ; c., visceral hump ; <f., the free margin of the 
 mantle-skirt, the middle letter d. points to that portion of the mantle-skirt 
 which is reflected over a part of the shell as seen in fig. 89, 6 ; the cup-like 
 fossa to which b. and d. point in the present figure is occupied by the coil of 
 the shell ; g.a. points to the lateral continuation of the nuchal plate 6. to 
 join the root of the mid-foot or siphon. 
 
 cephalic region, to which an olfactory function is ascribed 
 both in Nautilus and in the other Siphonopoda. 
 
 The gonads are always separated in male and female 
 individuals. The genital aperture and duct is sometimes 
 single, when it is the left ; sometimes the typical pair is 
 developed right and left of the anus. The males of nearly 
 all Siphonopoda have been shown to be characterized by a 
 peculiar modification of the arm-like processes or lobes of 
 the fore-foot, connected with the copulative function. The 
 
 FIG. 91. Lateral view of the same specimen as that drawn in fig. 90. Letters 
 as in that figure with the following additions e points to the concave margin 
 of the mantle-skirt leading into the sub-pallial chamber ; g, the mid-foot or 
 siphon ; i% the superficial origin of its retractor muscles closely applied to 
 the shell and serving to hold the animal in its place ; i, the siphuncular pedicle 
 of the visceral hump broken off short ; r, r, the superior and inferior ophthal- 
 mic tentacles. 
 
 mantle as in the Dibranch Spirula. The shell consists of 
 a series of chambers, the last formed of which is occupied 
 by the body of the animal, the hinder ones (successively 
 deserted) containing gas (fig. 89). 
 
 The pair of cephalic eyes are hollow chambers (fig. 118, 
 A) opening to the exterior by minute orifices (pinhole 
 camera), and devoid of refractive structures. A pair of 
 osphradia are present at the base of the gills (fig. 101, off). 
 Salivary glands are wanting. An ink-sac is not present. 
 Branchial hearts are not developed on the branchial adve- 
 hent vessels. 
 
132 
 
 MOLLUSCA 
 
 Family 1. Nautilidse. 
 Genera : [Orthoceras], Breyn. ; [Cyrtoceras], Goldfuss ; [Gompho- 
 
 ceras], Munster ; [Phragmoceras], Brod. ; [Gijroccras], Meyer ; 
 
 [Ascoceras], Barraude ; [Oncoccms], Hall ; [Lituitcs], Breyn. ; 
 
 [Trochoceras], Barraude; Nautilus, L. (figs. 88, 89, 90, &c.) ; 
 
 [Cfymenia], Miiiist. ; [Nothoceras], Barraude. 
 Family 2. Ammonitidee. 
 
 Genera : [Bactrites], Sanderg. ; [Goniatitcs], de Haan ; [Khabdo- 
 
 ceras], Hauer ; [Clydonitcs], Hauer ; [CoMoceras], Hauer ; 
 
 [Baculina], d'Orb. ; [Ceratites], de Haan ; [Baculites], Lam. ; 
 
 [Toxoceras], d'Orb.; [Criocems], Leveille ; [Ptychoceras],&'0rb. ; 
 
 [Hamites], Parkinson ; [Ancyloceras], d'Orb. ; [Scaphites], 
 
 Parkinson ; [Ammonites], Breyn.; [ Tu rrilites], Lam. ; [Helio- 
 
 ceras], d'Orb.; [Heteroceras], d'Orb. 
 
 N.B. The names in brackets are those of extinct genera. 
 
 Order 2. Dibranchiata ( = Holosiphona, Acetabulifera). 
 Characters. Siphonopodous Cephalopoda in which the 
 inflected lateral margins of the mid-foot are fused so as to 
 form a complete tubular siphon (fig. 96, i). The circum- 
 oral lobes of the fore-foot carry suckers disposed upon them 
 in rows (as in the Pteropod Pneumodermon), not tentacles 
 (see figs. 92, 95, 96). There is a single pair of typical 
 ctenidia (fig. 103) acting as gills (hence Dibranchiata), and 
 
 Fio. 92. Sepia offidnalis, L., half the natural size, as seen when dead, the long 
 prehensile arms being withdrawn from the pouches at the side of the head, 
 in which they are carried during life when not actually in use. a, neck ; 
 6, lateral fin of the mantle-sac ; c, the eight shorter anus of the fore-foot ; d, 
 the two long prehensile arms ; e, the eyes. 
 
 a single pair of nephridia opening by apertures right and 
 left of the median anus (fig. 103, r), and by similar internal 
 pores into the pericardial chamber, which consequently does 
 not open directly to the surface as in Nautilus. The ovi- 
 ducts are sometimes paired right and left (Octopoda), 
 sometimes that of one side only is developed (Decapoda, 
 except Ommastrephes). The sperm-duct is always single 
 except, according to Keferstein, in Eledone moschata. 
 
 A plate-like shell is developed in a closed sac formed by 
 the mantle (figs. 98, 99), except in the Octopoda, which have 
 none, and in Spirula (fig. 100, D) and the extinct Belemni- 
 tidse, which have a small chambered shell resembling that 
 
 of Nautilus with or without the addition of plate-like and 
 cylindrical accessory developments (fig. 100, C). 
 
 The pair of cephalic eyes are highly-developed vesicles 
 with a refractive lens (fig. 1 20), cornea, and lid-folds, the 
 vesicle being in the embryo an open sac like that of Nautilus 
 (fig. 119). Osphradia are not present, but cephalic olfac- 
 tory organs are recognized. One or two pairs of large 
 salivary glands with long ducts are present. An ink-sac 
 formed as a diverticulum of the rectum and opening near 
 the anus is present in all Dibranchiata (fig. 103, i), and has 
 been detected even in the fossil Belemnitidae. Branchial 
 hearts are developed on the two branchial advehent blood- 
 vessels (fig. 104, vc', vf). 
 
 The Dibranchiata are divisible into two sub-orders, accord- 
 ing to the number and character of the arm-like sucker- 
 bearing processes of the fore-foot. 
 
 FIG. 93. Decapodous Siphonopods; one-fourth the natural size linear. A. 
 Cheiroteuthis Veranyi, d'Orb. (from the Mediterranean). B. Thysanoteuthis 
 rhombus, Troschel (from Messina). C. Loligopsis cyclura, Per. and d'Orb. 
 (from the Atlantic Ocean). 
 
 Sub-order 1. Decapoda. 
 
 Characters. Dibranchiata with the fore-foot drawn out into 
 eight shorter and two longer arms (prehensile arms), the latter being 
 placed right and left between the third and fourth shorter arms. 
 The suckers are stalked and strengthened by a horny ring. The 
 eyes are large and have a horizontal in place of a sphincter-like lid. 
 The body is elongated and provided with lateral fins (lamelliform 
 expansions of the mantle). The mouth has a buccal membrane. 
 The mantle-margin is locked to the base of the siphon by a specially- 
 developed cartilaginous apparatus. Numerous water-pores are pre- 
 sent in the head and anterior region of the body, leading into re- 
 cesses of the integument of unknown significance. The oviduct is 
 single ; large uidamental glands are present. The viscero-pericar- 
 dial space is large, and lodges the ovary (Sepia). There is always 
 a shell present which is enclosed by the upgrowth of the mantle, 
 so as to become " internal." 
 
 Section a. Decapoda Calciphora. 
 Character. Internal shell calcareous. 
 Family 1. Spirulidee, 
 
 Genus: Spirilla, Lam. (fig. 100, D). 
 Family 2. Bclcmnitidx. 
 
 Genera : [Spirulirostra], d'Orb. (fig. 100, C) ; [Beloptcra], Desh. ; 
 [Belemnosis], Edw. ; [Coiwtcuthis], d'Orb. (fig. 100, A) ; [Acan- 
 thoteuthis], R. Wag.; [Bclcmnitcs], Lister, 1678; [Belemnitella], 
 d'Orb.; [Xiphoteuthis], Huxley. 
 Family 3. Sepiadse. 
 
 Genera: Sepia, L. (figs. 92, 98, &c.); [Bclosepia], Voltz ; Coeco- 
 teuthis, Owen. 
 
MOLLUSCA 
 
 133 
 
 Section b. Dccapoda Owitdrophora. 
 Character. Internal shell horny. 
 
 Sub-section o. Myopsidx (d'Orb.). 
 
 Eye with closed cornea, so that the surrounding water does not 
 touch the lens ; mostly frequenters of the coast. 
 Family 1. Loligidse. 
 
 Genera : Loligo, Schneid. (figs. 99, &c. ) ; Loliolus, Steenstrup ; 
 Sqnoteuthis, Blv. ; [Teuthopsis], Desl. ; [Leptoteuthis], Meyer; 
 [Selemnosepia], Ag. ; [Eelotevthis], Munst. 
 Family 2. Sepiolidx. 
 Genera : Sepiola, Schneid. ; Bossia, Owen. 
 
 Sub-section fi.Oigopsida (d'Orb.). 
 
 Eye with open cornea, so that the surrounding water bathes the 
 anterior surface of the lens ; mostly pelagic animals. 
 Family 3. Crandiiadas. 
 
 Genus : Cmnchia, Leach (fig. 94, C). 
 Family 4. Loligopsidse. 
 
 Genus : Loligopsis, Lam. (fig. 93, C). 
 Family 5. Cheiroteuthidee. 
 
 Genera : Cheiroteuthis, d'Orb. (fig. 93. A) ; Histioleuthis, d'Orb. 
 Family 6. Thysanoteutiiidse. 
 
 Genus : Thysanoteuthis, Trosehel (fig. 93, B). 
 Family 7. Onydwteuthidss. 
 
 Genera : Gonatus, Gray ; OnychoteuUiis, Lichtenst (fig. 97) ; Ony- 
 cJiia, Lesneur ; EnaplotevOiis, d'Orb. , Veranya, Krohn ; [Plesio- 
 teuthis], A. Wag. ; [Cel&ito], Miinst. ; Dosidicus, Steenstrnp ; 
 Ommastrephes, d'Orb. 
 
 Sub-order 2. Octopoda. 
 
 Characters. Dibranchiata with the fore-foot drawn out into eight 
 arms only; suckers sessile, devoid of horny ring; eyes small, the 
 
 D 
 
 FIG. 94. Octopodous Siphonopods ; one-fourth the natural size linear. A. 
 Pinruxioput cordiformis, Qaoy and Gain (from New Zealand). B. Tremac- 
 topus tialaixas, Ver. (from the Mediterranean). C. Cranchia soabra, Owen 
 (from the Atlantic Ocean ; one of the Decapoda). D. Cirrhoteuthis iKUtri, 
 Esch. (from the Greenland coast). 
 
 outer skin can be closed over them by a sphincter-like movement. 
 The body is short and rounded ; the mantle has no cartilaginous 
 locking apparatus, and is always fused to the head dorsally by a 
 broad nuchal band. Ko buccal membrane surrounds the mouth. 
 The siphon is devoid of valves. The oviducts are paired ; there are 
 no nidamental glands. The viscero-perieardial space is reduced to 
 two narrow canals, passing from the nephridia to the capsule of the 
 genital gland. There is no shell on or in the visceral hump. 
 Family 1. CirrJiokv.(hid&. 
 
 Genus : CirrhoteutJiis, Esch. (Sciadtphorus, Reinh.) (fig. 94, D). 
 Family 2. Octopodidas. 
 
 Genera : Pinnoctopus, d'Orb. (fig. 94, A) ; Octopiis, Lam. (fig. 95) ; 
 Soeurgitf, Trosch. ; Eltdone, Leach ; Bolitxna, Steenstrup. 
 
 Family 3. Pkilonexidas. 
 
 Genera: Tremoctopus, Delle Chiaje (Pliilonexui, d'Orb.) (fig. 94, 
 B) ; Parasira, Steenstrup (Octopus catenulatus, Fer., is the 
 female, and Octopus carena, Ver. , is the male of the one species 
 of this genus according to Steenstrup (fig. 96) ) ; Argonauta, L. 
 (the shell of this genus is formed only in the female by the 
 expanded ends of the two large " arms " of the fore-foot). 
 
 B 
 
 FIG. 95. A. Male specimen of Octopta yranlandieut, with the third arm of the 
 right side hectocotylized. B. Enlarged view of the hectocotylized arm of 
 Sepia. 
 
 Further Remarks on the Cephalopoda. In order to give 
 a more precise conception of the organization of the Cephalo- 
 poda in a concrete form we select the Pearly Nautilus for 
 further description, and in pass- 
 ing its structure in review we 
 shall take the opportunity of 
 comparing here and there the 
 peculiarities presented by that 
 animal with those obtaining in 
 allied forms. In the last edition 
 of this work the Pearly Nautilus 
 was made the subject of a de- 
 tailed exposition by Professor 
 Owen, and it has seemed accord- 
 ingly appropriate that it should 
 be somewhat fully treated on 
 the present occasion also. The 
 figures which illustrate the pre- 
 sent description are (excepting 
 fig. 89) original, and prepared 
 from dissections (made under the 
 direction of the writer) of a male 
 and female Nautilus pompUius, 
 lately purchased for the Museum 
 of University College, London. 
 
 Visceral Hump and ShtU. 
 The visceral hump of Nautilus 
 (if we exclude from considera- 
 tion the fine siphuncular pedicle FIG %. Male of Parasim mttnu- 
 Which it trails, as it were, behind **- Steenstrup, (Orfp arena, 
 
 it) is very little, if at all, affected 
 by the coiled form of the shell 
 which it carries, since the animal 
 always slips forward in the shell 
 as it grows, and inhabits a cham- 
 ber which is practically cylindri- 
 cal (fig. 89). Were the deserted chambers thrown off instead 
 of being accumulated behind the inhabited chamber as a 
 coiled series of air-chambers, we should have a more correct 
 indication in the shell of the extent and form of the animal's 
 
 Ver.), showing the hectocotylized 
 arm. ft, (2, f>, t*, the first, second, 
 third, and fourth arms or pro- 
 cesses of the fore-foot; A, the 
 third arm of the right side hecto- 
 cotylized; i, the apical sac of the 
 hectocotylized arm; y, the fila- 
 ment which issues from the sac 
 when development is complete ; 
 f, the siphon. (From Gegenbaur.) 
 
134 
 
 MOLLUSCA 
 
 body. Amongst Gastropods it is not very unusual to find 
 the animal slipping forward in its shell as growth advances 
 and leaving an unoccupied chamber in the apex of the shell. 
 This may indeed become shut off from the occupied cavity 
 by a transverse septum, and a series of such septa may be 
 formed (fig. 42), but in no Gastropod are these apical 
 chambers known to contain a 
 gas during the life of the 
 animal in whose shell they 
 occur. A further peculiarity 
 of the Nautilus shell and of 
 that of the allied extinct Am- 
 monites, Scaphites, Orthoceras, 
 &c., and of the living Spirula, 
 is that the series of deserted 
 air-chambers are traversed by 
 a cord -like pedicle extending 
 from the centro-dorsal area of 
 the visceral hump to the small- 
 est and first-formed chamber of 
 the series. No structure com- 
 parable to this siphuncular 
 pedicle is known in any other 
 Mollusca. Its closest repre- 
 sentative is found in the so- 
 called " contractile cord " of 
 the remarkable form Rhabdo- 
 pleura, referred according to 
 present knowledge to the Poly- 
 zoa. There appears to be no 
 doubt that the deserted cham- 
 bers of the Nautilus shell con- 
 tain in the healthy living 
 animal a gas which serves to 
 lessen the specific gravity of 
 the whole organism. The gas 
 is said to be of the same com- 
 position as the atmosphere, 
 with a larger proportion of 
 nitrogen. With regard to its 
 origin we have only conjec- 
 tures. Each septum shutting 
 off an air-containing chamber 
 is formed during a period of 
 quiescence, probably after the 
 reproductive act, when the vis- 
 ceral mass of the Nautilus may 
 be slightly shrunk, and gas is 
 secreted from the dorsal inte- 
 gument so as to fill up the 
 space previously occupied by 
 the animal. A certain stage 
 is reached in the growth of 
 the animal when no new cham- 
 bers are formed. The whole 
 process of the loosening of the 
 animal in its chamber and of 
 its slipping forward when a 
 new septum is formed, as well as the mode in which the 
 air-chambers may be used as a hydrostatic apparatus, and 
 the relation to this use, if any, of the siphuncular pedicle, 
 is involved in obscurity, and is the subject of much in- 
 genious speculation. In connexion with the secretion of 
 gas by the animal, besides the parallel cases ranging from 
 the Protozoon Arcella to the Physoclistic Fishes, from 
 the Hydroid Siphonophora to the insect-larva Corethra, 
 we have the identical phenomenon observed in the closely- 
 allied Sepia when recently hatched. Here, in the pores 
 of the internal rudimentary shell, gas is observable, which 
 has necessarily been liberated by the tissues which secrete 
 
 Fin. 97 Head and circum-oral pro- 
 cesses of the fore-foot of Onycho- 
 
 prehensile arms, the clavate extre- 
 mities of which are provided with 
 suckers at e, and with a double row 
 of hooks beyond at/. The tempo- 
 rary conjunction of the arms by 
 means of the suckers enables them 
 to act in combination. 
 
 the shell, and not derived from any external source 
 (Huxley). 
 
 The coiled shell of Nautilus, and by analogy that of the 
 Ammonites, is peculiar in its relation to the body of the 
 animal, inasmuch as the curvature of the coil proceeding 
 
 Fig. 98. Fig. 99. 
 
 FIG. 98. The calcareous internal shell of Sepia officinalis, the so-called cuttle- 
 bone, a, lateral expansion ; b, anterior cancellated region ; c, laminated 
 region, the laminre enclosing air. 
 
 FIQ. 99. The horny internal shell or gladius or pen of Loligo. 
 
 from the centro-dorsal area is towards the head or forward, 
 instead of away from the head and backwards as in other 
 discoid coiled shells such as Planorbis ; the coil is in fact 
 absolutely reversed in the two cases. Amongst the extinct 
 allies of the Nauti- 
 lus (Tetrabranch- 
 iata) we find shells 
 of a variety of 
 shapes, open coils 
 such as Scaphites, 
 leading on to per- 
 fectly cylindrical 
 shells with chamber 
 succeeding cham- 
 ber in a straight 
 line (Orthoceras), 
 whence again we 
 may pass to the 
 cork-screw spires 
 formed by the shell 
 of Turrilites. 
 
 Whilst the Tetra- 
 branchiata, so far as 
 we can recognize 
 their remains, are 
 characterized by 
 these large chambered shells, which, as in Nautilus, were 
 with the exception of some narrow-mouthed forms such 
 as Gomphoceras but very partially covered by reflexions 
 of the mantle-skirt (fig. 89, b), the Dibranchiata present 
 an interesting series of gradations, in which we trace 
 (a) the diminution in relative size of the chambered 
 shell ; (b) its complete investiture by reflected folds of 
 the mantle (Spirula, fig. 100, D) ; (<) the concrescence 
 
 Fio. 100. Internal shells of Cephalopoda Siphono- 
 poda. A. Shell of Conotcuthis dvpiniana, d'Orb. 
 (from the Neocomian of France). B. Shell of 
 Sepia orbigniana, Fer. (Mediterranean). C. Shell 
 of Spirtilirostra Bellardii, d'Orb. (from the Mio- 
 cene of Turin). The specimen is cut so as to show 
 in section the chambered shell and the laminated 
 " guard " deposited upon its surface. D. Shell of 
 Spinila leevis, Gray (New Zealand). 
 
MOLLUSCA 
 
 135 
 
 of the folds of the mantle to form a definitely -closed 
 shell-sac ; (d) the secretion by these mantle-folds or walls 
 of the shell -sac of additional laminae of calcareous shell- 
 substance, which invest the original shell and completely 
 alter its appearance (Spirulirostra, fig. 100, C; Belemnites); 
 (e) the gradual dwindling and total disappearance of the 
 original chambered shell, and survival alone of the calcare- 
 ous laminae deposited by the inner walls of the sac (Sepia, 
 fig. 100, B) ; (/) the disappearance of all calcareous sub- 
 stance from the pen or plate which now represents the 
 contents of the shell-sac, and its persistence as a horny 
 body simply (Loligo, fig. 99); (<?) the total disappearance 
 of the shell-sac itself, and consequently of its pen or plate, 
 nevertheless the rudiments of the shell-sac appearing in 
 the embryo and then evanescing (Octopus). The early 
 appearance of the sac of the mantle in which the shell is 
 enclosed, in Dibranchiata, has led to an erroneous identifi- 
 cation of this sac with the primitive shell-sac of the archi- 
 Mollusc (fig. 1), of Chiton (fig. 10, A), of Arion (fig. 69, 
 D, a), and of the normally -developing Molluscan embryo 
 (figs. 68 and 72***, sk). The first appearance of the shell- 
 sac of Dibranchiata is seen in figs. 121 and 122, its forma- 
 tion as an open upgrowth of the centro-dorsal area of the 
 embryo having been demonstrated by Lankester (34) in 
 1873, who subsequently showed (35) that the same shell-sac 
 appears and disappears without closing up in Argonauta 
 and Octopus, and pointed out the distinctness of this sac 
 and the primitive shell-gland. The shell of the female 
 Argonauta is not formed by the visceral hump, but by the 
 enlarged arms of the foot, which are in life always closely 
 applied to it. 
 
 The shell of such Pteropoda as have shells (the Thecoso- 
 mata) is excessively light, and fits close to the animal, no 
 air-chambers being formed. It is important to note that 
 in this division of the Cephalopoda there is the same tend- 
 ency, which is carried so far in the Dibranchiate Siphono- 
 pods, for the mantle-skirt to be reflected over and closely 
 applied to the shell (e.g., Cavolinia, figs. 79 and 80). But 
 in Pteropoda there is no complete formation of a closed 
 sac by the reflected mantle, no thickening of the enclosed 
 shell, no dwindling of the original shell and substitution 
 for it of a laminated plate. The variety of form of 
 the glass-like shells of Pteropoda is a peculiarity of that 
 group. 
 
 Head, Foot, Mantle-skirt, and Sub-pallial Chamber. In 
 the Pearly Nautilus the ovoid visceral hump is completely 
 encircled by the free flap of integument known as mantle- 
 skirt (fig. 91, d, e). In the antero-dorsal region this flap 
 is enlarged so as to be reflected a little over the coil of the 
 shell which rests on it. In the postero-ventral region the 
 flap is deepest, forming an extensive sub-pallial chamber, 
 at the entrance of which e is placed in fig. 91. A view of 
 the interior of the sub-pallial chamber, as seen when the 
 mantle-skirt is retroverted and the observer faces in the 
 direction indicated by the reference line passing from e in 
 fig. 91, is given in fig. 101. With this should be com- 
 pared the similar view of the sub-pallial chamber of the 
 Dibranchiate Sepia (fig. 103). It should be noted as a 
 difference between Nautilus and the Dibranchiates that in 
 the former the nidamental gland (in the female) lies on 
 that surface of the pallial chamber formed by the dependent 
 mantle-flap (figs. 101, g.n. ; 89, F), whilst in the latter it lies 
 on the surface formed by the body-wall ; in fact in the 
 former the base of the fold forming the mantle-skirt com- 
 prises in its area a part of what is unreflected visceral 
 hump in the latter. 
 
 The apertures of the two pairs of nephridia, of the vis- 
 cero-pericardial sac, of the genital ducts, and of the anus 
 are shown in position on the body-wall of the pallial cham- 
 ber of Nautilus in figs. 101, 102. There are nine apertures 
 
 in all, one median (the anus), and four paired. Besides 
 these apertures we notice two pairs of gill-plumes which 
 are undoubtedly typical ctenidia, and a short papilla (the 
 
 nepk.f 
 'ISCffT. 
 
 Via. 101. View of the postero- ventral surface of a female Pearly Nautilus, the 
 mantle-skirt (c) being completely reflected so as to show the inner wall of 
 the sub-pallial chamber (drawn from nature by A. G. Bourne), a, muscu- 
 lar band passing from the mid-foot to the integument ; b. the valve on the 
 surface of the funnel-like mid-foot, partially concealed by the inrolled lateral 
 margin of the Utter ; e, the mantle-skirt retroverted ; an, the median anus ; 
 x, post-anal papilla of unknown significance ; g.n., nidamental gland ; r.or., 
 aperture of the right oviduct ; ?,or., aperture of the rudimentary left oviduct 
 (pyrifonn sac of Owen) ; nepli.a., aperture of the left anterior nephridium ; 
 <w}*.J>, aperture of the left posterior nephridinm ; tiscper., left aperture of 
 the viscero-pericardial sac ; olf, the left osphradium placed near the base of 
 the anterior gill-plume. The four gill-plumes (ctenidia) are not lettered. 
 
 osphradium) between each anterior and posterior gill-plume 
 (see figs. 101, 102, and explanation). As compared with 
 this in a Dibranchiate, we find (fig. 103) only four aper- 
 
 FIG. 102. View of the postero-ventral surface of a male Pearly Nautilus, th 
 mantle-skirt (c) being completely reflected so as to show the inner wall of 
 the sub-pallial chamber, and the four ctenidia and the foot cut short (drawn 
 from nature by A. G. Bourne), pe., penis, being the enlarged termination 
 of the right spermatic duct ; Lsp., aperture of the rudimentary left spermatic 
 duct (pyrifonn sac of Owen). Other letters as in fig. 101. 
 
 tures, viz., the median anus with adjacent orifice of the 
 ink-sac, the single pair of nephridial apertures, and one 
 asymmetrical genital aperture (on the left side), except in 
 female Octopoda and a few others where the genital 
 ducts and their apertures are paired. No viseero-peri- 
 cardial pores are present on the surface of the pallial 
 chamber, since in the Dibranchiata the viscero-pericardial 
 
136 
 
 MOLLUSCA 
 
 sac opens by a pore into each nephridium instead of 
 directly to the surface. A single pair of ctenidia (gill- 
 plumes) is present instead of the two pairs in Nautilus. 
 The existence of two pairs of ctenidia and of two pairs 
 of nephridia in Nautilus, placed one behind the other, is 
 highly remarkable. The interest of this arrangement is in 
 relation to the general morphology of the Mollusca, for 
 it is impossible to view this repetition of organs in a linear 
 series as anything else than an instance of metameric seg- 
 mentation, comparable to the segmentation of the ringed 
 worms and Arthropods. The only other example which 
 we have of this metamerism in the Mollusca is presented 
 by the Chitons. There we find not two pairs of ctenidia 
 merely, but sixteen pairs (in some species more) accom- 
 
 v.br 
 
 Fio. 103. View of the postero-ventral surface of a male Sepia, obtained by 
 cutting longitudinally the firm mantle-skirt and drawing the divided halves 
 apart. This figure is strictly comparable with fig. 101. C, the head ; J, the 
 mid-foot or siphon, which has been cut open so as to display the valve i ; R, 
 the glandular tissue of the left nephridium or renal-sac, which has been cut 
 open (see fig. 108) ; P, P, the lateral fins of the mantle-skirt ; Br, the single 
 pair of branchije (ctenidia) ; a, the anus, immediately below it is the open- 
 ing of the ink-bag ; c, cartilaginous socket in the siphon to receive d, the 
 cartilaginous knob of the mantle-skirt, the two constituting the "pallial 
 hinge apparatus " characteristic of Decapoda, not found in Octopoda ; g, the 
 azygos genital papilla and aperture ; 'i, valve of the siphon (possibly the rudi- 
 mentary hind-foot) ; m, muscular band connected with the fore-foot and 
 mid-foot (siphon) and identical with the muscular mass k in fig. 91 ; r, renal 
 papillae, carrying the apertures of the nephridia; v.br, branchial efferent 
 blood-vessel ; v .br f , bulbous enlargements of the branchial blood-vessels (see 
 figs. 104, 108) ; , ink-bag. (From Gegenbaur.) 
 
 panied by a similar metamerism of the dorsal integument, 
 which carries eight shells. In Chiton the nephridia are 
 not affected by the metamerism as they are in Nautilus. 
 It is impossible on the present occasion to discuss in the 
 way which their importance demands the significance of 
 these two instances among Mollusca of incomplete or partial 
 metamerism ; but it would be wrong to pass them by with- 
 out insisting upon the great importance which the occur- 
 rence of these isolated instances of metameric segmentation 
 in a group of otherwise unsegmented organisms possesses, 
 and the light which they may be made to throw upon the 
 nature of metameric segmentation in general. 
 
 The foot and head of Nautilus are in the adult inex- 
 tricably grown together, the eye being the only part belong- 
 ing primarily to the head which projects from the all- 
 embracing foot. The fore-foot or front portion of the foot 
 
 in Nautilus has the form of a number of lobes carrying 
 tentacles and completely surrounding the mouth (figs. 88, 
 89, 91). The mid-foot is a broad median muscular process 
 which exhibits in the most interesting manner a curling in 
 of its margins so as to form an incomplete siphon (fig. 
 101), a condition which is completed and rendered per- 
 manent in the tubular funnel, which is the form presented 
 by the corresponding part of Dibranchiata (fig. 96). The 
 hind-foot possibly is represented by the valvular fold on the 
 surface of the siphon-like mid-foot. In the Pteropoda the 
 wing-like swimming lobes (epipodia or pteropodia) corre- 
 spond to the two halves of the siphon, and are much the 
 largest element of the foot. The fore-foot surrounding 
 the head is often quite small, but in Clione and Pneumo- 
 dermon carries lobes and suckers. A hind-foot is in Ptero- 
 poda often distinctly present ; it is open to doubt as to 
 whether the corresponding region of the foot in Siphono- 
 poda is developed at all. 
 
 The lobes of the fore-foot of Nautilus and of the other 
 Siphonopoda require further description. It has been 
 doubted whether these lobes were rightly referred (by 
 Huxley) to the fore-foot, and it has been maintained by some 
 zoologists (Grenadier, Jhering) that they are truly processes 
 of the head. It appears to the present writer to be im- 
 possible to doubt that the lobes in question are the fore- 
 portion of the foot when their development is examined 
 (see fig. 121, and especially fig. 72**), further, when the fact 
 is considered that they are innervated by the pedal ganglion, 
 and, lastly, when the comparison of such a Siphonopod as 
 Sepia is made with such a Pteropod as Pneumodermon in its 
 larval (fig. 84) as well as in its adult condition (fig. 85). The 
 
 FIG. 104. Circulatory and excretory organs of Sepia (from Gegenbaur, after 
 John Hunter), br, branchiae (ctenidia) ; c, ventricle of the heart ; a, anterior 
 artery (aorta) ; a', posterior artery ; v, the right and left auricles (enlarge- 
 ments of the efferent branchial veins) ; v', efferent branchial vein on the free 
 face of the gill-plume ; v.c, vena cava ; vi, vc', advehent branchial vessels 
 (branches of the vena cava, see fig. 108) ; vc", abdominal veins ; x, branchial 
 hearts and appendages ; re, e, glandular substance of the nephridia developed 
 on the wall of the great veins on their way to the gills. The arrows indicate 
 the direction of the blood-current. 
 
 larval Pneumodermon shows clearly that the sucker-bearing 
 processes of that Mollusc are originally far removed from 
 the head and close in position to the pteropodial lobes of 
 the foot. By differential growth they gradually embrace 
 and obliterate the head, as do the similar sucker-bearing 
 processes of Sepia. In both cases the sucker-bearing pro- 
 cesses are "fore-foot." The fore-foot of Nautilus completely 
 surrounds the buccal cone (fig. 88, e), so as to present an 
 appearance with its expanded tentacles similar to that of the 
 disc of a sea-anemone (Actinia). No figure has hitherto 
 been published exhibiting this circum-oral disc with its 
 tentacles in natural position as when the animal is alive and 
 swimming, the small figure of Valenciennes being deficient 
 in detail. All the published figures represent the actual 
 appearance of the contracted spirit-specimens. Mr A. G. 
 
MOLLUSCA 
 
 137 
 
 Bourne, B.Sc., of University College, has prepared from 
 actual specimens the drawings of this part in the male and 
 female Xautilus reproduced in fig. 88, and has restored the 
 parts to their natural form when expanded. The drawings 
 show very strikingly the difference between male and female. 
 In the female (lower figure), we observe in the centre of 
 the disc the buccal cone e carrying the beak-like pair of 
 jaws which project from the finely papillate buccal membrane. 
 Three tentaculiferous lobes of the fore-foot are in immediate 
 contact with this buccal cone ; they are the right and left 
 (e, c) inner lobes, as we propose to call them, and the in- 
 ferior inner lobe (<t), called inferior because it really lies 
 ventralwards of the mouth. This inner inferior lobe is 
 clearly a double one, representing a right and left inner 
 inferior lobe fused into one. A lamellated organ on its sur- 
 face, probably olfactory in function (),marks the separation 
 of the constituent halves of this double lobe. Each half 
 carries a group of fourteen tentacles. The right and the 
 left inner lobes (c, c) each carry twelve tentacles. Ex- 
 
 FIG. 105. Diagram to show the relations of the heart in the Kollnsca (from 
 Gegenbaur). A. Part of the dorsal vascular tronk and transverse trunks of 
 a worm. B. Ventricle and auricles of Nautilus. C. Of a Lamellibranch, of 
 Chiton, or of Loligo. D. Of Octopus. E. Of a Gastropod, a, auricle ; r, 
 ventricle ; of, arteria cephalica (aorta) ; ai, arteria abdominalis. The arrows 
 show the direction of the blood-current. 
 
 ternal to these three lobes the muscular substance of the 
 mouth-embracing foot is raised into a wide ring, which 
 becomes especially thick and large in the dorsal region 
 where it is notably modified in form, offering a concavity 
 into which the coil of the shell is received, and furnish- 
 ing a protective roof to the retracted mass of tentacles. 
 This part of the external annular lobe of the fore-foot is 
 called the "hood" (figs. 90, 91, m.). The median antero- 
 posterior line traversing this hood exactly corresponds to 
 the line of concrescence of the two halves of the fore-foot, 
 which primitively grew forward one on each side of the 
 head, and finally fused together along this line in front of 
 the mouth. The tentacles carried by the great annular 
 lobe are nineteen on each side, thirty-eight in alL They 
 are somewhat larger than the tentacles carried on the three 
 inner lobes. The dorsalmost pair of tentacles (marked 
 g in fig. 88) are the only ones which actually belong to 
 that part of the disc which forms the great dorsal hood m. 
 The hood is, in fact, to a large extent formed by the enlarged 
 sheaths of these two tentacles. In the Ammonites (fossil 
 Tetrabranchiata allied to Nautilus) the dorsal surface of 
 the hood secreted a shelly plate in two pieces, known to 
 palaeontologists as Trigonellites and Aptychus. Possibly, 
 however, this double plate was carried on the surface of 
 the bilobed nidamental gland with the form and sculptur- 
 ing of which, in Xautilus, it closely agrees. All the ten- 
 tacles of the circum-oral disc are set in remarkable tubular 
 sheaths, into which they can be drawn. The sheaths of 
 some of those belonging to the external or annular lobe are 
 seen in fig. 91, marked n. The sheaths are muscular as 
 well as the tentacles, and are simply tubes from the base 
 of which the solid tentacle grows. The functional signifi- 
 cance of this sheathing arrangement is as obscure as its 
 morphological origin. With reference to the latter, it 
 appears highly probable that the tubular sheath represents 
 the cup of a sucker such as is found on the fore-foot of the 
 
 Dibranchiata. In any case, it seems to the writer impos- 
 sible to doubt that each tentacle, and its sheath on a lobe 
 of the circum-oral disc of Nautilus, corresponds to a sucker 
 on such a lobe of a Dibranchiate. Keferstein follows Owen 
 in strongly opposing this identification, and in regarding 
 such tentacle as the equivalent of a whole lobe or arm of a 
 Decapod or Octopod Dibraneh. We find in the details of 
 these structures, especially in the facts concerning the 
 hectocotylus and spadix, the most conclusive reasons for 
 dissenting from Owen's view. We have so far enumer- 
 ated in the female Xautilus ninety tentacles. Four more 
 remain which have a very peculiar position, and almost 
 lead to the suggestion that the eye itself is a modified 
 tentacle. These remaining tentacles are placed one above 
 (before) and one below (behind) each eye, and bring up 
 the total to ninety-four (fig. 91, r, v). They must be con- 
 sidered as also belonging to the fore-foot which thus sur- 
 rounds the eye. 
 
 In the adult male Xautilus we find the following im- 
 portant differences in the tentaculiferous disc as compared 
 with the female (see upper drawing in fig. 88). The 
 inner inferior lobe is rudimentary, and carries no tentacles. 
 It is represented by three groups of lamellae (d), which are 
 not fully exposed in the drawing. The right and left inner 
 lobes are subdivided each into two portions. The right 
 shows a larger portion carrying eight tentacles, and smaller 
 detached groups (q) of four tentacles, of which three have 
 their sheaths united whilst one stands alone. These four 
 tentacles may be called the " anti-spadix." The left inner 
 lobe shows a similar larger portion carrying eight tentacles, 
 and a curious conical body in front of it corresponding to 
 the anti-spadix. This is the " spadix " of Van der Hoeven 
 (36). It carries no tentacles, but is terminated by imbri- 
 cated lamellae. These lamellae appear to represent the four 
 tentacles of the anti-spadix of the right internal lobe, and 
 are generally regarded as corresponding to that modification 
 of the sucker-bearing arms of male Dibranchiate Siphono- 
 pods to which the name " hectocotylus " is applied. The 
 spadix is in fact the hectocotylized portion of the fore- 
 foot of the male Xautilus. The hectocotylized arm or lobe 
 of male Dibranchiata is connected with the process of copu- 
 lation, and in the male Xautilus the spadix has probably a 
 similar significance, though it is not possible to suggest 
 how it acts in this relation. It is important to observe 
 that the modification of the fore-foot in the male as com- 
 pared with the female Xautilus is not confined to the 
 existence of the spadix. The anti-spadix and the reduction 
 of the inner inferior lobe are also male peculiarities. The 
 external annular lobe in the male does not differ from that 
 of the female ; it carries nineteen tentacles on each side. 
 The four ophthalmic tentacles are also present. Thus in 
 the male Xautilus we find altogether sixty-two tentacles, 
 the thirty-two additional tentacles of the female being repre- 
 sented by lamelliform structures. 
 
 If we now compare the fore-foot of the Dibranchiata with 
 that of Xautilus, we find in the first place a more simple 
 arrangement of its lobes, which are either four or five pairs 
 of tapering processes (called " arms ") arranged in a series 
 around the buccal cone, and a substitution of suckers for 
 tentacles on the surface of these lobes (figs. 92, 95, 96). 
 The most dorsally-placed pair of arms, corresponding to the 
 two sides of the hood of Xautilus, are in reality the most 
 anterior (see fig. 75, (6) ), and are termed the first pair. In 
 the Octopoda there are four pairs of these arms (figs. 94, 
 95), in the Decapoda five pairs, of which the fourth is 
 greatly elongated (figs. 92, 93). In Sepia and other Deca- 
 poda (not all) each of these long arms is withdrawn into a 
 pouch beside the head, and is only ejected for the purpose 
 of prehension. The figures referred to show some of the 
 variations in form which these arms may assume. In the 
 
 S 
 
138 
 
 MOLLUSCA 
 
 Octopoda they are not unfrequently connected by a web, 
 and form an efficient swimming-bell. The suckers are placed 
 on the ad-oral surface of the arms, and may be in one, 
 two, or four rows, and very numerous. In place of suckers 
 in some genera we find on certain arms or parts of the 
 arms horny hooks ; in other cases a hook rises from the 
 centre of each sucker. The hooks on the long arms of 
 Onychoteuthis are drawn in fig. 97. The fore-foot, with 
 its apparatus of suckers and hooks, is in the Dibranchiata 
 essentially a prehensile apparatus, though the whole series 
 of arms in the Octopoda serve as swimming organs, and in 
 many (e.g., the Common Octopus or Poulp) the sucker- 
 bearing surface is used as a crawling organ. 
 
 In the males of the Dibranchiata one of the arms is 
 more or less modified in connexion with the reproductive 
 function, and is called the " hectocotylized arm." This 
 name is derived from the condition assumed by the arm 
 in those cases in which its modification is carried out to 
 the greatest extent. These cases are those of the Octo- 
 pods Argonauta argo and Parasira catenulata (fig. 96). 
 In the males of these the third arm (on the left side in 
 Argonauta, on the right side in Parasira) is found before 
 the breeding season to be represented by a globular sac of 
 integument. This sac bursts, and from it issues an arm 
 larger than its neighbours, having a small sac at its extremity 
 in Parasira (fig. 96, x), from which subsequently a long 
 filament issues. Before copulation the male charges this 
 arm with the spermatophores or packets of spermatozoa 
 removed from its generative orifice beneath the mantle-skirt, 
 and during coitus the arm becomes detached and is left- 
 adhering to the female by means of its suckers. A new arm 
 is formed at the cicatrix before the next breeding season. 
 The female, being much larger than the male, swims away 
 with the detached arm lodged beneath her mantle-skirt. 
 There, in a way which is not understood, the fertilization 
 of the eggs is effected. Specimens of the female Parasira 
 with the detached arm adherent were examined by Cuvier, 
 who mistook the arm for a parasitic worm and gave to it 
 the name Hectocotylus. Accordingly, the correspondingly 
 modified arms of other Siphonopoda are said to be hecto- 
 cotylized. Steenstrup has determined the hectocotylized 
 condition of one or other of the arms in a number of male 
 Dibranchs as follows : in all, excepting Argonauta and 
 Parasira, the modification of the arm is slight, consisting in 
 a small enlargement of part or the whole of the arm, and 
 the obliteration of some of its suckers, as shown in fig. 95, 
 A, B ; in Octopus and Eledone the third right arm is 
 hectocotylized ; in Kossia the first left arm is hectocotylized 
 along its whole length, and the first right arm also in the 
 middle only ; in Sepiola only the first left arm along its 
 whole length ; in Sepia it is the fourth left arm which is 
 modified, and at its base only ; in Sepioteuthis, the same at 
 its apex; in Loligo, the same also at its apex; in Loliolus, 
 the same along its whole length ; in Ommastrephes, 
 Onychoteuthis, and Loligopsis no hectocotylized arm has 
 hitherto been observed. 
 
 In the females of several Dibranchs (Sepia, &c.) the 
 packets of spermatozoa or spermatophores received from 
 the male have been observed adhering to the smaller arms. 
 How they are passed in this case by the female to the ova 
 in order to fertilize them is unknown. 
 
 Musculature, Fins, and Cartilaginous Skeleton. Without 
 entering into a detailed account of the musculature of 
 Nautilus, we may point out that the great muscular masses 
 of the fore-foot and of the mid-foot (siphon) are ultimately 
 traceable to a large transverse mass of muscular tissue, 
 the ends of which are visible through the integument on 
 the right and left surfaces of the body dorsal of the 
 free flap of the mantle-skirt (fig. 89, I, I, and fig. 91, k). 
 These muscular areas have a certain adhesion to the shell, 
 
 and serve both to hold the animal in its shell and as the 
 fixed supports for the various movements of the tentaculi- 
 ferous lobes and the siphon. They are to be identified 
 with the ring-like area of adhesion by which the foot-muscle 
 of the Limpet is attached to the shell of that animal (see 
 fig. 27). In the Dibranchs a similar origin of the muscular 
 masses of the fore-foot and mid-foot from the sides of the 
 shell modified, as this is, in position and relations can be 
 traced. 
 
 In Nautilus there are no fin-like expansions of the integu- 
 ment, whereas such occur in the Decapod Dibranchs along 
 the sides of the visceral hump (figs. 92, 93). As an excep- 
 tion among Octopoda lateral fins occur in Pinnoctopus (fig. 
 94, A), and in Cirrhoteuthis (fig. 94, D). In the Ptero- 
 podous division of the Cephalopoda such fin-like expansions 
 of the dorsal integument do not occur, which is to be con- 
 nected with the fact that another region, the mid-foot, which 
 in Siphonopods is converted into a siphon, is in them 
 expanded as a pair of fins. 
 
 In Nautilus there is a curious plate-like expansion of 
 integument in the mid-dorsal region just behind the hood, 
 lying between that structure and the portion of mantle- 
 skirt which is reflected over the shell. This is shown in 
 fig. 90, b. If we trace out the margin of this plate we 
 find that it becomes continuous on each side with the 
 sides of the siphon or mid-foot. In Sepia and other Deca- 
 pods (not in Octopods) a closely similar plate exists in an 
 exactly corresponding position (see b in figs. 110, 111). In 
 Sepia a cartilaginous development occurs here immediately 
 below the integument forming the so-called " nuchal plate," 
 drawn in fig. 116, D. The morphological significance of 
 this nuchal lamella, as seen both in Nautilus and in Sepia, 
 is not obvious. Cartilage having the structure shown in 
 fig. 117 occurs in various regions of the body of Siphono- 
 poda. In all Glossophorous Mollusca the lingual apparatus 
 is supported by internal skeletal pieces, having the char- 
 acter of cartilage ; but in the Siphonopodous Cephalopoda 
 such cartilage has a wider range. 
 
 In Nautilus a large H-shaped piece of cartilage is found 
 forming the axis of the mid-foot or siphon (fig. 116, A, 
 B). Its hinder part extends up into the head and supports 
 the peri-oesophageal nerve-mass (a), whilst its two anterior 
 rami extend into the tongue-like siphon. In Sepia, and 
 Dibranchs generally, the cartilage takes a different form, 
 as shown in fig. 116, C. The processes of this cartilage 
 cannot be identified in any way with those of the capito- 
 pedal cartilage of Nautilus. The lower larger portion of 
 this cartilage in Sepia is called the cephalic cartilage, and 
 forms a complete ring round the oesophagus ; it completely 
 invests also the ganglionic nerve-collar, so that all the 
 nerves from the latter have to pass through foramina in 
 the cartilage. The outer angles of this cartilage spread 
 out on each side so as to form a cup-like receptacle for the 
 eyes. The two processes springing right and left from this 
 large cartilage in the median line (fig. 116, C) are the 
 " prse-orbital cartilages;" in front of these, again, there is 
 seen a piece like an inverted T, which forms a support to 
 the base of the "arms" of the fore-foot, and is the "basi- 
 brachial " cartilage. The Decapod Dibranchs have, further, 
 the " nuchal cartilage " already mentioned, and in Sepia, a 
 thin plate-like " sub-ostracal " or (so-called) dorsal cartilage, 
 the anterior end of which rests on and fits into the concave 
 nuchal cartilage. In Octopoda there is no nuchal cartilage, 
 but two band-like " dorsal cartilages." In Decapods there 
 are also two cartilaginous sockets on the sides of the funnel 
 " siphon-hinge cartilages " into which fleshy knobs of 
 the mantle-skirt are loosely fitted. In Sepia, along the 
 whole base-line of each lateral fin of the mantle (fig. 92), 
 is a " basi-pterygial cartilage." It is worthy of remark that 
 we have, thus developed, in Dibranch Siphonopods a more 
 
MOLLUSCA 
 
 139 
 
 complete internal cartilaginous skeleton than is to be found 
 in some of the lower Vertebrates. There are other instances 
 of cartilaginous endo-skeleton in groups other than the 
 Vertebrata. Thus in some capito-branchiate Chaetopods 
 cartilage forms a skeletal support for the gill-plumes, whilst 
 in the Arachnids (Mygale, Scorpio) and in Lunulus a large 
 internal cartilaginous plate the ento-sternite is devel- 
 oped as a support for a large series of muscles. 
 
 Alimentary Tract. The buccal cone of Nautilus is ter- 
 minated by a villous margin (buccal membrane) surround- 
 ing the pair of beak-like jaws. These are very strong and 
 dense in Nautilus, being calcified. Fossilized beaks of Tetra- 
 branchiata are known under the name of Rhyncholites. 
 In Dibranchs the beaks are horny, but similar in shape to 
 those of Nautilus. They resemble in general those of a 
 parrot, the lower beak being the 
 larger, and overlapping the upper or 
 dorsal beak. The lingual ribbon and 
 odontophoral apparatus has the struc- 
 ture which is typical for Glosso- 
 phorous Mollusca. In fig. 107, A is 
 represented a single row of teeth 
 from the lingual ribbon of Nautilus, 
 and in fig. 107, B, C, of other Si- 
 phonopoda. 
 
 In Nautilus a long and wide crop 
 or dilated oesophagus (cr, fig. 110) 
 passes from the muscular buccal mass, 
 and at the apex of the visceral hump 
 passes into a highly muscular stom- 
 ach, resembling the gizzard of a bird 
 (gizz, fig. 110). A nearly straight 
 intestine passes from the muscular 
 stomach to the anus, near which it 
 develops a small cascum. In other 
 Siphonopods the cesophagus is usually FIG. m Aiimimtery canal 
 narrower (fig. 106, ), and the mus- 
 
 Clllar Stomach more Capacious (fig. 
 
 106, v), whilst a very important 
 
 feature in the alimentary tract is 
 
 formed by the caecum. In all but 
 
 Nautilus the caecum lies near the 
 
 stomach, and may be very capacious 
 
 much larger than the stomach in Loligo vulgaris or 
 
 elongated into a spiral coil, as in fig. 106, e. The simple 
 
 is omitted. Of, oesophagus ; 
 r.the stomach opened long- 
 itudinally; z, probe passed 
 through the pylorus ; c, 
 commencement of the cse- 
 cum ; , its spiral portion ; 
 i, intestine ; a, ink-bag ; b, 
 itsopening into the rectum. 
 
 Fio. 107. Lingual dentition of Siphonopoda. A. A single row of lingual teeth 
 of A'au/Uiis pompilius (after Keferstein). B. Two rows of lingual teeth of 
 Sepia oflcinalis (after Troschel). C. Lingual teeth of Ekdonc tirrhosa (after 
 Loven). 
 
 U-shaped flexure of the alimentary tract as seen in fig. 
 106, and in fig. 110, is the only important one which it 
 exhibits in the Cephalopoda, the Pteropoda (except the 
 Limacinida) agreeing with the Siphonopoda in this sim- 
 
 plicity in consequence of their visceral hump being un- 
 twisted. The acini of the large liver of Nautilus are 
 compacted into a solid reddish -brown mass by a firm 
 membrane, as also is the case in the Dibranchiata. 
 The liver has four paired lobes in Nautilus, which open 
 by two bile-ducts into the alimentary canal at the com- 
 mencement of the intestine. The bile-ducts unite before 
 entering the intestine. In Dibranchiata the two large 
 lobes of the liver are placed antero-dorsally (beneath 
 the shell in Decapoda), and the bile-ducts open into the 
 caecum. Upon the bile-ducts in Dibranchiata are deve- 
 loped yellowish glandular diverticula, which are known 
 as " pancreas," though neither physiologically nor morpho- 
 logically is there any ground for considering either the so- 
 called liver or the so-called pancreas as strictly equivalent 
 to the glands so denominated in the Vertebrata. In Nauti- 
 lus the equivalents of the pancreatic diverticula of the 
 Dibranchs can be traced upon the relatively shorter bile- 
 ducts. 
 
 Salivary Glands are not developed in Nautilus unless a 
 pair of glandular masses lying on the buccal cavity are to 
 be considered as such. In the Dibranchs, on the contrary, 
 one (Sepia, Loligo) or two pairs of large salivary glands 
 are present, an anterior and a posterior (Octopus, Eledone, 
 Onychoteuthis). Each pair of salivary glands has its 
 paired ducts united to form a single duct, which runs 
 forward from the glands and opens into the buccal cavity 
 
 a.r 
 
 FIG. 108. Diagram of the nephridial sacs, and the veins which run through 
 them, in Sepia officinalis (after VigeliusX The nephridial sacs are supposed 
 to have their upper walls removed, r.c, vena cava ; r.d.r.c, right descending 
 branch of the same ; T.S.V.C, left descending branch of the same ; t.b.a., vein 
 from the ink-bag ; r.m, mesenteric vein ; r.g, genital vein ; r.o.d, right 
 abdominal vein ; r.o-s, left abdominal vein ; r.p.d, right pallia] vein ; v.p.s, 
 left pallia! vein ; c.6, branchial heart ; x, appendage of the same ; c.r, capsule 
 of the branchial heart ; np, external aperture of the right nephridial sac ; y, 
 reno-pericardial orifice placing the left renal sac or nephridium in communi- 
 cation with the viscero-pericardial sac, the course of which below the nephri- 
 dial sac is indicated by dotted lines ; y', the similar orifice of the right side ; 
 o.r, glandular renal outgrowths ; w.fc, viscero-pericardial sac (dotted outline). 
 
 near the radula. The anterior pair of glands when present 
 lie in the head near the buccal mass, the posterior pair lie 
 much farther back beneath the liver, at the sides of the 
 oesophagus. It is the posterior pair which alone are pre- 
 sent in Sepia and Loligo. The ink-bag is to be considered 
 as an appendage of the rectum. It is not developed in 
 Nautilus, nor in the Pteropoda ; in all Dibranchiata (even in 
 the fossil Belemnites) it is present (fig. 106, a ; fig. 103, t), 
 and has been observed to develop as a diverticulum of the 
 rectum, with spirally plaited walls which very early secrete 
 a black pigment. The spiral plaitings of the walls diminish 
 
140 
 
 MOLLUSCA 
 
 in relative size as the volume of the sac increases. Its 
 outer surface acquires a metallic iridescence similar to that 
 of the integuments of many fishes. The opening of the 
 ink-sac is in the adult sometimes distinct from but near to 
 
 vent 
 
 FIG. 109. Diagram to show the relations of the four nephridial sacs, the viscero- 
 pericardial sac, and the heart and large vessels in Nautilus (drawn by A. G. 
 Bourne), neph, neph, on the right side point to the two ncphridia of that 
 side (the two of the opposite side are not lettered), each is seen to have an 
 independent aperture ; x is the viscero-pericardial sac, the dotted line indicat- 
 ing its backward extension ; visc.per.apert marks an arrow introduced into 
 the right aperture of the viscero-pericardial sac ; r.e., r.e., point to the 
 glandular enlarged walls of the advelient branchial vessels, two small 
 glandular bodies of the kind are seen to project into each nephridial sac, 
 whilst a larger body of the same kind depends from each of the four branchial 
 advehent vessels into the viscero-pericardial sac ; v.c., vena cava ; vent, 
 ventricle of the heart; oo., cephalic aorta (the small abdominal aorta not 
 drawn) ; a.b.v, advehent branchial vessel ; e.v.b., efferent branchial vessel. 
 
 the anus (Sepia) ; in other cases it opens into the rectum 
 near the anus. The ink-bag of Dibranch Siphonopoda is 
 possibly to be identified with the adrectal (purpuriparous) 
 gland of some Gastropoda. 
 
 Coelom, Blood-vascular System, and Excretory Organs. 
 Nautilus and the other Siphonopoda conform to the 
 
 , Vxn, 
 n a * 
 
 FIG. 110. Diagram representing a vertical approximately median antoo 
 posterior section of Nautilus pompilius (from a drawing by A. G. Bourne). 
 The parts which are quite black are the cut muscular surfaces of the foot and 
 buccal mass, a, the shell ; 6, the nuchal plate identical with the nuchal 
 cartilage of Sepia (see fig. 90, b) ; c, the integument covering the visceral 
 hump ; ri, the mantle flap or skirt in the dorsal region where it rests against 
 the coil of the shell ; e, the inferior margin of the mantle-skirt resting on the 
 lip of the shell represented by the dotted line ; /, the pallial chamber with 
 two of the four gills ; g, the vertically cut median portion of the mid-foot 
 (siphon); h, the capito-pedal cartilage (see fig. 116); i, the valve of the 
 siphon ; I, the siphuncular pedicle (cut short) ; m, the hood or dorsal enlarge- 
 ment of the annular lobe of the fore-foot ; , tentacles of the annular lobe ; 
 p, tentacles of the inner inferior lobe ; q, buccal membrane ; r, upper jaw or 
 beak ; s, lower jaw or beak ; (, lingual ribbon ; x, the viscero-pericardial sac ; 
 n.c, nerve-collar ; oe, oesophagus ; cr, crop ; fjizz, gizzard ; int, intestine ; an, 
 anus ; ni, nidamental gland ; nept, aperture of a nephridial sac ; r.e, renal 
 glandular masses on the walls of the afferent branchial veins (see fig. 109); 
 a.b.v., afferent branchial vessel ; e.b.v, efferent branchial vessel ; vt, ventricle 
 of the heart. 
 
 general Molluscan characters in regard to these organs. 
 Whilst the general body-cavity or coelom forms a lacunar 
 
 blood-system or series of narrow spaces, connected with 
 the trunks of a well-developed vascular system, that part 
 of the original ccelom surrounding the heart and known 
 as the Molluscan pericardium becomes shut off from this 
 general blood-lymph system, and communicates, directly in 
 Nautilus, in the rest through the nephridia, with the exte- 
 rior. In the Siphonopoda this specialized pericardial cavity 
 is particularly large, and has been recognized as distinct 
 from the blood-carrying spaces, even by anatomists who 
 have not considered the pericardial space of other Mollusca 
 to be thus isolated. The enlarged pericardium, which may 
 even take the form of a pair of sacs, has been variously 
 named, but is best known as the viscero-pericardial sac or 
 chamber. In Nautilus this sac occupies the whole of the 
 postero-dorsal surface and a part of the antero-dorsal (see 
 fig. 110, x), investing the genital and other viscera which 
 lie below it, and having the ventricle of the heart sus- 
 pended in it. Certain membranes forming incomplete 
 septa, and a curious muscular band the pallio-cardiac 
 band traverse the sac. The four branchial advehent veins, 
 which in traversing the walls of the four nephridial sacs 
 give off, as it were, glandular diverticula into those sacs, 
 also give off at the same points four much larger glandular 
 
 e.t. 
 
 aperk. 
 
 Fid. 111. Diagram representing a vertical approximately median antero- 
 posterior section of Septa officiualis (from a drawing by A. G. Bourne). The 
 lettering corresponds with that of fig. 110, with which this drawing is intended 
 to be compared, a, shell (here enclosed by a growth of the mantle) ; b, the 
 nuchal plate (here a cartilage); c (the reference line should be continued 
 through the black area representing the shell to the outline below it), the 
 integument covering the visceral hump; d, the reflected portion of the 
 mantle-skirt forming the sac which encloses the shell ; e, the inferior margin 
 of the mantle-skirt (mouth of the pallial chamber) ; /, the pallial chamber ; 
 g, the vertically cut median portion of the mid-foot (siphon) ; i, the valve of 
 the siphon ; m, the two upper lobes of the fore-foot ; n, the long prehensile 
 arms of the same ; o, the fifth or lowermost lobe of the fore-foot ; p, the third 
 lobe of the fore-foot ; g, the buccal membrane ; v, the upper beak or jaw ; s, 
 the lower beak or jaw ; (, the lingual ribbon ; x, the viscero-pericardial sac ; 
 n.c, the nerve-collar ; cr., the crop ; gizz., the gizzard ; an, the anus ; c.t., the 
 left ctenidium or gill-plume ; vent, ventricle of the heart ; a.b.v., afferent 
 branchial vessel ; e.b.v, efferent branchial vessel ; re, renal glandular mass ; 
 n.n.a, left nephridial aperture; visc.per.apert., viscero-pericardial aperture 
 (see fig. 108) ; br.b., branchial heart ; app., appendage of the same ; i.s., ink- 
 bag. 
 
 masses, which hang freely into the viscero-pericardial 
 chamber (fig. 109, r.e). In Nautilus the viscero-pericardial 
 sac opens to the exterior directly by a pair of apertures, one 
 placed close to the right and one close to the left posterior 
 nephridial aperture (fig. 101, viscper.). This direct opening 
 of the pericardial sac to the exterior is an exception to what 
 occurs in all other Mollusca. In all other Molluscs the 
 pericardial sac opens into the nephridia, and through them 
 or the one nephridium to the exterior. In Nautilus there 
 is no opening from the viscero-pericardial sac into the 
 nephridia. Therefore the external pore of the viscero-peri- 
 cardial sac may possibly be regarded as a shifting of the 
 reno-pericardial orifice from the actual wall of the nephridial 
 sac to a position alongside of its orifice. Parallel cases 
 of such shifting are seen in the varying position of the 
 orifice of the ink-bag in Dibranchiata, and in the orifice 
 of the genital ducts of Mollusca, which in some few cases 
 (e.g., Spondylus) open into the nephridia, whilst in other 
 cases they open close by the side of the nephridia on the 
 surface of the body. The viscero-pericardial sac of the 
 
MOLLUSCA 
 
 141 
 
 Dibranchs is very large also, and extends into the dorsal 
 region. It varies in shape that is to say, in the extensions 
 of its area right and left between the various viscera in 
 different genera, but in the Decapods is largest. In an ex- 
 tension of this chamber is placed the ovary of Sepia, whilst 
 the ventricle of the heart and the branchial hearts and their 
 appendages also lie in it. It is probable that water is 
 drawn into this chamber through the nephridia, since sand 
 and other foreign matters are found in it. In all it opens 
 into the pair of nephridial sacs by an orifice on the wall of 
 each, not far from the external orifice (fig. 108, y, y'). 
 There does not seem any room for doubting that each orifice 
 corresponds to the reno-pericardial orifice which we have 
 seen in the Gastropoda, and shall find again in the Lamelli- 
 branchia. The single tube-like nephridium and the peri- 
 cardium of the Pteropoda also communicate by an aperture. 
 
 The circulatory organs, blood-vessels, and blood of Nauti- 
 lus do not differ greatly from those of Gastropoda. The 
 ventricle of the heart is a four-cornered body, receiving a 
 dilated branchial efferent vessel (auricle) at each corner 
 (fig. 109). It gives off a cephalic aorta anteriorly, and 
 a smaller abdominal aorta posteriorly. The diagram, fig. 
 105, serves to show how this simple form of heart is related 
 to the dorsal vessel of a worm or of an Arthropod, and how 
 by a simple flexure 'of the ventricle (D) and a subsequent 
 suppression of one auricle, following on the suppression of 
 one branchia, one may obtain the form of heart charac- 
 teristic of the Anisopleurous Gastropoda (excepting the 
 Zygobranchia). The flexed condition of the heart is seen 
 in Octopus, and is to some extent approached by Nautilus, 
 the median vessels not presenting that perfect parallelism 
 which is shown in the figure (B). The most remarkable 
 feature presented by the heart of Nautilus is the possession 
 of four instead of two auricles, a feature which is simply 
 related to the metamerism of the branchiae. By the left 
 side of the heart of Nautilus, attached to it by a membrane, 
 and hanging loosely in the viscero-pericardial chamber, is 
 the pyriform sac of Owen. This has recently been shown 
 to be the rudimentary left oviduct or sperm-duct, as the 
 case may be (Lankester and Bourne, 37), the functional 
 right ovi-sac and its duct being attached by a membrane 
 to the opposite side of the heart. 
 
 The cephalic and abdominal aorta? of Nautilus appear, 
 after running to the anterior and posterior extremes of the 
 animal respectively, to open into sinus-like spaces surround- 
 ing the viscera, muscular masses, &c. These spaces are 
 not large, but confined and shallow. Capillaries are stated 
 to occur in the integument. In the Dibranchs the arterial 
 system is very much more complete ; it appears in some 
 cases to end in irregular lacunae or sinuses, in other cases 
 in true capillaries which lead on into veins. An investiga- 
 tion of these capillaries in the light of modern histological 
 knowledge is much needed. From the sinuses and capil- 
 laries the veins take origin, collecting into a large median 
 trunk (the vena cava), which in the Dibranchs as well as in 
 Nautilus has a ventral (postero-ventral) position, and runs 
 parallel to the long axis of the animal. In Nautilus this 
 vena cava gives off at the level of the gills four branchial 
 advehent veins (fig. 109, >.<.), which pass into the four 
 gills without dilating. In the Dibranchs at a similar posi- 
 tion the vena cava gives off a right and a left branchial 
 advehent vein (fig. 108, r.s.v.c, r.d.v.c), each of which, 
 traversing the wall of the corresponding nephridial sac and 
 receiving additional factors (fig. 108, v.g, v.p.d, v.a.d, v.b.a), 
 dilates at the base of the corresponding branchial plume, 
 forming there a pulsating sac the branchial heart (fig. 104, 
 x; and fig 108, c.b). Attached to each branchial heart is a 
 curious glandular body, which may possibly be related to 
 the larger masses (r.e in fig. 109) which depend into the 
 viscero-pericardial cavity from the branchial advehent veins 
 
 of Nautilus. From the dilated branchial heart the bran- 
 chial advehent vessel proceeds, running up the ad-pallial 
 face of the gill-plume (vi, vc, fig. 104). From each gill- 
 plume the blood passes by the branchial efferent vessels 
 (v, fig. 104) to the heart, the two auricles being formed 
 by the dilatation of these vessels (v, v in fig. 104). 
 
 The blood of Siphonopoda contains the usual amoaboid cor- 
 puscles, and a diffused colouring matter the haemocyanin 
 of Fredericque which has been found also in the blood of 
 Helix, and in that of the Arthropods Homarus and Limulus. 
 It is colourless in the oxidized, blue in the deoxidized state, 
 and contains copper as a chemical constituent. 
 
 The nephridial sacs and renal glandular tissue are closely 
 connected with the branchial advehent vessels in Nautilus 
 and in the other Siphonopoda. The arrangement is such 
 as to render the typical relations and form of a nephridium 
 difficult to trace. In accordance with the metamerism of 
 Nautilus already noticed, there are two pairs of nephridia. 
 Each nephridium assumes the form of a sac opening by a 
 pore to the exterior. As is usual in nephridia, a glandular 
 and a non-glandular portion are distinguished in each sac ; 
 these portions, however, are not successive parts of a tube, as 
 happens in other cases, but they are localized areas of the wall 
 of the sac. The glandular renal tissue is, in fact, confined 
 to a tract extending along that part of the sac's wall which 
 immediately invests the great branchial advehent vein. 
 The vein in this region gives off directly from its wall a 
 complete herbage of little venules, which branch and ana- 
 stomose with one another, and are clothed by the glandular 
 epithelium of the nephridial sac. The secretion is accumu- 
 lated in the sac and passed by its aperture to the exterior. 
 Probably the nitrogenous excretory product is very rapidly 
 discharged ; in Nautilus a pink-coloured powder is found 
 accumulated in the nephridial sacs, consisting of calcium 
 phosphate. The presence of this 
 phosphatic calculus by no means 
 proves that such was the sole ex- 
 cretion of the renal glandular tis- 
 sue. In Nautilus a glandular 
 growth like that rising from the 
 wall of the branchial vessel into 
 its corresponding nephridial sac, 
 but larger in size, depends from 
 each branchial advehent vessel into 
 the viscero-pericardial sac, prob- 
 ably identical with the "append- 
 age" of the branchial hearts of 
 Dibranchs. 
 
 The chief difference, other than 
 that of number between the ne- 
 phridia of the Dibranchs and those 
 of Nautilus, is the absence of the 
 accessory growths depending into 
 the viscero-pericardial space just 
 mentioned, and, of more import- 
 ance, the presence in the former of 
 a pore leading from the nephridial 
 sac into the viscero-pericardial sac 
 (y, y' in fig. 108). The external 
 orifices of the nephridia are also 
 more prominent in Dibranchs than 
 in Nautilus, being raised on papillae 
 (np in fig. 108 ; r in fig. 103). In 
 Sepia, according to Vigelius (38), 
 the two nephridia give off each 
 a diverticulum dorsalwards, which 
 unites with its fellows and forms 
 a great median renal chamber, 
 lying between the ventral portions of the nephridia and 
 the viscero-pericardial chamber. In Loligo the fusion 
 
 IG. 112. Nervous system of 
 Nautilus pompilivs (from Ge- 
 genbaur, after OwenX *, *, 
 ganglion-like enlargements on 
 nerves passing from the pedal 
 ganglion to the inner series of 
 tentacles; f, nerves to the ten- 
 tacles of the outer or annular 
 lobe ; 6, pedal ganglion-pair ; 
 a, cerebral ganglion-pair); c, 
 pleuro - visceral ganglionic 
 band (fused pleural and visce- 
 ral ganglion-pairs) ; d, genital 
 ganglion placed on the course 
 of the large visceral nerve, just 
 before it gives off its branchial 
 and its osphradial branches ; 
 m, nerves from the pleural 
 ganglion to the mantle-skirt. 
 
142 
 
 MOLLUSCA 
 
 of the two nephridia to form one sac is still more obvious, 
 since the ventral portions are united. In Octopus the 
 nephridia are quite separate. 
 
 Tegumental pores have not been described in Nautilus, 
 but exist in Dibranchiata, and have been (probably 
 erroneously, but further investigation is needed) supposed 
 to introduce water into the vascular system. A pair of 
 
 CtK 
 
 Fig. 114. 
 
 Fios. 113, 114. Nerve-centres of Octopus. Figure 113 gives a view from the 
 dorsal aspect, figure 114 one from the ventral aspect, hue, the buccal mass ; 
 fed, pedal ganglion ; opt, optic ganglion ; cer, cerebral ganglion ; pi, pleura! 
 ganglion ; vise, visceral ganglion ; ois, oesophagus ; /, foramen in the nerve- 
 mass formed by pedal, pleural, and visceral ganglion-pairs, traversed by a 
 blood-vessel. 
 
 such pores leading into sub-tegumental spaces of consider- 
 
 able area, the nature of which is imperfectly known, exist 
 
 on the back of the head in Philonexis, Tremoctopus, and 
 
 Argonauta. At the base of the arms and mouth four such 
 
 pores are found in Histioteuthis and Ommastrephes, six 
 
 in Sepia, Loligo, Onychoteuthis. Lastly, a pair of such 
 
 pores are found in the 
 
 Decapoda at the base 
 
 of the long arms, lead- 
 
 ing into an extensive 
 
 sub-tegumental pouch 
 
 on each side of the head 
 
 into which the long 
 
 arms can be, and usually 
 
 are, withdrawn. In 
 
 Sepia, Sepiola, and Ros- 
 
 sia the whole arm is 
 
 coiled up in these sacs ; 
 
 in Loligo only a part 
 
 of it is so ; in Histio- 
 
 teuthis, Ommastrephes, 
 
 and Onychoteuthis, the 
 
 sacs are quite small 
 
 and do not admit the 
 
 arms. 
 
 Nervous System. 
 Nautilus, like the other 
 Cephalopoda (e.g., Pneu- 
 modermon, fig. 87 ; 
 Octopus, fig. 113), ex- 
 hibits a great concentra- 
 tion of the typical Mol- _ 
 
 ..* " , Fro. 115. Lateral view of the nervous centres 
 
 lUSCan ganglia, as Shown and nerves of the right side of Octopus vul- 
 
 in fit* 112 The gan- 9 (from a drawing by A. G Bourne). %, 
 
 n . iiw. buccal ganglion; cer., cerebral ganglion; 
 
 glia take On a band-like ptd., pedal ganglion ; pi, pleural, and vise., 
 
 c j r, 4. i-4.ii visceral region of the pleuro-visceral ganglion; 
 lOrm, and are but little 
 
 gang . ste i(^ the right stellate ganglion of the 
 mantle connected by a nerve to the pleural 
 portion ; n.visc., the right visceral nerve ; 
 n.olf., its (probably) olfactory branches; 
 
 n - br " its brauchial tranches. 
 
 differentiated from their 
 Commissures and COn- 
 
 nectives, an archaic 
 condition reminding us of Chiton. The special optic out- 
 growth of the cerebral ganglion, the optic ganglion (fig. 
 112, o), is characteristic of the big-eyed Siphonopoda. 
 The cerebral ganglion-pair (a) lying above the oesophagus 
 
 is connected with two sub-cesophageal ganglion -pairs of 
 band-like form. The anterior of these is the pedal b, b, 
 and supplies the fore-foot with nerves t', t, as also the 
 mid-foot (siphon). The hinder band is the visceral and 
 [>leural pair fused (compare fig. 112 with fig. 87, and 
 especially with the typical Euthyneurous nervous system 
 of Limnseus, fig. 22) ; from its pleural portion nerves pass 
 to the mantle, from its visceral portion nerves to the 
 branchiae and genital ganglion (d in fig. 112), and in 
 immediate connexion with the latter is a nerve to the 
 osphradium or olfactory papilla. No buccal ganglia have 
 been observed in Nautilus, nor has an enteric nervous system 
 been described in this animal, though both attain a special 
 development in the Dibranchiata. The figures (114 and 
 115) representing the nerve-centres of Octopus serve to 
 exhibit the disposition of these parts in the Dibranchiata. 
 The ganglia are more distinctly swollen than in Nautilus. 
 In Octopus an infra-buccal ganglion-pair are present cor- 
 responding to the buccal ganglion-pair of Gastropoda. In 
 Decapoda a supra-buccal ganglion -pair connected with 
 these are also developed. Instead of the numerous radi- 
 ating pallial nerves of Nautilus, we have in the Dibran- 
 chiata on each side (right and left) a large pleural 
 erve passing from the pleural portion of the pleuro- 
 visceral ganglion to the mantle, where it enlarges to 
 form the stellate ganglion. From each stellate ganglion 
 nerves radiate to supply the powerful muscles of the 
 mantle-skirt. The nerves from the visceral portion of the 
 pleuro-visceral ganglion have the same course as in Nautilus, 
 but no osphradial papilla is present. An enteric nervous 
 system is richly developed in the Dibranchiata, connected 
 with the somatic nervous centres through the buccal 
 ganglia, as in the Arthropoda through the stomato-gastric 
 ganglia, and anastomosing with deep branches of the vis- 
 ceral nerves of the viscero-pleural ganglion-pair. It has 
 been especially described by Hancock (39) in Omma- 
 strephes. Upon the stomach it forms a single large and 
 readily-detected gastric ganglion. It is questionable as to 
 how far this and the " caval ganglion " formed in some 
 Decapoda by branches of the visceral nerves which accom- 
 pany the vena cava are to be considered as the equivalents 
 of the "abdominal ganglion," which in a typical Gastropod 
 nervous system lies in the middle of the visceral nerve-loop 
 or commissure, having the right and left visceral ganglia 
 on either side of it, separated by a greater or less length 
 of visceral nerve-cord (see figs. 20, 21, 22). There can be 
 little doubt that the enteric nervous system is much more 
 developed in the Dibranchiata than in other Mollusca, and 
 that it effects a fusion with the typical " visceral " cords 
 more extensive than obtains even in Gastropoda, where 
 such a fusion no doubt must also be admitted. 
 
 Special Sense-Organs. Nautilus possesses a pair of 
 osphradial papilla (fig. 101, olf) corresponding in position 
 and innervation to Spengel's organ placed at the base of the 
 ctenidia (branchiae) in all classes of Mollusca. This organ 
 has not been detected in other Siphonopoda. In Ptero- 
 poda it is well developed as a single ciliated pit, although 
 the ctenidia are in that group aborted (fig. 87, Osp.). 
 Nautilus possesses other olfactory organs in the region 
 of the head. Just below the eye is a small triangular 
 process (not seen in our figures), having the structure of a 
 shortened and highly-modified tentacle and sheath. By 
 Valenciennes, who is followed by Keferstein, this is regarded 
 as an olfactory organ. The large nerve which runs to this 
 organ originates from the point of juncture of the pedal 
 with the optic ganglion. The lamelliform organ upon the 
 inner inferior tentacular lobe of Nautilus is possibly also 
 olfactory in function. In Dibranchs behind the eye is a 
 pit or open canal supplied by a nerve corresponding in 
 origin to the olfactory nerve of Nautilus above mentioned. 
 
MOLLUSCA 
 
 143 
 
 Possibly the sense of taste resides in certain processes 
 within the mouth of Nautilus and other Siphonopoda, 
 
 
 Flo. 116. Cartilaginous skeleton of Siphonopoda (after Keferetein). A. Capito- 
 pedal cartilage of Xautilus pompUius ; a points to the ridge which supports 
 the pedal portion of the nerve-centre. B. Lateral view of the same, the 
 large anterior processes are sunk in the muscular substance of the siphon. 
 C. Cephalic cartilages of Sepia ofcinalis. D. Nuchal cartilage of Sepia offici- 
 aite, 
 
 The otocysts of Nautilus were discovered by Macdonald 
 (40). Each lies at the side of the head, ventral of 
 the eye, resting on the capito-pedal cartilage, and supported 
 by the large auditory 
 nerve which arises 
 from the pedal gan- 
 glion. It has the 
 form of a small sac, 
 1 to 2 mm. in dia- 
 meter, and contains 
 whetstone - shaped 
 crystals, such as are 
 known to form the 
 otoliths of other Mol- 
 lusca. The otocysts 
 of Dibranchiata are 
 larger and deeply 
 sunk in the cephalic 
 
 cartilage. It has FIG. 117. Minute structure of the cartilage of 
 
 been shown by Lan- ^^ ( m . Gfgenbaur, after FurbringerjL a, 
 
 ; "J ~~ simple, b, dividing, cells: c, canaliculi; <f, an 
 
 kester that they de- empty cartilage capsule with its pores ; f.canali- 
 
 velop as open pits ^^^ on - 
 
 (fig. 121, (5), (6), o), which gradually close up, the com- 
 munication with the exterior becoming narrowed into a 
 fine canal, which is reflected over one end of the sac, and 
 finally has its external opening obliterated. A single 
 otolith only is found in all Dibranchiata. 
 
 The eye of Nautilus is among the most interesting struc- 
 tures of that remarkable animal. No other animal which 
 has the same bulk and general elaboration of organization 
 has so simple an eye as that of Nautilus. When looked 
 at from the surface no metallic lustre, no transparent 
 coverings, are presented by it. It is simply a slightly pro- 
 jecting hemispherical box like a kettle-drum, half an inch 
 in diameter, its surface looking like that of the surrounding 
 integument, whilst in the middle of the drum-membrane is 
 a minute hole (fig. 91, u). Owen very naturally thought 
 that some membrane had covered this hole in life, and had 
 been ruptured in the specimen studied by him. It, how- 
 ever, appears from the researches of Hensen (41) that the 
 hole is a normal aperture leading into the globe of the eye, 
 which is accordingly filled by sea-water during life. There 
 is no dioptric apparatus in Nautilus, and in place of refract- 
 ing lens and cornea we have actually here an arrangement 
 for forming an image on the principle of "the pin-hole 
 camera." There is no other eye known in the whole animal 
 kingdom which is so constructed. The wall of the eye- 
 
 globe is tough, and the cavity is lined solely by the naked 
 retina, which is bathed by sea-water on one surface and 
 receives the fibres of the optic nerve on the other (see fig. 
 118, A). As in other Siphonopods (e.g., fig. 120, Hi, Re, 
 p), the retina consists of two layers of cells separated by a 
 layer of dark pigment The most interesting consideration 
 connected with this eye of Nautilus is found when the 
 further facts are noted (1) that the elaborate lens-bearing 
 eyes of Dibranchiata pass through a stage of development 
 in which they have the same structure as the eye of Nautilus 
 namely, are open sacs (fig. 119) ; and (2), that amongst 
 other Mollusca examples of cephalic eyes can be found which 
 in the adult condition are, like the eye of Nautilus and the 
 developing eye of Dibranchs, simple pits of the integument, 
 the cells of which are surrounded by pigment and connected 
 with the filaments of an optic nerve. Such is the structure 
 
 Int* 
 
 Co.ep C 
 
 Ir 
 
 Tnt 3 
 
 Int 
 
 Jf.qp 
 
 N.qp 
 
 Fie. 118. Diagrams of sections of the eyes of Molluscs. A. Nautilus (and 
 PatellaX B. Gastropod (Limax or Helix). C. Dibranchiate Siphonopod 
 (OigopsidX Pol, eyelid (outermost fold) ; Co, cornea (second fold) ; Ir, iris 
 (third fold) ; Jnt 1 , *, 3, *, different parts of the integument ; /, deep portion 
 of the lens ; P, outer portion of the lens ; Co.ep, ciliary body; A, retina ; 
 K.op, optic nerve ; G.op, optic ganglion ; z, inner layer of the retina ; K.S, 
 nervous stratum of the retina. (From Balfonr, after Grenacher.) 
 
 of the eye of the Limpet (Patella) ; and in such a simple eye 
 we obtain the clearest demonstration of the fact that the 
 retina of the MoUuscan cephalic eye, like that of the 
 Arthropod cephalic eye and unlike that of the Vertebrate 
 myelonic eye, is essentially a modified area of the general 
 epiderm, and that the sensitiveness of its cells to the action 
 of light and their relation to nerve-filaments is only a 
 specialization and intensifying of a property common to the 
 whole epiderm of the surface of the body. What, however, 
 strikes us as especially remarkable is that the simple form 
 of a pit, which in Patella serves to accumulate a secretion 
 which acts as a refractive body, should in Nautilus be 
 glorified and raised to the dignity of an efficient optical 
 apparatus. Natural selection has had an altogether excep- 
 tional opportunity in the ancestors of Nautilus. In all other 
 Mollusca, starting as we may suppose from the follicular or 
 pit-like condition, the eye has proceeded to acquire the form 
 of a dosed sac, the cavity of the closed vesicle being then 
 filled partially or completely by a refractive body (lens) 
 secreted by its walls (fig. 118, B). This is the condition 
 attained in most Gastropoda. It presents a striking contrast 
 to the simple Arthropod eye, where, in consequence of the 
 existence of a dense exterior cuticle, the eye does not form 
 a vesicle, and the lens is always part of that cuticle. 
 
 In the Dibranchiate division of the Siphonopoda the 
 greatest elaboration of the dioptric apparatus of the eye 
 is attained, so that we have in one sub-class the extremes 
 of the two lines of development of the Molluscan eye, those 
 two lines being the punctigerous and the lentigerous. The 
 structure of the Dibranchiate's eye is shown in section in fig. 
 118, C, and in fig. 120, and its development in fig. 119 and 
 fig. 123. The open sac which forms the retina of the young 
 Dibranchiate closes up, and constitutes the posterior chamber 
 of the eye, or primitive optic vesicle (fig. 123, A,poc). The 
 
144 
 
 MOLLUSCA 
 
 lens forms as a structureless growth, projecting inwards from 
 the front wall of this vesicle (fig. 123, B, I). The integument 
 around the primitive optic vesicle which has sunk below 
 
 A 
 
 Fro. 119. Diagrams of se etions showing the early stage of development of the 
 eye of Loligo when it is, like the permanent eye of Nautilus and of Patella, 
 an open sac. A. First appearance of the eye as a ring-like upgrowth. B. 
 Ingrowth of the ring-like wall so as to form a sac, the primitive optic vesicle 
 of Loligo. (From Lankester.) 
 
 the surface now rises up and forms firstly nearest the axis 
 of the eye the iridian folds (if in B, fig. 123 ; ik in fig. 120 ; 
 Ir in fig. 118), and then secondly an outer circular fold 
 grows up like a wall and completely closes over the iridian 
 folds and the axis of the primitive vesicle (fig. 120, C). 
 This covering is transparent, and is the cornea, In the 
 oceanic Decapoda the cornea does not completely close, 
 but leaves a central aperture traversed by the optic axis. 
 These forms are termed Oigopsidse by d'Orbigny (42), whilst 
 the Decapoda with closed cornea are termed Myopsidse. 
 In the Octopoda the cornea is closed, and there is yet 
 another fold thrown over the eye. The skin surrounding 
 the cornea presents a free circular margin, and can be drawn 
 over the surface of the cornea by a sphincter muscle. It 
 thus acts as an adjustable diaphragm, exactly similar in 
 
 KK, 
 
 FIG. 120. Horizontal section of the eye of Sepia (Myopsid). KK, cephalic 
 cartilages (see fig. 116) ; C, cornea (closed) ; L, lens ; ci, ciliary body ; Ri, 
 internal layer of the retina ; Re, external layer of the retina ; p, pigment 
 lietween these ; o, optic nerve ; go, optic ganglion ; fc and K, capsular cartilage ; 
 ik, cartilage of the iris ; w, white body ; ae, argentine integument. (From 
 Gegenbaur, after Hensen.) 
 
 movement to the iris of Vertebrates. Sepia and allied Deca- 
 pods have a horizontal lower eyelid, that is to say, only 
 one-half of the sphincter-like fold of integument is movable. 
 The exact history of the later growth of the lens in the 
 Dibranchs' eye is not clear. As seen in fig. 120, it appears, 
 after attaining a certain size, to push through the front 
 wall of the primitive optic vesicle at the point correspond- 
 ing to its centre of closure, and to project a little into the 
 anterior chamber formed by the cornea. The wall of the 
 
 primitive optic vesicle adjacent to the embedded lens (L) 
 now becomes modified, forming a so-called " ciliary body," 
 in which muscular tissue is present, serving to regulate the 
 focus of the lens (a in fig. 120). Bobretzky (43) differs 
 from Lankester, whose view is above given, in assigning a 
 distinct origin to the protruding anterior segment of the 
 lens (I 1 in fig. 118). The optic ganglion, as well as the 
 other large ganglia of the Dibranchiata, originate in the 
 mesoblast of the embryo. The connexion between the cells 
 of the retina and the nerve- fibres proceeding from the optic 
 ganglion must therefore be a secondary one. 
 
 Chromatophores. In Nautilus these remarkable struc- 
 tures, which we mention here as being intimately asso- 
 ciated with the nervous system, appear to be absent. In 
 Dibranchiata they play an important part in the economy, 
 enabling their possessor, in conjunction with the discharge 
 of the contents of the ink-bag, to elude the observation of 
 either prey or foe. They consist of large vesicular cells 
 (true nucleated cells converted into vesicles), arranged in 
 a layer immediately below the epidermis. Each chroma- 
 tophore-cell has from six to ten muscular bands attached 
 to its walls, radiating from it star-wise. The contraction 
 of these fibres causes the chromatophore-cell to widen 
 out; it returns to its spherical resting state by its own 
 elasticity. In the spherical resting state such a cell may 
 measure '01 mm., whilst when fully stretched by its radiat- 
 ing muscles it covers an area of '5 mm. The substance 
 of the chromatophore-cells is intensely coloured with one 
 of the following colours scarlet, yellow, blue, brown 
 which are usually of the greatest purity and brilliance. The 
 action of the chromatophores may be watched most readily 
 in young Loligo, either under the microscope or with the 
 naked eye. The chromatophores are suddenly expanded, 
 and more slowly retracted with rapidly-recurring alter- 
 nation. All the blue, or all the red, or all the yellow 
 may be expanded and the other colours left quiescent. 
 Thus the animal can assume any particular hue, and 
 change its appearance in a dazzling way with extraordinary 
 rapidity. There is a definite adaptation of the colour 
 assumed in the case of Sepia and others to the colour of 
 the surrounding rock and bottom. 
 
 Gonads and Genital Ducts. In Nautilus it has recently 
 been shown by Lankester and Bourne (37) that the genital 
 ducts of both sexes are paired right and left, the left duct 
 being rudimentary and forming the "pyriform appendage," 
 described by Owen as adhering by membranous attach- 
 ment to the ventricle of the heart, and shown by Kefer- 
 stein to communicate by a pore with the exterior. Thus 
 the Cephalopoda agree with our archi-Mollusc in having 
 bilaterally symmetrical genital ducts in the case of the 
 most archaic member of the class. The ovary (female 
 gonad) or the testis (male gonad) lies in Nautilus as in 
 the Dibranchs in a distinct cavity walled off from the 
 other viscera, near the centro-dorsal region. This chamber 
 is formed by the coelomic or peritoneal wall the space 
 enclosed is originally part of the coelom, and in Sepia 
 and Loligo is, in the adult, part of the viscero-pericardial 
 chamber. In Octopus it is this genital chamber which 
 communicates by a right and a left canal with the nephrid- 
 ium, and is the only representative of pericardium. The 
 ovary or testis is itself a growth from the inner wall of this 
 chamber, which it only partly fills. In Nautilus the right 
 genital duct, which is functional, is a simple continuation 
 to the pore on the postero-dorsal surface of the membran- 
 ous walls of the capsule in which lies the ovary or the 
 testis, as the case may be. The gonad itself appears to 
 represent a single median or bilateral organ. 
 
 The true morphological nature of the genital ducts of the 
 Cephalopoda and of other Mollusca is a subject which invites 
 speculation and inquiry. In all the cases in which such 
 
MOLLUSCA 
 
 145 
 
 ducts continuous with the tunic of the gonad itself occur 
 viz., in Nematoid worms, in Arthropoda, and in Teleostean 
 fishes, besides Mollusca there is an absence of definite 
 knowledge as to the mode of development of the duct. 
 It seems, however, from such facts as have been ascer- 
 tained that the gonad lies at first freely in the ccelom, 
 and that the duct develops in connexion with the genital 
 pore, and attaches itself to the embryonic gonad, or to the 
 capsule which grows around it. The question then arises 
 as to the nature of the pore. In other groups of animals 
 we find that the pore, and funnel or tube connected with 
 it by which the genital products are conveyed to the 
 exterior, is a modified nephridium (usually a pair, one 
 right and one left). Is it possible that this is also the 
 case where the duct very early becomes united to the 
 gonad, and even gives rise to the appearance of a tubular 
 ovary or testis ? Probably this is the case in Teleostean 
 fishes (see Huxley's observations on the oviducts of the 
 smelt, 44) ; but it seems to be a tenable position that in other 
 cases, including the Mollusca, the genital pore is a simple 
 opening in the body-wall leading into the body-cavity 
 or crelom, such as we find on the dorsal surface of the 
 earth-worm, which has become specialized for the extrusion 
 of the genital products. Possibly, as in Nemertine and 
 Chaetopod worms, the condition preceding the development 
 of these definite genital pores was one in w"hich a temporary 
 rupture of the body-wall occurred at the breeding season, 
 and this temporary aperture has gradually become perma- 
 nent. The absence of genital pores in Patella, and some 
 Lamellibranchs which make use of the nephridia for the 
 extrusion of their genital products, suggests that the very 
 earliest Mollusca or their forefathers were devoid of genital 
 ducts and pores. In no Mollusca, however, is the nephrid- 
 iurn used in the same way as a genital duct as it is in the 
 Chsetopoda, the Gephyrsea, and the Vertebrata; for the 
 open mouth of the nephridium in Mollusca leads into the 
 pericardial space, and it is not through this space and this 
 mouth that the genital products of any Mollusca enter 
 the nephridium (except perhaps in Neomenia), although 
 it is by this mouth that the genital products enter the 
 nephridium in the former classes above named. Hence 
 the arrangement in Patella, &c., is to be looked upon as a 
 special development from the simpler condition when the 
 Mollusca brought forth by rupture ( = schizodinic, from cioYs, 
 travail), and not as derived from the common arrangement 
 of adaptation of a nephridium to the genital efferent func- 
 tion ( = nephrodinic). 1 
 
 The functional oviduct of Nautilus forms an albumini- 
 parous gland as a diverticulum, which appears to corre- 
 spond to a dilatation in the male duct, which succeeds the 
 testis itself, and is called the " accessory gland." The male 
 duct has a second dilatation (Needham's sac), and then is 
 produced in the form of a large papilla. In Dibranchs 
 the genital ducts are but little more elaborated. They are 
 ciliated internally. In female Octopoda, in Ommastrephes, 
 and in one male Octopod (Eledvne moschata) the genital 
 ducts are paired, opening right and left of the anus. But 
 in all other Dibranchs a single genital duct only is deve- 
 loped, viz., that of the left side, and leads from the genital 
 capsule or chamber of the gonad to an asymmetrically-placed 
 pore. In the male Dibranchs the genital duct is coiled 
 and provided with a series of glandular dilatations and 
 
 1 Coelomate animals are, according to this nomenclature, either 
 Schizodinic or Porodiuic. The Porodinic gronp is divisible into Ne- 
 phrodinic and Idiodinic, in the former the nephridium serving as a 
 pore, in the latter a special (Wios) pore being developed. In each of 
 these latter groups the pore may be (1) devoid of a duct, (2) provided 
 with a duct which is unattached to the gonad and opens into the body- 
 cavity, (3) provided with a duct which fuses with the gonad. The 
 genital ducts of Idiodinic forms may be called Idiogonaducts, as dis- 
 tinguished from the Nephrogouaducts of nephrodinic forms. 
 
 receptacles. These are connected with the formation of 
 the spermatophores. In the Siphonopoda the spermatic 
 fluid does not flow as a liquid from the genital pore, but 
 the spermatozoa are made up into little packets before 
 extrusion. In other Mollusca (Pulmonata) and in other 
 animals (Chaetopoda) this formation of " sperm-ropes " is 
 known, but in the Siphonopoda it attains its highest 
 development. Exceedingly complicated structures of a 
 cylindrical form (sometimes an inch in length) are formed 
 in the male genital duct by a secretion which embeds and 
 cements together the spermatozoa. They are formed in 
 Nautilus as well as in Dibranchs, the actual manner in which 
 their complicated structure is produced being not easily con- 
 jectured. Accessory glands not forming part of the oviduct, 
 but furnishing the material for enclosing the eggs in an elastic 
 envelope, are found as paired structures, opening some way 
 behind the anus in Nautilus (101, g.n.) and in the Di- 
 branchs. They are known as the nidamental glands. In 
 the female Sepia they are particularly large and prominent, 
 and are accompanied by a second smaller pair. 
 
 Reproduction and Development. The details of sexual 
 congress and of the actual fertilization of the egg are quite 
 unknown in Nautilus, and imperfectly in the Dibranchs 
 and the Pteropoda. Allusion has already been made to 
 the subject in connexion with the hectocotylized arm. The 
 mature eggs of Nautilus are unknown, as well as the appear- 
 ance which they present when deposited. In the Dibranchs 
 the eggs are always very large ; in some cases the amount 
 of food-yelk infused into the original egg-cell is so great as 
 to give it the size of a large pea. This results in that 
 mode of development which is only known outside this 
 class among the Vertebrata ; it is discoblastic. The proto- 
 plasm of the fertilized egg-cell segregates to one pole of 
 the egg, and there undergoes cell-division, resulting in the 
 formation of a disc of cleavage cells (fig. 121, (1)) resem- 
 bling the cicatricula of the hen's egg, which subsequently 
 spreads over and invests the whole egg (fig. 121, (2)). For 
 details of this process we must refer the reader to other 
 works (45, 46) ; but it may here be noted that in addition to 
 the layer of cleavage cells, which consists of more than one 
 stratum of cells in the future embryonic area as opposed 
 to the yelk-sac area, additional cells are formed in the 
 mass of residual yelk apparently by an independent process 
 of segregation, each cell having a separate origin, whence 
 they are termed "autoplasts." The autoplasts eventually 
 form a layer of fusiform cells (fig. 121, (7), h; fig. 122, m, 
 and fig. 123, ps), the "yelk-membrane" which everywhere 
 rests upon and encloses the residual yelk. The cleavage 
 cells form a single layer on the yelk-sac area and two layers 
 on the embryonic area, an outer layer one cell deep (fig. 1 22, 
 ep), and an inner the middle layer of the three which 
 is often thick and many cells deep (fig. 122, m). There is 
 great difficulty here in identifying the layers with the three 
 typical layers of other animal embryos, except in regard 
 to the outermost, which corresponds with the epiblast of 
 Vertebrates in many respects. The middle layer, however, 
 gives rise to the nerve-ganglia as well as to the muscles, 
 coelom, and skeleto-trophic tissues, and to the mid-portion 
 of the alimentary canal with its hepatic diverticula, the 
 liver (see fig. 121, (7) and explanation, where the origin of 
 the mid-gut as a vesicle r is seen). It is clearly, therefore, 
 something more than the mesoblast of the Vertebrate, 
 giving rise, as it does, to important organs formed both by 
 epiblast and hypoblast in other animals. Lastly, the yelk- 
 membrane, though corresponding to the Vertebrate hypo- 
 blast in position and structure, furnishes no part of the 
 alimentary tract, but disappears when the yelk is com- 
 pletely absorbed. In fact, the developmental phenomena 
 in Sepia, Loligo, and Octopus are profoundly perturbed by 
 the excessive proportion of food-yelk. Balfour has shown 
 
 T 
 
146 
 
 MOLLUSC A 
 
 that in the chick the orifice of closure of the overspreading 
 blastoderm does not represent the whole of the blastopore, 
 
 T> 
 
 if 
 
 (1) 
 
 (7) 
 
 Fro. 121. Development of Loligo. (1) View of the cleavage of the egg during 
 the first formation of embryonic cells. (2) Lateral view of the egg at a little 
 later stage, a, limit to which the layer of cleavage-cells has spread over the 
 egg ; ft, portion of the egg (shaded) as yet uncovered by cleavage-cells ; ap, the 
 autoplasts ; kp, cleavage-pole where first cells were formed. (3) Later stage, 
 the limit a now extended so as to leave but little of the egg-surface (6) unen- 
 closed. The eyes (d\ mouth (e), and mantle-sac (?() have appeared. (4) Later 
 stage, anterior surface, the embryo is becoming nipped off from the yelk 
 sac ((/). (5) View of an embryo similar to (3) from the cleavage-pole or 
 centro-dorsal area. (6) Later stage, posterior surface. (7) Section in a 
 median dorso-ventral and antero-posterior plane of an embryo of the same 
 age as (4). (8) View of the anterior face of an older embryo. (9) View of the 
 posterior face of an embryo of the same age as (8). Letters in (3) to (9) : a, 
 lateral fins of the mantle ; 6, mantle-skirt ; c, supra-ocular invagination to 
 form the " white body " ; d, the eye ; e, the mouth ; /, 2, 3, 4 t s : the five paired 
 processes of the fore-foot ; g t rhythmically contractile area of the yelk-sac, 
 which is itself a hernia-like protrusion of the median portion of the fore-foot 
 (see fig. 72**) ; A, dotted line showing internal area occupied by yelk (food- 
 material of the egg) ; k, first rudiment of the mid-foot (paired ridges which 
 unite to form the siphon or funnel) ; J, sac of the radula or lingual ribbon ; 
 m, stomach ; n, rudiments of the gills (paired ctenidia); o, the otocysts, a 
 pair of invaginations of the surface of the mid-foot ; p, the optic ganglion ; 
 q, the distal portion of the ridges which form the siphon or mid-foot, k being 
 the basal portion of the same structure ; r, the vesicle-like rudiment of the in- 
 testine formed independently of the parts connected with the mouth, s, k, m, 
 and without invagination ; s, rudiment of the salivary glands ; ( in (7), the 
 shell-sac at an earlier stage open (see lig. 122), now closed up ; u, the open 
 shell-sac formed by an uprising ring-like growth of the centro-dorsal area ; 
 w in (5), the mantle-skirt commencing to be raised up around the area of the 
 shell-sac. In (7) me.s points to the middle cell-layer of the embryo, ep to the 
 outer layer, and h to the deep layer of fusiform cells which separates every- 
 where the embryo from the yelk or food-material lying within it. (Original.) 
 
 but that this is in part to be sought in the widely-separated 
 primitive streak. The present writer has little doubt that 
 
 a structure corresponding to the primitive streak of the 
 chick, and lying near the klastic pole, will be found in 
 Sepia and Loligo, and the strange vesicular origin of the 
 mid-gut will be traced to and explained by it. 
 
 Leaving this difficult question of the cell-layers of the 
 embryo, we would draw the reader's attention to the series 
 of sketches representing the semi-transparent embryo of 
 Loligo, drawn in fig. 121. When the cleavage cells have 
 nearly enclosed the yelk, the upper or embryonic area 
 shows the rudiments of the centro-dorsal mantle-sac or 
 pen-sac, the mouth, the paired optic pits, and the paired 
 
 FIR. 122. Section through the still open shell-sac occupying the centro-dorsal 
 area of an embryo of Loligo ; the position is inverted as compared with fig. 
 121 (3) and (7). ep, outer cell-layer; m, middle cell-layer; m', deep cell- 
 layer of fusiform culls ; y, the granular yelk or food-material of the egg ; 
 shs, the still open shell-sac. (From Lankester.) 
 
 otic pits (fig. 121, (3), (5)). The eye-pits close up (fig. 
 119), the orifice of the mantle-sac narrows, and its margin 
 becomes raised and freely produced as mantle-skirt ; 
 at the same time an hour-glass-like pinching in of the 
 whole embryo commences, separating the embryo proper 
 from the so-called yelk-sac (fig. 121, (4)). Around the 
 " waist " of constriction, pair by pair, ten lobes arise (fig. 
 121, (8)), the arms of the fore-foot. It now becomes 
 obvious that the yelk-sac is but the median surface of the 
 fore-foot bulged out inordinately by food-yelk, just as the 
 hind region of the foot is in the embryo slug (see fig. 72**, 
 and explanation). Just as in the slug, this dilated yelk- 
 holding foot is rhythmically contractile, and pulsates 
 steadily over the area g in fig. 121, (4). At this stage, 
 and long subsequently, the mouth of the young Cephalopod 
 is in no way surrounded by the fore-foot, but lies well 
 above its nascent lobes (e in fig. 121). Subsequently it 
 sinks, as it were, between the right and left most anterior 
 pair of the series, which grow towards one another and 
 fuse above it, and leave no trace of their original position 
 and relations. Fig. 121, (6) gives a view of the postero- 
 dorsal surface of an embryo, in which the important fact 
 is seen of the formation of the funnel or siphon by the 
 union of two pieces (<?), which grow up each independently, 
 one right and one left, like the sides of the siphon of 
 Nautilus or the swimming lobes of a Pteropod, and subse- 
 quently come together, as shown in (9), where the same 
 letter q indicates the same part. The explanations of figs. 
 121 and 123 are given very full, and here, therefore, we 
 shall only allude to two additional points. A curious mass 
 of tissue of unknown significance occurs in the orbit of 
 Dibranchs, known as the white body (w in fig. 120). A 
 strongly-marked invagination just above the orbit is a very 
 prominent feature in the embryo of Loligo, Sepia, and 
 Octopus, and appears to give rise to this so-called white 
 body. This invaginated portion of the outer cell-layer is 
 seen in fig. 121, (8) and (9), lettered c ; in fig. 123, A and 
 B, it is lettered wb. 
 
 Lastly, in fig. 123, A, the origin of the optic nerve- 
 ganglion ng from the cells of the middle layer should be 
 especially noticed. In some other Molluscs the nerve- 
 ganglia have been definitely traced to the outer cell-layer, 
 
MOLLUSCA 
 
 147 
 
 whilst in some Gastropods, according to Bobretzky, they 
 originate, as here shown, for Loligo. 
 
 The egg-coverings of the Dibranehiate are very complete. 
 Argonauta and Octopus deposit each egg in a firm oval 
 case, thin and transparent, which has a long stalk by 
 which (in Octopus) the egg is fixed in company with two 
 or three hundred others to some foreign object. Sepia 
 encloses each egg in a thick envelope of many layers 
 resembling india-rubber. Loligo encloses many rows of 
 eggs in a copious tough jelly, and anises a dozen or twenty 
 such egg-strings to one spot. Sepia and Loligo desert 
 their eggs when laid. The female Octopus most jealously 
 
 net 
 
 poc 
 
 .ym 
 
 mtf 
 
 FIG. 123. Right and left sections through embryos of Loligo. A. Same stage 
 as fig. 121 (4). B. Same stage as fig. 121 (8) ; only the left side of the sections 
 is drawn, and the food-material which occupies the space internal to the 
 membrane ym is omitted. a(, rectum ; u, ink-sac ; ep, outer cell-layer; mes, 
 middle cell-layer ; ym, deep cell-layer of fusiform cells (yelk-membrane); ng, 
 optic nerve-ganglion ; of, otocyst ; ir&, the " white body " of the adult ocular 
 capsule forming as an imagination of the outer cell-layer ; mtf, mantle-skirt ; 
 g, gill ; ps, pen-sac or shell-sac, now closed ; dg, dorsal groove ; poc, primitive 
 optic vesicle, now closed (see fig. 1 19) ; /, lens ; r, retina ; soc, second or anterior 
 optic chamber still open ; if, iridean folds. C. The primitive invagination to 
 form one of the otocysts, as seen in fig. 121 (5) and (6). (After Lankester.) 
 
 guards them, building a nest of stones and incubating. 
 Argonauta carries hers with her in a special brood-holding 
 shell. 
 
 The development of the Pteropoda, so far as is known, 
 presents no points of contact with that of the Siphonopoda 
 rather than with that of the Gastropoda, owing to the fact 
 that in them the egg has not an excess of food-yelk. Con- 
 sequently, we find typical trochosphere and veliger larvae 
 among the Thecosomata (fig. 8, C, and fig. 81), whilst the 
 Isolated observation of Gegenbaur has made known very 
 remarkable larvje referable to the Gymnosomata, and with 
 little doubt to Pneumodermon (fig. 84). The former set of 
 larva: are sufiicient to demolish once for all the view which 
 has been entertained by some zoologists, viz., that the velar 
 disc of the veliger larva is the same thing as the ptero- 
 podial lobes of the mid-foot of Pteropoda. The latter 
 brae are of importance in showing that, as in embryo 
 Siphonopods so in embryo Pteropods, the sucker-bearing 
 lobes of the fore-foot are truly podia! structures, and only 
 embrace the head and surround the mouth as the result of 
 late embryonic growth. 
 
 BRANCH K. LIPOCEPHALA. 
 
 Characters. Mollusca with the head region undeveloped. 
 No cephalic eyes are present ; the buccal cavity is devoid 
 
 of biting, rasping, or prehensile organs. The animal is 
 sessile, or endowed with very feeble locomotive powers. 
 The Lipocephala comprise but one class, the Lamelli- 
 branchia, also known as Elatobranchia and Conchifera. 
 
 Class LAMELLIBRANCHIA 
 
 Characters. Lipocephala in which the archaic BILA- 
 TERAL SYMMETRY of the Mollusca is usually fully retained, 
 and raised to a dominant feature of the organization by the 
 lateral compression of the body and the development of the 
 shell as two bilaterally symmetrical plates or valves cover- 
 ing each one side of the animal. The FOOT is commonly a 
 simple cylindrical or ploughshare-shaped organ, used for 
 boring in sand and mud, and more rarely presents a crawl- 
 ing disc similar to that of Gastropoda ; in some forms it is 
 aborted. The paired CTENIDIA are very greatly developed 
 right and left of the elongated body, and form the most 
 prominent organ of the group. Their function is chiefly 
 not respiratory but nutritive, since it is by the currents 
 produced by their ciliated surface that food -particles are 
 brought to the feebly-developed mouth and buccal cavity. 
 
 The Lamellibranchia present as a whole a somewhat 
 uniform structure, so that, although they are very numerous, 
 it is not possible to divide them into well-marked sub-classes 
 or sections, and orders. The chief points in which they 
 vary are (1) in the structure of the ctenidia or branchial 
 plates ; (2) in the presence of one or of two chief muscles, 
 the fibres of which run across the animal's body from one 
 valve of the shell to the other (adductors) ; (3) in the greater 
 or less elaboration of the posterior portion of the mantle- 
 skirt so as to form a pair of tubes, by one of which water 
 is introduced into the sub-pallial chamber, whilst by the 
 other it is expelled ; (4) in the perfect or deficient symmetry 
 of the two valves of the shell and the connected soft parts, 
 as compared with one another ; (5) in the development of 
 the foot as a disc-like crawling organ (Area, Nucula, Pectun- 
 culus, Trigonia, Lepton, Galeomma), as a simple plough- 
 like or tongue-shaped organ (Unionacea, <fcc.), as a re-curved 
 saltatory organ (Cardium, <tc.), as a long burrowing cylin- 
 der (Solenacea, <fec.), or its partial (Mytilacea) or even com- 
 plete abortion (Ostracea). 
 
 The essential Molluscan organs are, with these excep- 
 tions, uniformly well developed. The MANTLE-SKIRT is 
 always long, and hides the rest of the animal from view, its 
 dependent margins meeting in the middle line below the 
 ventral surface when the animal is retracted it is, as it 
 were, slit in the median line before and behind so as to 
 form two flaps, a right and a left ; on these the right and 
 the left calcareous valves of the shell are borne respectively, 
 connected by an uncalcified part of the shell called the 
 ligament. In many embryo Lamellibranchs a centre-dorsal 
 PRIMITIVE SHELL-GLAXD or follicle has been detected (figs. 
 8 and 151). The MOUTH lies in the median line anteriorly, 
 the AXUS in the median line posteriorly. 
 
 Both CTENIDIA right and left are invariably present, the 
 fl-ris of each taking origin from the side of the body as in 
 the schematic archi-Molluse (see fig. 1 and fig. 131). A 
 pair of NEPHRIDIA opening right and left, rather far forward 
 on the sides of the body, are always present. Each opens 
 by its internal extremity into the pericardium. A pair of 
 GENITAL APERTURES, connected by genital ducts with the 
 paired gonads, are found right and left near the nephridial 
 pores, except in a few cases where the genital duct joins 
 that of the nephridium (Spondylus). The sexes are often, 
 but not always, distinct. No accessory glands or copulatory 
 organs are ever present in Lamellibranchs. The ctenidia 
 often act as brood-pouches. 
 
 A dorsal contractile HEART, with symmetrical right and 
 left auricles (fig. 143, A) receiving aerated blood from the 
 ctenidia and mantle-skirt, is present, being unequally de- 
 
148 
 
 MOLLUSCA 
 
 veloped only in those few forms which are inequivalve. 
 The typical PERICARDIUM is well developed. It appears, 
 as in other Mollusca, not to be a blood-space although 
 developed from the ccelom, and it communicates with the 
 exterior by the pair of nephridia. As in Cephalopoda (and 
 possibly other Mollusca) water can be introduced through 
 the nephridia into this space. The ALIMENTARY CANAL 
 keeps very nearly to the median vertical plane whilst ex- 
 hibiting a number of flexures and loopings in this plane. 
 A pair of large glandular outgrowths, the so-called "liver" 
 or great digestive gland, exists as in other Molluscs. A 
 pair of pedal OTOCYSTS, and a pair of OSPHRADIA at the 
 base of the gills, appear to be always present. A typical 
 NERVOUS SYSTEM is present (fig. 144), consisting of a 
 cerebro-pleuro-visceral ganglion-pair, united by connectives 
 to a pedal ganglion-pair and an osphradial ganglion-pair 
 (parieto-splanchnic). 
 
 A special caecum connected with the pharynx is some- 
 times found, containing a tough flexible cylinder of trans- 
 parent cartilaginous appearance and unknown significance, 
 called the "crystalline style" (Mactra), which possibly 
 represents the radular sac of Glossophora. In many Lamelli- 
 branchs a gland is found on the hinder surface of the foot 
 in the mid line, which secretes a substance which sets into 
 the form of threads the so-called " byssus " by means of 
 which the animal can fix itself. Sometimes this gland is 
 found in the young and not in the adult (Anodon, Unio, 
 Cyclas). In some Lamellibranchs (Pecten, Spondylus, 
 Pholas, Mactra, Tellina, Pectunculus, Galeomma, &c.), 
 although cephalic eyes are always absent, special eyes 
 are developed on the free margin of the mantle-skirt, 
 apparently by the modification of tentacles which are 
 commonly found there (fig. 145). The existence of pores 
 in the foot and elsewhere in Lamellibranchia by which liquid 
 can pass into and out of the vascular system, although 
 asserted as in the case of other Mollusca, appears to be 
 improbable. It has yet to be shown by satisfactory micro- 
 scopic sections that the supposed pores are anything but 
 epidermal glands. 
 
 The Lamellibranchia live chiefly in the sea, some in 
 fresh waters. A very few have the power of swimming by 
 opening and shutting the valves of the shell (Pecten, Lima); 
 most can slowly crawl or rapidly burrow; others are, when 
 adult, permanently fixed to stones or rocks either by the 
 shell or the byssus. In development some Lamellibranchia 
 pass through a free-swimming trochosphere stage with prse- 
 oral ciliated band ; other fresh-water forms which carry the 
 young in brood-pouches formed by the ctenidia have sup- 
 pressed this larval phase. 
 
 The following classification and enumeration of genera 
 are based primarily upon the characters of the adductor 
 muscles. The Heteromya and Monomya must be conceived 
 of as derived from forms resembling such Gastropodous 
 Isomya as Nucula and Trigonia, which undoubtedly are 
 the nearest living representatives of the ancestral Lipo- 
 cephala, and bring us nearest to the other branch of the 
 Mollusca, the Glossophora. 
 
 Order 1. Isomya. 
 
 Character. Anterior and posterior adductor muscles of approxi- 
 mately equal size. 
 
 Sub-order 1. Integripallia. 
 
 Characters. Marginal attachment of the mantle to the shell not 
 inflected to form a sinus ; siphons not developed in some, present 
 in most. 
 Family 1. Artacea. 
 
 Genera: Ana, L. (fig. 132) ; Cucullsea, Lam. ; Pectunculus, Lain. ; 
 Linwpsis, Sassi; Nucula, Lam. (fig. 134) ; Isoarca, Miinster ; 
 Lcda, Schu. ; Yoldia, Moll. ; SolenMa, Sowerby, &c. 
 Family 2. Trigoniacea. 
 
 Genera : Trigonia, Brug. ; Axinus, Sow. ; Lyrodesma, Conrad. 
 
 Family 3. Unionacea. 
 
 Genera : Unio, Retz. ; Castalia, Lam. ; Anodon, Cuv. (figs. 124, 
 
 &c.) ; Iridina, Lam. ; Mycetopus, d'Orb., &c. 
 Family 4. Lucinacea. 
 
 Genera : Lucina, Brng. ; Corbis, Cuv. ; Diplodonta, Brown ; 
 Kellia, Turton ; Montacuta, Turton ; Lepton, Turton ; Gale- 
 omma, Turton ; Astarte, Sow. ; Crassatella, Lain. ; Cardinia, 
 Ag. ; Cardita, Brug., &c. 
 Family 5. Cyprinacea. 
 
 Genera : Tridacna, Da C. ; Chaina, L. ; Dimya, Ron. ; Diceras, 
 Lk. ; Isocardia, Lam. ; Hippopodium, Sow. ; Cardium, L. ; 
 CorWcula, Meg. ; Cyrena, Lk. ; Cyclas, Brug. (fig. 146) ; Pisid- 
 ium, Pfr. (figs. ] 48-153) ; Cyprina, Lam., &o. 
 
 Sub-order 2. Sinupallia. 
 
 Characters. Marginal attachment of the mantle to the shell in- 
 flected so as to form a sinus into which the pallial siphons can be 
 withdrawn ; siphons always present, and large. 
 Family 6. Veneracea. 
 
 Genera : Cypricardia, Lam. ; Tapes, Megl. ; Cyclina, Desh. ; 
 Cytherea, Lam. (figs. 125, &c.) ; Chionc, Megl. ; Venus, L. ; 
 LiLcinopsis, F. H. ; Sanguinolaria, Lam. ; Psammobia, Lam. 
 (fig. 130) ; Tellina, L. ; Donax, L. ; Scrobicularia, Schu. ; 
 Cumingia, Sow. ; Rangia, DsmL. ; Mactra, L. (fig. 140) ; Trigo- 
 nella, Da C. ; Vaganella, Gr. ; Lutraria, Lam. 
 Family 7. Myacea. 
 
 Genera : Myochama, Stb. ; Chamostrea, Rois ; Pandora, Sol. ; 
 Thracia, Leach ; Thetis, Sow. ; Pholadomya, Sow. ; Corbula, 
 Brug. ; Mya, Lam. ; Saxicava, Fleur ; Panopsea, Ad. ; Glyci- 
 meris, Lam. ; Siligua, Mhlf., &c. ; Solen, L. 
 Family 8. Pholadacca. 
 
 Genera : Clavagella, Lam. ; Aspergillum, Lam. (figs. 128, 129) ; 
 Humphreyia, Gr. ; Pholas, L. ; Pholadidea, Turt. ; Teredo, L. ; 
 Teredina, Lam. ; Furcella, Oken, &c. 
 
 Order 2. Heteromya. 
 
 Characters. Anterior adductor (pallial adductor) much smaller 
 than the posterior adductor (pedal adductor) ; siphons rarely present. 
 Family 1. Mytilacea. 
 
 Genera: Mytilus, L. (fig. 133); Mudiola, Lam.; Crenclla, Brown ; 
 Lithodomtts, Cuv. ; Dreissena, Beii. (fig. 136) ; Modiolarca, 
 Gr., &c. 
 Family 2. Mulleriacea. 
 
 Genera : Aetheria, Lam. ; MuHeria, Fer. 
 
 Order 3. Monomya. 
 
 Characters. Anterior adductor absent iu the adult; siphons 
 never developed. 
 Family 1. Aviculacea. 
 
 Genera: Cardiola, Brdp. ; Avicula, Kl. ; Malleus, Lam. ; Ino- 
 
 ceramiis, Sow. ; Crenatula, Lam. ; Perna, Brug., &c. 
 Family 2. Ostracea. 
 
 Genera: Ostrea, L. (fig. 6); Anomia, L. ; Spondylus, L. ; Plicatula, 
 Lam. ; Vulsella, Lam. ; Lima, Brug. ; Pecten, L. ; Hiunites, 
 Dfr., &c. 
 
 Further Remarks on the Lamellibranchia. The Lamelli- 
 branchia are the only members of the Lipocephalous branch 
 of Mollusca existing at the present day ; and we must 
 suppose that, whilst on the one hand the earliest Glosso- 
 phorous forms were developing from the archi-Mollusca by 
 the elaboration of the buccal apparatus, the bivalved sessile 
 Lamellibranchs were developing in another direction from 
 univalve cephalophorous ancestors. The large bilobed 
 mantle-flap with its pair of shells covering in the whole 
 animal, the current-producing largely-expanded ctenidia, 
 and the reduced cephalic region are characters which go 
 hand in hand, and were simultaneously acquired, each being 
 related to the development of the others. Unless the 
 " crystalline style " of Lamellibranchs is to be considered 
 as the rudiment of the " radular sac " of Glossophora, as 
 suggested by Balfour, there is no indication whatever that 
 the ancestors of the Lamellibranchia had acquired a repre- 
 sentative of the buccal apparatus so highly developed in 
 Glossophora before diverging from the archi-Mollusca ; 
 that is to say, the common ancestors of the two great 
 branches of Mollusca presented the distinctive character 
 of neither branch they had not an aborted cephalic region, 
 and they had not a lingual ribbon. 
 
 As an example of the organization of a Lamellibranch, 
 we shall review the structure of the Common Pond-Mussel 
 (Anodonta cygnea), comparing its structure with those of 
 
MOLLUSCA 
 
 149 
 
 other Lamellibrancliia. Tlie Swan Mussel has superficially 
 a perfectly-developed bilateral symmetry. The left side of 
 the animal is seen as when removed from its shell in fig. 
 124 (1). The valves of the shell have been removed by 
 severing their adhesions to the muscular areae k, i, i; I, m, it. 
 
 (1) 
 
 fU. 
 
 FIG. 124. Diagrams of the external form and anatomy of AiutJonta c.vjnra, the 
 Pond-Mussel ; in all the figures the animal is seen from the left side, the 
 centro-dorsal region uppermost, as in the drawings of fig. 75, which compare. 
 (1) Animal removed from its shell, a probe g passed into the sub-pallia! 
 chamber through the excurrent siphonal notch. (2) View from the ventral 
 surface of an Anodon with its foot expanded and issning from between the 
 gaping shells. (3) The left mantle-flap reflected upwards so as to expose the 
 sides of the b nly. (4) Diagrammatic section of Anodon to show the course of 
 the alimentary canal. (5) The two gill-plates of the left side reflected upwards 
 so as to expose the fissure between foot and gill where the probe g passes, 
 (ti) Diagram to show the positions of the nerve-ganglia, heart, and nephridia. 
 Letters in all the figures as follows : a, centro-dorsal area ; 6, margin of 
 the left mantle-flap ; c, margin of the right mantle-flap ; d, excurrent siphonal 
 notch of the mantle margin ; r, incurrent siphonal notch of the mantle 
 margin ; f, foot ; g, probe passed into the superior division of the sub-pallial 
 chamber through the excurrent siphonal notch, and issuing by the side of 
 the foot into tie inferior division of the sub-pallial chamber ; h, anterior 
 (pallial) adductor muscle of the shells ; i, anterior retractor muscle of the 
 foot ; t, protractor muscle of the foot ; I, posterior (pedal) adductor muscle 
 of the shells ; a, posterior retractor muscle of the foot ; , anterior labial 
 tentacle ; o, posterior labial tentacle ; p, base-line of origin of the reflected 
 mantle-flap from the side of the body ; g, left external gill-plate ; r, left in- 
 ternal gill-plate ; rr, inner lamella of the right inner gill-plate ; rg, right outer 
 gill-plate ; s, line of concrescence of the outer lamella of the left outer gill- 
 plate with the left mantle-flap ; I, pallial tentacles ; , the thickened mus- 
 cular pallial margin which adheres to the shell and forms the pallial line of 
 the left side ; r, that of the right side ; r, the month ; x, aperture of the left 
 organ of Bojanus (nephridium) exposed by cutting the attachment of the 
 inner lamella of the inner gill-plate ; y, aperture of the genital duct ; 2, fissure 
 between the free edge of the inner lamella of the inner gill-plate and the side 
 of the foot, through which the probe g passes into the upper division of the 
 sub-pallial space ; aa, line of concrescence of the inner lamella of the right 
 inner gill-plate with the inner lamella of the left inner gill-plate ; ab, ac, ad, 
 three pit-like depressions in the median line of the foot supposed by some 
 writers to be pores admitting water into the vascular system ; at, left shell 
 valve ; af t space occupied by liver ; ag, space occupied by gonad ; oA, muscular 
 substance of the foot : ai, duct of the liver on the wall of the stomach ; at, 
 stomach ; al, rectum traversing the ventricle of the heart ; am, pericardium ; 
 an, glandular portion of the left nephridinm ; ap, ventricle of the heart ; aq, 
 aperture by which the left auricle joined the ventricle ; or, non-glandular por- 
 tion of the left nephridium; as, anus; at, pore leading from the pericardium into 
 the glandular sac of the left nephridium ; aw, pore leading from the glandular 
 into the non-glandular portion of the left nephridium ; or, internal pore lead- 
 ing from the non-glandular portion of the left nephridium to the external 
 pore x; air, left cerebro-pleuro-visceral ganglion; ox, left pedal ganglion; 
 ay, left otocyst ; _-, left olfactory ganglion (parieto-splanchnic) ; 66, floor of 
 the pericardium separating that space from tie non-glandular portion of the 
 nephridia. 
 
 The free edge of the left half of the mantle-skirt b is repre- 
 sented as a little contracted in order to show the exactly simi- 
 lar free edge of the right half of the mantle-skirt c. These 
 edges are not attached to, although they touch, one another ; 
 each flap (right or left) can be freely thrown back in the way 
 which has been carried out in fig. 1 24, (3) for that of the 
 left side. This is not always the case with Lamellibranchs ; 
 there is in the group a tendency for the corresponding 
 edges of the mantle-skirt to fuse together by concrescence, 
 
 and so to form a more or less completely closed bag, as in 
 the Scaphopoda (Dentalium). In this way the notches 
 d,eo{ the hinder part of the mantle-skirt of Anodon are in 
 the Siphonate forms converted into two separate holes, the 
 edges of the mantle being elsewhere fused together along 
 this hinder margin. Further than this, the part of the 
 mantle-skirt bounding the two holes is frequently drawn out 
 so as to form a pair of tubes which project from the shell (figs. 
 130, 141). In such Lamellibranchs as the oysters, scallops, 
 and many others which have the edges of the mantle-skirt 
 quite free, there are numerous tentacles upon those edges. 
 In Anodon these pallial tentacles are confined to a small area 
 surrounding the inferior siphonal notch (fig. 124, (3), t). 
 
 The centro-dorsal point a of the animal of Anodonta 
 (fig. 124, (1 )) is called the umbonal area ; the great anterior 
 muscular surface k is that of the anterior adductor muscle, 
 the posterior similar surface t is that of the posterior 
 adductor muscle ; the long line of attachment M is the 
 simple " pallial muscle,"- a thickened ridge which is seen 
 to run parallel to the margin of the mantle-skirt in this 
 Lamellibranch. In some of the Siphonate Isomya, which 
 are hence termed " Sinupallia," the pallial muscle is not 
 simple but deeply incurved at the posterior region so as to 
 allow of the large pallial siphons being retracted within the 
 shell or expanded at will (fig. 127, and figs. 140, 141). 
 
 It is the approximate equality 
 in the size of the anterior and 
 posterior adductor muscles which 
 has led to the name Isoyma for 
 the group to which Anodon be- 
 longs. The hinder adductor 
 muscle may be considered as re- 
 presenting morphologically the 
 transverse fibres of the root of 
 the foot of Nautilus by which it 
 adheres to its shell (fig. 91, ), the 
 annular muscular area of Patella 
 (fig. 2 7,c), and the columella muscle 
 of the Gastropods generally. It 
 is always large in Lamellibranchs, 
 but the anterior adductor may 
 be very small (Heteromya), or FlG : . 
 
 . J ..' iir ofthe shell of Cytherea (one of 
 
 absent altogether (Monomya). the Sinupalliate Isomya), from 
 
 The anterior adductor muscle is >e dorsal aspect, 
 in front of the mouth and alimentary tract altogether, 
 and must be regarded as a special and peculiar deve- 
 lopment of the median anterior part of the mantle-flap 
 
 'lorinfenfT ' 
 FIG. 126. Right valve of the same shell from the outer face. 
 
 in Heteromya and Isomya. Amongst those Lamelli- 
 branchs which have only a posterior adductor (Monomya), 
 it is remarkable that the oyster has been found (by 
 Huxley) to possess, when the young shells and muscles 
 first develop, a well-marked anterior adductor as well as a 
 posterior one. Accordingly there is ground for supposing 
 
150 
 
 MOLLUSCA 
 
 that the Monomya have been developed from Isomya- 
 like ancestors, and have lost by atrophy their anterior 
 adductor. The single adductor muscle of the Monomya 
 is separated by a z^ment 
 
 difference of fibre ?/* ^ 
 
 into two portions, ^^--si^S^iPsK;"'"" 7 
 
 but neitherof these ^"^ lunuL 
 
 can be regarded as 
 possibly represent 
 ing the anterior 
 adductor of the 
 other Lamelli- 
 branchs. One of 
 these portions is 
 more ligamentous, 
 and serves to keep 
 the two shells con- 
 stantly attached to 
 
 One another, whilst Fro. 127. Left valve of the same shell from the inner 
 
 the more fleshypor- face ' < Figs ' 125 > 126 ' 127 ^ Owen ' ) 
 
 tion serves to close the shell rapidly when it has been gaping. 
 In removing the valves of the shell from an Anodon, it 
 is necessary not only to cut through the muscular attach- 
 ments of the body-wall to the shell but to sever also a 
 strong elastic ligament, or spring resembling india-rubber, 
 joining the two shells about the umbonal area. The shell 
 of Anodon does not present these parts in the most strongly 
 marked condition, and accordingly our figures (figs. 125, 
 126, 127) represent the valves of the Sinupalliate genus 
 Cytherea. The corresponding parts are recognizable in 
 Anodon. Referring to the figures (125, 126) for an ex- 
 planation of terms applicable to the parts of the valve and 
 the markings on its inner surface corresponding to the 
 muscular area which we have already noted on the surface 
 of the animal's body we must specially note here the posi- 
 tion of that denticulated thickening of the dorsal margin 
 of the valve which is called the hinge (fig. 127). By this 
 hinge one valve is closely fitted to the other. Below this 
 hinge each shell becomes concave, above it each shell rises a 
 little to form the umbo, and it is into this ridge-like upgrowth 
 of each valve that the elastic ligament or spring is fixed (fig. 
 127). As shown in the diagram (fig. 
 127*) representing a transverse sec- 
 tion of the two valves of a Lamelli- 
 branch, the two shells form a double 
 lever, of which the toothed-hinged is 
 the fulcrum. The adductor muscles 
 placed in the concavity of the shells 
 act upon the long arms of the lever 
 at a mechanical advantage ; their con- 
 traction keeps the shells shut, and 
 stretches the ligament or spring h. 
 On the other hand, the ligament h 
 acts upon the short arm formed by 
 the umbonal ridge of the shells ; when- 
 ever the adductors relax, the elastic 
 substance of the ligament contracts, 
 and the shells gape. It is on this 
 account that the valves of a dead La- 
 mellibranch always gape ; the elastic 
 ligament is no longer counteracted by 
 the effort of the adductors. The state 
 of closure of the valves of the shell is 
 not, therefore, one of rest ; when it is 
 at rest that is, when there is no 
 muscular effort the valves of a Lamellibranch are slightly 
 gaping, and are closed by the action of the adductors when 
 the animal is disturbed. The ligament is simple in Anodon ; 
 in many Lamellibranchs it is separated into two layers, an 
 outer and an inner (thicker and denser). That the condition 
 
 shells, ligament, and ad- 
 ductor muscle, a, b, right 
 and left valves of the 
 shell ; c, d, the umbones 
 or short ai ms of the lever ; 
 e, f, the long arms of the 
 lever ; (7, the hinge ; h, the 
 ligament ; i, the adductor 
 muscle. 
 
 of gaping of the shell-valves is essential to the life of the 
 Lamellibranch appears from the fact that food to nourish 
 it, water to aerate its blood, and spermatozoa to fertilize 
 its eggs, are all introduced into this gaping chamber by 
 currents of water, which are set going by the highly- 
 developed ctenidia. The current of water enters into the 
 sub-pallial space at the spot marked e in fig. 124, (1), 
 and, after passing as far forward as the mouth w in fig. 124, 
 (5), takes an outward course and leaves the sub-pallial 
 space by the upper notch d. These notches are known 
 in Anodon as the afferent and efferent siphonal notches 
 respectively, and correspond to the long tube-like afferent 
 inferior and efferent superior " siphons " formed by the 
 mantle in many other Lamellibranchs (fig. 130). 
 
 Whilst the valves of the shell are equal in Anodon we 
 find in many Lamellibranchs (Ostrsea, Chama, Corbula, &c.) 
 one valve larger, and the other smaller and sometimes flat, 
 whilst the larger shell may be fixed to rock or to stones 
 (Ostrsea, &c.). A further variation consists in the develop- 
 ment of additional shelly plates upon the dorsal line be- 
 tween the two large valves (Pholadidse). In Pholas dactylus 
 we find a pair of umbonal plates, a dors-umbonal plate and 
 a dorsal plate. It is to be remembered that the whole of the 
 cuticular hard product produced on the dorsal surface and 
 on the mantle-flaps is to be regarded as the " shell," of 
 which a median band-like area, the ligament, usually remains 
 uncalcified, so as to result in the production of two valves 
 united by the elastic ligament. But the shelly substance 
 does not always in boring forms adhere to this form after 
 its first growth. In Aspergillum the whole of the tubular 
 mantle area secretes a continuous shelly 
 tube, although in the young condition two 
 valves were present. These are seen (fig. 
 129) set in the firm substance of the adult 
 tubular shell, which has even replaced the 
 ligament, so that the tube is complete. In 
 Teredo a similar tube is formed as the animal 
 elongates (boring in wood), the original shell- 
 valves not adhering to it but remaining mov- 
 able and provided with a special muscular 
 apparatus in place of a ligament. 
 
 Let us now examine the organs which lie 
 beneath the mantle-skirt of Anodon, and are 
 bathed by the current of water which cir- 
 
 a 
 
 Fig. 128. Fig- 129. 
 
 Fio. 128. Shell of Aspergillum vaginiferum (from Owen). 
 
 Fio. 129. Shell of Aspergillum vaginiferum to show the original valves a, now 
 embedded in a continuous calcification of tubular form (from Owen). 
 
 culates through it. This can be done by lifting up and 
 throwing back the left half of the mantle-skirt as is re- 
 presented in fig. 124, (3). We thus expose the plough- 
 like foot (/), the two left labial tentacles, and the two 
 left gill-plates or left ctenidium. In fig. 124, (5), one of 
 the labial tentacles n is also thrown back so as to show 
 
MOLLUSCA 
 
 151 
 
 and the posterior a continuation of the inner gill-plate. 
 There is no embryological evidence to support this sug- 
 gested connexion, and, as will appear immediately, the 
 history of the gill -plates in various forms of Lamelli- 
 branchs does not directly favour it. Yet it is very prob- 
 able that the labial tentacles and gill -plates are modi- 
 fications of a double horseshoe -shaped area of ciliated 
 filamentous processes which existed in ancestral Mollusca 
 much as in Phoronis and the Polyzoa, and is to be com- 
 pared with the continuous prae- and post-oral ciliated band 
 of the Echinid larva Pluteus and of Tornaria (49). 
 
 The gill-plates have a structure very different from that 
 of the labial tentacles, and one which in Anodon is singu- 
 larly complicated as compared with the condition presented 
 by these organs in some other Lamellibranchs, and with 
 what must have been their original condition in the ances- 
 tors of the whole series of living Lamellibranchia. The 
 phenomenon of " concrescence " which we have already had 
 to note as showing itself so importantly in regard to the 
 free edges of the mantle-skirt and the formation of the 
 siphons, is what, above all things, has complicated the 
 structure of the Lamellibranch ctenidium. Our present 
 knowledge of the interesting series of modifications through 
 which the Lamellibranch gill-plates have developed to their 
 most complicated form is due to R. Holman Peck (50) 
 and to Mitsukuri (51). The Molluscan ctenidium is typi- 
 cally, as shown in fig. 2, a plume-like struc- 
 ture, consisting of a vascular axis, on each 
 side of which is set a row of numerous la- 
 melliform or filamentous processes. These 
 processes are hollow, and receive the venous 
 blood from, and return it again aerated into, 
 the hollow axis, in which an afferent and an 
 efferent blood-vessel may be differentiated. 
 In the genus Nucula (fig. 134), one of the 
 
 FIG 130. Psammabiajtorida, right side, showing expanded foot e, and g incnrrent and g 1 exenrrent Arcaceae, we have an example of a Lamelli- 
 
 branch retaining this plume-like form of gill 
 
 the mouth w, and the two left gill-plates are reflected 
 so as to show the gill-plates of the right side (rr, rq) pro- 
 jecting behind the foot, the inner or median plate of each 
 side being united by concrescence to its fellow of the 
 opposite side along a continuous line (aa). The left inner 
 gill-plate is also snipped so as to show the subjacent orifices 
 of the left nephridium x, and of the genital gland (testis or 
 ovary) y. The foot thus exposed in Anodon is a simple 
 muscular tongue-like organ. It can be protruded between 
 the flaps of the mantle (fig. 124, (1), (2)) so as to issue 
 from the shell, and by its action the Anodon can slowly 
 crawl, or burrow in soft mud or sand. It has been sup- 
 posed that water is taken into the blood-vessels of the 
 Anodon through pores in the foot, and in spite of opposi- 
 tion this view is still maintained (Griesbach, 47). In fig. 
 124, (2) the letters ab, ac, ad, point to three pit-like depres- 
 sions, supposed by Griesbach to be pores leading into the 
 blood-system. According to Carriere (48) these pits are 
 nothing but irregularities of the surface ; in some cases 
 they are the entrances to ramified glands. Other Lamelli- 
 branchs may have a larger foot relatively than has Anodon. 
 In Area it has a sole-like surface. In Area too and many 
 others it carries a byssus-forming gland and a byssus- 
 cementing gland. In the Cockles, in Cardium, and in 
 Trigonia, it is capable of a sudden stroke, which causes 
 the animal to jump when out of the water, in the latter 
 
 genus to a height of four feet. In Mytilus the foot is 
 reduced to little more than a tubercle carrying the aper- 
 tures of these glands. In the Oyster it is absent alto- 
 gether. 
 
 The labial tentacles of Anodon (n, o in fig. 124, (3), (5) ) 
 are highly vascular 
 flat processes richly 
 supplied with nerves. 
 The left anterior ten- 
 tacle (seen in the 
 figure) is joined at 
 its base in front of 
 the mouth () to the 
 right anterior ten- 
 tacle, and similarly 
 the left (o) and right 
 posterior tentacles 
 are joined behind the 
 mouth. Those of 
 Area (t, k in fig. 132) 
 show this relation to 
 the mouth (a). These 
 organs are character- 
 istic of all Lamelli- 
 branchs ; they do not 
 vary except in size, 
 being sometimes 
 drawn out to 
 streamer-like dimen- 
 sions. Their appear- 
 ance and position suggest that they are in some way 
 related morphologically to the gill-plates, the anterior 
 labial tentacle being a continuation of the outer gill-plate, 
 
 FIG. 131. Diagram of a view from the left side of 
 the animal of AnodotUa cygiwea, from which the 
 mantle-skirt, the labial tentacles, and the gill- 
 filaments have been entirely removed so as to 
 show the relations of the axis of the gill-plumes 
 or ctenidia g, A. a, centre-dorsal area ; ft, ante- 
 rior addnctor muscle; c, posterior addnctor 
 muscle ; d, month ; t, anus ; /, foot ; g, free por- 
 tion of the aids of left ctenidium; k, axis of 
 right ctenidium ; fc, portion of the axis of the 
 left etenidium which is fused with the base of 
 the foot, the two dotted lines indicating the 
 origins of the two rows of gill-filaments ; m, line 
 of origin of the anterior labial tentacle ; n, ne- 
 phridial aperture ; o, genital aperture ; r, line 
 of origin of the posterior labial tentacle. (Ori- 
 ginal.) 
 
 In other Arcaceaa (e.g., Area and Pectunculus) the lateral 
 processes which are set on the axis of the ctenidium are not 
 lamellae, but are slightly-flattened, very long tubes or hol- 
 low filaments. These fila- 
 ments are so fine and are 
 set so closely together 
 that they appear to form 
 a continuous membrane 
 until examined with a 
 lens. The microscope 
 shows that the neighbour- 
 ing filaments are held to- 
 gether by patches of cilia, 
 called " ciliated junc- 
 tions," which interlock 
 with one another just as 
 two brushes may be made 
 to do. In fig. 133, A a 
 portion of four filaments 
 of a ctenidium of the Sea- 
 Mussel (Mytilus) is repre- 
 sented, having precisely } 
 the same structure as 
 those of Area. The fila- 
 ments of the gill (cteni- 
 dium) of Mytilus and 
 Area thus form two 
 closely set rows which 
 depend from the axis of 
 the gill like two parallel 
 plates. Further, their structure is profoundly modified by 
 the curious condition of the free ends of the depending 
 filaments. These are actually reflected at a sharp angle 
 
 /r 
 
 10. 132. View from the ventral (pedal) as- 
 pect of the animal of A rca A'oar, the mantle- 
 flap and gill- filaments having been cut away, 
 a, mouth ; b, anus ; c, free spirally turned 
 extremity of the gill-axis or ctenidial axis 
 of the right side ; d, do. of the left side ; 
 e, / anterior portions of these axes fused 
 by concrescence to the wall of the body ; 
 g, anterior adductor muscle ; ft, posterior 
 adductor; f, anterior labial tentacle; t, 
 posterior labial tentacle : J, base line of the 
 foot ; m, sole of the foot ; 11, callosity. 
 (Original.) 
 
152 
 
 MOLLUSCA 
 
 doubled on themselves in fact and thus form an additional 
 row of filaments (see fig. 133, B). Consequently, each primi- 
 tive filament has a descending and an ascending ramus, and 
 instead of each row forming a simple plate, the plate is 
 double, consisting of a descending and an ascending lamella. 
 As the axis of the ctenidium lies by the side of the body, 
 and is very frequently connate with the body, as so often 
 happens in Gastropods also, we find it convenient to speak 
 of the two plate-like structures formed on each ctenidial 
 axis as the outer and the inner gill-plate ; each of these is 
 
 Bate 
 
 FIG. 133. Filaments of the ctenidium of Mytilus edulis (after Holman Peck). 
 A. Part of four filaments seen from the outer face in order to show the ciliated 
 junctions c.j. B. Diagram of the posterior face of a single complete filament 
 with descending ramus and ascending ramus ending in a hook-like process. 
 ep., ep., the ciliated junctions ; il.j., inter-lamellar junction. C. Transverse 
 section of a filament taken so as to cut neither a ciliated junction nor an 
 inter-lamellar junction. /.., frontal epithelium ; i./.e'., l.f.e"., the two rows 
 of latero-frontal epithelial cells with long cilia ; ch, chitonous tubular lining 
 of the filament ; lac., blood lacuna traversed by a few processes of connective 
 tissue cells ; b.c., blood-corpuscle. 
 
 composed of two lamellae, an outer (the reflected) and an 
 adaxial in the case of the outer gill-plate, and an adaxial and 
 an inner (the reflected) in the case of the inner gill-plate. 
 This is the condition seen in Area and Mytilus, the so- 
 called plates dividing upon the slightest touch into their 
 constituent filaments, which are but loosely conjoined by 
 their "ciliated junctions." Complications follow upon 
 this in other forms. Even in Mytilus and Area a con- 
 nexion is here and there formed between the ascending 
 and descending rami of a filament by hollow extensible 
 outgrowths called " interlamellar junctions" (ilj in B, fig. 
 133). Nevertheless the filament is a complete tube formed 
 of chitonous substance and clothed externally by ciliated 
 epithelium, internally by endothelium and lacunar tissue 
 a form of connective tissue as shown in fig. 133, C. 
 Now let us suppose, as happens in the genus Dreissena 
 a genus not far removed from Mytilus that the ciliated 
 inter-filamentar junctions (fig. 136) give place to solid 
 permanent inter-filamentar junctions, so that the filaments 
 are converted, as it were, into a trellis-work. Then let us 
 suppose that the inter-lamellar junctions which we have 
 already noted in Mytilus become very numerous, large, 
 and irregular ; by them the two trellis-works of filaments 
 would be united so as to leave only a sponge-like set 
 of spaces between them. Within the trabeculse of the 
 sponge-work blood circulates, and between the trabeculae 
 the water passes, having entered by the apertures left 
 
 in the trellis-work formed by the united gill-filaments 
 (fig. 138, A, B). The larger the intra-lamellar spongy 
 
 Fro. 134. Structure of the ctenidia of Nucula (after Mitstikuri) ; see also 
 fig. 2. A. Section across the axis of a ctenidium with a pair of plates- 
 flattened and shortened filaments attached, i, j, k, g are placed on or near 
 the membrane which attaches the axis of the ctenidium to the side of the 
 body ; a, b, free extremities of the plates (filaments) ; d, mid-line of the 
 inferior border ; e, surface of the plate ; t, its upper border ; h, chitonous 
 lining of the plate ; r, dilated blood-space ; it, fibrous tract ; o, upper blood- 
 vessel of the axis ; n, lower blood-vessel of the axis ; s, chitonous framework 
 of the axis ; cp, canal in the same ; A, B, line along which the cross-section 
 C of the plate is taken. B. Animal of a male Nmula proxima, Say, as seen 
 when the left valve of the shell and the left half of the mantle-skirt are re- 
 moved, a.a., anterior adductor muscle ; p.a., posterior adductor muscle ; 
 v.m, visceral mass ; f, foot ; g, gill ; I, labial tentacle ; La., filamentous 
 appendage of the labial tentacle ; Ib, hood-like appendage of the labial ten- 
 tacle ; m, membrane suspending the gill and attached to the body along the 
 line x, y, z, w ; p, posterior end of the gill (ctenidium). C. Section across 
 one of the gill-plates (A, B, in A) comparable with fig. 133, C. i.a., outer 
 border ; d.a., axial border ; /./., latero-frontal epithelium ; e, epithelium of 
 general surface ; r, dilated blood-space ; h, chitonous lining (compare A). 
 
 growth becomes, the more do the original gill-filaments 
 lose the character of blood-holding tubes and tend to 
 become dense elastic rods for the simple purpose of sup- 
 porting the spongy growth. This is seen both in the 
 section of Dreissena gill (fig. 136) and in those of Anodon 
 (fig. 137, A, B, C). In the drawing of Dreissena the 
 individual filaments /, /, / are cut across in one lamella at 
 the horizon of an inter-filamentar junction, in the other 
 (lower in the figure) at a point where they are free. The 
 chitonous substance ch is observed to be greatly thickened 
 as compared with what it is in fig. 133, C, tending in 
 fact to obliterate altogether the lumen of the filament. 
 And in Anodon (fig. 1 37, C) this obliteration is effected. In 
 Anodon, besides being thickened, the skeletal substance of 
 the filament develops a specially dense rod-like body on 
 each side of each filament. Although the structure of the 
 ctenidium is thus highly complicated in Anodon, it is yet 
 more so in some of the Siphonate genera of Lamellibranchs. 
 The filaments take on a secondary grouping, the surface of 
 the lamella being thrown into a series of half-cylindrical 
 ridges, each consisting of ten or twenty filaments; a filament 
 
MOLLUSCA 
 
 153 
 
 of much greater strength and thickness than the others may 
 be placed between each pair of groups. In Anodon, as in 
 
 FIG. 135. Diagrams of transverse sections of a Lainellibranch to show the 
 adhesion, by concrescence, of the gill-lamella; to the mantle-flaps, to the font, 
 and to one another. A shows two conditions with free gill-axis ; B, con- 
 lition at foremost region in Anodon ; C, hind region of foot in Anodon ; D, 
 region altogether posterior to the foot in Anodon. a, visceral mass ; b, foot ; 
 c, mantle flap ; d, axis of gill or ctenidinm ; , adaxial lamella of outer gill- 
 plate ; er, reflected lamella of outer gill-plat* ; / adaxial lamella of inner 
 gill-plate ; fr, reflected lamella of inner gill-plate ; y, line of concrescence of 
 the reflected lamella; of the two inner gill-plates ; A, rectum ; i, supra-branchial 
 space of the sub-pallial chamber. (Original.) 
 
 many other Lamellibranchs, the ova and hatched embryos 
 are carried for a time in the ctenidia or gill apparatus, and 
 in this particular case the space between the two lamellae 
 
 
 chitonous substance; toe, lacunar tissue; pig, 'pigment-cells- 6r blood- 
 corpuscles ; Jt, frontal epithelium ; Iff, (ft", two rows of latero-frontal epi- 
 thelial cells with long cilia ; Irf, fibrous, possibly muscular, substance of the 
 inter-filamenter junctions. 
 
 of the outer gill-plate is that which serves to receive the 
 ova (fig. 137, A). The young are nourished by a substance 
 
 formed by the cells which cover the spongy inter-lamellar 
 outgrowths. 
 
 There are certain other points in the modification of the 
 typical ctenidium which must be noted in order to under- 
 stand the ctenidium of Anodon. The axis of each ctenid- 
 ium, right and left, starts from a point well forward near 
 the labial tentacles, but it is at first only a ridge, and does 
 not project as a free cylindrical axis until the back part of 
 
 o.l A 
 
 oil 
 
 FIG. 137. Transverse sections of gill-plates of Anodon (after Peck). A. Outer 
 gill-plate. B. Inner gill-plate. C. A portion of B more highly magnified. 
 o.l, outer lamella ; i.f, inner lamella ; r, blood-vessel : / constituent fila- 
 ments ; lac, lacunar tissue ; c*, chitonous substance of the filament dkr 
 chitonous rod embedded in the softer substance c*. 
 
 the foot is reached. This is difficult to see at all in Ano- 
 don, but if the mantle-skirt be entirely cleared away, and 
 if the dependent lamellae which spring from the ctenidial 
 axis be carefully cropped away so as to leave the axis itself 
 intact, we obtain the form shown in fig. 131, where g and 
 h are respectively the left and the right ctenidial axes pro- 
 jecting freely beyond the body. In Area this can be seen 
 with far less trouble, for the filaments are more easily re- 
 moved than are the consolidated lamellae formed by the 
 filaments of Anodon, and in Area the free axes of the 
 ctenidia are large and firm in texture (fig. 132, c, d). 
 
 If we were to make a vertical section across the long 
 axis of a Lamellibranch which had the axis of its ctenidium 
 free from its origin onwards, we should find such relations 
 as are shown in the diagram fig. 135, A. The gill axis d 
 is seen lying in the sub-pallial chamber between the foot 
 b and the mantle c. From it depend the gill-filaments or 
 lamellae formed by united filaments drawn as black lines 
 /. On the left side these lamellae are represented as hav- 
 ing only a small reflected growth, on the right side the 
 reflected ramus or lamella is complete (fr and er). The 
 actual condition in Anodon at the region where the gills 
 commence anteriorly is shown in fig. 135, B. The axis of 
 the ctenidium is seen to be adherent to, or fused by con- 
 crescence with, the body-wall, and moreover on each side 
 the outer lamella of the outer gill-plate is fused to the 
 nantle, whilst the inner lamella of the inner gill-plate is 
 fused to the foot. If we pass a little backwards and take 
 another section nearer the hinder margin of the foot, we 
 
 u 
 
154 
 
 MOLLUSCA 
 
 get the arrangement shown diagrammatically in fig. 135, 
 C, and more correctly in fig. 142. In this region the inner 
 lamellae of the inner gill-plates are no longer affixed to the 
 foot. Passing still further back behind the foot, we find 
 
 FIG. 138. Gill-lamellse of Anodon (after Peck). A. Fragment of the outer 
 lamella of an inner gill-plate torn from the connected inner lamella, the sub- 
 filamentar tissue also partly cut away round the edges so as to expose the 
 filaments, their transverse junctions tr, and the "windows" left in the lattice- 
 work ; sfe, internal surface of the lamella ; v, vessel. B. Diagram of a block 
 cut from the outer lamella of the outer gill-plate and seen from the inter- 
 lamellar surface (after Peck). /, constituent filaments ; trf, fibrous tissue of 
 the transverse inter-filamentar junctions; v, blood-vessel; ilj, inter-lamellar 
 junction. The series of oval holes on the back of the lamella are the water- 
 pores which open between the filaments in irregular rows separated horizon- 
 tally by the transverse inter- filamentar junctions. 
 
 in Anodon the condition shown in the section D, fig. 135. 
 The axes i are now free ; the outer lamellae of the outer 
 gill-plates (er) still adhere by concrescence to the mantle- 
 skirt, whilst the inner lamellae of the inner gill-plates meet 
 one another and 
 fuse by concres- 
 cence at g. In 
 the lateral view of 
 the animal with 
 reflected mantle - 
 skirt and gill- 
 plates, the line of 
 concrescence of the _ 
 
 , ,. - Fro. ISO. Transverse sections of A, a Lamellibranch, 
 
 inner lamellae Ot and B, an Isopleurous Gastropod (Chiton), to show 
 fh inner trill t' ie relations of j>, the foot; !-, the branchiae ; and 
 ' m, the mantle. (From Gegenbaur.) 
 
 plates is readily 
 
 seen; it is marked aa in fig. 124, (5). In the same 
 figure the free part of the inner lamella of the inner 
 gill-plate resting on the foot is marked z, whilst the 
 attached part the most anterior has been snipped 
 with scissors so as to show the genital and nephridial 
 apertures x and y. The concrescence, then, of the 
 free edge of the reflected lamellae of the gill-plates of 
 Anodon is very extensive. It is important, because such 
 a concrescence is by no means universal, and does not 
 occur, for example, in Mytilus or in Area ; further, because 
 
 when its occurrence is once appreciated, the reduction of 
 the gill-plates of Anodon to the plume-type of the simplest 
 ctenidium presents no difficulty ; and, lastly, it has import- 
 ance in reference to its physio- 
 logical significance. The me- 
 chanical result of the concres- 
 cence of the outer lamellae to 
 the mantle-flap, and of the 
 inner lamellae to one another 
 as shown in section D, fig. 
 135, is that the sub-pallial 
 space is divided into two 
 
 spaces by a horizontal sep- FIG. HO. Lateral view of a Mactra, 
 tum. The Upper Space (i) 'e nght^valve of the shell and right 
 
 communicates with the outer 
 world by the excurrent or su- 
 perior siphonal notch of the 
 mantle (fig. 1 24, d) ; the lower 
 space communicates by the 
 lower siphonal notch (e in fig. 124). 
 
 man tie -flap removed, and the si- 
 phons retracted. &r, &r', outer and 
 inner gill-plates ; t, labial tentacle ; 
 ta, tr t upper and lower siphons ; ms, 
 siphonal muscle of the mantle-flap ; 
 ma, anterior adductor muscle ; mp, 
 posterior adductor muscle ; p, foot ; 
 c, umbo. (From Gegenbaur.) 
 
 The only communica- 
 tion between the two spaces, excepting through the trellis- 
 work of the gill-plates, is by the slit (z in fig. 124, (5)) left 
 by the non-concrescence of a part of the inner lamella of the 
 inner gill-plate with the foot. A probe (<?) is introduced 
 through this slit-like passage, and it is seen to pass out by 
 the excurrent siphonal notch. It is through this passage, 
 or indirectly through the pores of the gill-plates, that the 
 water introduced into the lower sub-pallial space must pass 
 on its way to the excurrent siphonal notch. Such a 
 subdivision of the pallial chamber, and direction of the 
 
 FIG. 141. The same animal as fig. 140, with its foot and siphons expanded. 
 Letters as in fig. 140. (From Gegenbaur.) 
 
 currents set up within it do not exist in a number of 
 Lamellibranchs which have the gill-lamellae comparatively 
 free (Mytilus, Area, Trigonia, ifec.), and it is in these forms 
 that there is least modification by concrescence of the pri- 
 mary filamentous elements of the lamellae. Probably the 
 gill -structure of Lamellibranchs will ultimately furnish 
 some classificatory characters of value when they have 
 been thoroughly investigated throughout the class. 
 
 The alimentary canal of Anodon is shown in fig. 124, (4). 
 The mouth is placed between the anterior adductor and 
 the foot; the anus opens on a median papilla overlying 
 the posterior adductor, and discharges into the superior 
 pallial chamber along which the excurrent stream passes. 
 The coil of the intestine in Anodon is similar to that of 
 other Lamellibranchs, but the crystalline style and its 
 diverticulum are not present here. The rectum traverses 
 the pericardium, and has the ventricle of the heart wrapped, 
 as it were, around it. This is not an unusual arrangement 
 in Lamellibranchs, and a similar disposition occurs in some 
 Gastropoda (Haliotis). A pair of ducts (ai) lead from the 
 first enlargement of the alimentary tract called stomach 
 into a pair of large digestive glands, the so-called liver, 
 the branches of which are closely packed in this region 
 (a/). The food of the Anodon, as of other Lamellibranchs, 
 consists of microscopic animal and vegetable organisms, 
 which are brought to the mouth by the stream which sets 
 into the sub-pallial chamber at the lower siphonal notch 
 (e in fig. 124). Probably a straining of water from solid 
 
MOLLUSCA 
 
 155 
 
 particles is effected by the lattice-work of the ctenidia or 
 gill-plates. 
 
 The heart of Anodon consists of a median ventricle em- 
 bracing the rectum (fig. 143, A), and giving off an anterior 
 and a posterior artery, and of two auricles which open into 
 the ventricle by orifices protected by valves. 
 
 The blood is colourless, and has colourless amoeboid 
 corpuscles floating in it. In two Lamellibranchs, Solen, 
 (Ceratifolen) legumen and Area Nox, the blood is crimson, 
 owing to the presence of corpuscles impregnated with 
 haemoglobin (Lankester, 31). In Anodon the blood is 
 driven by the ventricle through the arteries into vessel- 
 like spaces, which soon become irregular lacunae surround- 
 ing the viscera, but in parts e.g., the labial tentacles and 
 walls of the gut very fine vessels with endothelial cell- 
 lining are found. The blood makes its way by large 
 veins to a venous sinus which lies in the middle line be- 
 low the heart, having the paired renal organs (nephridia) 
 placed between it and that organ. Hence it passes 
 through the vessels of the glandular walls of the nephridia 
 right and left into the gill-lameLUe, whence it returns 
 through many openings into the widely-stretched auricles. 
 A great deal more pre- 
 cision has been given to 
 accounts of the structure 
 of arteries, veins, and 
 capillaries in Anodon 
 than the facts warrant. 
 The course of the blood- 
 stream can only be some- 
 what vaguely inferred ex- 
 cept in its largest out- 
 lines. Distinct arterial 
 and venous channels can- 
 not be distinguished in 
 the gill-lamellae, in spite 
 of what Langer (52) has 
 written on the subject, 
 though it is highly prob- 
 able that there is some 
 
 kind of Circulation in the 
 
 mlla Tn trip filampnt 
 glllS. in me mamentS 
 
 of the gill of Mytilus the 
 . , .. j- -j j 
 
 tubular cavity is divided 
 by a more or less complete fibrous septum into two 
 channels, presumably for an ascending and a descend- 
 ing blood-current. The ventricle and auricles of Anodon 
 lie in a pericardium which is clothed with a pave- 
 ment endothelium (d, fig. 143). Veins are said by Keber 
 and others to open anteriorly into it, but this appears to 
 be an error. It does not contain blood or communicate 
 directly with the blood-system ; this isolation of the peri- 
 cardium we have noted already in Gastropods and Cephalo- 
 pods. A good case for the examination of the question as 
 to whether blood enters the pericardium of Lamellibranchs, 
 or escapes from the foot, or by the renal organs when the 
 animal suddenly contracts, is furnished by the Solen legu- 
 men, which has red blood-corpuscles. According to ob- 
 servations made by Penrose (53) on an uninjured Solen 
 legumen, no red corpuscles are to be seen in the pericardial 
 space, although the heart is filled with them, and no such 
 corpuscles are ever discharged by the animal when it is 
 irritated. 
 
 The pair of nephridia of Anodon, called in Lamelli- 
 branchs the organ of Bojanus, lie below the membranous 
 floor of the pericardium, and open into it by two well- 
 marked apertures (e and/ in fig. 143). Each nephridium, 
 after being bent upon itself as shown in fig. 143, C, D, 
 opens to the exterior by a pore placed at the point marked 
 x in fig. 124, (5), (6). It is no doubt possible, as in the 
 
 142 ._ Vertiall ^^ Oaaagl 
 
 donta, about the mid-region of the foot, w, 
 mantle-flap ; br, onter, bY, inner gill-plate 
 _e^ composed of two lamellae;/, foot ; r, 
 ventricle of the heart ; a, auricle ; p, p', 
 pericardial cavity ; i, intestine. 
 
 Gastropoda and Cephalopoda, for water to enter from the 
 exterior by the nephridia into the pericardium, but that 
 it ever does so is as yet not proved. What is certain 
 from the set of the ciliary currents is that liquid generally 
 
 ae 
 
 FIG. 148. Diagrams showing the relations of pericardium and nephridia in a 
 Lamellibranch such as Anodon. A. Pericardium opened dorsally so ao to 
 expose the heart and the Boor of the pericardial chamber d. B. Heart 
 removed and floor of the pericardium cut away on the left side so as to open 
 the non-glandular sac of the nephridium, exposing the glandular sac 6, 
 which is also cut into so as to show the probe /. C. Ideal pericardium and 
 nephridium viewed laterally. D. Lateral view showing the actual relation 
 of the glandular and non-glandular sacs of the nephridium. The arrows 
 indicate the course of fluid from the pericardium outwards, o, ventricle of 
 the heart ; 6, auricle ; 66, cut remnant of the auricle ; c. dorsal wall of the 
 pericardium cut and reflected ; c, renp-pericardial orifice ; /, probe intro- 
 duced into the left reno-pericardial orifice ; g, non-glandular sac of the left 
 nephridium ; A, glandular sac of the left nephridium ; t, pore leading from 
 the glandular into the non-glandular sac of the left nephridium ; t, pore 
 leading from the non-glandular sac to the exterior ; ac, anterior, 06, posterior, 
 cut remnants of the intestine and ventricle, 
 
 passes out of the pericardium by the nephridia. One half 
 of each nephridium is of a dark-green colour and glandular 
 (h in fig. 143). This opens into the reflected portion which 
 overlies it as shown in the diagram fig. 143, D, t ; the latter 
 has non-glandular walls, and opens by the pore k to the 
 exterior. The nephridia may be more ramified in other 
 Lamellibranchs than they are in Anodon. In some they 
 are difficult to discover. That of the common oyster 
 has recently (1882) been detected by Hoek (54). Each 
 nephridium in the oyster is a pyriform sac, which commu- 
 nicates by a narrow canal with the urino-genital groove 
 placed to the front of the great adductor muscle ; by a 
 second narrow canal it communicates with the pericardium. 
 From all parts of the pyriform sac narrow stalk-like tubes 
 are given off, ending in abundant widely-spread branching 
 glandular caeca, which form the essential renal secreting 
 apparatus. The genital duct opens by a pore into the 
 urino-genital groove of the oyster (the same arrangement 
 being repeated on each side of the body) close to but distinct 
 from the aperture of the nephridial canal Hence, except 
 for the formation of a urino-genital groove, the apertures 
 are placed as they are in Anodon. Previously to Hoek's 
 discovery a brown-coloured investment of the auricles of 
 the heart of the oyster had been supposed to represent 
 the nephridia in a rudimentary state. This investment, 
 which occurs also in Mytilus but not in Anodon, may pos- 
 sibly consist of secreting cells, and may be comparable to 
 the pericardial accessory glandular growths of Cephalopoda. 
 Nervous System and Sense-organs. In Anodon there are 
 three well-developed pairs of nerve-ganglia (fig. 1 44, B and 
 fig. 1 24, (6)). An anterior pair, lying one on each side of the 
 
156 
 
 MOLLUSCA 
 
 mouth (fig. 144, B, a) and connected in front of it by a 
 commissure, are the representatives of the cerebral, pleural, 
 and visceral ganglia of the typical Mollusc, which are not 
 here differentiated as they are in Gastropods (compare, 
 however, fig. 67). A pair placed close together in the foot 
 (fig. 144, B, b, and fig. 
 124, (6), ax) are the typ- 
 ical pedal ganglia ; they 
 are joined to the cerebro- 
 pleuro - visceral ganglia 
 by connectives. 
 
 Posteriorly beneath 
 the posterior adductors, 
 and covered only by a 
 thin layer of elongated 
 epidermal cells, are the 
 olfactory ganglia, their 
 epidermal clothing con- 
 stituting the pair of os- 
 phradia, which are thus 
 seen in Lamellibranchs 
 to occupy their typical 
 position and to have the 
 typical innervation, the 
 
 TIPT-VP fn pa nil nsnlirarl Fla 144. Nerve-ganglia and cords of three 
 
 OSpJiraa- La memt , ranchs (from Gegenbaur): A, of 
 
 ium being given off by Teredo ; B, of Anodonta ; C, of Pecten. a, 
 
 ,1 i v cerebral ganglion-pair ( = cerebro-pleuro- 
 
 tne Visceral ganglion visceral) ; 6, pedal ganglion-pair ; c, olfac- 
 
 that is to say, by the tory (osphradial) ganglion-pair. 
 
 undifferentiated cerebro-pleuro- visceral ganglion of its 
 proper side. This identification of the posterior ganglion- 
 pair of Lamellibranchs is due to Spengel (11). Other 
 
 \ 
 
 
 FIG. 145. Pallial eye of Spondylus (from Hickson). a, pree-corneal epithe- 
 lium ; b, cellular lens ; c, retinal body ; d, tapetum ; e, pigment ; /, retinal 
 nerve ; g, complementary nerve ; h, epithelial cells filled with pigment ; k, 
 tentacle 
 
 anatomists have considered this ganglion-pair as corre- 
 sponding to either the pleural or the visceral of Gastropoda, 
 or to both, and very usually it is termed "the parieto- 
 splanchnic " (Huxley). 
 
 The sense-organs of Anodon other than the osphradia 
 consist of a pair of otocysts attached to the pedal ganglia 
 (fig. 124, (6), ay). The otocysts of Cyclas are peculiarly 
 favourable for study on account of the transparency of the 
 small foot in which they lie, and may be taken as typical 
 of those of Lamellibranchs generally. The structure of 
 one is exhibited in fig. 146. A single otolith is present 
 as in the veliger embryos of Opisthobranchia. In adult 
 Gastropoda there are frequently a large number of rod-like 
 otoliths instead of one. 
 
 Anodon has no eyes of any sort, and the tentacles on the 
 mantle edge are limited to its posterior border. This 
 deficiency is very usual in the class; at the same time, many 
 Lamellibranchs have tentacles on the edge of the mantle 
 supplied by a pair of large well-developed nerves, which 
 are given off from the cerebro-pleuro-visceral ganglion-pair, 
 
 and very frequently some of these tentacles have undergone 
 a special metamorphosis converting them into highly- 
 organized eyes. Such eyes on the mantle-edge are found 
 in Pecten, Spondylus, Lima, Ostrea (?), Pinna, Pectunculus, 
 Modiola, Mytilus (?), Cardium, Tellina, 
 Mactra, Venus, Solen, Pholas, and Ga- 
 leomma. They are totally distinct from 
 the cephalic eyes of typical Mollusca, and 
 have a different structure and historical de- 
 velopment. They have not originated as 
 pits but as tentacles. They agree with the Fm 146 _ oto t 
 dorsal eyes of Onchidium (Pulmonata) in of Cyclas (from 
 the curious fact that the optic nerve pene- ra^uie^fciiiated 
 trates the capsule of the eye and passes in ceils lining the 
 front of the retinal body (fig. 145), so that 
 its fibres join the anterior faces of the nerve-end cells as 
 in Vertebrates, instead of their posterior faces as in the 
 cephalic eyes of Mollusca and Arthropoda ; moreover, the 
 lens is not a cuticular product but a cellular structure, 
 which, again, is a feature of agreement with the Vertebrate 
 eye. It must, however, be distinctly borne in mind that 
 there is a fundamental difference between the eye of Verte- 
 brates and of all other groups in the fact that in the 
 Vertebrata the retinal body is itself a part of the central 
 nervous system, and not a separate modification of the 
 epidermis myelonic as opposed to epidermic. The struc- 
 ture of the reputed eyes of several of the above-named 
 genera has not been carefully examined. In Pecten and 
 Spondylus, however, they have been fully studied (see fig. 
 145, and explanation). 
 
 The gonads of Anodon are placed in distinct male and 
 female individuals. In some Lamellibranchs for instance, 
 the European Oyster and the Pisidium pusillum the sexes 
 are united in the same individual; but here, as in most 
 hermaphrodite animals, the two sexual elements are not 
 ripe in the same individual at the same moment. It has 
 been conclusively shown that the Ostrea edulis does not 
 fertilize itself. The American Oyster (0. virginiana) and 
 the Portuguese Oyster (0. angulata) have the sexes sepa- 
 rate, and fertilization is effected in the open water after 
 the discharge of the ova and the spermatozoa from the 
 females and males respectively. In the Ostrea edulis fertil- 
 ization of the eggs is effected at the moment of their escape 
 from the uro-genital groove, or even before, by means of 
 spermatozoa drawn into the sub-pallial chamber by the in- 
 current ciliary stream, and the embryos pass through the 
 early stages of development whilst entangled between the 
 gill-lamellae of the female parent (fig. 6). In Anodon the 
 eggs pass into the space between the two lamella? of the 
 outer gill-plate, and are there fertilized, and advance whilst 
 A ,.. B 
 
 p-ad 
 
 at 
 
 FIG. 147. Two stages in the development of Anodonta (from Balfour). Both 
 figures represent the glocliidium stage. A, when free swimming, shows the 
 two dentigerous valves widely open. B, a later stage, after fixture to the fin 
 of a fish, sh, shell ; ad, adductor muscle ; s, teeth of the shell ; by, byssus ; 
 a. ad, anterior adductor ; p.ad, posterior adductor ; mt, mantle-flap ; /, foot ; 
 br, branchial filaments ; au.v, otocyst ; a!, alimentary canal. 
 
 still in this position to the glochidium phase of develop- 
 ment (fig. 147). They may be found here in thousands 
 in the summer and autumn months. The gonads them- 
 selves are extremely simple arborescent glands which open 
 to the exterior by two simple ducts, one right and one 
 
MOLLUSCA 
 
 157 
 
 left, continuous with the wall of the tubular branches of 
 the gland (fig. 124, (5), (6), y). In no Lamellibranch is 
 there a divergence from this structure, excepting that in 
 some (Ostrea) the contiguous nephridial and the genital 
 aperture are sunk in a urino-genital groove, which in other 
 cases (Spondylus ?) may partially close up so as to con- 
 stitute a single pore for the nephridial and genital ducts. 
 No accessory genital glands are present. 
 
 The development of Anodon is remarkable for the curious 
 larval form known as Glochidium (fig. 147). TheGlochidium 
 
 il 
 
 B 
 
 Fig. 118. Embryos of Puuiivn piuiflua (after Lankester). A. Oniy four 
 embryoniewlls are present, still enclosed in the egg envelope. B. The cells 
 have multiplied and commenced to invaginate, forming a blastopore or orifice 
 of imagination, bL 
 
 quits the gill-pouch of its parent and swims by alternate 
 opening and shutting of the valves of its shell, as do 
 adult Pectcn and Lima, trailing at the same time a long 
 
 
 Fig. 150. 
 
 FIG. 149. Embryo of Pisid Sum putinum in the diblastula stage, surface view 
 (after Lankester). The embryo has increased in size by accumulation of 
 liquid between the outer and the iuvaginated cells. The blastopore has 
 closed. 
 
 Fio. 150. B. Same embryo as fig. 143, in optical median section, showing the 
 invaginated cells liy which form the arch-enteron, and the mesoblastic cells 
 me which are budded off from the surface of the mass tiy, and apply them- 
 selves to the inner surface of the dcric or epiblastic cell-layer ep. C. The 
 same embryo focused so as to show the mesoblastic cells which immediately 
 underlie the outer cell-layer. 
 
 byssus thread. By this it is brought into contact with the 
 fin of a fish, such as Perch, Stickleback, or others, and effects 
 
 a hold thereon by means of the toothed edge of its shells. 
 Here it becomes encysted, and is nourished by the exuda- 
 tions of the fish. A distinct development of its internal 
 organs has been traced by the late Professor Balfour, but no 
 one has followed it to the moment at which it drops from 
 the fish's fin and assumes the form of shell characteristic of 
 the parent. Other Lamellibranchs exhibit either a trocho- 
 sphere larva which becomes a Veliger, differing only from 
 the Gastropod's and Pteropod's Veliger in having bilateral 
 shell-calcifications instead of a single central one ; or, like 
 Anodon, they may develop within the gill-plates of the 
 mother, though without presenting such a specialized larva 
 as the Glochidium. An example of the former is seen in the 
 
 A 
 
 FIG. 151. Further stages in the development of Pitic/ivn pvsittum (after 
 Lankester). A- Optical section of an embryo in which the foot has begun to 
 develop. B. The same embryo focused to its surface plane to show the 
 month o. C. Later embryo, showing the shell-gland M. D. Lateral view of 
 the same embryo. E. Later stage, with rudiments of the mantle-flap, lateral 
 view. F. Still later stage, with shell-valves and branchial filaments, ep, 
 epiblast ; me-, mesoblast ; oi, met-enteron ; rp, rectal peduncle or pedicle of 
 imagination connecting the met-enteron with the cicatrix of the blastopore ; 
 o, mouth ; pk, pharynx ; sh, shell-gland ; win, mantle-flap ; 6r, branchial 
 filaments ; y, granular cells of doubtful significance ; r, vesicular structure 
 of unknown significance. 
 
 development of the European Oyster, to the figure of which 
 and its explanation the reader is specially referred (fig. 6). 
 An example of the latter is seen in a common little 
 fresh-water bivalve, the Piridium pusillum, which has been 
 studied by Lankester (12). The successive stages of the 
 development of this Lamellibranch are illustrated in the 
 woodcuts figs. 148 to 153 inclusive. These should be 
 compared with the figures of Gastropod development 
 (figs. 3, 4, 5, 7, and 72***). Fig. 148 shows the cleavage 
 of the egg-cell into four (A), and at a later stage the tucking 
 in of some of the cells to form an invaginated series (B). 
 
158 
 
 MOLLUSCA 
 
 The embryonic cells continue to divide, and form an oval 
 vesicle containing liquid (fig. 149); within this, at one pole, 
 is seen the mass of invaginated cells (fig. 150, hy). These 
 invaginated cells are the arch-enteron ; they proliferate and 
 give off branching cells, which apply themselves (fig. 150, 
 C) to the inner face of the vesicle, thus forming the meso- 
 
 Fio. 152. Diagram of embryo of Pisidium in the same stage as E in fig. 151. 
 m, mouth ; /, foot ; ph, pharynx ; gs, met-enteron ; pi, rectal peduncle or 
 pedicle of invaginatiou ; shs, shell-gland. (From Lankester.) 
 
 blast or ccelomic outgrowths. The outer single layer of 
 cells which constitutes the surface of the vesicle (fig. 
 147) is the ectoderm or epiblast or deric cell-layer. The 
 little mass of hypoblast or 
 enteric cell-mass now en- 
 larges, but remains con- 
 nected with the cicatrix of \ V >n 
 the blastopore or orifice of 
 invagination by a stalk, the 
 rectal peduncle (fig. 151, A, 
 rp). The enteron itself be- 
 comes bilobed and is joined 
 by a new invagination, that 
 of the mouth and stomo- 
 dseum, ph. Fig. 151, B 
 shows the origin of the 
 mouth o, being a deeper 
 
 view Of the Same specimen Fio. 153.-Diagram of embryo of Pisidinm, 
 
 . . r , . , in same stage as F in iig. 151 (after Lan- 
 
 m the same position Which kester). m, mouth ; x, anus ; /, foot ; br, 
 
 rtra-nrti in fio- 1^1 A branchial filaments ; mn, margin of the 
 
 III iig. iux, A. man tie-skirt ; B, organ of Bojanus (ne- 
 
 Tll6 mesoblast multiplies phridium). The unshaded area gives 
 
 its Cells, Which become the position of the shell-valve. 
 
 partly muscular and partly skeleto-trophic. Centro-dor- 
 sally now appears the embryonic shell-gland (fig. 151, 
 C, sh). The pharynx or stomodeeum is still small, the 
 foot not yet prominent. A later stage is seen in fig. 
 152, where the pharynx is widely open and the foot pro- 
 minent. No ciliated velum or prie-oral (cephalic) lobe 
 ever develops. The shell-gland disappears, the mantle- 
 skirt is raised as a ridge (fig. 151, E, mn), the paired 
 shell-valves are secreted, the anus opens by a proctodseal 
 ingrowth into the rectal peduncle, and the rudiments of 
 the gills (br) and of the nephridia (B) appear (figs. 151, 
 F, and 153, dorsal and lateral views of same stage), and 
 thus the chief organs and general form of the adult are 
 
 acquired. Later changes, not drawn here, consist in the 
 growth of the shell-valves over the whole area of the 
 mantle-flaps, and in the multiplication of the gill-fila- 
 ments and their consolidation to form gill -plates. It 
 is important to note that the gill-filaments are formed 
 one by one posteriorly. The labial tentacles are formed 
 late. In the allied genus Cyclas, a byssus gland is formed 
 in the foot and subsequently disappears, but no such gland 
 occurs in Pisidium. The nerve-ganglia and the otocysts 
 probably form from thickenings of the epiblast, but detailed 
 observation on this and other points of histogenesis in the 
 Lamellibranchia is still wanting. 
 
 List of Memoirs, c., referred to by numbers in the preceding article. (1) G. 
 Cuvier, Memoires pour servir a I'histoire et a I'anatomie des Mollusques, Paris, 
 1816. (2) J. Poll, Testacea utriusque Sicilies, eorumque historia et anatome, 
 tabulis aeneis 49 illustrata, vols. i.-iii., fol., Parma, 1791-1795 and 1826-1827. 
 
 (3) St delle Chiaje, Memarie sulla storia e notomia degli animali senza vertebre 
 del regno di Napoli, Naples, 1823-1829 ; new edit, with 172 plates, fol., 1843. 
 
 (4) J. Vaughan Thompson, Zoologiml Kesearch.es, Cork, 1830 ; memoir iv., "On 
 the Cirripedes or Barnacles, demonstrating their deceptive character." (5) A. 
 Kowalewsky, " Entwickelungsgeschichte der einfachen Ascidien," in Mfm. de 
 I'Acad. des Sciences de St Pettrsbourg, 1860, and " Entwickelungsgeschichte 
 des Amphiaxus lanceolatus," iliil., 1807. (6) J. Vaughan Thompson, Zoological 
 Researches, Cork, 1830 ; memoir v., " Polyzoa, a new animal discovered as 
 an inhabitant of some Zoophytes." (7) C. G. Ehrenberg, Die Korallenthiere des 
 Rothen Meeres, Berlin, 1834 (Abhand. d. k. Akad. d. Wissenschaften in Berlin, 
 1S32). (8) H. Milne-Edwards, Recherches anatomiques physiologiques et zpolo- 
 
 branchiaten u. Gastropoderi," Zoolog. Anzeiger, 1881, No. 90. (15) E. Kay 
 Lankester, " Development of the Pond-Snail," Quart. Journ. Mic 
 
 " 
 
 . 
 
 icrosc. Sc., 1874, 
 
 , 
 
 and "Shell-gland of Cyclas and Planula of Limnaeus," ibid., 1876. 
 Horst, "Development of the European Oyster," Quart. Journ. Microsc. Sc., 
 
 (16) R. 
 
 1S82, p. 341. (17) E. Ray Laukester, "Coincidence of the blastopore and 
 anus in Paludina," Quart. Journ. Microsc. Sc., 1870. (18) Id., "Zoological 
 Observations made at Naples," Annals and Mag. Nat. Hist., February, 1873. 
 (19) W. K. Brooks, "Development of the American Oyster," Report of the 
 Commissioners of Fisheries of Maryland, 1860. (20) Henri Milne-Edwards, 
 
 Vermetus " (1800). (22) A. Kolliker, Entwickelungsgeschichle der Cepltalopoden, 
 Zurich, 1844. (23) C. Gegenbaur, Untersucliungen uber Pteropoden und Hetero- 
 poden, Leipsic, 1855. (24) J. W. Spengel, " Die Geruchsorgane und das Nerven- 
 system der Mollusken," Zeitschr. f. wiss. Zool., 1881. (25) A. A. W. Hubrecht, 
 
 jf Chiton," Proc. Roy. Soc. Land., 1881. (27) E. Ray Lankester, "On some 
 undescribed points in the anatomy of the Limpet," Annals and Mag. Nat. 
 History, 1867 ; J. T. Cunningham, "The Renal Organs of Patella," Quart. Journ. 
 Microsc. Sc., 1883. (28) P. Fraisse, " Ueber Molluskenaugen mit embryonalem 
 Typus," Zeitschr. f. mss. Zool, 1881. (29) L. v. Graff, " Ueber Rhodope Veranit, 
 Koll.," Morpholog. Jahrb., vol. viii. (30) H. Simroth, " Das Fussnervcnsystem 
 der Paludina vivipara," Zeitschr. f. wiss. Zool., 1881. (31) E. Ray Lankester, 
 "A contribution to the knowledge of Hemoglobin," Proc. Roy. Soc. Land., 
 1873. (32) H. de Lacaze Duthicrs, "Du systeme nerveux des Mollusques 
 Gasteropodes Pulmones aquatiques et d'un nouvelorgane d'innervation," Anh. 
 de Zoologie eipcrimentale, vol. i. (33) C. Semper, Animal Life (for eye of 
 Onchidium, p. 371), International Scientific scries, 1881. (34) Same as number 
 18. (35) E. Ray Lankester, " Observations on the development of the Cephalo- 
 poda," Quart. Journ. Microsc. Sc., 1875. (36) J. van der Hoeven, " Bijdrage 
 tot de outleedkundige kennis aangaaende Nautilus pompilius," Verliandl. d.. 
 K. Akad. v. Wet. Naturk., Amsterdam, 1856. (37) E. Ray Lankester and A. G. 
 Bourne, "On the existence of Spongers olfactory organ and of paired genital 
 ducts in the Pearly Nautilus," Quaff. Journ. Microsc. Sc., 1883. (38) J. W. 
 Vi^elius "Ueber das Excretions-System der Cephalopoden," Niederldndisches 
 Archiv fiir Zoologie, bd. v., 1880. (39) Albany Hancock, "On the nervous 
 system of Ommastrephes todarus," Annals and Mag. Nat. Hist., 1852. (40) 
 J. D. Macdonald, "On the anatomy of Nautilus umbilicatvs," Phil. Trans, oj 
 Roy. Soc. Lond., 1S55. (41) V. Hensen, " Ueber das Auge einiger Cephalopoden," 
 Zeitschr. f. wiss. Zool., 1865. (42) A. d'Orbigny, Mollwstpies vivants et fossiles, 
 t. i. (Cephalopodes acetabuliferes), Paris, 1845 (with 36 plates). (43) Bobretzky, 
 "On the development of the Cephalopoda," Trans, of Soc. of friends of Nat. 
 Hist, of Moscow, vol. xxiv. (Russian). () T. H. Huxley, " Oviducts of the 
 Smelt," Proc. Zool. Soc. Lond., 1SS3. (45) Same as 35. (46) F. M. Balfour, 
 Comparative Embryology, vols. i. and ii., London, 1881-1882. (47) H. Gries- 
 bach " Ueber das Gefass-System und die Wasseraufnahme bei den Najaden und 
 Mytiliden," Zeitschr. f. wiss. Zool., 1SS3. (48) Same as 14. (49) Same as 13. 
 
 Akad. d. Wiss'ensch., Vienna, 1805-1866. (53) J. Penrpse, in "Report of the 
 Committee on the Zoological Station of Naples," British Assoc. Report, 1882. 
 
 (54) P. P. C. Hoek, "Les orgaiics de la generation de 1'huitre," Journ. de la Soc. 
 Neerlandaise ae Zool, 1883. (E. R. L.) 
 
P L Y Z A 
 
 T)OLYZOA is the name applied by J. Vaughan Thompson 
 \_ in 1 730 (I) 1 to a group of minute polyp-like organisms 
 which were subsequently (1834) termed "Bryozoa" by 
 Ehrenberg (2). The forms included in this group were 
 stated by Thompson to be "in a general way the whole 
 of the Flustraceae, in many of which I have clearly ascer- 
 tained the animals to be Polyzoae," they having been pre- 
 viously considered by zoologists to be allied to the Hydra- 
 like polyps. These organisms had previously been known 
 by the hard corneous " cells " or chambers which are formed 
 by the animals on the surface of their bodies, and build up, 
 
 in consequence of the formation of dense colonies by bud- 
 ding, complex aggregates known as "sea mats " and "sea 
 mosses." Thompson expressly stated the opinion that the 
 organization of the animals detected by him led to the 
 conclusion that " they must be considered as a new type of 
 the Mollusca Acephala." 
 
 Subsequently (1844) Henri Milne-Edwards (3) pointed 
 out the relationship of Thompson's Polyzoa to the Brachio- 
 poda, and, adopting the tetter's view as to their Molluscan 
 affinities, proposed to unite these two classes with the 
 Tunicata in a group to be called " Molluscoidea." Recent 
 
 FIG. IA. Forms connecting the EupdljOM and the Gephyraea. 
 
 1. Ptoronii autlmlit. Hasirell. 
 
 2. One of the two nephridia of the same : ext, external aperture ; in*, inf, the two internal funnel- like apertures. 
 
 3. View of the tentacular area of Phormis auiiralit the tentacles cut to their bases, tx, outer line of tentacles ; it, inner line of tentacles ; m, month ; 
 
 tp, epistome ; -r, gap in the inner series of tentacles : neph, nepliridio-pores ; an, anus ; gt, glandular pit. 
 (After Benham, Quart. Jotirn. liter. Sri., vol. 30. 1889.) 
 
 4. Goljutyia tflntotiiii, Lankester. Specimen in which the introvert is telescoped into the body, o, the sderorhynchus, which with b, the scleropyge, 
 
 represents the hard zooecium of Enpolyzoa ; r, anus. 
 
 5. View of the same in an expanded condition, a, sclerorhynchus : b, scleropyge ; rf, the soft introvert carrying mouth, surrounded by six pinnate tentacle*. 
 
 (After Lankester, Tnmt. Linn. Soc., 2nd ser., "Zoology," voL ii., 1885.) 
 
 6. Atpidfifiphon Stfenttrvpii, Diesing. a, anterior corneous plate ; 6, terminal posterior plate ; d, introvert. 
 
 (After Selenka, Die Sipvnculidm, 1883.) 
 
 researches have entirely separated the Tunicata from this 
 association, and have demonstrated that they belong to 
 the great phylum of Yertebrata. On the other hand, the 
 association of the Polyzoa with the Brachiopoda appears 
 at present to be confirmed, though the relationship of 
 these two classes to the Mollusca has been shown to rest 
 
 1 These numbers refer to the bibliography which will be found in 
 page 171. 
 
 on mistaken identification of parts; see, however, Harmer 
 (18). 
 
 The Polyzoa appear to be related to the Sipunculoid 
 Gephyraean worms (Gephyraea inermia) more nearly than 
 to any other class of the animal kingdom. The study and 
 interpretation of the facts of their ontogeny (growth from 
 the egg) presents such extreme difficulty that in the pre- 
 sent state of our knowledge it is necessary to regard them 
 
160 
 
 P O L Y Z A 
 
 ad interim as forming with the Brachiopoda and Sipuncu- 
 loids an isolated group, to which the name " Podaxonia " 
 may be applied, pending the decision of their affinities by 
 the increase of our knowledge of the embryology of import- 
 ant members of the group. 1 
 
 The forms included at the present day in Thompson's 
 class of " Polyzoa " may then be thus classified : 
 
 PHYLUM PODAXONIA. 
 CLASS l.SIPUNCULOIDEA. 
 CLASS II. BRACHIOPODA. 
 CLASS III. POLYZOA. 
 
 Section 1. VERMIFORMIA. 
 
 Sole genus : Phoronis (figs. 4 and 51 
 Section 2. PTEROBRANCHIA. 
 Genus 1 : Rhabdopleura (fig. 7). 
 Genus 2 : Cephalodiscus (figs. 8. 9, 10). 
 Section 3. EUPOLYZOA. 
 Sub-class 1. Ectoprocta. 
 
 Order 1. PHYLACTOL^MA. 
 Examples : Lophopus, Plumatdla (fig. 2, B), Cristatella 
 
 (tig. 3), Fredericella. 
 Order 2. GYMNOL^MA. 
 Sub-order 1. Cyclostoma. 
 Examples : Crisia (fig. 13, A), ffornera, Tubulipora, 
 
 Discopnrella. 
 Sub-order 2. Ctenostoma. 
 
 Examples : A Icyonidium, Vesicidaria, Serialaria, Bower- 
 bankia (fig. 1, A), Paludicella (fig. 1, E and fig. 2, A). 
 Sub-order 3. Chilostoma. 
 
 Examples : Celhdaria, Scrupocellaria, Kinetoskias (fig. 
 14), Bugula, Bicellaria, Flustra (fig. 1, G), Afucro- 
 nella (fig. 1, C, D, F), Membranipora, Lepralia, 
 Eschara, Cellepora, Retepora. 
 Sub-class 2. Entoprocta. 
 
 Genera: Pedicelliiia (fig. 15), Loxosoma (fig. 16), Una- 
 
 tella, Ascopodaria. 
 
 We shall most readily arrive at a conception of the 
 essential structure of a Polyzoon, and of the variations to 
 which that essential structure is subject within the class, 
 by first examining one member of the group in detail and 
 subsequently reviewing the characters presented by the 
 divergent sub-classes, orders, <fec., above indicated. 
 
 The most convenient form for our purpose is Paludicella 
 Ehrenbergii (fig. 2, A), belonging to the typical section of 
 the class (the Eupolyzoa) and to the order Gymnolaema. 
 The organism occurs as minute tree-like growths (figs. 2, 
 A and 1, E) attached to stones in freshwater streams and 
 canals. The branches of the little tree are rarely more than 
 an inch in length, and are regularly swollen and jointed at 
 intervals. Each of the very numerous joints is about one- 
 fifth of an inch long, and is in reality a tubular horny box 
 attached above and below to the preceding and succeeding 
 joints, and having on one side of it a spout-like aperture 
 from which a crown of tentacles can be protruded. Each 
 joint is thus inhabited by a distinct animal which is more 
 or less completely shut off from the one in front of it and 
 the one behind it, although it originated from the hinder 
 and has given rise to the fore-lying individual by a process 
 of budding, and retains a continuity of substance with both. 
 A single cell or joint with its contained animal is repre- 
 sented in fig. 2, A. 
 
 Paludicella produces an arboriform colony, the main 
 trunk or stolon being adherent to some stone or piece of 
 wood. The substance of the wall of the cells is formed 
 by a chemical body allied to chitin. Other Polyzoa may 
 form mat-like expansions the cells being placed in one 
 plane, side by side (fig. 1, C, D, F, G), as well as in linear 
 series ; others again form solid masses, whilst many agree 
 with Paludicella in the simple linear arrangement of their 
 units. Phoronis and Loxosoma, on the other hand, do 
 not form colonies at all the former because it does not 
 
 1 The research of Harmer (18) on Loxosoma is published too late 
 for due notice in this article. It tends to the conclusion that the 
 Eupolyzoa are after all degraded Mollusca, and have no connexion 
 with the Vermiformia, Pterobranchia, Brachiopoda, and Sipunculoidea. 
 The reader is referred to Mr Harmer's memoir. 
 
 bud, the latter because the buds become detached from 
 their parent as soon as formed, as do the buds of the 
 Hydrozoon Hydra. 
 
 On the whole Paludicella presents us with a very simple 
 form of Polyzoon-colony (technically termed a "zoarium"), 
 in which the aggregate of budded persons, each of which 
 
 FIG. 1. Various forms of zoaria of Eupolyzoa. 
 
 A. Bowerbankia pustulosa, one of the Ctenostoma ; natural size. 
 
 B. A cluster of polypides of Bowerbankia pustulosa, some with expanded 
 
 tentacles ; more highly magnified. 
 
 C. Zocecia of Mucronella, pavonella (Chilostoma); highly magnified. 
 
 D. Zoarium of Mucronella pavonella, forming a disk-like encrustation on a 
 
 piece of stone; natural size. 
 
 E. Zoarium of Paludicella Ehrenbergii (Ctenostoma) , natural size. 
 
 F. Zocecia of Mucronella Peachii ; highly magnified. Compare with C in order 
 
 to note specific characters. 
 
 G. Zoarium of Flustra securifrons; natural size. 
 
 is called a "polypide," does not exhibit any marked indi- 
 viduation, but is irregular and tree-like. But, just as in 
 the Hydrozoa we find the Siphonophora presenting us with 
 a very definite shape and individuality of the aggregate or 
 colony, so in the Polyzoa we find instances of high indi- 
 
P L T Z A 
 
 161 
 
 viduation of the zoarium of a similar kind. The most 
 remarkable example is afforded by the locomotive zoarium 
 or colony of Cristatella (fig. 3) ; and another very striking 
 instance is that of the stalked zoaria of Kinetoskias 
 (fig. 14) and Adeona. 
 
 The horny consistence of the cells which are produced 
 by Paludicella is very usual in other Polyzoa ; but we find 
 frequently that the substance which forms the cells is 
 gelatinous and soft instead of being horny, or again may 
 be strongly calcareous. The term ccenaecium is applied to 
 the mass of cells belonging to a colony or zoarium when 
 considered apart from the living polypides which form it. 
 Often such coencecia are found retaining form and structure 
 when the soft living polypides have decomposed and dis- 
 appeared. A single cell of the cceno3cium, corresponding 
 to a single polypide, is called by the special students of 
 the Polyzoa a zocedum. 
 
 If we examine a single cell or zocecium of Paludicella 
 more carefully whilst 
 its polypide is alive, 
 we 'discover that the 
 horny cell is nothing 
 more than the cuticle 
 of the polypide itself, 
 to which it is absol- 
 utely adherent. At 
 the so-called " mouth " 
 or spout of the cell 
 the cuticle suddenly 
 changes its character 
 and becomes a very 
 delicate and soft pel- 
 licle instead of being 
 thickandhorny. There 
 is no real discontinuity 
 of the cuticle at this 
 region, but merely a 
 change in its qualities. 
 This gives to that por- 
 tion of the body of the 
 polypide which lies 
 beyond the spout a 
 mobility and capacity 
 for folding and pleat- 
 ing which is entirely 
 denied to that part 
 where the cuticle is 
 more dense (fig. 2, A). 
 Accordingly we find 
 that the anterior por- 
 tion of the body of the 
 polypide can be pulled 
 into the hinder part as 
 the finger of a glove 
 may be tucked into the 
 hand. It is, in fact, an 
 " introvert " (for the 
 use of this term see 
 MOLLUSCA, vol. xvi. 
 p. 652). This arrange- 
 ment is universal in the Ectoproctous Eupolyzoa, but does 
 not obtain either in the Entoprocta, the Pterobranchia, or 
 the Vermiformia. la Phoronis, Rhabdopleura, and Cepha- 
 lodiscus the anterior part of the body can not be tucked or 
 telescoped into the hinder part as it can in typical Eu- 
 polyzoa. On the other hand it is very important to note 
 that the Sipunculoid Gephyrasans are all pre-eminently 
 characterized by possessing identically this arrangement. 
 The introversion is effected in Paludicella (as in other Eu- 
 polyzoa) by a series of long detached retractor muscles of 
 
 FIG. 2. A. PolypWe of Paludicella Ehrmbergii, 
 seen as a transparent object in optical section 
 and highly magnified (from Gegcnbaur, after 
 Allman). For natural size see fig. 1, E. a, 
 anus; br, peristomial circlet of ciliated ten- 
 tacles; i, thickened cuticle of the body-wall, 
 forming the horny cell or zooecium ; m, 
 median retractor muscle of the introversible 
 part of the body ; r'. anterior retractor of the 
 same ; mr, great retractor muscle of the same; 
 o, ovary, passing from which to the stomach is 
 the anterior mesentery or funiculus ; /, testis ; 
 o?, oesophagus ; r, stomach ; x, posterior mes- 
 entery or funiculus; x f . anterior mesentery or 
 funiculus. Observe at the right upper corner 
 of the figure the base of a second polypide and 
 the "rosette-plate" of separation. 
 
 B. Diagram of a polypide of Plumatella. 
 Letters as above. 
 
 considerable power (fig. 2, A, mr, r', m) ; the same is true 
 of Sipunculus. 
 
 The view has been advanced by Allman (4) that the re- 
 tractile part of the polypide is to be considered as a distinct 
 individual budded from the basal portion, which is regarded 
 as an equivalent individual. It does not appear to the 
 present writer that such a theoretical conception tends to 
 facilitate the understanding of the structure and relations 
 of these animals. 
 
 An "ectocyst" and "endocyst" have also been distin- 
 guished in former treatises, and these terms form part of a 
 special " polyzoarial " nomenclature, but do not appear to 
 be any longer needful. Equally undesirable is the misap- 
 plied term "endosarc" lately introduced by Jolliet (5) to 
 denote a certain portion of the Polyzoon structure which 
 will not be referred to here by that name. 
 
 The retractile or introversible portion of the body of the 
 polypide of Paludicella is terminated by a crown of sixteen 
 stiff non-contractile tentacles (fig. 2, A, br) which form a 
 circle around a central aperture the animal's mouth. 
 These tentacles are hollow and beset with vibratile cilia. 
 The beating of the cilia causes a powerful current in the 
 water by which food is brought to the animal's mouth. 
 Each tentacle is also muscular, and can be bent and 
 straightened at will. The tentacles not only serve to 
 bring food into the mouth, but they are efficient as gill- 
 filaments, being possibly homologous with (as well as func- 
 tionally similar to) the gill-filaments of Lamellibranch 
 Molluscs. They also serve as delicate tactile organs, and 
 are the only sense organs possessed by the Eupolyzoa. 
 
 In Paludicella the platform around the mouth from 
 which the tentacles arise, or lophophore, as it is termed, is 
 circular. This is the case in all members of the large 
 group of Gymnolaema and in the Entoprocta. But in 
 the Phylactolaama the lophophore is drawn out on each 
 side, right and left, so as to present a hprse-shoe shape 
 (fig. 2, B), and in some forms, notably Lophopus and 
 Alcyonella, the two arms or diverging rami of the horse- 
 shoe are very strongly developed. 
 
 In the Pterobranchia the tentacles are confined in one 
 genus (Rhabdopleura) to the two arm-like outgrowths of 
 the lophophore, and are not simply hollow but contain a 
 well-developed cartilaginoid skeleton (fig. 7). In the allied 
 genus Cephalodiscus there are not merely a single pair of 
 such arm-like processes, each bearing two rows of tentacles, 
 but the lophophore is developed into twelve arm-like pro- 
 cesses (fig. 9), which form a dense tuft of filaments around 
 the anterior extremity of the animal. 
 
 In the Vermiformia (Phoronis) we again meet with a 
 very perfect horse-shoe-shaped lophophore (fig. 4). The 
 tentacles upon the crescentic or otherwise lobed circumoral 
 region of the Sipunculoids are the representatives of the 
 tentacles of the Polyzoa ; whilst the tentaculiferous 
 " arms " of the Brachiopoda appear to be the equivalents 
 of the Polyzoon's lophophore much drawn out and in most 
 cases spirally rolled. 
 
 Just below the circular crown of tentacles in Paludicella 
 we find an aperture which the study of internal anatomy 
 proves to be the anus. In all Polyzoa the anus has this 
 position near the mouth ; and in this respect we again 
 note an agreement with Sipunculus and the other so-called 
 Gephyrsea inermia. In one division of the Polyzoa alone 
 is there any noteworthy variation in the position of the 
 anus, namely, in the Entoprocta (sub-class of the section 
 Eupolyzoa). In these forms the anus, instead of lying 
 just below the lophophore or platform from which the 
 tentacles spring, is included like the mouth within its 
 area (fig. 15, C). 
 
 Passing now to the deeper structure of Paludicella, we 
 
 find that it is a Ccelomate animal ; that is to say, there 
 
 X 
 
162 
 
 P O L Y Z O A 
 
 exists between the body-wall and the wall of the aliment- 
 ary tract a distinct space termed " perigastric space, " 
 "body-cavity," or "coelom." This is true of all Polyzoa, 
 though it has been erroneously stated by G. 0. Sars that 
 Rhabdopleura does not possess such a ccelom. In Eu- 
 polyzoa (excepting the Entoprocta) the ccelom is very 
 capacious ; it is occupied by a coagulable hsemolymph in 
 which float cellular corpuscles, and also the generative 
 products, detached, as is usual in Coslomata, from definite 
 "gonads" developed on its lining membrane (fig. 2, A, o, t). 
 This lining membrane or " ccelomic epithelium " is ciliated 
 in the Phylaetolaema, but its characters appear not to have 
 been definitely determined in other Eupolyzoa. The 
 ccelomic space and the tissues bounding it are continuous 
 throughout the colony or zoarium of a Polyzoon either 
 directly without any constriction marking off one polypide 
 from another, or through perforate septum-like structures 
 as in Paludicella (see right-hand upper process of fig. 2, A), 
 which form incomplete barriers between juxtaposed zooecia, 
 and are termed " rosette-plates " or " communication-plates. " 
 The ccelomic cavity is continued in Paludicella and probably 
 in all Polyzoa into the tentacles, so that these organs expose 
 the haemolymph fluid to a respiratory action, and hence 
 may be called branchial. 
 
 The body-wall of Paludicella consists, alike in the 
 anterior introversible region and in the posterior region, of 
 an outer cuticle which has already been spoken of as 
 thickened around the base of the polypide so as to become 
 there the hard tube-like zooacium. Beneath this is the 
 delicate layer of living epidermic cells which are the 
 mother-cells or matrix of that cuticle. Beneath this again 
 are a few scattered annuli of muscular fibre-cells arranged 
 ring - wise around the cylindrical body ; more deeply 
 placed than these are five large bundles of longitudinally 
 placed muscular fibre-cells which are attached at three 
 different levels to the soft introversible portion of the 
 body, and by their retraction pull it in three folds or tele- 
 scopic joints into the capacious hinder part of the body. 
 In some Polyzoa the muscular fibre-cells present trans- 
 verse striations. These folds are shown in fig. 2, A ; 
 
 FIG. 3. The locomotive zoarium of the freshwater Phylactolsemous Polyzoon 
 Cristatella mucedo; magnified six times linear (after Allman). a, individual 
 polypides with their horse-shoe-shaped crown of tentacles exserted ; b, stato- 
 blasts seen through the transparent tissues ; c, the muscular foot or base of the 
 colony by means of which it crawls ; d, portion of water-weed upon which the 
 Cristatella is crawling. 
 
 but when the longitudinal muscles are completely con- 
 tracted the tentacular crown would be pulled down far out 
 of sight into the midst of the body by the great longitu- 
 dinal muscle mr. Deeper than the longitudinal muscles, 
 and clothing them and everything else which projects into 
 the ccelom, is the ccelomic epithelium, not easily observed, 
 and sufficiently known only in the Phylactolsema. Part of it 
 gives rise to the generative products (fig. 2 A, o, t). 
 Other Eupolyzoa have a similar but not identical arrange- 
 ment of the longitudinal muscles acting essentially as 
 retractors of the " introvert " or soft anterior region of the 
 body and a similar structure of the body-wall which is in 
 
 essential features identical with that of the Sipunculoid 
 worms, the Chaetopod worms, and other typical Ccelomate 
 animals. 
 
 The alimentary canal of Paludicella forms a closely com- 
 pressed U-shaped loop depending from the closely approxi- 
 mated mouth and anus into the capacious ccelom. It is 
 clothed on its coelomic surface (in Phylactolsema at any 
 rate) with ccelomic epithelium, and beneath this are 
 extremely delicate muscular layers. Within it is lined, 
 except in the immediate region of the mouth (which is 
 lined by the in-pushed outer cell-layer), by the enteric cell- 
 layer the digestive cells derived from the archenteron of 
 the embryo. We can distinguish in Paludicella a contrac- 
 tile pharyngo-cesopha'gus (fig. 2, A, ce), a digestive stomach 
 v (the lining cells of which have a yellow colour), and an 
 intestine which forms that arm of the loop connected with 
 the anus. This simple form of alimentary canal is uni- 
 formly present in Polyzoa. In Bowerbankia and its allies 
 a muscular gizzard with horny teeth is interposed between 
 oesophagus and digestive stomach. 
 
 The alimentary canal of Paludicella does not hang quite 
 freely in the ccelomic cavity, but, as is usually the case in 
 other classes where the ccelom is large, mesenteries are 
 present in the form of fibrous (muscular 1) bands clothed 
 with ccelomic epithelium and suspending the gut to the 
 body-wall. In Paludicella there are two of these mesen- 
 teries, an anterior (x) and a posterior (x). The presence 
 of two mesenteric bands is exceptional. Usually in the 
 Eupolyzoa we find one such mesentery only, corresponding 
 to the hinder of the two in Paludicella. The special name 
 funiculus (Huxley) is applied to this mesenteric band, and 
 it is noteworthy that the cells of the ccelomic epithelium, 
 either upon its surface or at its point of insertion into the 
 body-wall, are modified as reproductive elements, forming 
 either the testis or ovary ; in the Phylactolaema they form 
 here also special asexual reproductive bodies, the stato- 
 blasts. The nervous tissue and organs of Paludicella have 
 not been specially investigated, but in many Eupolyzoa 
 an oval mass of nerve-ganglion cells is found lying between 
 the mouth and anus, and there is no doubt that it is 
 present in this case. In Plumatella nerve-fibres have 
 been traced from this ganglion to the tentacles and other 
 parts around the mouth (fig. 11, w, x, y). A "colonial 
 nervous system " was described some years ago by Fr. 
 Miiller in Serialaria ; but modern histologists do not 
 admit that the tissue so named by Miiller is nerve-tissue. 
 The ganglion above mentioned is the only nervous tissue 
 at present known in Polyzoa (but see fig. 17, x). 
 
 No heart or blood-vessels of any kind exist in Paludi- 
 cella nor in any of the Eupolyzoa or Pterobranchia. On 
 the other hand the isolated vermiform genus Phoronis 
 presents a closed contractile system of longitudinal vessels 
 (dorsal and ventral) which contain nucleated corpuscles 
 coloured red by haemoglobin (figs. 4, 5). 
 
 No excretory organs (nephridia) or genital ducts have 
 been observed in Paludicella, nor have such organs been 
 detected in the majority of the Polyzoa which have been 
 studied. In the Entoprocta, however, a pair of minute 
 ciliated canals are found in the nearly obliterated body- 
 cavity opening to the exterior near the tentacular crown in 
 both Pedicellina and Loxosoma, which represent the cephalic 
 nephridia of worms. A definite pair of nephridia occur in 
 Phoronis. A similar significance is perhaps to be attributed 
 to the " intertentacular organ " of Farre a ciliated pas- 
 sage opening between two tentacles of the lophophore in 
 Membranipora, Alcyonidium, and other forms through 
 which Hincks has observed the spermatozoa to escape in 
 large numbers. This organ occurs equally in female speci- 
 mens of Membranipora, and is not therefore simply a sper- 
 matic duct. 
 
P O L Y Z O A 
 
 163 
 
 Paludicella, as we have seen, develops both ova and 
 spermatozoa in one and the same polypide. The details 
 of impregnation and development have not been followed 
 in this instance, but in some of the marine Eupolyzoa 
 (Gymnolaema) remarkable bud-like structures termed oaecia 
 are developed for the special reception of the ova, and in 
 these organs fertilization takes place. In the Entoprocta 
 there is a peculiar brood-pouch. The spermatozoa of one 
 polypide probably in all cases fertilize the ova of another, 
 but we have not yet in many cases a knowledge of how 
 the spermatozoa get to the eggs, or how the eggs escape 
 from the body-cavity of the parent. In the hippocrepian 
 freshwater Polyzoa (Phylactolsema) the ova appear to be 
 fertilized and undergo the early stages of development 
 within the body-cavity of the parent or in a hernia-like 
 protrusion of it. Probably in such cases the embryos 
 escape by the death of the parent and rupture of the 
 parental tissues, as do also the peculiar asexual internal 
 buds or statoblasts of these forms. 
 
 The embryo Polyzoon or "larva" swims freely in its 
 early condition by means of cilia, and is in this condition 
 a single polypide or "person." The forms assumed by 
 these ciliated larvae in different Polyzoa are very various 
 and exceedingly difficult of interpretation. We shall have 
 more to say with regard to them below (see figs. 19, 20, 
 21). The ciliated larva then fixes itself and commences 
 to produce polypides by a process of budding, the buds 
 remaining not merely in contact but in organic continuity, 
 and increasing continually in number so as to form a large 
 colony or zoarium. In Paludicella we have seen that this 
 colony has a simple tree-like form. The new buds form 
 as wart-like growths, usually one, sometimes two in number, 
 at the free end of a cell or zocecium near the spout-like 
 process from which the tentacular crown is everted. In 
 Paludicella all the polypides of a colony are alike ; there 
 is no differentiation of form or distribution of function 
 amongst the members of the colony. In many Eupolyzoa 
 this simplicity is by no means maintained, but a great 
 variety of form and function is assumed by various 
 members of the aggregate. The only approach to a 
 differentiation of the polypides in Paludicella is in the 
 arrest of growth of some of the buds of a colony in 
 autumn, which, instead of advancing to maturity, become 
 conical and invested with a dark-coloured cuticle. They 
 are termed hybernacida. Should the rest of the poly- 
 pides die down in winter, these arrested buds survive 
 and go on to complete development on the return of 
 spring. 
 
 In Paludicella we have thus seen a fairly simple and 
 central example of Polyzoon structure and life-history. 
 The variations upon this theme presented in different 
 groups of Polyzoa have been to some small extent noted 
 in the preceding account, but we shall now be able to 
 indicate them more precisely by considering the various 
 groups of Polyzoa in succession. The limit assigned to 
 this article necessitates very large omissions. The reader 
 who wishes to have the fullest information on the many 
 difficult and uncertain matters connected with this subject 
 is referred to Allman, Freshieater Polyzoa (Ray Society, 
 1856) ; Hincks, British Marine Polysoa (Van Voorst, 
 1880); Haddon, "Budding in Polyzoa," Quart. Journ. 
 J/tcr. Set., 1883 ; Balfour, Embryology, voL L p. 242 ; and 
 the original memoirs cited by these writers. 
 
 THE VERMIFORMLL 
 
 The first section of the Polyzoa comprises but a single 
 genus, Phoronis. It differs from all other Polyzoa first 
 in its greater size (species 2 inches long are known) 
 and elaboration of organization, and correlatively with 
 that in the fact that it does not produce buds. Further, 
 
 it does not produce a closely adherent cuticular zooecium 
 as do Paludicella and the Eupolyzoa generally, but a 
 leathery tube in 
 which the animal 
 freely moves, resem- 
 bling that of some 
 Chastopods (Sabel- 
 la). Like some 
 Sabellse, Phoronis 
 forms closely packed 
 aggregates of indi- 
 viduals not brought 
 together by any 
 process of budding, 
 but each separately 
 developed from an 
 egg. Phoronis has 
 an elongate, worm- 
 like, unsegmented 
 body, with a conical 
 posterior termina- 
 tion (like Sipuncu- 
 1ns), and anteriorly 
 provided with a 
 horse - shoe - shaped 
 crown of tentacles 
 surrounding the 
 mouth (figs. 4, 5). 
 There is an inter- 
 tentacular " web " 
 between the bases 
 of the tentacles as 
 in the Phylactolae- 
 ma. Caldwell (6) 
 has recently shown 
 that the tentacles 
 are supported by a 
 
 mesoblastic skele- F '- *->* hippotrepia, Wright; magnified 
 
 . sii times linear (from Allman). a, horse-shoe- 
 
 ton, as IS also the shaped lophophore with tentacles; c, epistome 
 
 Aflp in TtViahHn- (P-oral lobe or prostomium); rf, (esophagus; /, 
 
 XVnaOQO- Tentra i Terae l: g, g, two anterior vessels which 
 
 pleura, but appar- nnite to form /; i, longitudinal muscular coat 
 
 . , of the body-wall ; t, intertentacular membrane. 
 
 ently not the case 
 
 in any other Polyzoa. Close to the mouth, as in all 
 Polyzoa, is placed the anus, outside the horse-shoe-shaped 
 lophophore or tenta- 
 cular platform (fig. 
 11, t). The tenta- 
 cular crown is not 
 introversible ; in this - 
 point Phoronis differs 
 from Paludicella and 
 the Ectoproctous Eu- 
 polyzoa, and agrees a _ 
 with the Entoprocta 
 and the Pterobranchia. 
 Overhanging the 
 mouth is a small pree- 
 oral lobe or " epi- 
 stome " (figs. 4, 5, c). 
 This organ is aborted 
 in Paludicella, and in- 
 deed in all the Gym- 
 nokema, but is present 
 in the other Polyzoa, 
 
 j ii ' Fio. 5. Lateral view of the anterior region of 
 
 and IS especially large Phoronis . The tentacles of the right arS of the 
 and well developed in lophophore are cut short in order to expose clearly 
 T>I ! j i j /-i the mouth ft and the overhanging "epistome" 
 
 Knabaopleura and Oe- or pite-oral lobe c. c, intestine ; A, dorsal Teasel. 
 phalodlSCUS. It has Other letters as in fig. 4. 
 
 been compared to the Molluscan foot, but undoubtedly in 
 Phoronis it is the persistent representative of the prae-oral 
 
 i . 
 
 1 
 
164 
 
 P O L Y Z O A 
 
 lobe of the larva (fig. 6), and therefore cannot be compared 
 to the Molluscan foot. If we are right in associating 
 Phoronis with the Polyzoa, this fact is sufficient to show 
 that the epistome of the Phylactokema (fig. 11, e) and the 
 buccal shield of Rhabdopleura (fig. 7, d) and of Cephalodis- 
 cus (fig. 9, b) are also cephalic in nature, and cannot rightly 
 be identified with the post-oral and ventral muscular lobe 
 known as the foot in Mollusca. A circum-oral nerve ring 
 occurs at the base of the tentacles and sends off a cord 
 which runs along the left side of the body. The alimen- 
 tary canal presents the same general form and regions as 
 in Paludicella. It hangs in the body-cavity, to the walls 
 of which it is suspended by definite mesenteries. 
 
 Phoronis presents a closed contractile vascular system 
 containing red-coloured blood-corpuscles (figs. 4, 5, /, </, 
 h). A pair of ciliated canals acting as genital pores is 
 found near the anus ; these have been shown by Caldwell 
 to be typical nephridia. 
 
 The development of Phoronis is remarkable. The egg 
 
 gives rise (after the usual phases of cleavage and gastrula- 
 
 tion) to the larval form known as Actinotrocha (fig. 
 
 6). This larva possesses a hood-like region overhanging 
 
 A A. 
 
 ^ -. 
 
 (3) 
 
 (10) 
 
 - D 
 
 FIG. 6. Development of Phoronis and typical dilate lame. (1), (2), (3), (8), 
 (9), (10), stages In the development of Phoronis (1), earliest larva; (2), lateral 
 view of the Actinotrocha; (3), ventral view of the same; (B), the ventral in- 
 vagination iv is formed; (9), the ventral invagination is everted, carrying with 
 it a loop of intestine ; (10), the permanent relations of mouth, anus, and body 
 (Podaxonia) are attained. (4), (5), Echinoderm larva with architroch, as in 
 Actinotrocha, but band-like, not digitate. (6), Echinoderm larva, with the 
 architroch divided into aprte-oral ccphalotroch (Molluscan and Rotifer's velum), 
 and a post-oral branchiotroch. (7), Chsetopod trochosphere larva with cephalo- 
 troch only, and elongation and segmentation of the oro-anal axis, a, anus; o, 
 mouth; pr, prostomium ; iv, ventral invagination of Phoronis larva. A B, oro- 
 anal axis; VD, dorso-ventnil axis. 
 
 the mouth and a number of ciliated post-oral processes 
 or tentacles. The anus is placed at the extremity of the 
 elongate body opposite to that bearing the mouth and 
 
 prse-oral hood. The prse-oral hood becomes the epistome, 
 and the tentacles, by further development (new tentacles 
 replacing the larval ones), become the horse-shoe-shaped 
 group of tentacles of the adult. A very curious process 
 of growth changes the long axis of the body and results 
 in the anus assuming its permanent position near the 
 mouth. An invagination appears on the ventral face of 
 the larva between the anus and mouth, and attains con- 
 siderable size. At a definite moment in the course of 
 growth this invagination is suddenly everted, carrying 
 with it in its cavity the intestine in the form of a loop. 
 Thus a new long axis is suddenly established at right 
 angles to the original oro-anal axis, and continues to de- 
 velop as the main portion of the body. The short area 
 extending from the prse-oral hood to the anus is thus the 
 true dorsal surface of Phoronis, whilst the elongated body 
 is an outgrowth of the ventral surface perpendicular to 
 the primary oro-anal axis, as conversely in many Mollusca 
 we find a short ventral area (the foot) between mouth and 
 anus, and an outgrowth of the dorsal surface (the visceral 
 hump) perpendicular to the primary oro-anal axis, forming 
 the chief body of the animal. In these relations Phoronis 
 (and with it the other Polyzoa) agrees with Sipunculus. 
 On the other hand Echiurus, the Chaetopods, Nemertine 
 worms, and some other groups which start from a simple 
 larval form not unlike that of Phoronis, present a continual 
 elongation of the original oro-anal axis, and no transference 
 of the long axis by the perpendicular or angular growth of 
 either the ventral or the dorsal surface of the larva. 
 
 Phoronis was discovered originally in the Firth of Forth 
 by Dr Strethill Wright. It occurs in the Mediterranean 
 and in Australian seas (Port Jackson). 
 
 THE PTEROBRANCHIA. 
 
 This section of the Polyzoa also comprises forms which 
 differ very widely from Paludicella. Inasmuch as their 
 development from the egg is at present quite unknown, 
 it may possibly prove that they have other affinities. 
 Only two genera are known, Rhabdopleura (Allman) and 
 Cephalodiscus (M'Intosh), the former dredged by Dr 
 Norman in deep water off the Shetlands (and subse- 
 quently in Norway), the latter taken by the " Challenger " 
 expedition in 250 fathoms off the coast of Patagonia. 
 
 The Pterobranchia have the mouth and anus closely 
 approximated, and immediately below the mouth are given 
 off a series of ciliated tentacles, but these do not form a 
 complete circle as in Paludicella, nor is the lophophore (the 
 platform of their origin) horse-shoe-shaped as in Phoronis. 
 The lophophore is drawn out into a right and a left arm in 
 Rhabdopleura (fig. 7), upon each of which are two rows 
 of ciliated tentacles ; no tentacles arc developed centrally 
 in the region between the two arms, so that the mouth is 
 not completely surrounded by these processes. The horse- 
 shoe-shaped lophophore of Phoronis could be modified so as 
 to represent the tentaculiferous arms of Rhabdopleura by 
 suppressing both rows of tentacles at the curve of the 
 horse-shoe, and leaving only those which occur on the 
 arms or rami of the horse shoe (see fig. 4). The lopho- 
 phore of Cephalodiscus presents us with twelve processes, 
 each carrying two rows of ciliated tentacles ; in fact we 
 have six pairs of tantaculiferous arms instead of a single 
 pair, and each of these arms is precisely similar to one 
 of the arms of Rhabdopleura (fig. 9), excepting that it 
 terminates in a knob instead of tapering. There is no 
 arrangement for introverting the anterior portion of the 
 body into the hinder portion in the Pterobranchia. 
 
 The little epistome or prse-oral lobe of Phoronis is repre- 
 sented in the Pterobranchia by a large muscular shield or 
 disk-like structure (fig. 7, d and fig. 9, b) which over- 
 hangs the mouth and has an actively secreting glandular 
 
P O L Y Z A 
 
 165 
 
 surface by which the tube or case (tubarium) in which the 
 polypide is enclosed is secreted. 
 
 Both Rhabdopleura and Cephalodiscus produce colonies 
 by budding; but the colonies of the former are large, 
 definite, and arborescent, whilst those of Cephalodiscus 
 are remarkable for the fact that the buds do not remain 
 long in organic continuity with their parent, but become 
 detached and nevertheless continue to be enclosed by the 
 same common envelope or secretion. The bud-formation 
 of Rhabdopleura recalls that of Paludicella in the fact that 
 it leads to the formation of continuous arboriform com- 
 munities. That of Cephalodiscus resembles the budding 
 of Loxosoma, since no two fully-formed individuals remain 
 
 '.I 
 
 FIG. T. Rhabdopltura yormani, Allman (original drawings, Lankester). A. 
 A single polypide removed from its tube and greatly magnified, a, mouth: b, 
 anns ; c, polypide-stalk or Kymnocanlus, the ' contractile cord " of Sara ; d, the 
 prae-oral lobe (buccal shield or disk of Allman); t, intestine: /, thoracic 
 region of the polypide ; g, one of the ciliated tentacles. B. Lateral view to 
 show the form of "the buccal shield and its pigment spot, g, ciliated tentacle 
 (in outline) ; h, basal ridge of the right arm of the lophophore. C. Lateral view 
 of a polypide. i, ciliated patch (Sars's organ) at the base of the lophophore-arm. 
 Other letters as above. D. Part of a lophophore-arm. with soft tissues rubbed 
 off to show the cartilaginoid skeleton, a, epithelium and soft tissues still 
 adherent at the tip of a tentacle; 6, skeleton of tentacle; e, skeleton of axis. 
 E. Portion of a colony of Rhabdopleura A'ormani, showing the branched tube- 
 like cases formed by the polypides. The black line within the tubes represents 
 the retracted polypides connected together by their common stalk, the pecto- 
 caulus. Magnified to three times the size of nature. 
 
 in organic continuity. Both Rhabdopleura and Cephalo- 
 discus (like Phoronis) produce cases or investments in 
 which they dwell. These are free secretions of the organ- 
 ism, and are not, like the ccenoecia of Eupolyzoa, cuticular 
 structures adherent to and part of the polypide's integu- 
 ment. The dwelling of Rhabdopleura is a branched 
 system of annulated tubes of a delicate membranous con- 
 sistency, each tube corresponding to a single polypide, the 
 rings of which it is built being successively produced at 
 the termination of the tube by the secreting activity of the 
 pree-oral disk (fig. 7, E). The polypides freely ascend and 
 descend in these tubes owing to the contractility of their 
 stalks. On the other hand the dwelling of Cephalodiscus j 
 
 is a gelatinous, irregularly branched, and fimbriated mass 
 
 (fig. 8), excavated by numerous cavities which communicate 
 
 with the exterior. In these 
 
 cavities are found the nu- 
 
 merous detached small 
 
 colonies of Cephalodiscus 
 
 (fig. 9), or we should rather 
 
 say the isolated budding 
 
 polypides. The remaining 
 
 important feature in the 
 
 organization of the Ptero- 
 
 branchia, namely, the parts 
 
 connected with the forma- 
 
 tion of buds, are best un- 
 
 derstood by first examining 
 
 Cephalodiscus. The body 
 
 of Cephalodiscus is seen 
 
 (fig. 9) to be an oval sac ; 
 
 in this is suspended the 
 
 U-shaped alimentary canal, 
 
 and from the -walk of its 
 
 cavity (ccelom) the ova and 
 
 the spermatozoa are de- 
 
 veloped. Projecting from 
 
 the ventral face of this 
 
 oval sac is a muscular cy- 
 
 lindrical stalk, into which 
 
 the viscera do not pass, 
 
 though the ccelom is con- 
 
 tinued into it (fig. 9, c). 
 
 This stalk is merely the 
 
 outdrawn termination of 
 
 thp hoHv Tr i ariont as FIG. 8. Dwelling of gelatinous consistence 
 
 DO -!- ' and brown colour formed by the polypides 
 
 long as the whole of the of Ccphalodiiau dodtmlojilwu, M'Intosh; 
 
 t ,, i i -. natural sire (from an original drawing 
 
 rest of the animal, and it kindly supplied by Prof. M-intosh,F.E 
 
 . 
 
 is from its extremity that o. polypide within the jelly;^, cavity once 
 , , , *, ._ occupied by polypides. 
 
 the buds are produced (fig. 
 
 9, a). Before the buds have attained half the size of their 
 parent they become detached, but continue to occupy some 
 portion of the common gelatinous dwelling. 
 
 FIG. 9. A polypide of Cephaloditmt dodecaiophut removed from the gelatinous 
 bouse (from an original drawing by Prof. M'lntosh). No organic connexion 
 has been severed in thus isolating this polypide with its attached bods a, a, 
 The figure represents the furthest point to which colony-formation attains in 
 this form, a, buds growing from the bas of the polypide-stalk ; 6, the pne- 
 oral lobe (bnccal shield or disk); c, the polypidc-stalk ; </, the ciliated tentacles 
 of the twelve lophophore arms (six pairs, each like the single pair of Rhab- 
 dopleura) inextricably matted and confused ; e, anterior margin of the pre- 
 oral lobe ; /, posterior margin of the same. Magnified about fifty times linear. 
 
 Turning to Rhabdopleura, we find that each polypide 
 has a body of similar shape and character to that described 
 for Cephalodiscus, and a similar ventrally developed 
 " stalk :l (fig. 7, A, c). But, inasmuch as the buds deve- 
 
166 
 
 P L Y Z O A 
 
 loped on the stalk of a Rhabdopleura polypide do not 
 detach themselves, we find that we can trace the stalk of 
 each polypide of a colony into connexion with the stalk of 
 the polypide from which it was originally budded, which 
 may now be considered as a " branch " bearing many- 
 stalked polypides upon its greatly extended length, and 
 such a " branch-stalk " may be further traced to its junc- 
 tion with the " stem-stalk " of the whole colony. The 
 stem-stalk was at one time the simple terminal stalk of a 
 single polypide, but by lateral budding it gave rise to 
 other polypides, and so became a gemmiferous " branch " ; 
 and further, when some of these in their turn budded and 
 became branches, it became the main " stem " of a copious 
 colony. 
 
 A serious error has been made in comparing the contrac- 
 tile stalk of the Pterobranchiate polypide to the "funi- 
 culus " or cord-like mesentery of Eupo- 
 lyzoa. With this it has morphologi- 
 cally nothing in common, since it is 
 not an internal organ, but simply the 
 elongated termination or stalk of the 
 body, comparable to the stalk of Pedi- 
 cellina (fig. 15) and Loxosoma (fig. 16), 
 or to the hydrocaulus of such a Hydro- 
 zoon colony as Cordylophora. The 
 stalk where it bears only very young 
 buds, or none at all, as is always its 
 condition in Cephalodiscus and in many 
 polypides of a Rhabdopleura colony, 
 may be called a "gymnocaulus"; when 
 once its buds have devel- 
 oped into full grown poly- 
 pides, and it has elongated 
 proportionally with their 
 growth, it becomes a "pec- Fla . IO.-A polypide of Cephalodiscus do- 
 
 tOCaulus"' that is tO Say it decalophus, from which the lophophore- 
 
 is to that part of it which 
 bears such polypides that 
 this term may be conveni- 
 ently applied. The pecto- 
 caulus of Rhabdopleura, both in the form of branch and 
 stem, undergoes remarkable change of appearance as com- 
 pared with the gymnocaulus. It loses its contractility, 
 shrinks, and develops on its surface a hard, dark, horny 
 cuticle (whence its name), comparable precisely in its nature 
 to the hardened cuticle which forms the zooecia of Eupo- 
 lyzoa. It now has the appearance of a black cord or 
 rod-like body lying within and adherent to the inner face 
 of the much wider tubular stem, and branches formed by 
 the gradual building up and arborescent extension of the 
 annulated tubarium secreted by the individual polypides. 
 It has been regarded both by Allman and by Sars as a 
 special structure, and called by the former " the chitinous 
 rod" or " blastophore," by the latter "the axial cord." 
 
 In reality it is the black-coloured pectocaulus of 
 Rhabdopleura which corresponds to the ccenoecium of an 
 ordinary Polyzoon ; whilst the term " ccenoecium " is 
 totally inapplicable morphologically to the annulated 
 branched tube in which the Rhabdopleura colony lives, 
 this having absolutely no parallel in the Eupolyzoa. 
 
 A sac-like testis has been discovered in Rhabdopleura 
 opening by the side of the anus (Lankester, 7) ; but the 
 ova have not yet been seen, nor is anything known of its 
 development. Similarly the eggs of Cephalodiscus are 
 observed within the body of the parent in the " Chal- 
 lenger" specimens, but nothing further is known of its 
 life-history. 
 
 A body-cavity is present (Lankester), though its exist- 
 ence has been denied by Sars and by M'lntosh. Neph- 
 ridia and nerve ganglia are not described. Cephalodkcus 
 
 tentacles and buccal shield have been 
 removed in order to show the remark- 
 able eyes, a, buds; c, stalk; ff, eyes; 
 A, post-oral collar, hidden by the buccal 
 shield in flg. 9. (Original drawing by 
 Prof. M'Intosh F.R.S.) 
 
 has two remarkable eye spots dorsal to the cephalic disk 
 (fig. 10, ff). 
 
 THE EUPOLYZOA. 
 
 Whilst it is necessary to include in the group Polyzoa 
 the forms we have already noticed as Vermiformia and 
 Pterobranchia, there can be no doubt that those organisms 
 to which we assign the name Eupolyzoa are primarily 
 those upon which naturalists have framed their concep- 
 tion of the group, and that they constitute a very con- 
 sistent assemblage, held together by well-defined characters, 
 and yet presenting an immense number of varied forms 
 showing a wide range of modifications. 
 
 All the Eupolyzoa have closely approximated mouth 
 and anus, and, like Paludicella, a complete range of hollow 
 ciliate tentacles, describing either a circle or a horse shoe, 
 surrounding the mouth. The anus as well as the mouth 
 is included in this area in a few exceptional forms (the 
 Entoprocta) ; it lies near but outside the lophophore (as the 
 area is termed) in the vast majority (the Ectoprocta). 
 Except in the Entoprocta, where the movement is limited, 
 the whole anterior portion of the body bearing the 
 lophophore can be invaginated into the hinder part (as 
 described above for the typical Eupolyzoon Paludicella). 
 This character distinguishes the Eupolyzoa from both 
 Vermiformia and Pterobranchiarr The polypides of all the 
 Eupolyzoa are minute, but all produce buds which remain 
 in organic continuity with their parent (except in Loxo- 
 soma) and build up very considerable and sometimes 
 massive colonies. 
 
 In all Eupolyzoa the cuticle of the hinder part of each 
 polypide is thick and dense, thus forming a hard-walled 
 sac, the zooecium. This is peculiar to and universal in 
 the Eupolyzoa (except Loxosoma), and is not to be 
 confounded with the non-adherent tubes of Phoronis and 
 Rhabdopleura or the jelly-house of Cephalodiscus. The 
 connected zocecia of a colony of Eupolyzoa constitute a 
 ccencecium. A simple nerve ganglion between mouth and 
 anus, a large body-cavity (except in Entoprocta), simple 
 gonads without accessory glands or ducts, usually testis 
 and ovary in the same polypide, absence of a blood-vascular 
 system, of any but the most rudimentary nephridia, and 
 of eyes, otocysts, or other special sense-organs, are features 
 characterizing all adult Eupolyzoa. 
 
 The section Eupolyzoa, with its vast number of species 
 and genera, requires a somewhat elaborate classification. 
 The forms in which the anus is enclosed within the 
 tentacular circle are very few, and are peculiar in other 
 respects. We follow Nitsche (8) in separating them as 
 the sub-class Entoprocta from the majority of Eupolyzoa 
 forming the sub-class Ectoprocta. 
 
 Sub-class 1. Ectoprocta, Nitsche. 
 
 Eupolyzoa with the anus not included within the area 
 of the lophophore. Anterior portion of the body of the 
 normal polypide introversible. Tentacles not individually 
 capable of being coiled or flexed. 
 
 Order 1. PHYLACTOL^MA, Allman. 
 
 Ectoproctous Eupolyzoa in which the polypide possesses 
 a prse-oral lobe or epistome, similar to that of Phoronis, 
 and comparable to the more highly developed buccal 
 shield or disk of the Pterobranchia. Lophophore (except in 
 Fredericella, where it is nearly circular) horse-shoe-shaped 
 (hippocrepian). Polypides of a colony equi-formal, that is, 
 not differentiated in structure and function. Neighbouring 
 zocecia are in free and open communication, the bud never 
 becoming shut off by a perforated cuticular plate from its 
 parent. Cuticle of the zocecia either gelatinous or horny, 
 forming massive or else arborescent ccenoecia, in one genus 
 
P O L Y Z O A 
 
 167 
 
 (Cristatella) having the form of a plano-convex ellipse and i 
 locomotive (fig. 3). In addition to the multiplication 
 of polypides in a colony by budding, and to the annual 
 production of new individuals from fertilized eggs which 
 initiate new colonies, a reproduction by internal buds 
 called " statoblasts," comparable to the gemmae of Spon- j 
 gilla, has been observed in all the genera (fig. 3, b). The 
 statoblasts are developed from the funiculus (mesentery), i 
 and are enclosed in ornate lenticular capsules of chitinous 
 substance, characteristic in form in each species. 
 
 The fertilized egg of the Phylactolserna does not give 
 rise to a zonociliate larva, but to a uniformly ciliate cyst- 
 like diblastula, which develops directly and produces 
 polypides by budding. The Phylactolaema are all inhabit- 
 ants of fresh water (lacustrine). 
 
 FIG. 11. Semi-ideal view of pait of the lophophore of Lopfiopui and its tentacles, 
 intended to show the nerve-ganglion, nerves, and parts around the month. 
 The tentacles have been cut away all along the right arm of the lophophore and 
 from the inner margin of the left arm. r, foramen placing the cavity of the 
 epistome in communication with the body-cavity ; r', body-wall ; d, month ; t, 
 the epistome or pile-oral lobe ; /, wall of the pharyni ; *, wall of the intestine ; 
 i, anns ; t, lophophore ; /, a ciliated tentacle ; r, elevator muscle of the epistome; 
 ir. the nerve-ganglion ; z, x 1 , nerves to lophophore and tentacles- jr, nerve to 
 pharyni. 
 
 The Phylactolaema include the genera Lophopus, Cristatella, 
 Alcyonella, Plumatella, and Fredericella, which have been beauti- 
 fully figured and described in Allman's classical Freshwater Polyzoa, 
 Kay Society, 1856. The colonies of Lophopus are small, consist- 
 ing of half a dozen polypides embedded in a massive glass-like 
 coaneecium. Cristatella (fig. 3) is remarkable amongst all Polyzoa 
 for its locomotive zoarium. Alcyonella forms massive coencecia of 
 many hundred polypides, as large as a man's fist. Plumatella and 
 Fredericella are delicate arborescent forms commonly encrusting 
 stones and the leaves of water-plants. All the genera known are 
 British. 
 
 The Phylactolaema furnish a remarkable instance of a well- 
 marked zoological group being confined to fresh water. Their 
 reproduction by statoblasts (not known in the marine Polyzoa) 
 appears to be related to the special conditions of lacustrine life, 
 since it is also observed under the same exceptional conditions in 
 the single freshwater genus of another great group of animals, viz., 
 Spongilla. Also related to their non-marine conditions of life is 
 the development of the fertilized egg, which, as in so many similar 
 cases, does not produce the remarkable banded forms of locomotive 
 larvae which are characteristic of their marine congeners. 
 
 Order 2. GYMXOLEMA, Allman. 
 
 Ectoproctous Eupolyzoa in which the polypide is devoid 
 of any trace of the pree-oral lobe or epistome, whilst the 
 
 lophophore is perfectly circular. The polypides of a colony 
 are frequently highly differentiated as avicnlaria, vibracu- 
 laria, ooecia (egg-receptacles), and even as root and stem 
 segments. The neighbouring polypides of a colony 
 communicate (?) with one another by "rosette-plates" or 
 " communication-plates " perforated areas in the walls of 
 contiguous zooecia. The greatest variety in the character 
 of the cuticle forming the zooecia (gelatinous, horny, 
 calcareous) and in the grouping of the polypides, as well as 
 in the shape of their zxxBcia, is observed in different 
 sub-orders and families. In addition to the ordinary 
 sexual reproduction, there are various modifications of the 
 process of budding, the full exposition of which would 
 necessitate more space than is here allotted, and is not 
 yet indeed within the possibilities of present knowledge. 
 The fertilized egg of the Gymnolaema gives rise to 
 remarkable ciliate larvas of various 
 forms (figs. 19, 20, 21), from which 
 the first polypide of a colony is 
 developed by an extraordinary and 
 unexplained series of changes. 
 The Gymnolaema are, with the 
 single exception of the genus Palu- 
 dicella, inhabitants of the sea. 
 
 The Gymnolsema are divided, accord- 
 ing to the system of Busk, into three 
 sub-orders characterized by the shape 
 of their zocecia, and the nature of the 
 mouth-like margin which it presents 
 when the exsertile portion of the poly- 
 pide is withdrawn within it. The 
 Cyclostoma have long tubular zooecia, 
 often of large size and often calcified, 
 placed side by side in cylindrical bun- 
 dles, or in other definite grouping ; the 
 month of the zocecium is circular and 
 devoid of processes. There is little or 
 no differentiation of the polypides con- 
 stituting a colony. Most of this group 
 are fossil, and the living genera belong 
 mostly to southern seas. The genera 
 Crisia (fig. 13, A), Diastopora, Tubuli- 
 pora, and Hornera are typical. The 
 Ctenostoma have usually a soft zooe- 
 cium ; its orifice is closed by the folds 
 of the retracted polypide or by a 
 circlet of bristles which surround it 
 Alcyonidium gclatinosum is the com- 
 monest representative of this group 
 on the British coasts. Bowerbankda 
 (fig. 1, A) and Paludicella (fig. 1, E) 
 also belong here. The Chilostoma 
 form the largest and most varied sub- 
 order of Gymnolsema. The zoo?cia are 
 horny or calcified ; their orifices can be 
 closed by a projecting lip in the form 
 of an operculum. The operculum is 
 a separable plate developed on the 
 cuticle of the retractile part of the 
 polypide, and has muscles attached 
 to it (fig. 13, B, C, D). The surface 
 of the zooecia is frequently sculptured, 
 and its orifice provided with processes 
 and spines (fig. 1, C, F). Very usually 
 some of the polypides of a colony are 
 modified asavicularia,vibracularia, radi- 
 cal fibres, and ooecia. The avicnlarium 
 is a polypide reduced to a simple muscu- 
 lar apparatus work ing upon the modified 
 operculum and zooecium so as to cause these hard parts to act as a 
 snapping apparatus comparable to a bird's head (fig. 12, o). They 
 are frequently found regularly distributed among the normal cells 
 of a colony, and probably have a cleansing function similar to that 
 attributed to the Pedieellarise of the Echinoderms. " Vibracularia" 
 are even more simplified polypides, being little more than motile 
 filaments, probably tactile in function. The opercnla of zocecia, 
 ooecia, and avicularia have recently been used by Busk in character- 
 izing genera and species, in a systematic way. Stem-building and 
 root-forming polypides are frequently found, being closed polypides 
 which subserve anchoring or supporting functions for the benefit of 
 the whole colony. The stem of Kinetoskias (fig. 14) is produced 
 
 FIG. 12. Two zooecia of Aea- 
 marchis (Bugula) aricularia, 
 Lmi. (Chilostoma), of which 
 the anterior contains a living 
 polypide, whilst the posterior 
 is empty. To each is attached 
 one of the characteristical- 
 ly modified polypides known 
 as an " avicularinm " o ; the 
 hinder of these has grasped 
 and holds in its beak a small 
 worm, a, anns ; i, intestine ; 
 , stomach; r, body-cavity 
 (coelom); ', tentacular crown 
 surrounding the month ; It, 
 testis cells developed on the 
 surface of the terminal mesen- 
 tery or " funiculus " ; o, o, avi- 
 cularia. 
 
168 
 
 P O L Y Z A 
 
 in this way. The Chilostoma include a large series of genera 
 arranged in the sections Cellularina, Flustrina, Escharina, and 
 
 FIG. 13. A. Cfenoccium of Crisia ettirnea, Lin., one of the Cyclostoma ; g, g, 
 tubular zooecia with circular terminal mouths ; x, ocecium, being a zooeciura 
 modified to serve as a brood-chamber. 
 
 B. Diagram of a single polypide of one of the Chilostoma in a state of expansion, 
 in order to show the position and action of the operculum. a, operculum, 
 a plate of thickened cuticle hinged or jointed to b, the main area of dense 
 cuticle of the antitentacular region known as the zooccium ; c, the soft-walled 
 portion of the polypide in expansion.' 
 
 C. The same zocecium with the polypide invaginated (telescoped) and the 
 operculum a shut down over the mouth of the zorecium. 
 
 D. Operculum detached, and seen from its inner face, to show the occlusor 
 muscles d d. 
 
 Celleporina. For the systematic description of the highly complex 
 and very varied colonial skeletons or coencecia of the Gynmoliema, 
 
 FIG. U.KineloMai (Naresia) cyathus (from Sir Wyville Thomson). The poly- 
 pides and zooecia are allied to Bugula, but the zoarium ns a whole is remarkable 
 for its definite shape, consisting of a number of slightly branched gracefully 
 bending filaments supported like the leaves of a palm on along transparent 
 stalk. (See Busk, in Quart. Journ. Micr. Kci., 1881, for further details.) 
 
 the reader is referred to the works of Busk (9), Hineks (10), Smitt 
 (11), and Heller (12). See also Ehlers (13) on Hypophorella. 
 
 Sub-class 2. Entoprocta, Nitsche. 
 
 Eupolyzoa in which the anal aperture lies close to the 
 mouth within the tentacular area or lophophore. Lopho- 
 phore sunk within a shallow basin formed by the inversion 
 of the broad truncated extremity of the cup-shaped body. 
 Tentacular crown not further introversible, the individual 
 tentacles (as in the Pterobranchia and unlike the Ecto- 
 procta) capable of being flexed and partially rolled up so 
 as to overhang the mouth (see fig. 15, B and C). Body- 
 cavity (ccelom) almost completely obliterated. The anti- 
 tentacular region of the polypide's body is drawn out to 
 form a stalk similar to the gymnocaulus of the Pterobran- 
 chia. The extremity of this stalk is provided with a 
 cement gland in the young condition which persists in the 
 adult of some species (Loxosoma neapolitanwn, fig. 16, 
 shs). Cuticular investment (zorecium) of the polypides 
 feebly developed. A pair of small nephridia are present. 
 
 The Entoprocta consist of the marine genera Pedi- 
 cellina (fig. 15), Loxosoma (fig. 16), and probably the 
 
 FIG. 15. A. Two polypides and buds of Pediceltina belgtca, Van B. (after 
 Van Beneden); greatly magnified, a, the polypide-stalk of a fully developed 
 polypide ; c, that of a less mature individual; b, a bud. All are connected by 
 a common stalk or stolon. li. and C. Two views of the body of the polypide 
 of Pedicellina (after Allman). a, cuticle ; 6, body-wall ; c, permanently in- 
 troverted anterior region of the body ; d, margin of the tentacular cup or 
 calyx thus formed; c, mouth; /, pharynx; g, stomach ; A, intestine; t, anus ; 
 t, epistome or prse-oral lobe ; /, nerve-ganglion ; m, gonad ; n, retractor 
 muscle of the lophophore ; o, lophophore. 
 
 insufficiently known freshwater American genus Urnatella 
 of Leidy. To these must be added Busk's new genus 
 Ascopodaria, as yet undescribed, based on a specimen 
 dredged by the " Challenger," showing a number of Pedi- 
 cellina-like polypides, carried as an umbel on a common 
 stalk of very peculiar structure. Pedicellina is found at- 
 tached to algiB, shells, zoophytes, &c., and to the integu- 
 ment of some Gephyraean worms (Sijwnculus punctatus) 
 and Annelids (Aphrodite) ; Loxosoma occurs on parlous 
 worms, ifec. Whilst the buds of Pedicellina remain connected 
 so as to constitute a colony, those produced by Loxosoma 
 are continually detached, so that the polypide is solitary. 
 Further, the cup-like body of Pedicellina is deciduous, and 
 frequently falls from the stalk and is replaced by new 
 growth. There is less distinction between body and stalk 
 in Loxosoma, and the former does not become detached. 
 Apparently a very important feature in the structure of 
 the Entoprocta is the absence of a body-cavity. This is, 
 however, more apparent than real. The Entoprocta are 
 true Coelomata, but the coelom is partially obliterated by 
 the growth of mesoblastic tissue. The nephridia presum- 
 ably lie in a space which, small as it is, represents the 
 coelom. See Harmer (18) for details. 
 
P L Y Z O A 
 
 169 
 
 Genealogical Relationships of the Groups of Polyzoa. 
 
 It is necessary that we should try to form some opinion 
 as to which of the various groups of Polyzoa are most like 
 the ancestral form from which they have all sprung, and 
 what are the probable lines of descent within the group. 
 Any attempt of the kind is speculative, but it is absolutely 
 needful since zoology has become a science that is to say, 
 an investigation of causes and not merely a record of 
 unexplained observations to enter upon such questions. 
 Colonial organisms have necessarily descended from soli- 
 tary ancestors, and it is probable that the ancestral form 
 of Polyzoa was not only solitary, as are Phoronis and 
 Loxosoma at the present day, but of relatively large size 
 and more elaborately organized than the majority of living 
 Polyzoa. Whilst the polypides have dwindled in size and 
 
 FIG. 16. Diagram of Lnrotoma Xeapolitaniim (after Koxralcwskv). A single 
 polypide devoid of buds, m, mouth: (, stomach; tfis, basaf gland of the 
 polypide-stalk. 
 
 lost some of their internal organs, the modern Polyzoa 
 have developed pnri passu with this degeneration an 
 elaborate system of bud-production and colony-formation. 
 The new individuality (the tertiary aggregate) attains a 
 high degree of development (Cristatella, Kinetoskias) in 
 proportion as the constituent units merged in this new 
 individuality have suffered a degeneration. The prae-oral 
 lobe (epistome, buccal disk) present in all Polyzoa except 
 the most minute and most elaborately colonial forms 
 namely, the Gymnolaema is to be regarded as an ancestral 
 structure which has been lost by the Gymnolaema. The 
 horse-shoe-shaped lophophore, such as we see it in Phoronis 
 and in Lophopus, is probably the ancestral form, and has 
 given rise to the two other extreme forms of lophophore, 
 namely, the " pterobranchiate," associated with a great 
 development of the epistome, and the " circular," associated 
 with a complete suppression of the epistome. The ento- 
 proctous lophophore is a special modification of the horse- 
 shoe-shaped, as shown in the diagram fig. 15, C. The 
 formation of zocecia, and so of an elaborate colonial 
 
 skeleton, was not a primary feature of the Polyzoa. Even 
 after budding and colony-formation had been established 
 zocecia were not at once produced, but possibly dwellings 
 of another kind (Pterobranchia). "We are thus led to look 
 upon the Gymnolaema as the extreme modification of the 
 Polyzoon type. Starting with an organism similar to 
 Phoronis, we may suppose the following branchings in the 
 pedigree to have occurred. 
 
 VERMIFORM IA 
 I 
 
 A. The complete hippocrepian 
 lophophore becomes specialized 
 in the form of ctenidia or gill- 
 plumes ; the epistome enlarged. 
 
 = PTEROBRASCHIA. 
 a. The anti-tentacular region of 
 the body elongated as a stalk 
 gives rise to one or two 
 rapidly detached buds (Ce- 
 phalodiscus). 
 
 0. The stalk gives rise to buds 
 which do not detach them- 
 selves, but remain in con- 
 tinuity so as to form a 
 colony of a hundred or 
 more individuals (Rhabdo- 
 plenra). 
 
 B. The complete hippocrepian 
 lophophore retains its form, but 
 acquires a gradually increasing 
 power of being telescoped into 
 the hinder part of the body. 
 = The Pro-Eupolyzoon. 
 
 A. The anti- tentacular region 
 of the body becomes stalk-like, 
 and develops buds which either 
 detach themselves as they form 
 (Loxosoma) or remain to form a 
 small colony (Pedicellina). The 
 telescopic introversibility of the 
 lophophore does not advance be- 
 yond an initial stage. The arms 
 of the lophophore grow round so 
 as to embrace the anus. 
 = Sub-class 1 (of the Eupoly- 
 zoa) Entoprocta. 
 
 B. The complete hippocrepian 
 lophophore remains in its origi- 
 nal form, and also the pne-or.il 
 epistome, but the telescopic in- 
 troversibility of the anterior 
 region of the body is greatly de- 
 veloped at the same time that 
 the cuticle of the hinder part of 
 the body is increased in thickness 
 and toughness. Bud production, 
 not from a stalk-like pedicle, but 
 from all parts of the liody, now 
 becomes characteristic, the buds, 
 which were at first deciduous, 
 now remaining in permanent 
 continuity so as to form colonies. 
 = The Pro-Ectoprocton. 
 
 A. The polypides acquire the 
 property of carrying their young 
 so as to avoid the disastrous 
 influences of fluviatile currents, 
 and also the property of produc- 
 ing resistent statoblasts, and 
 thus are enabled to become 
 isolated and to persist in the 
 peculiar conditions of fresh 
 waters. 
 
 = The 1st order (of Ectoprocta) 
 Phylactolaema. 
 
 R The polypides forming 
 relatively larger colonies, and 
 themselves becoming relatively 
 more minute, lose by atrophy the 
 prse-oral epistome ; and simul- 
 taneously the arms of the hippo- 
 crepian lophophore dwindle, and 
 a simple circum-oral circlet of 
 tentacles is the result The 
 cuticle of the hinder part of the 
 polypide becomes more and more 
 specialized as the cell or zooe- 
 cmm, and in different polypides 
 in various parts of the colony 
 acquires special forms as egg- 
 cases, snappers (avicularia), ten- 
 tacles, stalk and root segments. 
 -The 2d order (of Ectoprocta) 
 Gymnolaema. 
 
 Distinctive Characters of the Polyzoa. 
 
 From all that has preceded it appears that the really 
 distinctive characters common to all the Polyzoa may be 
 summed up as follows : 
 
 Coelomata with closely approximated mouth and anus, 
 the bulk of the body forming a more or less elongate 
 growth at right angles to the original (ancestral) oro-anal 
 axis, and starting from the original ventral (i.e., oral) sur- 
 face. A variously modified group of ciliated tentacles is 
 disposed around the mouth, being essentially the develop- 
 ment by digitiform upgrowth of a post-oral ciliated band. 
 
170 
 
 POLYZOA 
 
 As negative characters it is important to note the absence 
 of all trace of metameric segmentation, of setae, and of 
 paired lateral (parapodia of Appendiculata) or median 
 ventral (podium of Mollusca) outgrowths of the body-wall. 
 Larval Forms of Polyzoa. 
 
 In the consideration of the probable pedigree and affinities of the 
 Polyzoa, we are not at present able to make use of the facts of 
 development from the egg, on account of the extreme difficulty 
 which the study of the young stages of these organisms presents. 
 In the case of Phoronis we have the only readily intelligible his- 
 tory. The larva, to start with, is of that form known as an archi- 
 troch (see Lankester, "Notes on Embryology and Classification," 
 Quart. Journ. Micr. Sci., 1876), having a prse-oral ciliated area 
 (velum or cephalotroch) continuous with a post-oral ciliated band 
 (the branehiotroch), which latter becomes developed into the ten- 
 tacular crown of the adult. 
 
 The actinotrocha (Phoronis) larva is readily comparable with the 
 trochosphere larva; of Echinoderms, Chsetopods, Gephyrseans, and 
 Molluscs. Its special character consists in the strong develop- 
 ment of the post-oral ciliated band, whereas the prse-oral ciliated 
 band is in most other classes (the Sipunculoids exccpted) the 
 predominant one. The Phoronis larva exhibits first of all an oro- 
 anal long axis, and this is suddenly abandoned for a new long axis 
 by the growth of the ventral surface of the larva at right angles to 
 the primary axis (hence the term Podaxouia). 
 
 In the other Polyzoa we do not at present know of any larva 
 which retains even in its earliest phases the original oro-anal long 
 axis. They all appear to start at once with the peculiar and 
 secondary long axis of the adult Phoronis, so that Balfour has 
 diagrammatically represented the Polyzoon larva by the sketch 
 given in fig. 19. This diagram applies, however, more especially to 
 the Entoprocta, since the anus is represented as included in the area 
 of the post-oral ciliated ring. The development of Pedicellina has 
 been very carefully followed by Hatschek, and may be said to be 
 
 Fig. 17. Fig. 18. 
 
 FIG. 17. Larva of Pedicellina (from Balfour, after Hatschek). r, vestibule 
 
 supposed by Balfour to be a bud, by Haimer (18) regarded as the cephalic 
 
 FIG. 18. Later stage of the same larva as fig. 17. Letters as before, with the 
 addition of nph, duct of the right nephridium ; a, anus ; hg, hind-gut 
 
 the only instance among the Eupolyzoa in which the growth of 
 the diflerent organs and the consequent relation of the form of the 
 larva to the form of the adult is understood (see figs. 17 and 18). 
 
 In the other Polyzoa, in spite of the painstaking and minute 
 studies of Barrois (14), the fact is that we do 
 not know what face of the larva corresponds to 
 the tentacular area, what to the stalk or anti- 
 tentacular extremity, what to the anterior and 
 what to the posterior surface. The conversion 
 of the larva into the first polypide has not st. 
 been observed in the case of these free-swim- 
 ming forms, and it is even probable that no 
 such conversion ever takes place, but that the 
 first polypide forms as a bud upon the body- 
 wall nf flip lirvn FlG - 19 - Diagram of an 
 wall ot the larva. jdea] Polyzoon larva 
 
 Two of the most remarkable forms of free- ( from Balfour). an, 
 swimming larvse of Gymnohiema are repre- 
 sented in figs. 20 and 21. In both, in addition 
 to the chief post-oral ciliated band, a smaller 
 ciliated ring is observed, which is identified 
 by Balfour with that which is found at the anti-tentacular extremity 
 (base of the stalk) in the Pedicellina larva. 
 
 anus ; m, mouth ; st, 
 stomach ; *, ciliated 
 disk (fg in fig!. 17, 18, 
 
 f'J 
 
 of Membrapora (,,, as Cy . 
 phonautcs). m, month ; a', anus ; fg, ciliated 
 body; x, problematical body, supposed by Bal- 
 four to be a bud, similnr to the dorsal organ 
 in figs. 17, 18, and to either st or m in fig. 'K>. 
 
 Thus tho 
 
 It does not seem justifiable, in the face of the existing uncertain- 
 ties as to identification of parts, and in view of the high probability 
 that the Gymno- 
 [sema are extremely 
 modified and degen- 
 erate forms (a con- 
 sideration which 
 applies in some re- 
 spects even more 
 strongly to the En- . 
 toprocta), to assume 
 that the larval form 
 schematized in fig. FlG 50. Larva of Ahyonidium mytili (from Balfour 
 19 represents an an- after Barrois). m 1, problematic structure ; si, oral 
 cestral condition of imagination (?) = Harmer's cephalic ganglion ; s, clli- 
 the Polyzoa Pro- ated disk (corresponding to /y in figs. 17, 18, and 21). 
 
 fessor Balfour (15) was, however, led to entertain such a view ; and, 
 assuming that the chief ciliated band (drawn as a broad black line) 
 
 corresponds to the single ~^___^ 
 
 praj-oral ciliated band of ^t* r~-~. 
 
 the trochosphere larva of 
 Echiurus, Polygordius, 
 Chffitopods, and Mollus- 
 ca, he pointed out that in 
 both cases the ciliated 
 girdle divides the larva 
 into a hemisphere in 
 which mouth and anus 
 lie and a hemisphere 
 which is the complement 
 of this ; in most classes 
 the first hemisphere 
 elongates and forms the 
 bulk of the body, whilst 
 the second hemisphere 
 forms the prostomium or 21 __. 
 prse-oral lobe. But, ac- 
 cording to Balfour's 
 theory, in Polyzoa it is 
 the second hemisphere 
 
 which enlarges and becomes the stalk-like body of the adult, whilst 
 the first hemisphere remains small and insignificant 
 Polyzoa would fix themselves in 
 later growth by what corresponds 
 to the head or prostomium of 
 other animals, as do the Bar- 
 nacles and the Ascidians. In- 
 genious as this speculation "is, we 
 must remember that it takes no 
 account of the facts known as 
 to Phoronis, nor of the Ptero- 
 branchia, and that it is con- 
 fessedly based upon the assump- 
 tion that the larvte of extremely 
 degenerate and peculiar members 
 of the group are not adaptive and 
 modified, but retain primary and 
 archaic characters. Further, it 
 is to be distinctly borne in mind 
 that the interpretation of parts 
 upon which this speculation 
 rests is, except in the case of 
 Pedicellina, altogether hypo- 
 thetical. 
 
 Relations of the Polyzoa to the 
 
 Srachiopoda. 
 
 The Polyzoa were first asso- 
 ciated with the Brachiopoda by 
 H. Milne-Edwards. The inves- 
 tigation of the development of 
 Terebratulina by Morse (16) led 
 to a further perception of the 
 points of agreement in struc- 
 ture between these two groups. 
 Lastly, Caldwell (6) has shown 
 that the mesenteries of Phoronis 
 
 have precisely similar relations ^ ^ _ you Terebratulina at a 5ta 
 to the lophophore, the nephndia, wllcn only 5ix tentacles are p,. es ent. 
 and the termination of the lutes- se, setae at the margin of the calyx ; p, 
 tine as have the gastro-parietal stalk comparable to the stalk of Pedi- 
 ; c +ol IIOY,<IC - TUPS cellina, Loxosoma, Cephalodiscus, and 
 and iho-panetal bands or mes- Rhal)lklpleu ,. a . pa ; cen ' len t gland at the 
 enterics of the Terebratulida!. apex of the stalk (after Morse). 
 The young Terebratulina (fig. 22) 
 
 may be readily compared with Loxosoma (fig. 16), the peduncle 
 with its cement glands in the former being identical with the stalk 
 and basal gland of the latter. The form of the alimentary canal 
 
P L Y Z A 
 
 171 
 
 and the disposition of the tentacular arms (fig. 23) is the same in 
 Brachiopoda and Polyzoa. The nephridia (oviducts) of Terebratula 
 have a position and relations similar to those of the nephridia (geni- 
 tal ducts) of Phoronis. The chief 
 
 difference between Polyzoa and 
 Bracliiopoda consists in the special 
 development of the margin of the 
 cupped end of the body, into which 
 the lophophore is sunk, as in Pedi- 
 cellina (see fig. 15, B, e). This 
 up-standing margin is enormously 
 
 Fig. 23. 
 
 Fig. 24. 
 
 FIG. 23. Lophophore and epistome of young TerebratuliDa, showing the hone- 
 shoe shape; the arms are turned in the direction the reverse of that taken by 
 the lophophore-arms in Polrzoa (see fig. 4). In later growth they will become 
 spirally coiled. (After Mo:se.) 
 
 Fio. 24. Larv.1 of the Brachiopod Argiope (from Gegenbaor, after Kowalewsky). 
 m, set geroua lube; b, setae ; d, enteron. 
 
 increased in the Brachiopoda, so as to form a voluminous hood or 
 collar, which surrounds the large tentacular arms, and forms a pro- 
 tective chamber for them. It is notched right and left so as to be 
 divided into two lobes, and on its surface is developed a horny or a 
 calcareous shell in two corresponding moieties. Until recently it 
 was held (see Lankester, 17) that both Brachiopoda and Polyzoa 
 were modifications of the Molluscan type, and the Brachiopods" 
 collar was identified with the pallial fold of Mollnsca. The resem- 
 blance of the two structures must now be considered as purely 
 homoplastie, and not as having any real morphological (homo- 
 genetic) significance. 
 
 The larvae of the Brachiopoda (figs. 24, 25) are as exceptional and 
 difficult of interpretation as those of Polyzoa, but no attempt has 
 been yet made to show that the one can be reduced to a common 
 form with the other. The three segments presented by some 
 Brachiopod larvae (fig. 25) have been compared to the segments of 
 Chaetopod worms by some writers ; and these, together with the 
 
 presence of set*, have been regarded as indicative of affinity between 
 the Brachiopoda and Chaetopoda (Morse). But it is sufficient, in 
 order to dispose of this suggestion, to point out that the segments 
 of the Chaetopoda follow one another along the primary oro-ana| 
 axis, whilst those of Brachiopoda are developed along an axis at 
 right angles to this (Caldwell). 
 The Brachiopoda must be classified together with the Polyzoa 
 
 Fio. 25. Snrf ace views of ten stages in the development of Terebratulina, showing 
 the free-swimming larva and its mode of fixation (after Morse), e, lophophoral 
 segment; th, thoracic segment; p, peduncular segment; ds, deciduous setae. 
 
 and Sipunculoidea in a phylum (Podaxonia) characterized by the 
 development of this secondary axis. 
 
 Bibliography. (I) 3. Vanghan Thompson, Zoological Researches, Memoir T., 
 "On Polyzoa, a new animal, an inhabitant of some Zoophytes," Ac., 1830; (2) 
 Ehrenberg, Abhandl. d. t. Atad. a. Katunciss. tu Berlin, 1*34; (3) Henri Milne- 
 Edwards, Rechercha anatomiquts, physiotogiques, ft toologiqua lur let Polypiert 
 de France, 8vo, 1841-44; (4) Altaian, The. British Frahxater Polyioa, Ray 
 Society, 18-56 ; (5) Jolllet, " Bryozoaires des cotes de France," Arch. d. Zool. 
 experim^vol. Ti., 1877; (6) Caldwell, Proceedings of the Royal Society. 1883; 
 (7) Lankester, " Rhabdopleura," Quart. Jour. Mitr. Sci., 1884; (8) Nitsche, 
 Zeilschr. far Kits. Zoologie, 1869, and supplement volume, 1876 ; (9) Busk, 
 Catalogue of the Marine Pofyzoa in the British tfuseum (1852X and Voyage 
 of the ' Challenger" " Report on the Polyzoa," vol. x. ; (10) Hincks. British 
 Marine Polyzoa, London, 1880; (11) Smitt, Kritist Fortectxing 6/rer Standi- 
 narient Haft Bryozoa, 1864-68; (13) Heller, Die Bryoioen d. Adrialischm 
 Ueeret, 1867: (13) Ehlers, " Hypophorella expanse," Abhandl. d. Umig. Geiellsch. 
 Gottingen. xxi., 1876; (14) Barrels, Annales dei Sciences Saturellet, vol. ix., 
 1880; (15) Balfour, Comparative Embryology, London. 1880, vol. i., p. 242 ; (16) 
 
 soma," Quart. Jour, tlicr, Sci., April 188S. 
 
 (E. R. L.) 
 
VERTEBRATA 
 
 TTERTEBRATA, the name of a great branch or phylum 
 
 V of the Animal Kingdom which comprises those ani- 
 
 La- mals having bony "vertebrae", or pieces of bone jointed 
 
 marck's so as to form a spinal column. The first recognition of 
 
 Jf d . , the group is due to Lamarck (1797), who united the four 
 
 Cuner s , . , & , T . 
 
 classifi- highest classes of Linnaeus s system as "animaux a verte- 
 
 cations. bres," whilst distinguishing the rest of the animal world as 
 "animaux sans vertebres." The same union of the four 
 Linnaean classes had been previously made by Batsch in 
 1788, who, however, proposed for the great division thus 
 constituted the name " Knochenthiere." The significance 
 of Lamarck's classification was materially altered, and the 
 foundation laid of our present attempts to represent by 
 our classifications the pedigree of the animal kingdom, 
 when Cuvier propounded his doctrine of " types," and re- 
 cognized the Vertebrata as one of four great types or plans 
 of structure to be distinguished in the animal world (/).* 
 
 The Vertebrata of Lamarck and Cuvier included beasts, 
 birds, reptiles, and fishes, and until recently the group 
 was considered as one of the most sharply limited in the 
 animal kingdom. The progress of anatomical studies 
 very soon rendered it clear that all Vertebrata did not 
 possess bony vertebrae ; for, besides the commoner sharks 
 and skates, with their purely cartilaginous skeletons, natu- 
 ralists became acquainted with the structure of fishes, such 
 as the sturgeons and the lampreys, which possess no verte- 
 brae at all, but merely a continuous elastic rod (the noto- 
 chord) in the place of the jointed spinal column. The 
 muscles and their skeletal septa were seen in these fishes 
 to be arranged in a series of segments attached to the sides 
 of this continuous rod ; and hence the structural character 
 of bony vertebrae, as distinguishing the Vertebrata, gave 
 place to the character of segmental arrangement of the 
 muscles of the body-wall, such muscles being supported 
 by a skeletal axis which might be itself unsegmented 
 (notochord), or replaced by segmental cartilaginous or bony 
 Essential vertebrae. The studies of embryologists furnished a sound 
 struc- foundation for this conception by demonstrating that in 
 tares of t j, e em bryos of Vertebrata with true vertebrae these struc- 
 '" tures are preceded by an unsegmented continuous noto- 
 chord. The inquiry into the structural characteristics of 
 Vertebrata led further to the recognition of several addi- 
 tional points of structure, the combination of which was 
 present only in the group which had been recognized by 
 Lamarck on superficial grounds. It was found that all 
 
 ^ These numerals refer to the bibliography at the end of the article. 
 
 Vertebrata possess laterally-placed passages leading from 
 the pharynx to the exterior, serving in the aquatic forms 
 as the exits for water taken in by the mouth, and provided 
 with vascular branchial processes, whilst in the embryos 
 of the higher air-breathing classes they appear only as 
 temporary structures. It was further established that the 
 great mass of nervous tissue lying dorsally above the 
 spinal column, and known as the cerebro- spinal nerve- 
 centre or brain and spinal cord, is in all cases a tube, 
 and originates as part of the dorsal surface of the embryo, 
 which becomes depressed in the form of a long groove and 
 finally closed in by the adhesion of its opposite edges, thus 
 forming a tube or canal. The three structures,- noto- 
 chord, gill-slits, and tubular dorsal nerve-cord, were more 
 than twenty years ago recognized as characterizing, together 
 with the metameric segmentation of the musculature of 
 the body-wall, all Vertebrata at some one or other period 
 of their existence. 
 
 The establishment by Darwin of the doctrine of organic Question 
 evolution in 1859 led naturalists consciously to make the of Verte ~ 
 attempt to determine the genetic affinities and the probable 
 ancestry of the various groups of animals, and enabled 
 I them to recognize in the classifications by "type", and 
 other such conceptions of earlier systematists, the uncon- 
 scious striving after genealogical representation of the 
 relationships of organic beings. The question naturally 
 arose in regard to the Vertebrata, as in regard to other 
 great divisions of the animal kingdom, What were the 
 characters of the earliest forms, the ancestors of those now 
 living? Then came the further questions as to whether 
 any surviving Vertebrata closely resemble the ancestral 
 form, and whether any animals are still in existence which 
 retain the general characters of those primeval forms 
 which were the common ancestors at once of Vertebrates 
 and of other large and equally well-marked phyla or 
 branches of the animal kingdom, such as the Molluscs, the 
 Annulates, <tc. This fascinating subject of inquiry received 
 its most important impulse from the embryological investi- 
 gations of the Russian naturalist Kowalewsky, and has 
 been for nearly a quarter of a century the fertile source of 
 speculation and its indispensable accompaniments, new 
 observation and research. Kowalewsky published in 1866 Kowa- 
 an account of the embryology of the lowest and simplest lewsky's 
 of then recognized Vertebrates, the lancelet (Amphioxus lab ms - 
 laneeolatits), in which he attempted to trace, cell for cell 
 from the fertilized egg-cell, the origin of the characteristic 
 
174 
 
 VERTEBRATA 
 
 Vertebrate organs of this animal (2). This work alone 
 would not have acquired historic importance, although it 
 is the starting-point of what may be called strict cellular 
 embryology, as compared with the less severely histological 
 works of previous students. But it was accompanied by 
 an account (j) of the development of Ascidia mamillata, 
 one of the so-called Tunicate Molluscs, in which it was 
 demonstrated by Kowalewsky, not only that this supposed 
 Mollusc possesses when first hatched from its egg-envelope 
 a notochord, pharyngeal gill -slits, and a tubular dorsal 
 nerve-cord and brain, but that these three characteristic- 
 ally Vertebrate features of organization originate from the 
 same cell-layers of the embryo, and in essentially the same 
 way as in Amphioxus, whilst the cell-layers themselves 
 originate from the egg-cell in the two animals by precisely 
 
 A "il a C D 
 
 6 ^BSx *v ^RBv V^Blltthv y. 
 
 
 
 FIG. 1. Early stages of Ascidia and Amphioxus. a, blastopore ; &, neural 
 groove; c, neural folds; d, closed portion of neural tube; e, commencing 
 oral invagination (stomodamm) of Ascidian tadpole ; /, right ami left cloaca! 
 imaginations of Ascidian tadpole ; g, anterior opening of neural tube of 
 Amphioxus, coincident with the later developed olfactory pit ; h, wall of jjne 
 of the series of paired outgrowths of archenteron or ccelomic pouches of 
 Amphioxus ; i, ectoderm ; fc, endoderm (of diblastula phase) ; /, notochord, 
 derived from endoderm ; m, cavity of gut ; n, cavity of nerve-tube ; o, wall 
 of nerve-tube, formed by upgrowth and union of neural folds ; p, mesoblast 
 of tail of Ascidian tadpole, derived from endoderm ; 7, lumen of ccelomic 
 pouches of Amphioxus, once continuous with m, but nipped off in the course 
 of development. 
 
 A, B, C, D. Four stages in development of Asciilia, surface views showing 
 gradual enclosure of neural area. E, F, G. Three similar stages in develop- 
 ment of Amphioxus. AA. Vertical antero-posterior median section of A. BB. 
 Similar section of B. DD. Similar section of a stage a little earlier than D. 
 EE. Similar section of 'E (Amphioxus). GG. Similar section of embryo repre- 
 sented in G. H. Transverse section of diblastula stage of Amphioxus, with 
 widely open blastopore (earlier than A or E). I. Transverse (right and left) 
 section about the middle of F, showing neural area. K. Transverse section 
 about middle of G, showing nerve-tube, notochord, and coalomic pouches or 
 inesoblastic somites q. L. Transverse section of a much further advanced 
 embryo of Amphioxus, showing nerve-tube, notochord, and gut ; the walls 
 of the ccelomic pouches are now converted into muscular tissue and the 
 pouch cavity (7) compressed, (All the figures after Kowalewsky, 77, 18.) 
 
 similar movements of cell division and invagination (see 
 figs. 1 and 2). Kowalewsky's discoveries established once 
 for all that the Ascidian tadpole is identical in three very 
 special and distinct features of structure with the frog's 
 
 tadpole. No classification which pretended to set forth 
 the genetic affinities of animals could henceforth separate 
 
 FIG. 2. Diagram illustrating relationship of tadpoles of Frog and Ascidian. 
 The two upper figures represent surface views of the tadpoles ; the two lower 
 ones show in place the chief Vertebrate organs, viz., notochord, gill-slits, 
 nerve-tube, and myclonic eye. (From Lankester's Degeneration.) 
 
 the Ascidian from the Vertebrata, and with it the Ascidian 
 brought the whole series of Tunicata. 
 
 The admission of Tunicata as a group of Vertebrata was Admis 
 proposed by the present writer as long ago as 1877 MJ^*J*J 
 but it required the intermediate proposition by Balfour of as ' y l e " 
 a group Chordata, to comprise the two divisions Tunicata brates. 
 and Vertebrata, in order to render the final admission of 
 Tunicata to their proper association with the Vertebrata 
 of Cuvier palatable to systematists. As an objection to 
 the simple inclusion of Tunicata in the great phylum Verte- 
 brata it has been urged that Tunicata do not possess verte- 
 bra?, a proposition which is equally true of Amphioxus 
 and of some Fishes. Shifting the objection, some writers 
 have maintained that the vertebration of the Vertebrata 
 may be understood as having reference to the segmenta- 
 tion of the muscles of the body-wall, which is exhibited 
 by all Cuvier's Vertebrata without exception, inclusive of 
 Amphioxus, though not by Tunicata. To this it may be 
 replied that the Ascidian tadpole, and more clearly the 
 free-swimming Tunicate Appendicularia (see fig. 9), do ex- 
 hibit a segmentation of the muscles of the hinder part of 
 the body- wall similar to and identical with that of Amphi- 
 oxus, whilst no such strict application of a name in its 
 original descriptive sense is desirable in systematic nomen- 
 clature. All Gastropoda (it has been pointed out) are not 
 gastropodous ; all Artkropoda, are not arthropodous ; and 
 many Echinoderma are not echinodermous. It is, in the 
 present writer's opinion, better to retain an historic and 
 familiar name for the great branch of the animal pedigree 
 to which it has become necessary to admit forms whose 
 affinities therewith were at one time unsuspected rather 
 than to sacrifice historical significance to a futile striving 
 after etymological accuracy. 
 
 The admission of Tunicata to association with Cuvier's Inclu- 
 Vertebrata has been followed by a further innovation. s ' "* 
 The remarkable marine worm Balanoglossm originally 
 described by Delia Chiaje at the end of the 1 8th century j n y e rt 
 was shown in 1866 by Kowalewsky (j) to possess a series brate 
 of pharyngeal gill-slits similar to those of Tunicata andP n >" lun: 
 Amphioxus. Later researches by Bateson (6) have de- 
 monstrated that Balanoglossus develops in embryonic life 
 a short notochord, whilst its nerve-cord is, in part at least, 
 tubular, and similar in position and relations to the median 
 
VERTEBRATA 
 
 175 
 
 epidermal tract by the infolding of which the nerve-tube 
 of Twiicata and the other Vertebrata is formed. Hence 
 it seems impossible to exclude Balanoglossus from a place 
 in the phylum Vertebrata. The possession of pharyngeal 
 gill-slits alone might not justify the association; but, -when 
 this is combined with the presence of the notochord, though 
 rudimentary and with a special condition and position of 
 the main nerve-centre, it becomes impossible to ignore 
 Balanoglossus in our conception of the Vertebrate phylum. 
 The recent discoveries of Harmer (7) with regard to Cephalo- 
 difcus will hereafter render it necessary to associate that 
 form, and in all pTobahility-Bkabdopleura also, with Balano- 
 glossus in the Vertebrate phylum. The further conse- 
 quences of such an association in regard especially to the 
 affinities of Polyzoa and of Gephyrxa open up a large field 
 of speculation and of consequent embryological and ana- 
 tomical research. 
 
 Ances- The Cuvierian Vertebrata, Amphioxus, Tunicate, and 
 tral form flajanoyiofsus being thus indisputably connected by a re- 
 brates *" niarkable combination of structural points, which admit of 
 no explanation consistent with the principles of evolutional 
 morphology except that of the genetic relationship of the 
 forms thus enumerated, we are at once confronted by those 
 questions as to the ancestral history of Vertebrata. which 
 have been already mentioned above as stimulated by 
 Kowalewsky's discoveries. Undoubtedly Amphioxus is 
 lower and simpler in structure than any Fish, Tunicata as 
 low as or lower than Amphioxus, and Balanoglossus, in 
 some respects, more archaic than either Amphioxus or the 
 Ascidian tadpole. The first tendency arising from the 
 discovery of the affinities of these simpler forms with the 
 Cuvierian Vertebrata was to see in them the representatives 
 of the ancestors of all Vertebrata. Amphioxus has been 
 pointed to by authorities in morphology as the living 
 presentation of our common Vertebrate ancestor ; a similar 
 position corresponding to an earlier stage of development 
 has been admitted by no less an authority than Darwin 
 for the Ascidian. It appears, nevertheless, that all such 
 simple solutions of the problem of Vertebrate ancestry are 
 without warrant. They arise from a very common tend- 
 ency of the mind, against which the naturalist has to 
 guard himself, a tendency which finds expression in the 
 very widespread notion that the existing anthropoid apes, 
 and more especially the gorilla, must be looked upon as 
 the ancestors of mankind, if once the doctrine of the descent 
 of man from ape-like forefathers is admitted. A little re- 
 flexion suffices to show that any given living form, such as 
 the gorilla, cannot possibly be the ancestral form from 
 which man was derived, since ex hypothesi that ancestral 
 form underwent modification and development, and in so 
 doing ceased to exist. The same considerations apply to 
 the question of the ancestry of Vertebrata. Probably no 
 existing low form of Vertebrate closely represents the an- 
 cestral form by the modification of which higher forms 
 have been developed. We have no justification for assum- 
 ing that such low forms do more than present to us a col- 
 lateral branch of the family, and that collateral branch 
 must, in all probability, have experienced its own special 
 Argn- series of modifications of structure. Not only this, but 
 ments vre have no sufficient ground for assuming that, even in 
 from sim- regret o f the simplicity of their structure, any given 
 stnic- animal forms at present existing exhibit a mere survival 
 ture. of a corresponding degree of simplicity in their remote 
 ancestors. Such an assumption was almost universally 
 made, until a more correct view was pressed on the atten- 
 tion of naturalists by Dr Anton Dohrn, the founder of the 
 zoological station of Naples (<?). So far from its being 
 the case that simplicity of organization necessarily implies 
 the continuous hereditary transmission of a low stage of 
 structural development from remote ancestors, there are 
 
 numerous instances in which it is certain that the existing 
 simplicity of structure is due to a process of degeneration, 
 and that an existing form of simple structure is thus de- 
 scended from ancestors of far higher complexity of organi- 
 zation than itself. Such are various parasitic worms and 
 Crustacea. The evidence in favour of the occurrence of 
 progressive simplification of structure or degeneration, in 
 place of progressive elaboration, depends (1) upon the com- 
 parison of the adult structure of the degenerate organism 
 with that of its nearest allies, by which it is often rendered 
 clear that the ensemble of the organization of the simpler 
 organism cannot be explained on the hypothesis that it 
 represents an ancestral or archaic condition common to it 
 and its more elaborate congeners, and (2) on the direct 
 evidence of individual development or life-history. The 
 latter evidence is conclusive, when we find, as in the case 
 of Cirrhipede Crustaceans and of Ascidian Tunica tes, that 
 the embryo on its way to the adult condition passes through 
 stages of development presenting a higher degree of or- 
 ganization than that ultimately reached, so that, as in the 
 Cirrhipede larva and the Ascidian tadpole, the young form 
 resembles allied organisms of a higher stage of develop- 
 ment, and subsequently degenerates from the point of 
 progressive elaboration to which it had attained, and be- 
 comes greatly simplified in the final stage of its growth. 
 Conclusive as such evidence is, there is no law of develop- 
 ment which necessitates its preservation. If it be an 
 advantage to the organism, the full force of heredity has 
 play, and what are called the " recapitulative phases " of 
 ancestral development are passed through by the indi- 
 vidual in the course of development from the egg. But 
 with remorseless thoroughness all such hereditary tenden- 
 cies may be removed when such removal is an advantage 
 to the organism, and the development from the egg may 
 proceed directly to the adult degenerate form. Such is 
 the case with many Tunicata, the young of which never 
 exhibit notochord and tadpole form ; indeed, were it not 
 for the preservation of a few exceptional cases, like that 
 of the Ascidian section of the group, we should have no 
 direct evidence of the degeneration of Tunicata, from tad- 
 pole-like ancestry. 
 
 The general result of the considerations which have Hypo- 
 been urged with regard to degeneration is this, that it is thesis of 
 prima facie as legitimate an hypothesis, that any existing de s en 
 animal has developed by progressive simplification from 
 more elaborate ancestors, as it is that such an animal has 
 developed by a continuous and unbroken progress in 
 elaboration from simpler ancestors ; and we are specially 
 called upon to apply the hypothesis of degeneration where 
 the animal under consideration is likely from its mode of 
 life to have undergone that process. Such modes of life, 
 tending to degeneration, are parasitism, sessile or adherent 
 habit, burrowing in the sea-bottom, and diffuse feeding. 
 The animal which pursues living prey, and contends with 
 other organisms for the dominion of the regions of earth and 
 water that are flooded with light and richly supplied with 
 oxygen gas, is the animal which represents the outcome of 
 a longer or shorter period of progressive elaboration. It 
 is worth while noting in parenthesis that in all cases the 
 "whirligig of time" has probably brought its revenges, 
 and that the ancestry of a form evolved through a long 
 period of progressive elaboration was at an antecedent 
 period subject to simplification and degeneration, whilst in 
 the past records of the present exemplars of the latter 
 process there must certainly have been long stretches of 
 continuous elaboration. 
 
 Applying these considerations to the construction of the Genea- 
 genealogical tree of Vertebrata, we find that the task is by logical 
 no means simplified. We cannot with the earliest evolu- tree- 
 tionists adopt a scale or ladder-like series, placing the 
 
176 
 
 VERTEBRATA 
 
 simplest form on the lowest step ; nor can we be satisfied 
 with a tree-like arrangement, in which the forms at the 
 ends of the branches are always more elaborate than those 
 nearer the trunk. Our genealogical tree will more strictly 
 conform to that of a parvenu human family, if we take 
 worldly prosperity in the latter case as corresponding to 
 elaboration of structure in the former. The strict family 
 genealogist will include in the successive ramifications of 
 the tree the five sons of the founder of the family, one of 
 whom remained an agricultural labourer, whilst two be- 
 came brewers and two emigrated. The cousins in the 
 next generation will be set forth in place, the sons of one 
 brewer becoming paupers, whilst those of the other advance 
 to the position of Government employes, and one to the 
 peerage. Thus in successive branchings of , 
 
 * . f, Ml' 1 rrtAKlWUKAU 
 
 the family history there may be alternate pro- SACS* SLITS. 
 gress and degeneration. And so it must be R y^is*>. 
 in the genealogical trees constructed by the B. E,..~ 
 naturalist : the fact that a branch is later in .<!*?.. 
 
 -n , I xv A >A t." 1 i Oral Saca & Slit. 
 
 origin will not imply that it is higher in ela- i. t po, t oral 
 boration than those below it, and accordingly 
 we must not expect to draw our tree so as to ~ 
 be able to trace all simpler forms to lower - 
 off-sets of the tree. 
 
 Divisions The structural features of those animals 
 of Verte- -which must be admitted to the Vertebrate 
 phylum in consequence of possessing noto- 
 
 phylum 
 
 laterally so as to enclose and protect the brain (hence Craniata}. 
 Cartilage is developed in other parts of the body as a skeletal sub- 
 stance, though it may be subsequently replaced in the cranium, as 
 elsewhere, by bone. The longitudinal muscles of the body-wall are 
 divided by transverse fibrous septa into a series of segments, vary- 
 ing in the adult from ten to one hundred or more in number. 
 Cartilaginous neural arches, corresponding in number and position 
 to the fibrous septa, and resting on the notochord, are developed 
 so as to protect the nerve-cord. Cartilaginous bars also pass out- 
 wards, with a direction at first horizontal and then ventral, from 
 the sides of the notochord into the intermuscular fibrous septa. 
 Very generally, but not .always, a tubular cartilaginous sheath 
 forms round the notochord ; this sheath with rare exceptions be- 
 comes segmented to form a series of vertebral bodies, which lie in 
 the planes of the fibrous intersegmental septa, and, increasing in 
 thickness by encroaching upon the substance of the notochord, 
 finally obliterate it almost entirely. 
 
 VASCULAR AKCBES. 
 
 THE CRANIAL' 
 
 Tll- NERVES. 
 
 . Olfactory. 
 
 3. Motor of | 
 
 Eye. 
 4. Pathetic of 
 
 By* 
 
 3- Tngeminus. 
 12' 
 
 *. 6. Abducnu of 
 Eye. 
 
 7. Faal. 
 
 8. Auditory. 
 9. Glossopharyn- 
 
 10. Vagu. or 
 
 chord, pharyngeal gill-slits, and dorsal nerve- 
 plate, tubular or unrolled, are such as enable ~ 
 us very readily to group them in four great 
 divisions, which appear to be equally distinct 
 from one another. As to what may be the 
 
 crpnptiV rplitinn* trv nnp inntVipr nf tlipsp fnnr Flc " 3 '~ Three llia srams showing characteristic disposition of gill-slits, skeletal arches, vascular 
 
 arches, and furcal nerves in a primitive ideal Craniate. The following abbreviations require 
 groups we will inquire Subsequently; for the explanation: pros., prosencephalon ; tfini., thalamencephalon ; mes., mesencephalon ; met., met 
 
 Pneumogastric 
 11. Spinal 
 
 Accessory. 
 12. Hypoglosaal 
 (Motor of Tongue 
 
 Charac- 
 ters of 
 Crani- 
 e,ta. 
 
 present we term these groups "branches. 
 They are as follows : 
 
 Phylum VERTEBRATA. 
 Branch a Craniata (Cuvierian Vcrtcbrata). 
 ,, b Cephalochorda (Amphioxiis). 
 ,, c Urochorda ( Tunicata). 
 ,, d Hemichorda (Balanoglossus}. 
 
 The Vertebrata thus limited may be defined as Coalomate 
 Enterozoa (Metazoa) with well-developed coelom. In all, 
 with the exception of the more degenerate members of 
 Urochorda, an elastic skeletal rod the notochord is de- 
 veloped dorsally by an outgrowth of cells forming the wall 
 of the primitive archenteron ; the notochord may or may 
 not persist in adult life. Pharyngeal gill-slits, which may 
 or may not persist in adult life, are developed in all Verte- 
 brates. In all, except in certain Urochorda, the chief nerve- 
 centre has the form of a dorsal, median, elongate tract, 
 derived from the epiblast, which becomes sunk below the 
 surface and invaginated so as to form a tube. In all there 
 is a tendency to metameric repetition of parts, which may 
 find its expression in a strongly-marked segmentation of 
 the musculature of the body-wall and its skeleton, or may 
 be recognizable only in a limited degree, as exhibited by 
 the successive gill-slits or successive gonads. 
 
 We shall now examine the distinctive features of each 
 of these large groups, and form an estimate of their rela- 
 tions to one another, and of their probable ancestry, this 
 being the task to which we must limit ourselves in the 
 brief space here afforded. 
 
 THE CRANIATA. 
 
 The Craniata are Vertebrata in which the tubular cerebro-spinal 
 nerve-mass is swollen anteriorly to form a brain, consisting primarily 
 of three successive vesicles, in connexion with the anterior of which 
 the special nerves of the olfactory organs and of the eyes originate. 
 The notochord, whilst extending posteriorly to the extremity of the 
 body, does not reach quite so far forward anteriorly as the termina- 
 tion of the nerve-tube. A cartilaginous cranium or brain-case de- 
 velops round the anterior extremity of the nerve-cord, and rises up 
 
 ncephalon ; and., otocyst or auditory sac ; tru. art., truncus arteriosu.s. (Original.) 
 
 The pharyngeal slits follow closely upon the mouth, and in ex- 
 isting Craniata never number more than eight pairs (see fig. 3). 
 They are separated from one another and their apertures strengthened 
 by a series of cartilaginous hoops, the first of which, that between 
 the mouth and the first gill-slit, forms the primitive upper and 
 lower jaw in all but the small and degenerate group Cyclostoma. 
 The gill-slits when functional are generally protected by an opereular 
 fold of the body-wall, which overhangs them and corresponds to 
 the epipleural fold of Amphioxus, the collar of Balanoglossus, and 
 doubtfully to the wall of the atrial chamber of Urochorda. The 
 extension of this fold along the sides of the middle third of the 
 body (between the pharyngenl region and the anus) acquired in 
 ancestral Craniata the function of a continuous right and left lateral 
 fin (see fig. 4). At the same time a continuous median fin, corre- 
 
 HrF 
 
 Bl'Jn 
 
 Fir.. 4. A. Hypothetical primitive Fish, with continuous lateral fins S, S (paired 
 right and left), confluent with median azygos fin (An), the post-anal part of 
 which is marked S', whilst its dorsal part is marked D. B. Actual Fish, 
 showing relation of isolated lateral and median fins to original hypothetical 
 fins of the upper figure. BrF, left pectoral fin (paired) ; BF, left pelvic fin 
 (paired); AF, anal (post-anal) azygos tin; SF, caudal azygos fin; RF and 
 FF, antcriorand posterior azygos dorsal fins ; An, anus. (From Wiedersheim.) 
 
 spending to the dorsal, caudal, and anal fins of existing Fishes, was 
 developed. In both lateral and median fins a cartilaginous 
 skeleton was developed, consisting of a basal longitudinal bar, 
 supporting a number of rods like the teeth of a comb. The 
 primitive form of fin skeleton is retained in the median fins of 
 some sharks ; the primitive lateral fin has in all cases either 
 
VERTEBRATA 
 
 177 
 
 entirely disappeared (Cydostoma) (as has their anterior extension, 
 the opereulum, in many cases) or it has become, together with its 
 skeletal elements, concentrated in two regions forming the pectoral 
 and the pelvic paired appendages or limbs, with their respective 
 girdles. 
 
 The cerebro-spinal nerve-centre and the disposition of the nerves 
 issuing from it present a remarkable complexity, and at the same 
 time uniformity, of structure in all Craniata (see fig. 5). The fore- 
 most of the three prim- _ 
 
 ary cerebral vesicles _' >^. * A 
 
 gives rise to paired an- 
 terior outgrowths, the 
 prosencephala, to a 
 median dorsal 
 growth, the 
 stalk of the 
 pineal eye 
 (rudimentary 
 in all exist- 
 ing Craniate), 
 and to a me- 
 dian ventral 
 outgrowth, 
 which is met 
 by an inva 
 gination 
 the epide 
 of the oral 
 cavity form- 
 ing the pitu- 
 itary body, 
 further, to a 
 pair of lateral 
 outgrowths, 
 which be- 
 come the 
 right and left 
 optic nerves 
 and retinae 
 respectively. 
 
 of FIG- 5. Diagrams of Craniate brain. A. Embryonic condition 
 of neural tube. G, Cerebral portion ; E, spinal cord ; I, 
 II, III, three primary cerebral vesicles. B. Longitudinal 
 section of adult brain, applicable to any and every Craniate 
 Vertebrate. Be, Floor of skull ; C*, notochord ; SD, roof or 
 skull ; If Hi, nasal cavity ; VH, fore-brain, prosencephalon, 
 or cerebrum ; (Af, olfactory lobe ; C, corpus striatum ; ZH, 
 thalamencephalon, corresponding to primary anterior vesicle, 
 from which the prosencephalon has grown out as well as Z, 
 the epiphysis or pineal body ; I, infnndibulum, with attached 
 hypophysis or pituitary body H; Opt, optic nerve; Tko, 
 thalamus options ; HC, posterior commissure ; MH, mesen- 
 cephalon (corpora quadrigmina or optic lobes or mid-brain); 
 HH, cerebellum or metencephalon ; SH, medulla oblongata 
 or epencephalon (the reference line touches the membranous 
 roof of the so-called "fourth ventricle" of the brain); Co, 
 canal of the spinal cord or myelon. (From Wiedersheim.) 
 
 The modifications of the hindmost of the three prim- 
 ary vesicles are also extremely definite and persistent throughout 
 the group: its anterior dorsal surface enlarges and becomes the 
 cerebellum (the metencephalon), whilst the cavity of its hinder 
 part (the medulla oblongata or the epencephalon) becomes compara- 
 tively wide, and is covered dorsally by a thin membrane only, in 
 which nervous tissue does not take a part. The intermediate 
 primary cerebral vesicle (the mesencephalon) does not give rise to 
 outgrowths. 
 
 In all Craniata nerves are given off from the cerebro-spinal cord 
 or tube with great regularity, one right and left in each success- 
 ive myomere or segment of the body -wall. Each nerve has two 
 roots, a dorsal (sensory) and a ventral (motor). A commissure 
 between each successive pair of deep or intestinal branches of the 
 spinal nerves forms the so-called sympathetic nerve-cords, one on 
 each side of the vertebral column. Nerves similar to the spinal 
 nerves, but not identical with them, are given off from the brain, 
 and perforate the cranial box right and left. In all Craniata there 
 are ten pairs of nerves which originate thus, and in the higher 
 forms two more pairs (elsewhere spinal) are included amongst those 
 which thus perforate the cranium (spinal accessory and hypoglossal). 
 The order and character of the cranial nerves are the same in all 
 Craniata. The first (olfactory) and the second (optic) are unlike 
 spinal nerves in both distribution and origin. As we pass back- 
 wards along the series, the cranial nerves are found to resemble more 
 and more the ordinary spinal nerves. Hence it has been inferred 
 that the cranial region consisted at one time of a number of distinct 
 myomeres (as many as uine), which have become fused and modi- 
 fied to form the typical craniate "head." The oculo-motor or 3d 
 nerve indicates the first of these segments, the trochlear or 4th 
 nerve the second, the abducens or 6th the third, the facial and 
 auditory (7th and 8th) the fourth, the glosso-pharyngeal or 9th the 
 fifth, and the vagus or 10th, with certain of its branches supposed 
 to have been originally distinct nerves, the sixth, seventh, eighth, 
 and ninth. It is probable that in Craniata the metamerism of the 
 gill-slits does not correspond to the metamerism of the body-wall. 
 The mouth and each successive gill-slit are related to a bifurcate 
 branch of a cranial nerve (furcal nerve) in lower Craniata (see fig. 
 3) ; these furcal nerves do not correspond, so far as we can at present 
 judge, with cranial myomeres. 
 
 The lateral pair of eyes (as opposed to the rudimentary pineal or 
 parietal eye) present a striking uniformity of origin and structure 
 throughout Craniata. Not only are they uniformly developed from 
 three elements, viz., the retinal cup which grows out from the 
 anterior of the primary cerebral vesicles, the epidermal lens which 
 grows inwards from the surface of the skin, and the connective 
 tissue between these two, but we find that the muscles attached to 
 the eyeball are identical throughout the series : that is, the superior, 
 
 inferior, and internal rectos, and the inferior obliqne mnscles repre- 
 sent the first cranial myomere, the superior oblique represent 
 the second, and the external rectus represent the third cranial 
 myomere. 
 
 The olfactory sacs are paired in all except Cydostoma, in which 
 they are represented by a single sac which may or may not be 
 archaic in its azygos character. The auditory sacs are paired 
 organs which develop as invaginations from the surface, the orifice 
 of invagination closing up, at the hinder part of the cranial region. 
 They present a gradually increasing complexity of form as we pass 
 from aquatic to terrestrial forms, but are identical in essential struc- 
 ture throughout. 
 
 All Craniata, except some Fishes, possess a muscular process on 
 the floor of the oral cavity which may carry teeth, or act as a lick- 
 ing organ, or assist in suction. This is the tongue. 
 
 All Craniata, with degradational exceptions, possess an outgrowth, 
 single or paired, of the post-pharyngeal region of the alimentary 
 canal, which is filled with gas. In many Fishes this becomes shut 
 off from the gut ; in others it remains in communication with the 
 gut by an open duct In Fishes it functions as a hydrostatic appar- 
 atus. In terrestrial Craniata it is subservient to the gas-exchange 
 of the blood and becomes the lungs. 
 
 All Craniata have a large and compact liver ; and a pancreas is 
 also uniformly present, except in Cydostoma, some bony Fishes, and 
 the lower Amphibia. 
 
 All Craniata have a thick-walled muscular heart, which appears 
 first as an "atrium," receiving the great veins, attached to a 
 "ventricle" by which the blood received from the atrium is pro- 
 pelled through a number of arteries, right and left, corresponding in 
 number to the pharyngeal gill-slits between which they pass. 
 A prseatrial chamber (the sinus) and an extra-ventricular chamber 
 (the conns) are added to the primitive chambers ; but the most 
 important modifications arise in consequence of the development 
 of pulmonary respiration and the gradual separation of the cavities 
 of the heart by median septa into a double series, a right and a left 
 The plan of the great arteries in all Craniata is in origin the same, 
 and is determined by the primitive existence of a branchial circula- 
 tion in the gill-slits, which is obliterated in higher forms. Simi- 
 larly the plan of the great veins is identical, the primitive posterior 
 vertebral veins of lower Craniata, though persistent in higher 
 members of the group, having their function gradually usurped by 
 the excessive development of the renal vein, and of renal-portal and 
 ultimately of iliac veina. 
 
 All Craniata have a lymphatic system or series of channels by 
 which the exudation from the capillary blood-vessels is returned 
 to the vascular system. It includes in its space-system the coelom 
 and a variety of irregular and canalicular spaces in the connective 
 tissues. Masses of spongy tissue (adenoid tissue, lymphatic glands) 
 exist, through which the lymph filters, and there acquires corpus- 
 cular elements as well as chemical elaboration. At various points 
 in various Craniata pulsating or simple communications are estab- 
 lished, between the lymphatic system and the veins. A special 
 and characteristic communication is established in the spleen, an 
 organ which is found in all Craniata, either as a single mass or as 
 scattered masses of spongy tissue in which blood-vessels and lymph- 
 atics unite. 
 
 The renal organs of Craniata are primitively a series of nephridia 
 corresponding in number to the myotomes of the mid-region of the 
 body in which they exist. They are connected in the simplest 
 Craniata by a right and a left archinephric duct, which appear to 
 be in origin lateral grooves of the epidermal surface. This primi- 
 tive renal system has been modified in some lower forms (Cydostoma 
 and Teleostean Fishes) by the atrophy of its anterior portion. But 
 in all other Craniata it acquires relations to the gonads or ovary 
 and testes, so that an anterior portion of the archinephros and a 
 corresponding longitudinal tract of the duct become separated to 
 serve as oviduct, a middle portion to serve as sperm-duct, while a 
 posterior portion retains exclusively or shares with the middle 
 portion the function of urinary excretion. The male and female 
 gonads are, with the rarest exceptions, developed in distinct indi- 
 viduals, though the rudiments of the suppressed gonad may in 
 some cases (Amphibia) be traceable in either sex. 
 
 The group of Craniate Vertebrata thus anatomically described, Sub- 
 whilst retaining the essential unity indicated, presents an immense divisions 
 variety of modifications. The chief modifications are distinctly of Crowi- 
 traceable to and accounted for by mechanical and physiological ata. 
 adaptation to a terrestrial and air-breathing life, as opposed to the 
 earlier aquatic and branchial condition. The existing forms of 
 Craniata have been arrested at several points, in the progress towards 
 the most extreme adaptation to terrestrial conditions, which is 
 presented by those forms that can not only breath air and live on 
 dry ground but fly habitually in the air. The organs most obviously 
 affected by this progressive adaptation are the skin, the skeleton, 
 especially of the limbs, the pharyngeal gills, and the air-bladder. 
 This fact will appear most clearly in the subjoined classification of 
 Craniata ; for space does not permit us to pursue further the 
 history of these modifications. 
 
178 
 
 VERTEBRATA 
 
 Classification of CRANIATA. J 
 Grade A. CYCLOSTOMA. 
 
 Class I. Myxinoidea. 
 
 II. Pctromyzontia. 
 Grade B. GNATHOSTOMA. 
 
 Grade a. Branchiata heterodactyla. 
 Class I. Pisces, 
 II. Dipnoi. 
 Grade b. Branchiata pentadactyla. 
 
 Class. Amphibia. 
 
 Grade c. Pentadactyla lipobranchia. 
 Branch a. Monocondyla. Branch, b. Amphicondyla. 
 
 Class I. Rcptilia. Class. Mamtnalia. 
 
 II. Aves. 
 
 Ances- If we now briefly consider what must have been the common 
 tral form ancestral form from which these Oraniata have proceeded, making 
 ofCVani-use of such internal evidence as their structure affords, we find 
 ate. that we get no further back than such an animal as would fit the 
 description given above, with the exception that we should be 
 warranted in substituting in the ancestor a pair of continuous 
 lateral fins, with comb-like cartilaginous skeleton, in place of the 
 two pairs of fins, or their total defect, seen in living Craniata. We 
 get no clear suggestion from the study of Craniata themselves as 
 to the meaning of the curious shape of the brain and its outgrowths 
 (though the pineal outgrowth has recently been explained as an 
 eye), nor as to the original genesis of the notochord. We should, 
 however, be justified in representing that region which now cor- 
 responds to the hinder part of the skull and brain as more fully 
 developed and segmented, so as to give a series of separate myotomes 
 and perhaps separate nerves corresponding to the several furcal 
 branches of the vagus ; and we may very well suppose that the num- 
 ber of pharyngeal gill-slits was larger in the ancestral than in any 
 living form, though it seems improbable that in any true Craniate 
 did each gill-slit correspond to a distinct muscular segment. 
 
 An attempt to go further than this has been made by Dr Anton 
 Dohrn by the method of hypothesis and subsequent corroborative 
 inquiry into facts of minute structure and embryological history. 
 Making use of the principle of degeneration, Dohrn started with 
 the legitimate hypothesis that the branches of Vertebrata other 
 than Craniata viz., Cephalochorda, Urochorda, and (though at the 
 time he commenced his work their structure was not fully under- 
 stood) Hemichorda were not to be regarded as permanent records 
 of steps in the evolution of Craniata, but rather as greatly de- 
 generate offshoots from the ancestors of that group, which could 
 throw but little light on the character of their non - degenerate 
 ancestors. A second fundamental assumption which led Dohrn to 
 his position was that the segmentation of the Craniates' body-wall 
 is a primitive and essential feature in their structure, and becomes 
 more and more fully expressed instead of less developed the further 
 we go back in their ancestry. Dohrn, in fact, assumes that what 
 is called metameric segmentation is a phenomenon of structure 
 which has occurred once only in the history of animal form, and 
 that all segmented animals are genetically related and descended 
 from a common segmented ancestor. Assuming this, he pointed 
 to the existing Chsetopod Worms as most nearly representing at 
 the present day the common ancestor of segmented animals. They 
 have, as he pointed out, a high organization, little inferior to that 
 of the lowest Craniata; they possess a well developed ccelom, 
 blood-vessels with red blood, a segmental series of nephridia 
 (modified in some as gonaducts), segmental branchiae, and lateral 
 locomotive organs ; not a few develop cartilage as a skeletal sup- 
 port ; and many show a concentration and fusion of segments to 
 form a complex head, which resembles, so far, that of Craniata. 
 The ventral in place of the dorsal position of the nerve-cord led 
 Dohrn to accept De Blainville's conception that the dorsal and 
 ventral surfaces are reversed in Vertebrata, as compared with 
 Annelids, Crustaceans, and Insects, so that the Vertebrate is com- 
 pared to an Insect walking with its ventral surface upward. This 
 led further to the notion that the mouth of the Ch&topod or 
 Annelid, which penetrates the nerve-cord, or rather passes between 
 its two divaricated lateral constituents in those animals, has in 
 Craniata disappeared, its place being taken by a new mouth de- 
 rived from the modification of a pair of gill-slits. The remnant of 
 the old mouth, which should, if the comparison instituted holds 
 good, lie in Craniata somewhere on the dorsal surface of the cranial 
 region, was sought by Dohrn in some of the peculiar and hitherto 
 unexplained median structures of the brain : at one time the fourth 
 ventricle with its deficient roof was suggested as thus to be explained, 
 whilst subsequently the curious median structures, the pineal and 
 pituitary bodies, were called in as possibly thus significant. 
 
 Without pursuing further the elaboration of Dohrn's views, it 
 must be at once noted that, whilst the legitimacy of the assumption 
 of degeneration must be admitted, the second assumption, viz., 
 that metameric segmentation is a character bringing all forms 
 showing it into a special genetic continuity, cannot be accepted. 
 1 The classes here enumerated are described in separate articles, whilst 
 Cydostoma and Dipnoi are included in the article ICHTHYOLOGY. 
 
 The property of repeating units of structure, so as to build up a 
 complex of many similar parts united to form one individual, is a 
 very general one in organic forms, and is exhibited in various con- 
 ditions by both animals and plants. Its simplest expression is 
 found in cell-structure and the binary division of cells. It shows 
 itself as affecting larger masses of structure in the arborescent 
 colonies of Calentera, in the radial or antimeric composition of 
 Echinoderms and of Compound Ascidians, and in the linear or 
 metameric segmentation of Worms, Arthropods, and Vertebrates. 
 There is abundant evidence that this property is a general one, 
 which may assert itself at any period in the history of a group of 
 animals, and does not imply special unity of origin in forms which 
 exhibit it. As pointed out in the article HYDKOZOA, merogenesis 
 the name applicable to this phenomenon generally may take 
 an extreme and complete character, leading to the separation and 
 independence of the units of structure produced ; in that case it 
 may be termed eumerogenesis. Or the process may be very partial, 
 occurring only during a period of embryonic growth, and subse- 
 quently ceasing, so that later growth obscures or obliterates it 
 altogether (dysmerogenesis). There is no ground for assuming 
 that either one of these extremes is fundamental or original. Any 
 mechanical or nutritional condition may lead to merogenesis in an 
 organism in which the tissues have a certain reproductive capacity, 
 or have not acquired final differentiation ; and it will depend upon 
 the balance of advantage, determined by natural selection, whether 
 the segmentation (supposing the merogenesis to take the linear 
 form) results in the separation of segment-buds, or in the formation 
 of an annulate body, or leaves traces of its occurrence only in certain 
 tissues and organs. The Cestoid Worms present within the range 
 of a single group almost every grade of eumerogenesis and dysmero- 
 genesis (CaryophyllsBus, Ligula, Teenia). In the otherwise amero- 
 genetic' Mollusca, Chiton and the pearly Nautilus show dysmero- 
 genesis in certain organs, whilst the Planarian Worms frequently 
 exhibit eumerogenesis in their bud-segmentation (to be compared 
 with that of the Annelid Gtenodrilus described by Zeppelin, 9) and 
 the elongated Neruertines only slight traces of dysmerogenesis. 
 
 If we deny Dohrn's assumption with reference to segmentation, 
 we are no longer led in the direction of the Annelids (Chsetopods) 
 in our search for the ancestry of the Craniate Vertebrata. 
 
 The fact that the notochord is the forerunner of the segmented 
 vertebral column, and is itself never segmental, instead of being a 
 difficulty, acquires directive significance. The fact that the nerve 
 tube is dorsal, and not ventral, no longer requires the large assump- 
 tion that animals have reversed their habitual carriage, but suggests 
 that the Craniates' ancestor had a dorsal median nerve, which has 
 increased in size and importance so as to become the nerve-tube 
 of existing forms. 
 The explanation of 
 the curious struc- 
 ture of the brain 
 will have to be 
 found otherwise 
 than in the assump- 
 tion of a perforating 
 pharynx, an as- 
 sumption which the 
 recent discovery of 
 the true nature of 
 the pineal body has 
 rendered untenable 
 in the latest form 
 advocated by its 
 ingenious author, 
 whose speculations, 
 nevertheless, de- 
 serve the fullest re- 
 cognition as having 
 stimulated inquiry 
 and guided observa- 
 tion. 
 
 Balfour (10) in 
 1878 refused to a- 
 dopt Dohrn's views, 
 and considered it 
 probable that the 
 dorsal position of 
 the nerve cord in 
 Vertebrata could be 
 accounted for, with- 
 out any assumption 
 of a substitution of 
 a pair of gill-slits for 
 the original mouth, 
 by assuming that 
 
 f) AMPHIOXUS 
 
 FIG. 6. Comparison of nervous systems of a Xemertine, 
 a prim itive Craniate, and A mphioxus. m, Median dor- 
 sal nerve, which becomes the myelon in the Craniate 
 and Amphioxus, acquiring an anterior enlargement in 
 the former ; I, lateral nerve (right and left), absent by 
 degeneration in Amphioxus ; lg, ganglia of lateral 
 nerve, forming a single large lobe on each side in 
 the Nemertine, and broken into a metameric series in 
 the Craniate ; v, roots of vagus nerve of the Craniate ; 
 dr, dorsal roots of nerves given off from myelon or 
 median dorsal nerve ; vr, ventral roots of these nerves, 
 here represented as separate nerves ; 0, mouth. (After 
 Hnbrecht.) 
 
 assuming 
 
 primitively the nerve-cord consisted of two lateral cords, as seen at 
 the present time in the Nemertine Worms, and that these cords 
 have coalesced dorsally in Vertebrata, just as it is clearly demon 
 
VERTEBRATA 
 
 179 
 
 strable that two originally lateral cords have coalesced tentrally to 
 form the Annelid's ventral nerve-chain. 
 
 The comparison of the Vertebrates' nervous system with that of 
 the Nemertines had already been made by the present writer, as 
 cited by Hubrecht (//) in connexion with the latter's discovery of 
 a complete sub-epidermal nerve-tunic in those worms. Hubrecht 
 has more recently on two occasions (12 and 13) developed an in- 
 teresting and important comparison of Nemertine and Vertebrate 
 structure. He has in the first place suggested that the notochord 
 of Vertebrata is nothing more than a modified survival of the pro- 
 boscidean sheath of the Nemertines, whilst the oral invagination of 
 the epidermis, in connexion with the hypophysis eerebri of the 
 Vertebrate, may be a last remnant of the proboscis itself. More 
 conclusively he has drawn attention to the median dorsal nerve of 
 Nemertines as corresponding ^o the Vertebrate cerebro-spinal nerve- 
 cord, whilst the great lateral nerve-cords of Nemertines, and the 
 lateral ganglia in which they expand anteriorly, are compared to 
 the lateral ganglia of the cephalic region of Craniate Vertebrata 
 and the nerve of the lateral line (see fig. 6). The comparison is 
 strengthened by the existence of a inetameric series of transverse 
 nerves in the Nemertine, which correspond in respect of their meta- 
 merism and their connexion with a dorsal median trunk, with the 
 spinal nerves of Craniata. Hubrecht is careful to insist that he 
 does not regard the Nemertines as representing the direct ancestry 
 of Vertebrata ; but he points out that from the primitive condition 
 of an elongate animal, with a plexiform nerve-tunic, it is readily 
 conceivable that a form was developed in which the nervous tissue 
 was concentrated in three cords, a median dorsal and two lateral, 
 and from such a form we can derive the Craniates' condition by 
 excessive development of the median tract and relatively small 
 development of the lateral cords, whilst the Nemertines' condition 
 would be attained by the converse process. The tubular condition 
 of the cerebro-spinal nerve-cord of Vertebrata, it may here be re- 
 marked, is now very generally regarded as being in its origin a 
 purely developmental feature. It was primitively separated from 
 the epidermis by delamination and in -sinking, and the mode of 
 formation by invagination of a canal has been substituted in accord- 
 ance with a general embryological law of growth, which is that 
 bulky structures originating beneath a surface from the cells form- 
 ing that surface take up their position in embryonic growth by in- 
 vagination of the parent surface. The tubular form, having thus 
 started, seems to have been utilized during one phase of Vertebrate 
 evolution for the respiration of the nervous tissue, by the introduc- 
 tion through an anterior unclosed pore of a current of water, which 
 escaped by the neuranal canal (as in larval Amphioxus). 
 
 There is a wide gap between any form presenting an approach to 
 a Nemertine Worm and the most simple Craniate Vertebrate which 
 can be imagined still provided with the organization characteristic 
 of all Craniata. To pass from such a Worm-like animal to a Craniate, 
 we have to account for and introduce, amongst other new develop- 
 ments, (1) a greatly increased metamerism, showing itself in the 
 segmentation of the muscles of the body-wall and in the repetition 
 of the nephridia ; (2) the characteristic sense organs; (3) the lateral 
 and median longitudinal folds or continuous fins ; (4) the carti- 
 laginous rods and bars of the skeleton ; (5) the gill-slits, even if 
 we admit the notochord to be represented by the proboscidean 
 sheath. 
 
 It remains to inquire whether the structure of the other Verte- 
 brata throws light on this long hypothetical passage from the 
 simple Worm phase to the elaborate Craniate, or suggests any other 
 ancestry. 
 
 THE CEPHALOCHORDA. 
 
 Char- CephalocJiorda are Vertebrata in which there is no anterior 
 
 acters of dilatation of the nerve-tube to form a brain (see fig. 6) aud no 
 Cephalo- specialized skeletal brain-case. The notochord extends from one 
 chorda, extremity of the elongate body to the other as a tapering nncon- 
 stricted rod, passing anteriorly some distance in front of the 
 nerve-cord. The longitudinal muscles of the body -wall are divided 
 by transverse fibrous septa into a series of segments (sixty-two in 
 Amphioxus laneeolatus), the more anterior of which are in front 
 of the mouth and not in any way fused to form a head or cranial 
 structure. Dense connective tissue (differing but little from car- 
 tilage) forms an unsegmented sheath to the notochord and an 
 unbroken neural canal above it, in which the nerve-cord lies. 
 The same tissue forms a series of metamerically repeated fin rays, 
 which support the base of a median fin extending along the entire 
 dorsal surface. The fin is continued ventrally from the caudal ex- 
 tremity as far forward as the anus, but without fin rays. Two 
 lateral up-growths of the body-wall (the epipleura) extend one on 
 either side from the head as far back as the anus. Each of these 
 is divided into three regions, (1) an anterior, which forms the 
 praeoral hood ; (2) a median, which forms the wall of the great 
 branchial chamber, the two folds meeting one another and coales- 
 cing in the ventral mid-line, excepting where they leave a posterior 
 median aperture, the atriopore ; and (3) the post-atrioporal pneanal 
 ventral fin (extending between atriopore and anus), which is formed 
 by the complete coalescence of the two folds behind the atriopore. 
 
 FlO. 7. Awtphiaxus Janaolatus, Yarrell (Branchiostoma lubrinm, Coste\ 
 (Original drawings.) (1) lateral view of adult, to show general form, the 
 myomeres, fin rays, and gonads. A, oral tentacles (28 to 32 in fall -grown 
 animals, 20 to 24 in half-grown specimens) ; B, prseoral hood or pneoral epi- 
 pleur ; C, plicated ventral surface of atrial chamber ; Di, D", D 5 *, gonads, 
 twenty-six pairs, coincident with myotomes 10 to 36 ; B, metapleor or lateral 
 ridge on atrial epiplenr ; P, atriopore, coincident with myotome 36 ; Gi, GH, 
 G**, double ventral fin rays, extending from myotomes 37 to 52, but hav- 
 ing no numerical relation to them ; H, position of anas, between myotomes 
 51 and 52 ; I, notochord, projecting beyond myotomes ; K^, K", K", myo- 
 tomes or muscular segments of body-wall, 62 in number ; LW*, L 2 **, L 253 , dor- 
 sal fin rays, about 250 in number, the hard substance of the ray being absent 
 st the extreme ends of the body (these have no constant numerical relation 
 to the myomeres); M, notochord as seen through the transparent myotomes, 
 the thin double-lined spaces being the connective-tissue septa and the 
 broader spaces the muscular tissue of the myotomes ; If, position of brown 
 funnel of left side (atrio-coclomic canal) ; O, nerve-tube resting on notochord. 
 (2) Dissection of Amphicana. By a horizontal incision on each side of the 
 body a large ventral area has been separated and turned over, as it were on 
 a hinge, to the animal's left side. The perforated pharyngeal region has then 
 been detached from the adherent epipleura or opercnlar folds (wall of atrial 
 or branchial chamber) by cutting the fluted pharyngo-pleural membrane rf, 
 and separated by a vertical cnt from the intestinal region, a. Edge of groove 
 formed by adhesion of median dorsal surface of alimentary canal to sheath 
 of notochord ; o, median dorsal surface of alimentary canal ; c, left dorsal 
 aorta ; oc, single dorsal aorta, formed by union of the two anterior vessels ; of, 
 same vessel resting on intestine ; d, cnt edge of pharyngo-plenral folds of 
 atrial tunic, really the original outer body-wall before the downgrowth of 
 epipleura ; d% atrial tunic (original body-wall) at non-perforate region, cut 
 and turned back so as to expose peri-enteric coelom and intestine r ; t, up- 
 standing folds of body-wall (pharyngo-pleural folds) on alternate bars of pep 
 forate region of body ; /, atrio-coelomic canals or brown funnels (collar-pores 
 of l!alanog!ossus) ; g, cavity of a gonad-sac ; m, cut musculature of body- 
 wall ; n, anus ; o, post-atrioporal extension of atrial chamber in form of 
 
180 
 
 VERTEBRATA 
 
 tubular csecum ; p, atriopore ; q, hepatic caecum ; r, intestine ; s, ccelom ; t, 
 area of adhesion between alimentary canal and sheath of notochord ; v, atrial 
 chamber or branchial cavity; TO, post-atrioporal portion of intestine; x, 
 canals of metapleura exposed by cutting ; E, probe passing through atrio- 
 pore into atrial or branchial chamber ; FF, probe passed from coelom, where 
 it expands behind the atriopore, into narrower perienteric ccelom of prse- 
 atrioporal region. 
 
 (3) Portion of (2) enlarged to show atrio-ccelomic canals (" brown funnels" 
 of Lankester). Lettering as in (2). 
 
 (4) Section taken transversely through prseoral region near termination of 
 nerve-tube, a, Olfactory ciliated pit on animal's left side, its wall confluent 
 with substance of nerve-tube ; b, pigment spot (rudimentary eye) on anterior 
 termination of nerve-tube ; c, first pair of nerves in section ; d, fin ray ; r , 
 myotome ; /, notochord ; g, space round myotome (? artifact or ccelom) ; ft, 
 subchordal canal (? blood-vessel); i, a symmetrical epipleura of prseoral hood. 
 
 The originally double character of this part of the ventral fin is 
 indicated by the double series of metameric fin rays which support 
 it. It is probable that these "epipleural" folds of Amphioxus 
 correspond to the opercular folds and lateral fins of Craniata. No 
 cartilaginous fin rays are developed in the atrio-pleural (opercular) 
 region of the epipleura ; but a longitudinal unsegmented bar of 
 cartilaginous consistency strengthens its side and bounds a lymph- 
 holding canal (x in iig. 8). 
 
 The gill-slits in Amphioxus are very numerous (one hundred or 
 more), and have no numerical relation to the metameres of the 
 muscular body -wall, though the first few which appear in the 
 embryo correspond at the time to successive myomeres, a relation 
 which they subsequently lose. The sides of the gill-slits are sup- 
 ported by ehitinous ( ?) bars, and each slit is divided into two equal 
 portions by a longitudinal tongue or bar, which grows out from 
 the dorsal margin of the slit soon after its first formation. The 
 number of gill-slits increases continually throughout the life of 
 Amphioxus by the formation of new ones at the posterior border of 
 the pharynx, whilst the myomeres do not increase in number after 
 early embryonic life. 
 
 The nerves given off from the dorsal nerve-cord of Amphioxus are 
 of two kinds, dorsal and ventral. The dorsal nerves correspond 
 in number and position to the myomeres, right and left, except in 
 the most anterior region of the body, where two larger pairs of 
 dorsal nerves are given off from near the extremity of the nerve- 
 cord, and pass forward, supplying the region which lies in front of 
 the termination of the musculature. The ventral nerves are 
 minute, and are given oif numerously, right and left, from the 
 nerve-tube throughout its length. The dorsal and ventral nerves 
 of a single myomere appear to correspond, respectively, to the 
 dorsal and ventral roots of a spinal nerve of a Craniate. 
 
 There is a single olfactory pit in Amphioxus, which rests upon 
 the left side of the anterior termination of the nerve-cord (see fig. 
 7, 4). Within the cavity of the nerve-cord at the same point a 
 patch of brown pigment is present (eye-spot). There are 110 repre- 
 sentatives of the lateral eyes of Craniata and no otocysts. 
 
 There is no representative of the Craniates' swim-bladder in 
 Amphioxus. A single wide diverticulum of the alimentary canal 
 represents the liver of Craniata ; the pancreas is unrepresented. 
 
 The vascular system is singularly incomplete : large trunks 
 exist, but few branches and no heart, whilst the blood itself is 
 colourless, and communicates (as in Craniata by the lymphatic 
 "hearts ") with the coelomic fluid at various points. A contractile 
 ventral trunk runs along the lower face of the slit pharynx, and 
 sends vessels right and left up the successive bars ; these vessels 
 unite above, as in Craniata, to form a double "dorsal aorta," which 
 posteriorly becomes a single vessel. A portal system of veins can 
 be traced in connexion with the hepatic csecum. 
 
 No system of lymphatic vessels, nor lymphatic "glands," nor a 
 spleen exist ; but the ccelom, and certain other spaces in the 
 connective tissue, contain coagulable lymph, and correspond to the 
 lymph spaces of Craniata. 
 
 There is no series of nephridia, nor a renal organ formed by the 
 coalescence of nephridia, nor are gonaducts present. The " brown 
 funnels," a pair of funnel-shaped tubes discovered by Lankester 
 (14), place the coelomic space of the opercular (epipleural) down- 
 growths of the body-wall in communication with the space which 
 these folds enclose. They appear to be identical with the "collar- 
 pores " of Balanoglossus, and it is doubtful whether they represent 
 nephridia. 
 
 In the larval Amphioxus there is developed from the left anterior 
 coelomic pouch a glandular tube and a sense-organ, which are re- 
 presented in the adult by the structures marked/ in fig. 8, B. This 
 tube is probably the same thing as the subneural gland of Ascidians 
 and as the proboscidean gland and pore of Balanoglossus. Quite 
 distinct from the foregoing is a nephridial tube lying on the left 
 side behind the mouth of the larval Amphioxus. All are probably 
 of the nature of nephridia. In the adult Amphioxus the nephridial 
 tube is in an atrophied condition, though large and active in a late 
 larval stage, when the olfactory pit opens into the neural canal. 
 Hatschek (75) describes this condition in "ausgebildete" but not 
 in adult examples. 
 
 The gonads are distinct ovaries and testes ; they are developed 
 in distinct male and female individuals in corresponding positions, 
 viz., in that part of the coslom which is carried downwards in the 
 
 U 
 
 d 
 
 fl/CHT 
 
 Fio. 8. Transverse sections of Amphioms. (Original.) A. Section through re- 
 gion of atrio-ccelomic canals, v. B. Section in front of mouth ; the right and 
 left sides are transposed, a, Cavity surrounding fin ray ; a', flu ray ; Z>, 
 muscular tissue of myotome ; c, nerve-cord ; d, notochord ; e, left aorta ; /, 
 thickened ridges of epithelium of prseoral chamber (Rader organ) ; g, coiled 
 tube lying in a coelomic space on right side of prEeoral hood, apparently an 
 artery ; ft, cuticle of notochord ; i, connective-tissue sheath of notochord ; fc, 
 median ridge of skeletal canal of nerve-cord ; I, skeletal canal protecting 
 nerve-cord ; m, inter-segmental skeletal septum of myotome ; n, subcutaneous 
 skeletal connective tissue ; o, ditto of metapleur (this should be relatively 
 thicker than it is) ; q, subcutaneous connective tissue of ventral surface of 
 atrial wall (not a canal, as supposed by Stieda and others) ; r, epiblastic epi- 
 thelium ; s, gonad-sac containing ova ; (, pharyngeal bar in section, one of 
 the "tongue" bars alternating with the main bai's and devoid of pharyngo- 
 pleural fold and ccelom ; r, atrio-ccelomic funnel ; w, so-called "dorsal "ccelom ; 
 x, lymphatic space or canal of metapleur ; y, sub-pharyngcal vascular trunk ; z, 
 blood-vessel (portal vein) on wall of hepatic cascum ; aa, space of atrial or 
 branchial chamber; bb, ventral groove of pharynx (anteriorly this takes the 
 form of a ridge) ; cc, hyperbi'anchial groove of pharynx ; drf, lumen or space of 
 hepatic caecum; ee, narrow ccelomic space surrounding hepatic caecum ; ff, 
 lining cell-layer of hepatic cojcum ; gg, inner face of a pharyngeal bar clothed 
 with hypoblast, the outer 'ace covered with epiblast (represented black) ; AA, a 
 main pharyngeal bar with projecting pharyngeal fold (on which the reference 
 line rests) in section, showing ccelomic space beneath the black epiblast ; ii, 
 transverse ventral muscle of epipleura ; kk, raphe or plane of fusion of two 
 down-grown epipleura; U, space and nucleated cells on dorsal face of noto- 
 chord ; mm, similar space and cells on its ventral face. 
 
 descending right and left outgrowth (epipleura, opercula) of the 
 body-wall, which encloses the atrial or branchial chamber. The 
 gonads are twenty-six pairs in number, corresponding to the 10-36 
 myomeres. They are devoid of ducts, and discharge their pro- 
 ducts by dehiscence into the atrial chamber, whence they pass to 
 the exterior, either by the atriopore or by entering the pharynx 
 through its slits, when they are ejected by the mouth. 
 
 In many respects Amphioxus, the only representative of Cephalo- 
 chorda, bears evidence of being derived from a more highly organized 
 
VERTEBRATA 
 
 181 
 
 Relation- ancestry. Its mode of life (burrowing in the sand in shallow water, 
 ship of whilst its general build is that of a swimming animal) and the 
 Cephalo- nature of its food (diatoms, &c., carried into the pharynx by ciliary 
 chorda to currents) in themselves suggest such a history. The vascular 
 Craniata system is elaborate in plan yet incomplete in detail, suggesting an 
 and the atrophy of its finer branches, which is consistent with the small 
 Verte- size of A mphioxus and the general principle- that a complex vascular 
 brate system can only be developed in an animal which has attained to 
 ancestry, a certain bulk. The absence of well-developed sense organs and of 
 " cephalization " in an animal which has attained to such elabora- 
 tion of structure as is shown by the pharynx and atrial chamber, 
 and which has such well-developed muscles to the body-wall, is an 
 inconsistency best explicable by degeneration ; so, too, the existence 
 of the elaborata series of fin rays, which are out of proportion to 
 the mechanical requirements of so small a form. 
 
 Degenerate though Amphioxus must be, the ancestor from 
 which it started on its retrogressive course was probably a long 
 way behind any living Craniate. There is no reason to suppose 
 that this ancestor had a cranium, or that the muscular segments 
 and segmental nerves in its cephalic region were fused and 
 welded. Amphioxus has probably lost, as compared with that 
 ancestor, lateral eyes and otocysts, nephridia, and, above all, size. 
 The epipleural folds which now form oral hood, branchial opercula, 
 and coalesced ventral fin were probably originally less developed 
 lateral ridges, protecting the gill-slits anteriorly and posteriorly, 
 serving by their undulations to assist in locomotion, whilst the 
 median fin and its rays were large and functional. 
 
 One of the most curious features in the structure of Amphioxus 
 is its asymmetry. The anus is on the animal's left side ; the nasal 
 pit upon its left ; the myomeres on the two sides of the noto- 
 cliord do not coincide ; and the right and left dorsal spinal nerves 
 do not arise vis-a-v is to one another. There is no conclusive reason 
 for regarding this as an ancestral feature, although the early larval 
 form is as curiously asymmetrical as the adult. Amphioxus habitu- 
 ally rests upon the sand, lying upon one side of the body, and it is 
 possible that the distortion is related to this habit, as in the case 
 of the Pleuronectid Fishes. 
 
 However we may estimate Amphioxus, we are not led by it, though 
 its muscular metamerism is so well marked, a single step in the 
 direction of the Annelids, neither are we led directly, it is true, in 
 the direction of Kemeriina, in connexion with those points, as to 
 relationship of notochord with proboscis sheath and nerve-cord 
 with median dorsal nerve, insisted on by Hubrecht But it will be 
 seen below that, by the agreement of Amphioxus with Balanoglossus 
 in the structure of the perforations of the pharynx, in the possession 
 of collar pores, and in the pneoral glandular body, we do arrive at 
 an important connexion with IT emertine-like forms. 
 
 THE UROCHORDA. 
 
 Uroehorda are Vertebrata which, with the exception of the group 
 Larvalia (Appendicularia, Fritillaria, CHkopleura), have receded 
 very far indeed from the characteristic Vertebrate structure, show- 
 ing neither notochord nor nerve-cord, and gill-slits only of the 
 most highly modified and aberrant form ; some, however (certain 
 Ascidians), pass through a larval condition in which these struc- 
 tures are present in the normal form. It is necessary for the pur- 
 poses of the present article to confine our attention to Larvalia and 
 to the larval forms which retain ancestral characters. (For a de- 
 scription of the whole group, see the article TUXICATA. ) In Uro- 
 ehorda thus signalized the notochord never reaches forward into the 
 anterior part of the body, but is confined to the tail (hence Uro- 
 chorda). The longitudinal muscles of the region traversed by the 
 notochord show traces of metameric segmentation, which are prob- 
 ably survivals of a more complete development of myomeres in 
 ancestral forms (16). There is no trace in Larvalia of fin rays 
 or other skeletal structure. Corresponding to the opercular folds 
 and epipleura of Craniaia and Cephaloehorda are ridges of the 
 body-wall, which protect the pharyngeal gill-slits, and may give 
 rise, as in Cephaloehorda, to an enclosed atrial chamber with 
 atriopore. The gill-slits iu these larval forms are few in number 
 (one or two pairs), but in many of the aberrant Uroehorda (by far 
 the majority of the group) they become excessively numerous and 
 complicated in structure, and are supported by a chitinous (?) 
 framework, as in Cephaloehorda. It has been suggested that the 
 fenestrated structure of the pharyngeal wall in Tunfcata does not 
 represent a series of gill-slits, but a single pair of slits subdivided. 
 This suggestion is worthy of further consideration. 
 
 The cerebro-spinal nerve-cord is tubular and presents itself as a 
 dilated cerebral vesicle in front of the notochord, and as a narrower 
 part running along the whole length of the notochord. 
 
 Sense organs are present a single eye with pigment and lens, 
 a single otocyst, and an olfactory pit (Larvalia). The mouth is 
 dorsal in position in the Ascidian tadpole, but subterminal in Lar- 
 valia. The pharynx is wide, and is followed by a narrow ojso- 
 phagus, stomach, and intestine, which does not open ventrally but 
 turns upwards to the anus. The Larvalia have a rudimentary 
 heart and no vascular system, a fact connected with their diniinu- 
 
 Char- 
 acters 
 of Uro- 
 ehorda. 
 
 tive size. For the same reason no vascular system develops in the 
 Ascidian tadpole until it has ceased to be locomotive and has entered 
 upon its later development ; but in the larger adult Uroehorda a 
 contractile heart and a well-developed vascular system are present. 
 
 No undeniable nephridia are present in Larvalia nor in the larval 
 Ascidian, and no structure comparable to the collar pores of Balano- 
 glossus or the atrio-ecelornie funnels of Amphioxus is known in them. 
 
 The subneural gland, however, a glandular tube opening anteriorly 
 near the mouth of the pharynx, appears to be identical with the 
 pneoral larval gland of Amphioxus and the proboscis pore and gland 
 of Balanoglassus. It is probably to be regarded as a nephridmm, 
 and has been compared by Jnlin and Van Beneden to the pituitary 
 body of Craniata, with which it corresponds in position and de- 
 velopment. 
 
 The gonads of Larvalia are developed in irregular masses on the 
 walls of the ccelom, ovaries and testes in the same individual. 
 
 As above indicated, there is a small section of Uroehorda which Classi- 
 retain in adult life the tadpole-like form and the essential Verte- fication 
 brate organs which are exhibited by the larvae only of other Uro- of Uro- 
 ehorda, and by a few only of these. This necessitates a primary chorda, 
 division of the branch into two grades. 
 
 Grade A. LAKVALIA (Appendicularia, Fritillaria, Oikopleura). 
 
 Grade B. SACCATA. 
 Class L Ascidise (Simplices, Sociales, Composite, Pyroso- 
 
 miidea). 
 Class II. Salpiformia (Salpiidea, Dolioliidea). 
 
 Fio. 9. FrittBorio (Jpptndicutaria)furaita, one of the tJrodkonla. (Original 
 drawings.) A. Lateral surface view, showing habitual carriage of " body " 
 at right angles to " tail." B. Organs of body as seen by transparency. C. 
 Lateral view of body with tail in morphological position, showing organs by 
 transparency. D. Surface view of animal from below to show apertures, a, 
 Otocyst in connexion with brain ; b, olfactory pit ; c, dorsal hood ; d, nerve- 
 tube passing from enlarged brain to caudal region, where it forms one true 
 ganglion and a series of minor enlargements, corresponding to the rudimentary 
 " myotomes " or " myomeres " of the tail ; e, stomach ; /, ovary ; g, testis ; *, 
 notochord (urochord) ; i, nerve- tube or myelon in tail ; 1% fifth myomere of 
 tail ; ?, anus ; m, heart ; n, gill-slit ; o, endostyle or hypobranchial groove ; 
 p, month. 
 
 Uroehorda are so extremely aberrant, and show so little more Relations 
 than a transient developmental indication of the essential Verte- of Uro- 
 brate organs, that we cannot hope to get much positive information cAordaio 
 from them on the subject of Vertebrate ancestry. Only the minute Verte- 
 Appendieularix (Larvalia) retain the Vertebrate structure through brate 
 life, and they are obviously, on account of their minute size, ex- ancestry, 
 tremely degenerate. It is possible to make hypotheses as to the 
 
182 
 
 VEETEBRATA 
 
 greater or less elaboration of the ancestors of Urochorda, and to 
 maintain even that their ancestry had reached as high a condition 
 as that shown by Craniata ; on the other hand, it does not seem 
 likely that their point of divergence from the main ancestral line 
 leading to Craniata was lower than, or even so low as, that at 
 which Amphioxus branched off. The differentiation of trunk and 
 tail by the limitation of the notochord anteriorly is a nearer ap- 
 proach to Craniate structure than that shown by Amphioxus, 
 whilst the definite development of a brain of considerable relative 
 size places Urochorda nearer to the Craniates than is Amphioxus. 
 The metameric myomeres so strongly developed in this last are 
 not absent in Urochorda, as is often maintained, but exist in a 
 rudimentary form, indicating that they had once a fuller develop- 
 ment. 
 
 THE HEMICHOEDA. 
 
 Char- Hemichorda comprise the single genus Balanoglossus 
 
 acters of formerly classified by Gegenbaur as Enteropneusta, an 
 
 Hemi- >_ 
 
 chorda. OK /r\\ Jk , Z 
 
 cc 
 
 II 
 
 FIG. 10. Balanoglossus, anatomy and development. (Modified from Bateson.) 
 A. Balanoglossus kowalewskii, Bateson; from the coast of Virginia, U.S.; 
 natural size; a, proboscis; &, collar; c, perforate region; rf, flattened diges- 
 tive region ; t , cylindrical hind region. J3. Diagram of dorsal view, showing 
 certain organs as though the body-wall were transparent. C. Diagram of a 
 vertical antero-posterior section. D. Diagram of a dorsal view to show 
 vessels and nerves by transparency. E. Diagram of a transverse section 
 through the collar. F. Larva of B. kowaleuskii diagram of horizontal sec- 
 tion. G. Vertical longitudinal section of an older larva of the same. Letter- 
 ing B-G : a, proboscis ; 6, collar ; /, nerve-tunic of proboscis ; j?, proboscis 
 pore (ciliated orifice) ; h, notochord (limited to a small tract of modified tissue 
 derived from prrcoral extension of alimentary canal) ; i, dorsal nerve-plate ; 
 k, collar-pore (right and left), opening to exterior from collar coelomjust be- 
 neath the collar ; I, continuation of dorsal nerve-plate as a nerve-cord ; ?, 
 pharyngeal perforations (gill-slits) ; ', ccelom of proboscis (anterior azygos 
 primitive coelomic pouch) ; n 2 , collar cojlom (right and left middle ccelomic 
 pouches of embryo) ; n 3 , body coelom (right and left posterior coelomic pouches 
 of embryo) ; 0, mouth ; p, ventral nerve-tunic of body-wall ; #, proboscis 
 gland ; r, strands connecting dorsal nerve-plate with outer wall of collar ; s, 
 cavity of pharynx in front of perforate region ; t, dilated part (heart) of dorsal 
 vessels within proboscis-gland; (', dorsal vessel ; , blood-vessels of body-wall 
 in section ; w, paired nerves of collar region in transverse section ; x, peri- 
 hffimal coelom, surrounding dorsal vessel in collar region; y, digestive region of 
 gut (in embryo) ; , mesoblast. H. Larva of another species of Balanoglossus, 
 known as the Tornaria larva of Johann Miiller, :md resembling an Echinoderm 
 larva, aa, pra?oral ciliated band of Toiiiaria ; &6, post-oral ditto ; cc, terminal 
 ditto; dd, mouth; ee, apical plate and sense organ;//', canal system and 
 pore; gg, gut; M,anus. 
 
 independent phylum of the animal kingdom. They are 
 Vertebrata of worm-like form, elongate and somewhat 
 
 flattened from above downwards. In front of the mouth 
 is a long cylindrical proboscis, and behind it a collar, the 
 free margin .of which is turned backwards, and corresponds 
 to the opercular epipleural folds of Cephalochorda and 
 Craniata. This agreement is supported by the existence 
 of a pair of collar pores opening into the coelom of the 
 collar, as the "brown funnels" of Amphioxus open into 
 the epipleural ccelom of that animal. A proboscis pore, 
 opening on the left side into the prseoral ccelom of the 
 proboscis (paired in B. kupfferi), is exactly representative 
 of the similarly placed pore which in the young Amphioxus 
 (according to Hatschek, 15) leads into the tubular organ 
 derived from the left ccelomic chamber of the praeoral 
 lobe of that animal. The whole surface of the body is 
 ciliated, as in Nemertines and Echinoderms, and as in no 
 other Vertebrates. Following the collar is a perforated 
 region of the body, gill-slits opening from the outer 
 surface into the pharynx. In the young form there is for 
 a time, as in Appendicularix and the Ascidian tadpole, 
 only one pair of gill-slits, but they subsequently increase 
 in number as the animal grows in length. They resemble 
 in form and structure those of Amphioxus. The notochord 
 (h in fig. 10) arises at the anterior end of the hypoblast 
 in the young, and grows forward, forming a support for 
 the base of the proboscis. It is limited to this very small 
 region. The cerebro-spinal nerve-cord originates by a 
 delamination of a solid cord of epiblast in the mid- 
 dorsal line of the middle third of the body; then by 
 invagination of its two ends it extends as a tube both 
 anteriorly and posteriorly. A general network of nerve- 
 fibres (and cells?) exists beneath the epidermis all over 
 the body. The blood-system is peculiar, consisting of an 
 anterior heart and a dorsal and ventral vessel ; these are 
 united by a plexus of subcutaneous vessels. The muscu- 
 lature of the body-wall is not broken into succsssive 
 myomeres ; but, on the other hand, the gonads (ovaries or 
 testes) are sac-like, and, as in Amphioxus, are repeated in 
 a series throughout a great length of the body. In the 
 pharyngeal region the gonad sacs agree in number with 
 the gill-slits. There are no nephridia (unless proboscis 
 pore and collar pores are to be so regarded) ; but the con- 
 nective-tissue cells of the body-cavity are active as excret- 
 ing agents, as in Echinoderms and in Urochorda, and a 
 large glandular organ in the proboscis attached to the end 
 of the notochord appears to have to do with this function. 
 Not the least remarkable fact about Hemichorda is the 
 nature of their larvae. No other Vertebrata present larval 
 forms which indicate the nature of the early ancestral 
 history in what we may call pras-chordal times ; however 
 interesting the Ascidian larva, or the young Amphioxus, 
 and the embryo dog-fish, they do not take us out of the 
 Vertebrate area. Some species of Balanoglossus (? B. minu- 
 tus), however, pass through a banded ciliate larval con- 
 dition, which was known as Tornaria, and was considered 
 to be an Echinoderm larva allied to Biplnnaria, before 
 its relation to Balanoglossus was discovered. It is not 
 possible to view the Tornaria larva of Balanoglossus as 
 otherwise than identical with Echinoderm larvce, and it 
 results that Balanoglossus and the Echinoderms have 
 remote genetic affinities of a special kind. 
 
 No classification of Hemichorda is possible beyond an Species 
 enumeration of the species : of Hem 
 
 1. Balanoglossus clavigerus (Delia Chiaje), Naples. 
 
 2. B. minutus (Kowalewsky), 
 
 3. B. kowalcwskii (Al. Agassiz), cast coast, United States. 
 
 4. B. brooksii (Bateson), ,, ,, 
 
 5. B, salmoneus (Giard), Brittany. 
 
 6. P. robinii (Giard), ,, 
 
 It seems that in Balanoylossus we at last find a form 
 which, though no doubt specialized for its burrowing 
 
 chorda. 
 
VERTEBRATA 
 
 183 
 
 Relations 
 of Hemi- 
 chorda 
 to Verte- 
 brate 
 ancestry. 
 
 sand-life, and possibly to some extent degenerate, yet has 
 not to any large extent fallen from an ancestral eminence. 
 The ciliated epidermis, the long Worm-like form, and the 
 complete absence of segmentation of the body-muscles lead 
 
 us to forms like the Nemertines. The great proboscis of 
 Balanoglossus may well be compared to the invaginable 
 organ similarly placed in the Nemertines. The collar is 
 the first commencement of a structure destined to assume 
 great importance in Cfphalockorda and Craniata, and 
 
 perhaps protective of a single gill-slit in Balanoglossus 
 before the number of those apertures had been extended. 
 Borrowing, as we may, the nephridia from the Nemertiues, 
 ! and the lateral in addition to the dorsal nerve, we find 
 that Balanoglossus gives the most hopeful hypothetical 
 solution of the pedigree of Vertebrates. Space has not 
 permitted us to go so fully into pros and cons as the 
 speculative nature of the subject requires ; but we give 
 the final conclusion to which our consideration of the 
 structure of the four great branches of the Vertebrata leads 
 in the form of the accompanying genealogical tree. 
 
 Bibliography. (/) Cuvier, "Surun Nouveau Rapprochement," 
 &c., in Ann. du Musee, 1812, voL xix. p. 73 ; (2) Kowalewsky, 
 "Devel. of Amphioxus," in Mem. dc VAcad. Imp., St Petersburg, 
 7th series, vol. xvi., No. 12, 1866 ; (j) Id., "Devel. of Ascidia," 
 ibid.; (4) Lankester (E. Bay), "Notes on Embryology and Classi- 
 fication," in Quart. Journ. Micr. Sci., vol. xvii., 1877; (5) Kowa- 
 lewsky, "Balanoglossus," in Mem. de VAcad. Imp., St Petersburg, 
 vol. x., No. 3, 1886; (6) Bateson, "Balanoglossus" in Quart. 
 Journ. Micr. Sci., April 1884 ; ibid., supplement number 1885, 
 ibid., June 1886 ; (7) Banner, " Cephalodiscus," in Challenger 
 Reports, vol. xviii. ; (<?) Dohrn, Ursprung der WirbelOiiere, 1875 ; 
 (9) Zepplin, " Budding in Ctenodrilus," in Zeitschr. wiss. Zoologie, 
 voL xx xix. ; (to) Balfour, Monograph of Development of Elasmo- 
 branch Fishes, 1878; (//) Hubrecht, "Nerve-Tunic of Nemer- 
 tines," in Quart. Journ. Micr. Sci., vol. xx., 1880 ; (12) and (jj) 
 Id., " Comparison of Nemertines and Vertebrates," in Quart. Journ. 
 Micr. Sci., vol. xxiii., 1883, and voL xxvii., 1887 ; (14) Lankester 
 (E. Ray), " Brown Funnels of Amphioxus," in Quart. Journ. Micr. 
 Sci., voL xv. p. 257, 1875; (/j) Hatschek, "Subncural Gland of 
 Amphioxus and Olfactory Pit," in Zool. Ameiger, p. 517, 1884 ; 
 (16) Lankester (E. Ray), " Vertebration of the Tail of Appcndicu- 
 laria," in Quart. Journ. Micr. Science, voL xxii., 1882 ; (77) 
 Kowalewsky, "Later Researches on Ascidian Development," in 
 Archiv fur mUcroskopische Anatomic, voL vii., 1871 ; (/<?) Id., 
 "Later Researches on Amphioxus Development," in Archiv fur 
 mikroskopische Anatomie, vol. xiii , 1876. (E. E. L.) 
 
TUNICATA 
 
 (By W. A. Herdman, D.Sc., Professor of Natural History, University College, Liverpool.) 
 
 Simple 
 Ascid- 
 ians. 
 
 Com- 
 pound 
 Ascid- 
 ians. 
 
 Cuvier 
 and 
 
 Savigny. 
 
 Lamarck 
 
 Cham- 
 isso and 
 alterna- 
 tion of 
 gensra- 
 tions. 
 Circula- 
 tion. 
 
 Milne- 
 Edwards. 
 
 Carl 
 Schmidt. 
 
 group of animals was formerly regarded as con- 
 _L stituting along with the Polyzoa and the Brachio- 
 poda the invertebrate class Molluscoidea. It is now known 
 to be a degenerate branch of the Chordata, and to be more 
 nearly related to the Vertebrata than to any group of the 
 Invertebrata. 
 
 HISTORY. 1 
 
 More than two thousand years ago Aristotle gave a 
 short account of a Simple Ascidian under the name of 
 Tethyum. He described the appearance and some of the 
 more important points in the anatomy of the animal. 
 From that time onwards to little more than a century ago, 
 although various forms of Ascidians had been briefly de- 
 scribed by writers on marine zoology, comparatively little 
 advance was made upon the knowledge of Aristotle. 
 Schlosser and Ellis, in a paper containing a description of 
 Botryllus, published in the Philosophical Transactions of 
 the Boyal Society for 1756, first brought the Compound 
 Ascidians into notice ; but it was not until the commence- 
 ment of the 19th century, as a result of the careful ana- 
 tomical investigations of Cuvier (/) upon the Simple 
 Ascidians and of Savigny (2) upon the Compound, that 
 the close relationship between these two groups of the 
 Tunicata was conclusively demonstrated. Up to 1816, 
 the date of publication of Savigny's great work (2), the 
 few Compound Ascidians then known had been generally 
 regarded as Alcyonaria or as Sponges; and, although 
 many new Simple Ascidians had been described by O. F. 
 Miiller (4) and others, their internal structure had not 
 been investigated. Lamarck (j) in 1816, chiefly as the 
 result of the anatomical discoveries of Savigny and Cuvier, 
 instituted the class Tunicata, which he placed between the 
 Badiata and the Vermes in his system of classification. 
 The Tunicata included at that time, besides the Simple 
 and the Compound Ascidians, the pelagic forms Pyrosoma, 
 which had been first made known by P6ron in 1804, and 
 Salpa, described by Forskal in 1775. 
 
 Chamisso in 1820 made the important discovery that 
 Salpa in its life-history passes through the series of changes 
 which were afterwards more fully described by Steenstrup 
 in 1842 as "alternation of generations"; and a few years 
 later Kuhl and Van Hasselt's investigations upon the same 
 animal resulted in the discovery of the alternation in the 
 directions in which the wave of contraction passes along 
 the heart and in which the blood circulates through the 
 body. It has since been found that this observation holds 
 good for all groups of the Tunicata. In 1826 H. Milne- 
 Echvards and Audouin made a series of observations on 
 living Compound Ascidians, and amongst other discoveries 
 they found the free-swimming tailed larva, and traced its 
 development into the young Ascidian. Milne -Edwards 
 (5) also founded the group of "Social" Ascidians, now 
 known as the Clavelinidx, and gave a classification of the 
 Compound Ascidians which was universally accepted for 
 many years. From the year 1826 onwards a number of 
 new and remarkable forms were discovered, as, for instance, 
 some of the Bolteninx (Macleay), Chelyosoma (Broderip and 
 Rowerby, and afterwards Eschricht), Oikopleura (Alertens), 
 Perophora (Lister), Pelonaia (Forbesand Goodsir), Chondro- 
 stachys and Diplosoma (Denis Macdonald), Diazona (Forbes 
 and Goodsir), and Rhodosoma (Ehrenberg, and afterwards 
 Lacaze-Duthiers). 
 
 In 1845 Carl Schmidt (6) first announced the presence 
 
 1 Only the more important works can be mentioned here. For a 
 more detailed account of the history of the group and a full biblio- 
 graphy, see (77) in the list of works at the eud of this article. 
 
 in the test of some Ascidians of " tunicine," a substance 
 very similar to cellulose, and in the following year Lowig 
 and Kolliker (7) confirmed the discovery and made some 
 additional observations upon this substance and upon the 
 structure of the test in general. Huxley (<?), in an im- Huxley, 
 portant series of papers published in the Transactions of 
 the Royal and Linnean Societies of London from 1851 on- 
 wards, discussed the structure, embryology, and affinities 
 of the pelagic Tunicates Pyrosoma, Salpa, Doliolum, and 
 Appendiadaria. These important forms were also investi- 
 gated about the same time by Gegenbaur, Vogt, H. Muller, 
 Krohn, and Leuckart. The most important epoch in the 
 history of the Tunicata is the date of the publication of 
 Kowalevsky's celebrated memoir upon the development of Kowa- 
 a Simple Ascidian (9). The tailed larva had been previously lev-sky. 
 discovered and investigated by several naturalists notably Tailed 
 H. Milne-Edwards (5), J. P. van Beneden (10), and Krohn 1 * 1 - 
 (//) ; but its minute structure had not been sufficiently 
 examined, and the meaning of what was known of it had 
 not been understood. It was reserved for Kowalevsky in Relation- 
 1866 to demonstrate the striking similarity in structure ship to 
 and in development between the larval Ascidian and the Verte- 
 vertebrate embryo. He showed that the relations between hrates> 
 the nervous system, the notochord, and the alimentary 
 canal are much the same in the two forms, and have been 
 brought about by a very similar course of embryonic de- 
 velopment. This discovery clearly indicated that the 
 Tunicata are closely allied to Amphioxus and the Verte- 
 brata, and that the tailed larva represents the primitive 
 or ancestral form from which the adult Ascidian has been 
 evolved by degeneration, and this led naturally to the view 
 usually accepted at the present day, that the group is a 
 degenerate side-branch from the lower end of the phylum 
 Chordata, which includes the Tunicata (Urochorda), Amphi- 
 oxus (Cephalochorda), and the Vertebrata. Kowalevsky's 
 great discovery has since been confirmed and extended to 
 all other groups of the Tunicata by Kupffer (12), Giard Knpffer, 
 (13 and fj), and others. Important observations upon Giard, &c. 
 the process of gemmation and the formation of colonies in Gemma- 
 various forms of Compound Ascidians have been made by tion. 
 Krohn, Metschnikoff, Kowalevsky, Ganin, Giard, Delia 
 Valle, and others, and have gradually led to the establish- 
 ment of the general principle, that all the more important 
 layers of the bud are derived more or less directly from 
 the corresponding regions in the body of the parent. 
 
 In 1872 Fol (14) added largely to the knowledge of the Fol, &c. 
 Appendiculariidx, and Giard (ij) to that of the Compound 
 Ascidians. The latter author described a number of new 
 forms and remodelled the classification of the group. The 
 most important additions which have been made to the 
 Compound Ascidians since Giard's work have been those 
 described by Yon Drasche (16) from the Adriatic and 
 those discovered by the "Challenger" expedition (17). 
 The structure and the systematic arrangement of the Simple 
 Ascidians have been mainly discussed of recent years by 
 Alder and Hancock (18), Heller (19), Lacaze-Duthiers 
 (20), Traustedt (21), and Herdman (17, 22). In 1874 
 Ussoff (2j) investigated the minute structure of the nervous sub- 
 system and of the underlying gland, which was first dis- neural 
 covered by Hancock, and showed that the gland has ag' and 
 duct which communicates with the front of the branchial * nd 
 sac or pharynx by an aperture in the dorsal (or "olfactory") t 
 tubercle. In an important paper published in 1880 Julin 
 (24) drew attention to the similarity in structure and rela- 
 tions between this gland and the hypophysis cerebri of the 
 vertebrate brain, and insisted upon their homology. He 
 suggests that they perform a renal function. The Thaliacea 
 
 2A 
 
186 
 
 TUNICATA 
 
 have of late years been the subject of several very import- 
 Thali- ant memoirs. The researches of Todaro, Brooks (23), 
 acea. Salensky (26), and others have elucidated the embryology, 
 the gemmation, and the life-history of the Salpidee ; and 
 Grobben, Barrois (27), and more especially Uljanin (28) 
 have elaborately worked out the structure and the details 
 of the complicated life-history of the Doliolidee. Finally, 
 in an important work published in 1886 on the morpho- 
 Vau logy of the Tunicata, E. van Beneden and Julin (jo) have, 
 Beneden mainly as the result of a close comparison of the embryo- 
 ~" A logy of Ascidians with that of Amphioxus and other 
 Chordata, added considerably to our knowledge of the 
 position and affinities of the Tunicata, and of the exact 
 relations of their organs to the corresponding parts of the 
 body in the Vertebrata. 
 
 ANATOMY. 
 
 and 
 Julin. 
 
 ters. 
 
 As a type of the Tunicata, Ascidia mentula, one of the 
 mentula. i ar g er species of the Simple Ascidians, may be taken. This 
 
 species is found in most of the European seas, generally in 
 External shallow water on a muddy bottom. It has an irregularly 
 eharac- ova te form, and is of a dull grey colour. It is attached to 
 
 some foreign object by one end (fig. 1). The opposite end 
 
 of the body is usually nar- br 
 
 row, and it has a terminal 
 
 opening surrounded by eight 
 
 rounded lobes. This is the 
 
 mouth or branchial aperture, 
 
 and it always indicates the 
 
 anterior end : of the animal. 
 
 About half-way back from 
 
 the anterior end, and on a 
 
 rounded projection, is the 
 
 atrial or cloacal aperture 
 
 an opening surrounded by 
 
 six lobes which is always 
 
 placed upon the dorsal re- 
 gion. When the Ascidian 
 
 is living and undisturbed, 
 
 water is being constantly 
 
 drawn in through the 
 
 branchial aperture and 
 
 passed out through' the 
 
 atrial. If coloured par- 
 ticles be placed in the 
 
 water near the apertures, 
 
 they are seen to be sucked 
 
 into the body through the 
 
 branchial aperture, and after 
 
 a short time some of them 
 
 are ejected with consider- 
 able force through the atrial 
 
 aperture. The current of 
 
 \ 
 
 Fio. 1. Ascidia mtntula from the right 
 
 water passing in is for re- side, at, atrial aperture ; 6r, branchial 
 , j . aperture; (.test. (Original.) 
 
 spiratory purposes, and it 
 
 also conveys food into the animal. The atrial current is 
 mainly the water which has been used in respiration, but 
 it also contains all excretions from the body, and at times 
 the ova and spermatozoa or the embryos. 
 
 The test. The outer grey part of the body, which is attached at 
 or near its posterior end and penetrated by the two aper- 
 tures, is the "test." This is a firm gelatinous cuticular 
 secretion from the outer surface of the ectoderm, which is 
 a layer of flat cells lining its inner surface. Although at 
 first produced as a cuticle, the test soon becomes organized 
 by the migration into it of cells derived from the ectoderm 
 (see fig. 2). These test cells may remain as rounded or 
 fusiform or stellate cells imbedded in the gelatinous matrix, 
 to which they are constantly adding by secretions on their 
 
 1 Some writers use a different nomenclature of regions ; see (77). 
 
 771 
 
 surfaces; or they may develop vacuoles in their proto- 
 plasm, which become larger and fuse to form a huge ovate 
 clear cavity f e e. s mo 
 
 (a " bladder 
 cell "), sur- 
 rounded by a 
 delicate film 
 of protoplasm 
 and having 
 the nucleus 
 still visible at 
 one point ; or 
 they may form 
 pigment gran- 
 ules in the pro- 
 toplasm ; or, 
 
 *' ' ? Fia. 2. Diagrammatic section of part of mantle and test 
 may deposit of an Ascidian, showing the formation of a vessel and 
 r.l-,r,Tin<.Q nf the structure of the test, m, mantle; e, ectoderm ; la. 
 test cell ; tm, matrix ; Uc, bladder cell ; s, s", blood sinus 
 lime SO that in mantle being drawn out into test ; me, mantle cells ; 
 , y, septum of vessel. (From Herdman, Challenger Report. 
 
 one or several 
 
 of them together produce a calcareous spicule in the test. 
 Only the unmodified test cells and the bladder cells are 
 found in Ascidia mentula. Calcareous spicules are found 
 chiefly in the Didemnidee, amongst Compound Ascidians ; 
 but pigmented cells may occur in the test of almost all 
 groups of Tunicata. The matrix in which these structures 
 are imbedded is usually clear and apparently homogeneous ; 
 but in some cases it becomes finely fibrillated, especially 
 in the family Cynthiidx. It is this matrix which contains 
 tunicine. At one point on the left side near the posterior 
 end a tube enters the test, and then splits up into a num- 
 ber of branches, which extend in all directions and finally 
 terminate in rounded enlargements or bulbs, situated chiefly 
 in the outer layer of the 
 test. These tubes are 
 known as the "vessels" of 
 the test, and they contain 
 blood. Each vessel is 
 bounded by a layer of ec- 
 toderm cells lined by con- 
 nective tissue (fig. 3, ), 
 and is divided into two 
 tubes by a septum of con- 
 nective tissue. The septum Flo S __ A< a vessel from the test, n, dia- 
 
 does not extend into the grammatic transverse section of a vcs- 
 
 , , sel ec, ectoderm ; ct, connective tissue ; 
 
 terminal bulb, and COnse- s ^ the two tubes ; y, septum ; ,-, ter- 
 
 quently the two tubes com- al bulb - (Original.) 
 municate at their ends (fig. 3, A). The vessels are formed 
 by an outgrowth of a blood sinus (derived originally from 
 the blastoccele of the embryo) from the body wall (mantle) 
 into the test, the wall of the sinus being formed by con- 
 nective tissue and pushing out a covering of ectoderm in 
 front of it (fig. 2, '). The test is turned inwards at the 
 branchial and atrial apertures to line two funnel-like tubes, 
 the branchial siphon leading to the branchial sac and the 
 atrial siphon leading to the atrial or peribranchial cavity. 
 
 The body wall, inside the test and the ectoderm, is formed Mant 
 of a layer (the somatic layer of mesoderm) of connective 
 tissue, inclosing muscle fibres, blood sinuses, and nerves. 
 This layer (the mantle) has very much the shape of the test 
 outside it, but at the two apertures it is drawn out to form 
 the branchial and atrial siphons (fig. 4). In the walls of 
 these siphons the muscle fibres form powerful circular 
 bands, the sphincter muscles. Throughout the rest of the 
 mantle the bands of muscle fibres form a rude irregular 
 network. They are numerous on the right side of the body, 
 and almost totally absent on the left. The muscles are all 
 formed of very long fusiform non-striped fibres. The con- 
 nective tissue of the mantle is chiefly a clear gelatinous 
 
TUNICATA 
 
 187 
 
 -k-s 
 
 end 
 
 Bran- 
 chial sac 
 
 neigh- 
 
 matrix, containing cells of various shapes ; it is frequently 
 
 pigmented and is penetrated by numerous lacunae, in which 
 
 the blood flows. In- 
 
 side the mantle, in 
 
 all parts of the body, 
 
 except along the ven- 
 
 tral edge, there is a 
 
 cavity, the atrial or 
 
 peribranchial cavity, 
 
 which opens to the 
 
 exterior by the atrial 
 
 aperture. This cavity 
 
 is lined by a layer of 
 
 cells derived origin- 
 
 ally from the ecto- 
 
 derm 1 and directly 
 
 continuous with that 
 
 layer through the vd- 
 
 atrial aperture (fig. 
 
 5) ; consequently the 
 
 mantle is covered both & a _____ 
 
 externally and inter- 
 
 nally by ectodermal 
 
 cells. 
 
 The branchial aper- 
 ture ( mout h) leads in- 
 * * e branchial si- 
 
 phon (buccal Cavity FIG. 4. Diagrammatic dissection of A. aunlvla to 
 
 Or Stomodaeum), and bra^chia? aperture"'; a, anus; b%, bran'chill 
 sac ; dl, dorsal lamina ; dt, dorsal tubercle ; 
 end, endostyle; *, heart; i, intestine; ra, 
 mantle ; nj, nerve ganglion ; or, oesophagus ; 
 aa, oesophageal aperture ; op, ovary ; pbr, 
 peribranchial cavity ; r, rectum ; rf, stomach ; 
 t, test ; (n, tentacles ; rd, vas deferens ; gl, 
 subneural gland. (Original.) 
 
 this opens into the 
 
 anterior end of a very 
 
 large cavity (the bran- 
 
 chial sac) which ex- 
 
 tends nearly to the 
 
 posterior end of the body (see figs. 4 and 5). This branchial 
 
 sac is an enlarged and modified pharynx, and is therefore 
 
 properly a part of the ali- 
 
 mentary canal. The oeso- 
 
 phagus opens from it far 
 
 back on the dorsal edge (see 
 
 below, p. 6 1 2). The wall of 
 
 the branchial sac is pierced 
 
 by a large number of ver- 
 
 tical slits, the stigmata, 
 
 placed in numerous trans- 
 
 verse rows. These slits 
 
 place the branchial sac in 
 
 communication with the 
 
 end 
 
 FIG. 5. Diagrammatic longitudinal (A) and transverse (B) sections through 
 Axulia to show the position of the ectoderm and the relations of the bran- 
 chial and peribranchial cavities. The lettering is the same as for fig. 4. B 
 represents a section taken along the dotted line A-B in A. (Original.) 
 
 peribranchial or atrial cavity, which lies outside it (fig. 5, 
 B). Between the stigmata the wall of the branchial sac 
 is traversed by blood-vessels, which are arranged in three 
 regular series (fig. 6), (1) the transverse vessels, which 
 run horizontally round the wall and open at their dorsal 
 and ventral ends into large longitudinal vessels, the dorsal 
 and ventral sinuses ; (2) the fine longitudinal vessels, which 
 run vertically between adjacent transverse vessels and open 
 into them, and which bound the stigmata ; and (3) the 
 internal longitudinal bars, which run vertically in a plane 
 
 1 According to E. van Beneden and Julin's recent investigations (jo) 
 only the outer wall of the atrium is lined with epiblast, the inner wall 
 being derived from the hypoblast of the primitive branchial sac. 
 
 internal to that of the transverse and fine longitudinal 
 
 vessels. These bars communicate with the transverse 
 
 vessels by short side J" !. 
 
 branches where they 
 
 cross, and at these 
 
 points are prolonged 
 
 into the lumen of the 
 
 sac in the form of 
 
 hollow papillae. The 
 
 edges of the stigmata 
 
 are richly set with 
 
 cilia, which drive the 
 
 water from the bran- 
 
 chial sac into the 
 
 peribranchial ca- 
 
 vity, and so cause 
 
 the currents that 
 
 flow in through 
 
 the branchial 
 
 aperture and 
 
 out through the 
 
 atrial 
 
 Fio. 6. A. Part of branchial sac of Ascidia from inside. 
 Along US Vent- B. Transverse section of same, tr, transverse vessel ; 
 <** connecting duct; *, horizontal membrane; il, 
 internal longitudinal bar ; Ir, fine longitudinal vessels; 
 
 p, ]f, papillae ; tg, stigmata. A a 
 different stales. (From Herdmau, 
 
 A and B are drawn to 
 Challenger Report.) 
 
 ral f>rlcrf> trip wall 
 M t> a l 
 
 of the branchial 
 sac is continn 
 
 ous externally with the mantle (fig. 5, B), while internally 
 it is thickened to form two parallel longitudinal folds 
 bounding a groove, the " endostyle," hypobrancbial groove, Endo- 
 or ventral furrow (figs. 4, 5, end). The endoderm cells style. 
 which line the endostyle are greatly enlarged at the 
 bottom and on parts of the sides of the furrow so as to 
 form projecting pads, which bear very long cilia. It is 
 generally supposed that this organ is a gland for the pro- 
 duction of the mucous secretion which is spread round the 
 edges of the branchial sac and catches the food particles in 
 the passing current of water; but it has recently been 
 pointed out that there are comparatively few gland cells in 
 the epithelium of the endostyle, and that it is more prob- 
 able that this furrow is merely a ciliated path along which 
 the mucous secretion (produced possibly by the subneural 
 gland) is conveyed posteriorly along the ventral edge of 
 the branchial sac. At its anterior end the edges of the Peri- 
 endostyle become continuous with the right and left halves pharyn- 
 of the posterior of two circular ciliated ridges, the peri- 
 pharyngeal bands, which run parallel to one another 
 round the front of the branchial sac. The dorsal ends of 
 the posterior peripharyngeal band bend posteriorly (en- Dorsal 
 closing the epibranchial groove), and then join to form lamina. 
 the anterior end of a fold which runs along the dorsal edge 
 of the branchial sac as far as the oesophageal aperture. 
 This fold is the dorsal lamina (figs. 4, 5, dt). It probably 
 serves to direct the stream of food particles entangled in 
 a string of mucus from the anterior part of the dorsal 
 lamina to the oesophagus. In many Ascidians this organ, Dorsal 
 instead of being a continuous membranous fold as in J.languet 
 mentula, is represented by a series of elongated triangular 
 processes the dorsal languets, one attached in the dorsal 
 median line opposite to each transverse vessel of the 
 branchial sac. The anterior peripharyngeal band is a 
 complete circular ridge, having no connexion with either 
 the endostyle or the dorsal lamina. In front of it lies the 
 prebranchial zone, which separates the branchial sac behind 
 from the branchial siphon in front. The prebranchial 
 zone is bounded anteriorly by a muscular band the pos- 
 terior edge of the sphincter muscle, which bears a circle 
 of long delicate processes, the tentacles (figs. 4, 7, 8, tn). Ten- 
 These project inwards at right angles so as to form a net- tades. 
 work across the entrance to the branchial sac. Each 
 tentacle consists of connective tissue covered with epithe- 
 
188 
 
 TUNICATA 
 
 Hum (endoderm), and contains two or more cavities which 
 are continuous with blood sinuses in the mantle. In the 
 
 Subneur- dorsal median line near the anterior end of the body, and 
 
 al gland, imbedded in the mantle on the ventral surface of the nerve 
 ganglion, there lies a small glandular mass the subneural 
 gland which, as Julin has shown (?./), there is reason to 
 regard as the homologue of the hypo- 
 physis cerebri of the vertebrate brain. 
 Julin and E. van Beneden have sug- 
 gested that the function of this organ 
 may possibly be renal. 1 The sub- 
 neural gland, which was first noticed 
 by Hancock, communicates anteriorly, 
 as Ussoff (23) pointed out, by means 
 of a narrow duct with the front of 
 the branchial sac (pharynx). The 
 opening of the duct is enlarged to 
 form a funnel-shaped cavity, which 
 may be folded upon itself, convoluted, 
 or even broken up into a number of 
 
 Dorsal smaller openings, so as to form a 
 
 tubercle, complicated projection, called the 
 dorsal tubercle, situated in the dorsal 
 part of the prebranchial zone (fig. 7). 
 The dorsal tubercle in A. mentula is 
 somewhat horse-shoe-shaped (fig. 8) ; 
 it varies in form in most Ascidians 
 according to the genus and species, 
 and in some cases in the individual 
 also. Possibly, besides being the 
 opening of the duct from the sub- 
 neural gland, it may be a sense-organ 
 for testing the quality of the water 
 entering the branchial sac. 
 
 Nervous The single elongated ganglion in 
 
 system. ^ e median dorsal line of the mantle 
 between the branchial and atrial si- 
 phons is the only nerve-centre in A. mentula and most other 
 Tunicata. It is the degenerate remains of the anterior 
 
 iq. 7. Diagrammatic sec- 
 tion through anterior dor- 
 sal part of A. mentula, 
 showing the relations of 
 the nerve ganglion, sub- 
 neural gland, &c. Letter- 
 ing as for fig. 4 ; , nerve ; 
 ', myelon ; pp, peripha- 
 ryngeal band ; sgl, sub- 
 neural gland ; sgil, its 
 duct ; t, test lining branch- 
 ial siphon. (Original.) 
 
 FIG. 8. Dorsal tubercle and neighbouring organs of A. mentula.. Lettering 
 as before ; egr, epibranchial groove ; 2, prebranchial zone. (Original.) 
 
 part of the cerebro-spinal nervous system of the tailed 
 larval Ascidian (see below, p. 614). The posterior or 
 spinal part has entirely disappeared in most Tunicata. 
 It persists, however, in the Appendiculariidx, and traces of 
 it are found in some Ascidians (e.g., Clavelina ; see Julin). 
 The ganglion gives off distributory nerves at both ends, 
 
 1 See also Herdman, Future, vol. xxviii. p. 284. 
 
 which run through the mantle to the neighbourhood of the Sense 
 apertures, where they divide and subdivide. The only or s an 
 sense-organs are the pigment spots between the branchial 
 and atrial lobes, the tentacles at the base of the branchial 
 siphon, and possibly the dorsal tubercle and the languets 
 or dorsal lamina. These are all in a lowly developed con- 
 dition. The larval Ascidians on the other hand have well- 
 developed intra-cerebral optic and auditory sense-organs ; 
 and in some of the pelagic Tunicata otocysts and pigment 
 spots are found in connexion with the ganglion. 
 
 The mouth and the pharynx (branchial sac) have already Alimi 
 been described. The remainder of the alimentary canal ar y 
 is a bent tube which in A. mentula and most other Ascid- canal 
 ians lies imbedded in the mantle on the left side of the 
 body, and projects into the peribranchial cavity. The 
 oesophagus leaves the branchial sac in the dorsal middle 
 line near the posterior end of the dorsal lamina (see fig. 
 4, ceo). It is a short curved tube which leads ventrally 
 to the large fusiform thick-walled stomach. The intestine 
 emerges from the ventral end of the stomach, and soon 
 turns anteriorly, then dorsally, and then posteriorly so as 
 to form a curve the intestinal loop open posteriorly. 
 The intestine now curves anteriorly again, and from this 
 point runs nearly straight forward as the rectum, thus com- 
 pleting a second curve the rectal loop open anteriorly 
 (see fig. 4). The wall of the intestine is thickened inter- 
 nally, to form the typhlosole, a pad which runs along its 
 entire length. The anus opens into the dorsal part of the 
 peribranchial cavity near to the atrial aperture. The walls 
 of the stomach are glandular; and a system of delicate 
 tubules with dilated ends, which ramifies over the outer wall 
 of the intestine and communicates with the cavity of the 
 stomach by means of a duct, is probably a digestive gland. 
 
 A mass of large clear vesicles which occupies the rectal Excr< 
 loop, and may extend over the adjacent walls of the in- tor y 
 testine, is a renal organ without a duct. Each vesicle is orgau 
 the modified remains of a part of the primitive crelom or 
 body-cavity, and is formed of cells which eliminate nitro- 
 genous waste matters from the blood circulating in the 
 neighbouring blood-lacuna? and deposit them in the cavity 
 of the vesicle, where they form a concentrically laminated 
 concretion of a yellowish or brown colour. These concre- 
 tions contain uric acid, and in a large Ascidian are very 
 numerous. The nitrogenous waste products are thus de- 
 posited and stored up in the renal vesicles in place of 
 being excreted from the body. In other Ascidians the 
 renal organ may differ from the above in its position and 
 structure ; but in no case has it an excretory duct, unless 
 the subneural gland is to be regarded as a renal organ. 
 
 The heart is an elongated fusiform tube placed on the Bloot 
 ventral and posterior edge of the stomach, in a space (the vascu 
 pericardium) which is part of the original ccelom or body- s ^* te 
 cavity, the rest of which exists merely in the form of lacunae cfle i ol: 
 and of the cavities of the reproductive organs and renal 
 vesicles in the adult Ascidian. The wall of the heart is 
 formed of a layer of epithelio-muscular cells, the inner 
 ends of which are cross-striated ; and waves of contraction 
 pass along it from end to end, first for a certain number of 
 beats in one direction and then in the other, so as to reverse 
 the course of circulation periodically. At each end the 
 heart is continued into a vessel (see fig. 9), a large sinus 
 or lacuna lined with a delicate endothelial layer. The 
 sinus leaving the ventral end of the heart is called the 
 branchio-cardiac vessel, 2 and the heart itself is merely the 
 differentiated posterior part of this sinus and is therefore 
 a ventral vessel. The branchio-cardiac vessel, after giving 
 off a branch which, along with a corresponding branch from 
 the cardio- visceral vessel, goes to the test, runs along the 
 
 2 On account of the periodic reversal of the circulation none of the 
 vessels can be called arteries or veins. 
 
TUNICATA 
 
 189 
 
 at 
 
 fore; be, branchio-cardiac or ventral vessel; cr, cardio- 
 visceral vessels; vb, viscero- branchial or dorsal vessel; 
 f(, vessels to test. (Original.) 
 
 ventral edge of the branchial sac externally to the endostyle, 
 and communicates laterally with the ventral ends of all the 
 transverse vessels of the branchial sac. The sinus leaving 
 the dorsal end of the heart is called the cardio-visceral 
 vessel, and this, after giving off to the test the branch 
 above mentioned, breaks up into a number of sinuses, 
 which ramify over the alimentary canal and the other 
 viscera. These visceral lacunre finally communicate with 
 a third great sinus, the viscera-branchial vessel, which runs 
 forward along the dorsal edge of the branchial sac exter- 
 nally to the dorsal lamina and joins the dorsal ends of all 
 the transverse vessels oL the branchial sac. Besides these 
 three chief systems there are numerous lacunas in all parts 
 of the body, by means of which anastomoses are established 
 between the different currents of blood. All these blood 
 spaces and lacunae are to be regarded as derived from the 
 blastocrele of the embryo, and not, as has been usually 
 Course of supposed, from the ceelom (jo). When the heart contracts 
 circula- ventro-dorsally, the course of the circulation is as follows : 
 tion - the blood which is flowing through the vessels of the 
 branchial sac is collected in an oxygenated condition in 
 the branchio-cardiac vessel, and, after receiving a stream 
 of blood from 
 the test, en- 
 ters the 
 heart. It 
 then pro- 
 pelled from 
 the dorsal j ''be 
 
 end of the Fio. 9- Diagram of circulation in Ascidia. Lettering as be- 
 
 heart into 
 the cardio- 
 visceral vessels, and so reaches the test and digestive and 
 other organs ; then, after circulating in the visceral lacunae, 
 it passes into the viscero-branchial vessel in an impure 
 condition, and is distributed to the branchial vessels to be 
 purified again. When the heart on the other hand contracts 
 dorso-ventrally, this course of circulation is reversed. As 
 the test receives a branch from each end of the heart, it 
 follows that it has afferent and efferent vessels which- 
 ever way the blood is flowing. In some Ascidians the 
 vessels in the test become very numerous and their end 
 branches terminate in swollen bulbs close under the outer 
 surface of the test. In this way an accessory respiratory 
 organ l is probably formed in the superficial layer of the 
 test. The blood corpuscles are chiefly colourless and 
 amoeboid ; but in most if not all Ascidians there are also 
 some pigmented corpuscles in the blood. These are gener- 
 ally of an orange or reddish brown tint, but may be opaque 
 white, dark indigo-blue, or of intermediate colours. Pre- 
 cisely similarly pigmented cells are found throughout the 
 connective tissue of the mantle and other parts of the body. 
 Repro- A. mentula is hermaphrodite, and the reproductive organs 
 ductive lie, with the alimentary canal, on the left side of the body, 
 organs. ij ne ovarv j s a ramified gland which occupies the greater 
 part of the intestinal loop (see fig. 4). It contains a cavity 
 which, along with the cavities of the testis, is derived from 
 a part of the original ceelom, and the ova are formed from 
 its walls and fall when mature into the cavity. The 
 oviduct is continuous with the cavity of the ovary and 
 leads forwards alongside the rectum, finally opening near 
 the anus into the peribranchial cavity. The testis is com- 
 posed of a great number of delicate branched tubules, 
 which ramify over the ovary and the adjacent parts of the 
 intestinal wall. Those tubules terminate in ovate swell- 
 ings. Near the commencement of the rectum the larger 
 tubules unite to form the vas deferens, a tube of consider- 
 able size, which runs forwards alongside the rectum, and, 
 like the oviduct, terminates by opening into the peri- 
 1 See Herdmau, A'ature, vol. xxii. p. 247. 
 
 branchial cavity close to the anus. The lumen of the 
 tubules of the testis, like the cavity of the ovary, is a part 
 of the original ceelom, and the spermatozoa are formed 
 from the cells lining the wall. In some Ascidians repro- 
 ductive organs are present on both sides of the body, and 
 in others (Polycarpa) there are many complete sets of both 
 male and female systems, attached to the inner surface of 
 the mantle on both sides of the body and projecting into 
 the peribranchial cavity. 2 
 
 EMBRYOLOGY* AND LIFE HISTORY. 
 
 In most Ascidians the eggs are fertilized in the peribranchial Embryo- 
 cavity, and undergo most of their development before leaving the logy, 
 parent ; in some cases, however, the eggs are laid, and fertilization 
 takes place in the surrounding water. The segmentation is com- 
 plete and regular (fig. 10, A) and results in the formation of a 
 spherical blastula, which then undergoes imagination (fig. 10, B). 
 The embryo elongates, and the blastopore or invagination opening 
 comes to be placed on the dorsal edge near the posterior end (fig. 
 10, C). The hypoblast cells lining the archenteron are columnar 
 in form, while the epiblast cells are more cubical (fig. 10, B, C, D). 
 The dorsal surface of the embryo now becomes flattened and then 
 depressed to form a longitudinal groove, extending forwards from 
 the blastopore to near the front of the body. This "medullary 
 groove" now becomes converted into a closed canal by its side 
 walls growing up, arching over, and coalescing in the median dorsal 
 
 FIG. 10. Stages in the embryology of a Simple Ascidian (after Kowalevsky). 
 A to F. Longitudinal vertical sections of embryos, all placed with the dorsal 
 surface uppermost and the anterior end at the right. A. Early blastula 
 stage, during segmentation. B. Early gastrula stage. C. Stage after gas- 
 trula, showing commencement of notochord. D. Later stage, showing forma- 
 tion of notochord and of neural canal. E. Embryo showing body and tail 
 and completely formed neural canal. F. Larva just hatched ; end of tail 
 cut off. G. Transverse section of tail of larva. 
 
 adp, adhering papilUe of larva ; at, epiblastic (atrial) involution ; au, auditory 
 organ of larva ; ar, archenteron ; be, blastocoele ; bp, blastopore ; ch, noto- 
 chord ; ep, epiblast ; hy, hypoblast ; we, neural canal ; nee, nenrenteric 
 canal ; oc, ocular organ of larva ; g, gelatinous investment of embryo ; , 
 muscle cells of tail ; mes, mesenternn ; me, mesoderm cells ; nr, cerebral 
 vesicle at anterior end of neural canal. 
 
 line (fig. 10, D). This union of the laminss dorsales to form the 
 neural canal commences at the posterior end behind the blastopore 
 and gradually extends forwards. Consequently the blastopore 
 comes to open into the posterior end of the neural canal (fig. 10, 
 D), while the anterior end of that cavity remains open to the 
 exterior. In tins way the archenteron communicates indirectly 
 with the exterior. The short canal leading from the neural canal 
 to the archenteron is known as the neurenteric canal (fig. 10, 
 
 2 For structure of other forms, see p. 614 sq. below. 
 s For reproduction by gemmation, see under "Classification," p. 
 614 sq. below. 
 
 X 
 
190 
 
 TUNICATA 
 
 D, nee). Previous to this stage some of the hypoblast cells at the 
 front edge of the blastopore and forming part of the dorsal wall of 
 the arehenteron (fig. 10, C, ch) have become separated off, and then 
 arranged to form an elongated band, two cells wide, underlying 
 the posterior half of the neural canal (fig. 10, D, E, ck. ). This 
 is the origin of the notochord. Outgrowths from the sides of the 
 arehenteron give rise to laterally placed masses of cells, which are 
 the origin of the mesoblast. These masses show no trace of meta- 
 meric segmentation. The cavities (reproductive and renal vesicles) 
 which are formed later in the mesoblast represent the ccelom. 
 Consequently the body-cavity of the Tunicata is a modified form 
 of enteroccele. The anterior part of the embryo, in front of the 
 notochord, now becomes enlarged to form the trunk, while the 
 posterior part elongates to form the tail (fig. 10, E). In the trunk 
 the anterior part of the arehenteron dilates to form the mescnteron, 
 the greater part of which becomes the branchial sac ; at the same 
 time the anterior part of the neural canal enlarges to form the 
 cerebral vesicle, and the opening to the exterior at the front end of 
 the canal now closes. In the tail part of the embryo the neural 
 canal remains as a narrow tube, while the remains of the wall of the 
 arehenteron the dorsal part of which becomes the notochord are 
 converted into lateral muscle bands (fig. 10, G) and a ventral cord 
 of cells, which eventually breaks up to form blood corpuscles. As 
 the tail grows longer, it becomes bent round the trunk of the embryo 
 inside the egg-membrane. About this period the epiblast cells 
 begin to form the test as a cuticular deposit upon their outer surface. 
 The test is at first devoid of cells and forms a delicate gelatinous 
 investment, but it shortly afterwards becomes cellular by the 
 migration into it of test cells formed by proliferation from the epi- 
 blast. 1 
 
 Larval The embryo is hatched about two or three days after fertilization, 
 stage. in the form of a tadpole-like larva, which swims actively through 
 the sea by vibrating its long tail. The anterior end of the body 
 is provided with three adhering papill* (fig. 10, F, adp) in the 
 form of epiblastic thickenings. In the free-swimming tailed larva 
 the nervous system, formed from the walls of the neural canal, 
 becomes considerably differentiated. The anterior part of the 
 cerebral vesicle remains thin-walled (fig. 10, F), and two unpaired 
 sense organs develop from its wall and project into the cavity. 
 These are a dorsally ami posteriorly placed optic organ, provided 
 with retina, pigment layer, lens, and cornea, and a ventrally placed 
 auditory organ, consisting of a large spherical partially pigmented 
 otolith, attached by delicate hair-like processes to the summit of a 
 hollow crista acoustica (fig. 10, F, au). The posterior part of the 
 cerebral vesicle thickens to form a solid ganglionic mass traversed 
 by a narrow central canal. The wall of the neural canal behind the 
 cerebral vesicle becomes differentiated into an anterior thicker 
 region, placed in the posterior part of the trunk and having a 
 superficial layer of nerve fibres, and a posterior narrower part which 
 traverses the tail, lying on the dorsal surface of the notochord, and 
 gives off several pairs of nerves to the muscles of the tail. Just in 
 front of the anterior end of the nervous system a dorsal involution 
 of the epiblast breaks through into the upturned anterior end of 
 the mesenteron and thus forms the mouth opening. Along the 
 ventral edge of the mesenteron, which becomes the branchial sac, 
 the endostyle is formed as a narrow groove with thickened side 
 walls. It probably corresponds to the median portion of the thyroid 
 body of Vertebrata. A curved outgrowth from the posterior end 
 of the mesenteron forms the alimentary canal (oesophagus, stomach, 
 and intestine), which at first ends blindly. An anus is formed 
 later by the intestine opening into the left of two lateral epiblastic 
 involutions (the atria), which rapidly become larger and fuse dorsally 
 to form the peribranchial cavity. Outgrowths from the wall of the 
 branchial sac meet these epiblastic involutions and fuse with them 
 to give rise to the first formed pair of stigmata, which thus come 
 to open into the peribranchial cavity ; and these alone correspond 
 to the gill clefts of Amphioxus and the Vertebrata. 
 
 Metamor. After a short free-swimming existence the fully developed tailed 
 phosis larva fixes itself by its anterior adhering papillre to some foreign 
 to adult object, and then undergoes a remarkable series of retrogressive 
 form. changes, which convert it into the adult Ascidian. The tail atro- 
 phios, until nothing is left but some fatty cells in the posterior 
 part of the trunk. The adhering papilla? disappear and are replaced 
 functionally by a growth of the test over neighbouring objects. 
 The nervous system with its sense organs atrophies until it is re- 
 duced to the single small ganglion, placed on the dorsal edge of the 
 pharynx, and a slight nerve cord running for some distance pos- 
 teriorly (Van Beneden and Julin). Slight changes in the shape 
 of the body and a further growth and differentiation of the branchial 
 sac, peribranchial cavity, and other organs now produce gradually 
 the structure found in the adult Ascidian. 
 
 The most important points in connexion with this process of 
 development and metamorphosis are the following. (1) In the 
 Ascidiau embryo all the more important organs (e.g., notochord, 
 neural canal, arehenteron) are formed in essentially the same 
 
 1 Some of the first test cells are also probably derived from the epithelium 
 of the egg follicle. 
 
 manner as they are in Amphioxus and other Chordata. (2) The 
 free-swimming tailed larva possesses the essential characters of the 
 Chordala, inasmuch as it has a longitudinal skeletal axis (the noto- 
 chord) separating a dorsally placed nervous system (the neural 
 canal) from a ventral alimentary canal (the arehenteron) ; and 
 therefore during this period of its life-history the animal belongs 
 to the Chordata. (3) The Chordate larva is more highly organized 
 than the adult Ascidian, and therefore the changes by which the 
 latter is produced from the former may be regarded as a process of 
 degeneration (31). The important conclusion drawn from all this 
 is that the Tunicata are the degenerate descendants of a group of 
 the primitive Chordata (see below p. 618). 
 
 CLASSIFICATION AND CHARACTERS OF GROUPS. 
 Order I. LAKVACEA. 
 
 Free-swimming pelagic forms provided with a large locomotory Char- 
 appendage (the tail), in which there is a skeletal axis (the urochord). acters 
 A relatively large test (the "Haus") is formed with Larva 
 great rapidity as a secretion from the ectoderm ; it is 
 merely a temporary structure, which is cast off and 
 replaced by another. The branchial sac is simply an 
 enlarged pharnyx with two ventral ciliated openings 
 (stigmata) leading to the exterior. There is no se- 
 i. parate peribranchial cavity. The nervous 
 * system consists of a large dorsally placed 
 ganglion and a long nerve cord, which 
 stretches backwards over the alimentary 
 canal to reach the tail, along which it 
 runs on the left side of the 
 urochord. The anus opens 
 ventrally on the surface of 
 the body in front of the stig- 
 mata. No reproduction by 
 gemmation or metamorphosis 
 is known in the life-history. 
 This is one of the most in- 
 teresting groups of the Tuni- 
 cata, as it shows more com- 
 pletely than any of the rest 
 the characters of the original 
 ancestral forms. It has un- 
 dergone little or no degen- 
 eration, and consequently 
 corresponds more nearly to 
 the tailed -larval condition 
 than to the adult forms of 
 the other groups. The order 
 
 FIG. ll.-Oikopleura copJtocerca in "Haus" includes a single family the 
 (after Fol), seen from right side, magnified APPENDICULARIIM), all the 
 six times. The arrows indicate the course members of which are minute 
 of the water ;x, lateral reticulated parts of and f re0 - swimming. They 
 
 occur on the surface of the 
 
 sea in most parts of the world. They possess the power to form Struc- 
 with great rapidity an enormously large investing gelatinous layer ture o 
 (fig. 11), which corresponds to the test of other groups. This was Appei 
 
 p,.p n.a dicula 
 
 n f<f ,y oi 
 
 /At 
 
 sir 
 
 tu 
 
 Fio. 12. Semi-diagrammatic view of Appendicularia from the right, a, anus ; 
 at, one of the atrial apertures ; app, tail ; br, branchial aperture ; brs, branchial 
 sac ; dt, dorsal tubercle ; end, endostyle ; h, heart ; i, intestine ; m, muscle 
 band of tail ; n, nerve cord in body ; ft', nerve cord in the tail ; o% cesophagus ; 
 ot, otocyst ; ov, ovary ; pp, perjpharyngeal band ; ijg, cerebral ganglion ; ng 1 , 
 caudal ganglion ; ng", enlargement of nerve cord in tail ; so, sense organ 
 (tactile) on lower lip ; sg, ciliated aperture iu pharynx ; st, stomach ; tes, tcstis ; 
 u, urochord ; u', its cut end. (Original.) 
 
 first described by Von Mertens and by him named "Haus." It 
 is only loosely attached to the body and is frequently thrown off 
 soon after its formation. The tail in the Appendicvlariidee is at- 
 tached to the ventral surface of the body (fig. 12), and usually 
 
TUNICATA 
 
 191 
 
 Thali- 
 acea. 
 
 Charac- 
 ters of 
 Cydo- 
 myaria. 
 
 Struc- 
 ture of 
 Dolio- 
 lum. 
 
 If 
 
 points more or less anteriorly. It shows distinct traces of aaeta- 
 meric segmentation, having its muscle bands broken up into myo- 
 tomes, while the nerve cord presents a series of enlargements from 
 which distributary nerves are given off (fig. 12, ng"). Near 
 the base of the tail there is a distinct elongated ganglion 
 (fig. 12, n<f). The anterior (cerebral) ganglion has connected 
 with it an otocyst, a pigment spot, and a tubular process 
 opening into the branchial sac and representing the dorsal 
 tubercle and associated parts of an ordinary Ascidian. The 
 branchial aperture or mouth leads into the branchial sac or 
 pharynx. There are no tentacles. The endostyle is short. 
 There is no dorsal lamina, and the peripharyngeal bands run 
 dorsally and posteriorly. The wall of the branchial sac has * ' 
 only two ciliated apertures. They are homologous with the 
 primary stigmata of the typical Ascidians and the gill 
 clefts of Vertebrates. They are placed far back on the -ven- 
 tral surface, one on each side of the middle line, and lead 
 into short funnel-shaped tubes which open on the surface of 
 the body behind the anus (fig. 12, at). These tubes corre- 
 spond to the right and left atrial involutions which, in an 
 ordinary Ascidian, fuse to form the peribranchial cavity. The 
 heart, according to Lankester, is formed of two cells, which 
 are placed at the opposite ends and connected by delicate con- 
 tractile protoplasmic fibrils. The large ovary and testis are placed 
 at the posterior end of the body. The remainder of the structural 
 details can be made out from fig. 12. 
 
 The family Appcnditulariidx comprises the genera, Oikoplfura 
 (Mertens), and Appendicularia (Cham.), in both which the body is 
 short and compact and the tail relatively long, while the endostyle 
 is straight ; FrUHlaria (Q. and G.), in which the body is long and 
 composed of anterior and posterior regions, the tail relatively short, 
 the endostyle recurved, and an ectodermal hood is formed over the 
 front of the body ; and KouxUevsJcia (Fol), a remarkable form de- 
 scribed by Fol (14), in which the heart, endostyle, and intestine 
 are said to be absent, while the branchial sac is provided with four 
 rows of ciliated tooth-like processes. 
 
 Order II. THALIACEA. 
 
 Free-swimming pelagic forms which may be either simple or 
 compound, and the adult of which is never provided with a tail or a 
 notochord. The test is permanent and may be either well developed 
 or very slight. The musculature of the mantle is in the form of 
 more or less complete circular bands, by the contraction of which 
 locomotion is effected. The branchial sac has either two large or 
 many small apertures, leading to a single peribranchial cavity, into 
 which the anus opens. Alternation of generations occurs in the life- 
 history, and may be complicated by polymorphism. The Thaliacca 
 comprises two groups, Cyclomyaria and Hemimyaria. 
 
 Sub-order 1. Cyclomyaria, 
 
 Free-swimming pelagic forms which exhibit alternation of genera- 
 tions in their life-history but never form permanent colonies. The 
 body is cask -shaped, with the branchial and atrial apertures at the 
 opposite ends. The test is more or less well developed. The 
 mantle has its musculature in the form of circular bands surrounding 
 the body. The branchial sac is fairly large, occupying the anterior 
 half or more of the body. Stigmata are usually present in its 
 posterior part only. The peribranchial cavity is mainly posterior 
 to the branchial sac. The alimentary canal is placed ventrally 
 close to the posterior end of the branchial sac. Hermaphrodite 
 reproductive organs are placed ventrally near the intestine. 
 
 This group forms one family, the DOLIOLID.E, including two 
 genera, Doliolum (Quoy and Gaimard) and Anchinia (C. Vogt). 
 
 Doliolum, of which several species are known from various 
 seas, has a cask-shaped body, usually from 1 to 2 cm. in length. 
 The terminal branchial and atrial apertures (fig. 13) are lobed, 
 and the lobes are provided with sense organs. The test is very 
 slightly developed and contains no cells. The mantle has eight or 
 nine circular muscle bands surrounding the body. The most 
 anterior and posterior of these form the branchial and atrial 
 sphincters. The wide branchial and atrial apertures lead into 
 large branchial and peribranchial cavities, separated by the pos- 
 terior wall of the branchial sac, which is pierced by stigmata ; con- 
 sequently there is a free passage for the water through the body 
 along its long axis, and the animal swims by contracting its ring- 
 like muscle-bands, so as to force out the contained water posteriorly. 
 Stigmata may also be found on the lateral walls of the branchial 
 sac, and in that case there are corresponding anteriorly directed 
 diverticula of the peribranchial cavity. There is a distinct endo- 
 style on the ventral edge of the branchial sac and a peripharyngeal 
 tand surrounding its anterior end, but there is no representative 
 of the dorsal lamina on its dorsal edge. The trsophagus com- 
 mences rather on the ventral edge of the posterior end of the 
 branchial sic, and runs backwards to open into the stomach, which 
 is followed by a curved intestine opening into the peribranchial 
 cavity. The alimentary canal as a whole is to the right of the 
 middle line. The hermaphrodite reproductive organs are to the 
 left of the middle line alongside the alimentary canal. They open 
 
 into the peribranchial cavity. The ovary is nearly spherical, while 
 the testis is elongated, and may be continued anteriorly for a long 
 distance. The heart is placed in the middle line ventrally, be- 
 
 at 
 
 at I 
 
 trs 
 
 Fro. IS. Doliolvm dfntievlatMm, sexual generation, from the left side. Letter- 
 ing as for fig. 12 ; m 1 m8, muscle Lands ; ng, nerve ganglion ; sg, stigmata ; 
 tgt, subneural gland : pbr, peribranchial cavity ; all, atrial lobes ; to, sense 
 organs ; brl, branchial lobes. (Original.) 
 
 tween the posterior end of the endostyle and the cesophageal aperture. 
 The nerve ganglion lies about the middle of the dorsal edge of the 
 body, and gives off many nerves. Under it is placed the subneural 
 gland, the duct of which runs forward and opens into the anterior 
 end of the branchial sac by a simple aperture, surrounded by the 
 spirally twisted dorsal end of the peripharyngeal band (fig. 13, dt). 
 
 The ova of the sexual generation produce tailed larvae ; these Develop- 
 develop into forms known as " nurses " (blastozooids), which are ment of 
 asexual, and are characterized by the possession of nine muscle Dolio- 
 bands, an auditory sac on the left side of the body, a ventrally- {urn. 
 placed stolon near the heart, upon which buds are produced, and 
 a dorsal outgrowth near the posterior end of the body. The buds 
 give rise eventually to the sexual generation, which is polymor- 
 phous, having three distinct forms, in two of which the reproduc- 
 tive organs remain undeveloped. The buds while still very young 
 migrate from their place of origin on the stolon, divide by fission, 
 and become attached to the dorsal outgrowth of the body of the 
 nurse, where they develop. The three forms produced are as follows. 
 (1) Nutritive forms (trophozooids), which remain permanently at- 
 tached to the nurse and serve to provide it with food ; they have 
 the body elongated dorso- ventrally, and the musculature is very 
 slightly developed. (2) Foster forms (phorozooids), which, like the 
 preceding, do not become sexually mature, but, unlike them, are 
 set free as cask-shaped bodies with eight muscle bands and a ventral 
 outgrowth, which is formed of the stalk by which the body was 
 formerly united to the nurse. On this outgrowth the (3) forms 
 (gonozooids) which become sexually mature are attached while still 
 young buds, and after the foster forms are set free these reproductive 
 forms gradually attain their complete development, and are event- 
 ually set free and lose all trace of their connexion with the foster 
 forms. They resemble the foster forms in having a cask-shaped 
 body with eight muscle bands, but differ in having no outgrowth 
 or process, and in having the reproductive organs Mly developed. 1 
 
 Anchinia, of which only one species is known, A. rubra, from Anchinia. 
 the Mediterranean, has the sexual forms permanently attached 
 to portions of the dorsal outgrowth from the body of the unknown 
 nurse. The body is elongated dorso- ventrally. The test is well 
 developed and contains branched cells. The musculature is not 
 so well developed as in Doliolum. There are two circular bands 
 at the anterior end and two at the posterior, and two on the 
 middle of the body. The stigmata are confined to the obliquely 
 placed posterior end of the branchial sac. The alimentary canal 
 forms a U-shaped curve. The reproductive organs are placed on 
 the right side of the body. The life-history is still imperfectly 
 known. As in the case of Doliolum the sexual generation is 
 polymorphous, and has three forms, two of which remain in a 
 rudimentary condition so far as the reproductive organs are con- 
 cerned. In Anthinia, however, the three forms do not occur to- 
 gether on one stolon or outgrowth, but are produced successively, 
 the reproductive forms of the sexual generation being independent 
 of the " foster forms " (see Barrois, 27}. 
 
 Sub-order 2. Hemimyaria, 
 
 Free-swimming pelagic forms which exhibit alternation of genera- Charac- 
 tions in their life-history and in the sexual condition form colonies, ters of 
 The body is more or less fusiform, with the long axis antero-posterior, Hemi- 
 and the branchial and atrial apertures nearly terminal. The test myaria. 
 is well developed. The musculature of the mantle is in the form 
 of a series of transversely -running bands, which do not form com- 
 plete independent rings as in the Cyclomyaria. The branchial and 
 
 1 For further details see Cljaniu (iS). 
 
192 
 
 TUNICATA 
 
 Struc- 
 ture of 
 Salpa. 
 
 peribranchial cavities form a continuous space in the interior of the 
 body, opening externally by the branchial and atrial apertures, and 
 traversed obliquely from the dorsal and anterior end to the ventral 
 and posterior by a long narrow vascular band, which represents the 
 dorsal lamina, the dorsal blood-vessel, and the neighbouring part 
 of the dorsal edge of the branchial sac of an ordinary Ascidian. 
 The alimentary canal is placed ventrally. It may either be stretched 
 out so as to extend for some distance anteriorly, or as is more 
 usual be concentrated to form along with the reproductive organs 
 a rounded opaque mass near the posterior end of the body, known 
 as the visceral mass or "nucleus." The embryonic development 
 is direct, no tailed larva being formed. 
 
 This sub-order contains two very distinct families, the SALPID^E, 
 which are the typical members, and the OCTACNKMID.S, including a 
 single very remarkable form (Octacnemus lylhius), which in some 
 respects does not conform with the characters given above. 
 
 The Salpidas includes the single genus Salpa (Forskal), which, 
 however, may be divided into two well-marked groups of species, ( 1 ) 
 those, such as S. pinnata, in which the alimentary canal is stretched 
 out along the ventral surface of the body, and (2) those, such as 
 S.fusiformis (fig. 14, A), in which the aliment- 
 ary canal forms a compact globular mass, the 
 " nucleus," near the posterior end of the body. 
 About fifteen species altogether are known ; 
 they are all pelagic forms and are found in 
 nearly all seas. Each species occurs in two 
 forms the solitary asex- 
 ual (proles solitaria) and 
 the aggregated sexual 
 (proles gregaria) which 
 are usually quite unlike 
 end - EH31- 3 ne another. The soli- 
 tary form (fig. 14, B) 
 gives rise by internal 
 d I gemmation to a complex 
 - tubular stolon, which 
 contains processes from 
 all the more important 
 organs of the parent body 
 and which becomes seg- 
 mented into a series of 
 Fio. 14. Salpa nmcinata-fusiformis. A. Aggre. buds or embryos. As 
 gated form. B. Solitary form. Lettering as the stolon elongates, the 
 before ; 1-9, muscle bands ; em, embryo ; gem, embryos near the f ree 
 geramiparous stolon ; in, mantle ; vise, visceral , > . . . , 
 mass (nucleus). (Original.) end which have become 
 
 advanced in their deve- 
 lopment are set free in groups, which remain attached together by 
 processes of the test, each enclosing a diverticulum from the mantle 
 so as to form "chains" (fig. 15). ' > 
 Each member of the chain is a Salpa 
 of the sexual or aggregated form, 
 and when mature may either still 
 attached to its neighbours or se- 
 parated from them (fig. 14, A) 
 produce one or several embryos, 
 which develop into the solitary 
 Salpa. Thus the two forms alter- 
 nate regularly. The more import- 
 ant points in the structure of a 
 typical Salpa are shown in fig. 16. 
 The branchial and atrial apertures 
 are at opposite ends of the body, 
 and each leads into a large cavity, 
 the branchial and peribrauchial Fio. 15. Posterior part of solitary 
 sacs, which are in free communiea- gi^owinga ^hain'of < efinbiyo*nearly 
 tiou at the sides of the obliquely- ready to be set free, gem, young 
 runnin" dorsal lamina or "gill." aggregated Salpie forming the chain ; 
 The test is well developed and s, stolon; m, muscle band of the 
 adheres closely to the surface of mantle ' < 0rl g' nal -> 
 the mantle. The muscle bands of the mantle do not completely 
 
 I'ISC 
 
 8 
 -9 
 
 -V gem 
 
 em b 
 
 br 
 
 encircle the 
 body. They are 
 present dorsally 
 and laterally, 
 but the major- 
 ity do not reach 
 the ventral sur- 
 face. In many 
 cases neigh- 
 bouring bands 
 join in the med- 
 ian dorsal line, Fia _ lg> _ g e mi-diagrammatic representation of Salpa from 
 (fig. 14). The leftside. Lettering as before ; emb, embryo ; m, mantle ; 
 anterior end of I, languet; sgd, duct of subneural gland; 1-11, muscle 
 tlin rlnrsil la bands of mantle ; f, thickening of test over nucleus ; ill, 
 
 . UUI . MJ gin or branchia. (Original.) 
 
 imna is pro- 
 
 longed to form a prominent tentacular organ, the languet, pro- 
 
 Cild' 
 
 jccting into the branchial sac. The nerve ganglion, subneural 
 gland, dorsal lamina, peripharyngeal bands, and endostyle are 
 placed in the usual positions. A pigment spot and an otocyst 
 are found in connection with the ganglion. The largo spaces at 
 the sides of the dorsal lamina (often called the gill or branchia 
 of Salpa), by means of which the cavity of the branchial sac is 
 placed in free communication with the peribranchial cavity, are to 
 be regarded as gigantic stigmata formed by the suppression of the 
 lateral walls of the branchial sac. Fig. 16 represents an aggre- 
 gated or sexual Salpa which was once a member of a chain, since 
 it shows a testis and a developing embryo. The ova (always few 
 in number, usually only one) appear at a very early period in the 
 developing chain Salpa, while it is still a part of the gemmiparous 
 stolon in the body of the solitary Salpa. This gave rise to the 
 view put forward by Brooks (25), that the ovary really belongs to 
 the solitary Salpa, which is therefore a female producing a series 
 of males by asexual gemmation, and depositing in each of these an 
 ovum, which will afterwards, when fertilized, develop in the body 
 of the male into a solitary or female Salpa. This idea would of 
 course entirely destroy the view that Salpa is an example of alterna- 
 tion of generations. The sexual or chain Salpa, although really 
 hermaphrodite, is always protogynous : i.e., the female elements or 
 ova are produced at an earlier period than the male organ or testis. 
 This prevents self-fertilization. The ovum is fertilized by the Devek 
 spermatozoa of an older Salpa belonging to another chain, and ment ( 
 the embryo is far advanced in its development before the testis is Salpa. 
 formed. At an early period in its development a part of the embryo 
 becomes separated off, along with a part of the wall of the cavity 
 in which it lies, to form the " placenta," in which the embryonic and 
 the maternal blood streams circulate in close proximity (or actually 
 coalesce during one period) and so allow of the passage of nutriment 
 to the developing embryo. At a somewhat later stage a number of 
 cells placed at the posterior end of the body alongside the future 
 nucleus become filled up with oil-globnles to form a mass of nutrient 
 material the elaeoblast which is used up later on in the develop- 
 ment. Many suggestions have been made as to the homology of 
 the elseoblast. The most probable is that it is the disappearing 
 rudiment of the tail found in the larval condition of most Ascidians. 
 
 The family Octacnemidee includes the single remarkable form Octa- 
 Odacnemus bythius, found during the " Challenger " expedition, and cnemic 
 first described by 
 Moseley (29). It is 
 apparently a deep- 
 sea representative 
 of the pelagic Sal- 
 pidse, and may pos- 
 sibly be fixed. The 
 body is somewhat 
 discoid, with its 
 margin prolonged 
 
 tnfnrm picrhttniipr Fl - 17. Diagrammatic vertical longitudinal section 
 ' of Octamemm lythtus (after Moseley). br, branchial 
 ing_ processes, onto aperture; m, opening of oesophagus ; r, rectum; at, 
 which the muscle atrial aperture ; rm, rm, radiating muscles ; n, nucleus ; 
 bands of the 
 mantle are con- 
 tinued. The ali- 
 mentary canal forms a compact nucleus (fig. 17) ; the endostyle is 
 very short ; and the dorsal lamina is apparently absent. The re- 
 production and life-history are entirely unknown. 
 
 Order III. ASCIDIACEA. 
 
 Fixed or free-swimming Simple or Compound Ascidians which in Ascid- 
 the adult are never provided with a tail and have no trace of a iacea. 
 notochord. The free-swimming forms are colonies, the Simple 
 Ascidians being always fixed. The test is permanent and well 
 developed ; as a rule it increases with the age of the individual. 
 The branchial sac is large and well developed. Its walls are per- 
 forated by numerous slits (stigmata) opening into the peribranchial 
 cavity, which communicates with the exterior by the atrial aperture. 
 Many of the forms reproduce by gemmation, and in most of them 
 the sexually -produced embryo develops into a tailed larva. 
 
 The Ascidiacea includes three groups, the Simple Ascidians, 
 the Compound Ascidians, and the free-swimming colonial Pyrosoina. 
 
 Sub-order 1. Ascidise Simplices. 
 
 Fixed Ascidians which are solitary and very rarely reproduce by Simple 
 gemmation ; if colonies are formed, the members are not buried in Ascid- 
 a common investing mass, but each has a distinct test of its own. ians. 
 No strict line of demarcation can be drawn between the Simple and 
 the Compound Ascidians, and one of the families of the former 
 group, the ClavclinidsR (the Social Ascidians), forms a transition 
 from the typical Simple forms, which never rejiroduce by gemmation, 
 to the Compound forms, which always do (see p. 618 below). The 
 Ascidise Simplices may be divided into the following families: 
 
 Family I. CLAVELINID.E. Simple Ascidians which reproduce by 
 gemmation to form small colonies in which each ascidiozooid has 
 a distinct test, but all are connected by a common blood-system. 
 
 mh, muscles of nucleus; g, respiratory membrane; 
 6, thickened margin of base of test ; pa, pedicle of 
 attachment. 
 
TUNICATA 
 
 193 
 
 Buds formed on stolons which are vascular outgrowths from the pos- 
 terior end of the body, containing prolongations from the ectoderm, 
 mesoderm, and endoderm of the ascidiozooid. Branchial sac not 
 folded ; internal longitudinal bars usually absent ; stigmata straight ; 
 tentacles simple. This family contains three genera : Ecteinascidia 
 (Herdman), with internal longitudinal bars in branchial sac ; Clavel- 
 ina (Savigny), with intestine extending behind branchial sac ; 
 and Perophora (Wiegmann), with intestine alongside branchial sac. 
 
 Family II. ASCIDIIDJE. Solitary fixed Ascidians with gelatinous 
 test ; branchial aperture usually eight-lobed, atrial aperture usually 
 six-lobed. Branchial sac not folded; internal longitudinal bars 
 usually present ; stigmata straight or curved ; tentacles simple. 
 This family is divided into three sections : 
 
 Sub-family 1. HYPOBYTHIN.E. Branchial sac with no internal 
 longitudinal bars. One genus, Hypobythius (Moseley). 
 
 Sub-family 2. AsciDlN.fi. Stigmata straight. Many genera, of 
 which the following are the more important : Ciona (Fleming), 
 dorsal languets present ; Ascidia (Linnseus, =Phallusia, Savigny), 
 dorsal lamina present (see figs. 1 to 10) ; Rhodosoma (Ehrenberg), 
 anterior part of test modified to form operculnm ; Abyssascidia 
 (Herdman), intestine on right side of branchial sac. 
 
 Sub-family 3. CORELLIJLE. Stigmata curved. Three genera : 
 Corella (Alder and Hancock), test gelatinous, body sessile ; Coryn- 
 cacidia (Herdman), test gelatinous, body pedunculated; Chelyo- 
 soma (Brod. and Sow.), test modified into horny plates. 
 
 Family III. CYNTHIID.S. Solitary fixed Ascidians, usually with 
 leathery test ; branchial and atrial apertures usually both four-lobed. 
 Branchial sac longitudinally folded ; stigmata straight ; tentacles 
 simple or compound. This family is divided into three sections : 
 
 Sub-family 1. STYELiN.fi, not more than four folds on each side 
 of branchial sac ; tentacles simple. The more important genera are 
 Styela (Maeleay), stigmata normal, and Bathyoncus (Herdman), 
 stigmata absent or modified. 
 
 Sub-family 2. CYNTHIN^B, more than eight folds in branchial 
 sac; tentacles compound; 
 body sessile. The chief 
 genus is Cynthia (Sa- 
 vigny), with a large 
 number of species. 
 
 Sub -family 3. BOL- 
 TENIX.E, more than eight 
 folds in branchial sac ; 
 tentacles compound ; 
 body peduneulated (fig. 
 
 18, A). The chief genera 
 are Boltenia (Savigny), 
 branchial aperture four- 
 lobed, stigmata normal ; brf , j 
 
 and Culeolus (Herd-^ 10 - 18. CiileoZm wlUemoai. A. Entire body, 
 manl ViranpViial anpr natural size. B. Part of branchial sac mag- 
 man;, Drancmal aper- nifled , atrjal aperture . ^ r> branchial aper- 
 
 ture with less than four tu re ; ped, peduncle ; brf, slight fold of branch- 
 lobes, stigmata absent or ial sac ; i I, internal longitudinal bar ; mh, mesh ; 
 modified (fie 18 B) *P> calcareous spicules in vessels ; tr, transverse 
 m . , . ' j ' '' vessels. (After Herdman, Challenger Report.) 
 1ms last is a deep-sea 
 
 genus discovered by the " Challenger " expedition (see IT). 
 
 Family IV. MOLGULID.E. Solitary Ascidians, sometimes not 
 fixed ; branchial aperture six-lobed, atrial four-lobed. Test usually 
 incrusted with sand. Branchial sac longitudinally folded ; stigmata 
 more or less curved, usually arranged in spirals; tentacles compound. 
 The chief genera are Molgula (Forbes), with distinct folds in the 
 branchial sac, and Eugyra (Aid. and Hanc.), with no distinct folds, 
 but merely broad internal longitudinal bars in the branchial sac. 
 In some of the Molgulidx (genus Anurella, Lacaze-Duthiers, 20) 
 the embryo does not become converted into a tailed larva, the 
 development being direct, without metamorphosis. The embryo 
 when hatched assumes gradually the adult structure, and never 
 shows the features characteristic of larval Ascidians, such as the 
 urochord and the median sense-organs. 
 
 Sub-order 2. Ascidiae Composite. 
 
 Com- Fixed Ascidians which reproduce by gemmation, so as to form 
 
 pound colonies in which the ascidiozooids are buried in a common invest- 
 
 Ascid- ing mass and have no separate tests. This is probably a somewhat 
 
 iaus. artificial assemblage formed of two or three groups of Ascidians 
 
 which produce colonies in which the ascidiozooids are so intimately 
 
 united that they possess a common test or investing mass. This 
 
 is the only character which distinguishes them from the Clavelinidse, 
 
 but the property of reproducing by gemmation separates them 
 
 from the rest of the Ascidix Simplices. The Ascidix Composite 
 
 may be divided into the following families : 
 
 Family I. DISTOMID.B. Ascidiozooids divided into two regions, 
 thorax and abdomen ; testes numerous ; vas deferens not spirally 
 coiled. The chief genera are Distoma (Gaertner) ; Distaplia (Delia 
 Valle) ; Colella (Herdman), forming a pedunculated colony (see fig. 
 
 19, A) in which the ascidiozooids develop incubatory pouches, 
 connected with the peribranchial cavity, in which the embryos 
 undergo their development (77) ; and Chondrostachys (Macdonald). 
 
 Family II. COJLOCORMIDJE. Colony not fixed, having a large axial 
 cavity with a terminal aperture. Branchial apertures five-lobed. 
 This includes one species, Ccelocormus huxleyi (Herdman), which is a 
 transition form between the ordinary Compound Ascidians (e.g., 
 Distomidai) and the Ascidise Salpiformes (Pyrosoma). 
 
 Family III. DIDEMXIDJB. Colony usually thin and incrusting 
 Test containing stel- 
 late calcareous spi- 
 cules. Testis single, 
 large ; vas deferens 
 spirally coiled. The 
 chief genera are Di- 
 demnum (Savigny), 
 in which the colony 
 is thick and fleshy A-\^ ^%f> S ' C D 
 
 and there are only FIO. 19.-Colonies of .4sctdi Composite (natural size), 
 three rows of stlg- A. Colella gnoyi. B. Leptodinum negkctum. C. Pha- 
 mata on each side of ryvgodictyon mirabile. D. Botryllvs, showing ar- 
 .r ^ v i rangement of ascidiozooids in circular systems each 
 
 BC > of which has a central common cloaca. (After Herd- 
 and Leptodinum man, Challenger Report.) 
 (Milne-Edwards), in 
 
 which the colony is thin and incrnsting (fig. 19, B) and there are 
 four rows of stigmata on each side of the branchial sac. 
 
 Family IV. DiPLOsoniD.fi. Test reduced in amount, rarely con- 
 taining spicules. Vas deferens not spirally coiled. In Diplosoma 
 (Macdonald), the most important genus, the larva is gemmiparons. 
 
 Family V. POLYCLINID.E. Ascidiozooids divided into three 
 regions, thorax, abdomen, and post-abdomen. Testes numerous ; 
 vas deferens not spirally coiled. The chief genera are Pharyngo- 
 diclyon (Herdman), with stigmata absent or modified, containing 
 one species, Ph. mirabile (fig. 19, C), the only Compound Ascidian 
 known from a depth of 1000 fathoms ; Polyclinum (Savigny), with 
 a smooth-walled stomach ; Aplidium (Savigny), with the stomach 
 wall longitudinally folded ; and Amaroucium (Milne-Edwards), in 
 which the ascidiozooid has a long post-abdomen and a large atrial 
 languet. 
 
 Family VI. BOTRYLLID.S. Ascidiozooids having the intestine 
 and reproductive organs alongside the branchial sac. Dorsal lamina 
 present ; internal longitudinal bars present in branchial sac. The 
 chief genera are Botryllus (Gaertn. and Pall. ), with simple stellate 
 systems (fig. 19, D), and Botrylloides (Milne -Ed wards), with 
 elongated or ramified systems. 
 
 Family VII. PoLYSTYELrD.fi. Ascidiozooids not grouped in 
 systems. Branchial and atrial apertures four-lobed. Branchial 
 sac may be folded ; internal longitudinal bars present. The chief 
 genera are Thylaeium (Cams), with ascidiozooids projecting above 
 general surface of colony ; Goodsiria (Cun- 
 ningham), with ascidiozooids completely 
 imbedded in investing mass ; and Chorizo- 
 cormus (Herdman), with ascidiozooids 
 united in little groups which are connected 
 by stolons. The last genus contains one 
 species, Ch. reticulatus, a transition form 
 between the other Polystyelidss and the 
 Styelinss amongst Simple Ascidians. 
 
 The methods of reproduction by gemma- 
 tion differ in their details in the various 
 groups of Compound Ascidians ; but in all 
 cases the process is essentially a giving off 
 from the parent body of groups of cells re- 
 presenting the ectoderm, the mesoderm, 
 and the endoderm, which develop into the 
 corresponding layers of the bud. The first 
 ascidiozooid of the colony produced by the 
 tailed larva does not form sexual repro- 
 ductive organs, but reproduces by gemma- 
 tion so as to make a colony. Thus there 
 is alternation of generations in the life- 
 history. In the most completely formed 
 colonies (e.g., Botryllus) the ascidiozooids 
 are arranged in groups (systems or coeno- 
 bii), and in each system are placed with 
 their atrial apertures towards one another, 
 and all communicating with a common 
 cloacal cavity which opens to the exterior 
 in the centre of the system (fig. 19 D). 
 
 Sub-order 3. Ascidise Salpiformes. 
 
 Free -swimming pelagic colonies having 
 the form of a hollow cylinder closed at one 
 end. The ascidiozooids forming the colony j,, 
 are imbedded in the common test in such a natural size. 
 
 Repro- 
 duction 
 by gem- 
 mation 
 
 of entire colony. 
 
 manner that the branchial apertures open 
 on the outer surface and theatrial apertures 
 on the inner surface next to the central 
 cavity of the colony. The ascidiozooids are produced by gemmation 
 from a rudimentary larva (the cyathozooid) developed sexually. 
 
 2B 
 
 Ascidim 
 
 Salpi- 
 
 elegant, for*- 
 Side view 
 B. End 
 
194 
 
 TUNICATA 
 
 Struc- This sub-order includes a single family, the PYEOSOMIDJB, con- 
 ture of taming one well-marked genus, Pyrosoma (Peron), with several 
 Pyro- species. They are found swimming near the surface of the sea, 
 soma. chiefly in tropical latitudes, and are brilliantly phosphorescent. 
 A fully developed Pyrosoma colony may be from an inch or two to 
 upwards of four feet in length. The shape of the colony is seen in 
 fig. 20. It tapers slightly towards the closed end, which is rounded. 
 The opening at the opposite end is reduced in size by the presence 
 of a membranous prolongation of the common test (fig. 20, B). 
 The branchial apertures of the ascidiozooids are placed upon short 
 papillae projecting from the general surface, and most of the ascidio- 
 zooids have long conical processes of the test projecting outwards 
 beyond their branchial apertures (figs. 20, 21, and 22). There is 
 only a single layer of ascidiozooids in the Pyrosoma colony, as all 
 the fully developed ascidiozooids are placed with their antero- 
 posterior axes at right angles to the surface and communicate by 
 their atrial apertures with the central cavity of the colony (fig. 21). 
 
 Fio. 21. Part of a longitudinal section through wall of Pyrotoma, showing 
 arrangement of ascidiozooids, magnified (partly after Savigny). at, atrial 
 apertures ; br, branchial apertures ; asc, young ascidiozooid of a future colony 
 produced by budding from cy, cyathozooid ; em, embryos in various stages ; 
 t, test ; tp, processes of test ; brs, branchial sac ; yas, young ascidiozooid. 
 
 Their dorsal surfaces are turned towards the open end of the colony. 
 The more important points in the structure of the ascidiozooid of 
 Pyrosoma are shown 
 in fig. 22. A circle of 
 tentacles, of which 
 one, placed ventrally 
 (fig. 22, in), is larger 
 than the rest, is 
 found just inside the 
 branchial aperture. 
 From this point a f 
 wide cavity, with a 
 few circularly-placed 
 muscle bands run- 
 ning round its walls, 
 leads back to the 
 large branchial sac, 
 which occupies the 
 greater part of the 
 body. The stigmata 
 are elongated trans- 
 versely and crossed 
 by internal longitu- 
 dinal bars. The dor- 
 sal lamina is repre- 
 sented by a series of 
 eight languets (I). 
 The nerve ganglion 
 (on which is placed 
 a small pigmented f 
 sense organ), the sub- 
 neural gland, the dor- 
 sal tubercle, the peri- 
 pharyngeal bands, 
 and the endostyle are 
 
 placed in the usual 
 
 <"> , i Fio. 22. Mature ascidiozooid of Pyrosoma, from left 
 positions. On each 6id e (partly after Keferstein). Lettering as before ; 
 side ot the anterior cm, cellular mass, the seat of phosphorescence; 
 end of the branchial c "*'. posterior cellular mass ; gs, gemmiparous 
 sar close to thp npri stolon ; mb, muscle band ; ngl, subneural gland ; 
 
 pharigeal bands, 7s *" pigment 8p0t a ganglion : * process f test ' 
 a mass of rounded gland cells which are the source of the phosphores- 
 cence. The alimentary canal is placed posteriorly to the branchial 
 eac, and the anus opens into a large peribranchial (or atrial) cavity, 
 of which only the median posterior part is shown (pbr) in fig. 22. 
 The reproductive organs are developed iu a diverticulum of the peri- 
 
 branchial cavity, and consist of a lobed testis and a single ovum at 
 a time. The development takes place in a part of the peribranchial Deve] 
 cavity (fig. 21, tm). The segmentation is meroblastic, and anment 
 elongated embryo is formed on the surface of a mass of yolk. The Pyro- 
 embryo, after the formation of an alimentary cavity, a tubular soma, 
 nervous system, and a pair of laterally placed atrial tubes, divides 
 into an anterior and a posterior part. The anterior part then 
 segments into four pieces, which afterwards develop into the first 
 ascidiozooids of the colony, while the posterior part remains in a 
 rudimentary condition, and was called by Huxley the "cyatho- 
 zooid " ; it eventually atrophies. As the four ascidiozooids increase 
 in size, they grow round the cyathozooid and soon encircle it (fig. 21, 
 asc and cy). The cyathozooid absorbs the nourishing yolk upon 
 which it lies, and distributes it to the aseidiozooids by means of a 
 heart and system of vessels which have been meanwhile formed. 
 When the cyathozooid atrophies and is absorbed, its original atrial 
 aperture remains and deepens to become the central cavity of the 
 young colony, which now consists of four ascidiozooids placed in a 
 ring, around where the cyathozooid was, and enveloped in a common 
 test. The colony gradually increases by the formation of buds from 
 these four original ascidiozooids. 
 
 PHYLOGENY, 
 
 The accompanying diagram shows graphically the pro- Phyl 
 bable origin and course of evolution of the various groups 
 of Tunicata, and therefore exhibits their relations to one 
 another much more correctly than any system of linear 
 classification can do. The ancestral Proto-Tunicata are 
 here regarded l as an offshoot from the Proto-Chordata 
 the common ancestors of the Tunicata (Urochorda), Am- 
 
 POLYST-., 
 
 POLYCL. 
 
 phioxus (Cephalochorda), and the Vertebrata. The ances- 
 tral Tunicata were probably free-swimming forms, not 
 very unlike the existing Append iculariidx, and are repre- 
 sented in the life-history of nearly all sections of the 
 Tunicata by the tailed larval stage. The Larvacea are the 
 first offshoot from the ancestral forms which gave rise to 
 the two lines of descendants, the Proto-Thaliacea and the 
 Proto-Ascidiacea. The Proto-Thaliacea then split into the 
 ancestors of the existing Cydomyaria and Hemimyaria. 
 The Proto-Ascidiacea gave up their pelagic mode of life 
 and became fixed. This ancestral process is repeated at 
 the present day when the free-swimming larva of the 
 Simple and Compound Ascidians becomes attached. The 
 Proto-Ascidiacea, after the change, are probably most 
 nearly represented by the existing genus Clavelina. They 
 have given rise directly or indirectly to the various groups 
 of Simple and Compound Ascidians and the Pyrosomidx. 
 These groups form two lines,, which appear to have diverged 
 close to the position of the family Clavelinidx. The 
 one line leads to the more typical Compound Ascidians, 
 and includes the Polyclintdse, Distomidx, Didemnidx, 
 Diplosomidx, Ccelocormidse, and finally the Ascidiee Sal- 
 piformes. The second line gave rise to the Simple 
 Ascidians, and to the Botryllidx and Polystyelidx, which 
 
 1 By Dohrn and others their point of origin is placed considerably 
 further up on the stem of the Chordata, thus causing the Tunicata 
 to be regarded as very degenerate Vertebrata (see 32). 
 
TUNICATA 
 
 195 
 
 are, therefore, not closely allied to the other Compound 
 Ascidians. The later Proto-Ascidiacea were probably 
 colonial forms, and gemmation was retained by the Clave- 
 linidx and by the typical Compound Ascidiana (Distomidx, 
 &c.) derived from them. The power of forming colonies 
 by budding was lost, however, by the primitive Simple 
 Ascidians, and must, therefore, have been regained inde- 
 pendently by the ancestral forms of the Botryttidx and 
 the Polystydidx. If this is a correct interpretation of the 
 course of evolution of the Tunicata, we arrive at the 
 following important conclusions. (1) The Tunicata, as a 
 whole, form a degenerate branch of the Proto-Chordata ; 
 (2) the Ascidix Salpiformes (Pyrosoma) are much more 
 closely related to the typical Compound Ascidians than 
 to the other pelagic Tunicata, viz., the Larvacea and 
 the Tkaliacea ; and (3) the Ascidix Composite form a 
 polyphyletic group, the sections of which have arisen 
 at several distinct points from the ancestral Simple 
 Ascidians. 
 
 Bibliography. (/) Cuvier, " Mem. s. les Ascidies," *c., in Mem. 
 d. Mus., vol. ii. p. 10, Paris, 1815 ; (2) Savigny, Memoires sur les 
 Animauxsans Vertebres, pt ii. fasc. L, Paris, 1816 ; (j) Lamarck, 
 Hist Nat. d. Anim, sans Vertebra, 1st ed., Paris, 1815-23 ; (4) 
 O. F. Miiller, Zool. Dcmica, vol. iv., 1806 ; (j) Milne-Edwards, 
 "Observ. s. les Ascidies Composees," &c., in Mem. Aead. Sei., 
 Paris, voL xviii., 1842 ; (6) Schmidt, Zur vergl. Physiol. d. wir- 
 bellos. Thiere, Brunswick, 1845 ; (7) Lowig and Kolliker, " De 
 la Compos., ic., d. EnveL d. Tun.," in Ann. Se. Nat., ser. 
 
 iii. (Zool.), voL v. f 1846 ; (8) Huxley, Phil. Trans., 1851 ; (9) 
 Kowalevsky, "EntwickeL d. einf. Ascid.," in Mem. St. Petersb. 
 Aead. Se., ser. vii, vol. x., 1866 ; (10) J. P. van Beneden, " Bech. 
 s. 1'Embryolog., Ac., d. Asc. Simp.," in Mem. Aead. Soy. Belg., 
 voL xx., 1847 ; (//) Krohn, in Weigmann and Miiller's Archiv, 
 1852 ; (12) Knpfler, Arch./. mOr. Anat., 1869, 1872 ; (13) Giard, 
 " fitude d. trav. Embryolog. d. Tun., &e.," in Arch. Zool. JSxper., 
 voL i, 1872 ; (14) Fol, " fitndea snr les Appendiculaires du Detroit 
 de Messine," in Mem. Soc. Phys. -Hist. Nat. Geneve, voL xxi. ; (tj) 
 Giard, " Reeherches s. L Asc. Comp.," in Arch. Zool. Exper., vol L. 
 1872; (16) Von Drasche, Die Synascidien der Bucht von Kovigno, 
 Vienna, 1883 ; (77) Herdman, " Report upon the Tunicata of the 
 Challenger Expedition," pt i in Zool. Chall. Exp., vol. vi., 1882 ; 
 pt ii in Zool. Chall. Exp., vol. ziv., 1886 ; pt iii. not yet pub- 
 lished ; (/<y) Alder and Hancock, in Ann. Mag. Nat. Hist., 1863, 
 1870 ; (79) Heller, "Untersuch. n. d. Tunic, d. Adriat. Meeres," 
 in Denkschr. d. k. Akad. Wiss., 1875-77 ; (20) Laeaze-Duthiers, 
 " Asc. Simp. d. Cotes d. 1. Manch," in Arch. Zool. Exper., 1874, 
 1877; (21) Transtedt, in Vidensk. Mcdd. Naturh. For., Copenhagen, 
 1881-84; (22) Herdman, "Notes on British Tunicata, &c.," in 
 Journ. Linn. Soc. Zool., voL xv., 1880 ; (23) Ussoff, in Proc. Imp. 
 Soc. Nat. Hist., Moscow, vol. xviiL, 1876 ; (24) John, "Rech. s. 
 1'Org. d. Asc. Simp.," in Arch. d. Biol., voL u., 1881 ; fey) Brooks, 
 " Development of Salpa," in Bull. Mus. Comp. Zool., Harvard, vol. 
 iii. p. 291 ; (26) Salensky, Ztschr. f. viss. Zool., 1877 ; (27) Barrois, 
 Journ. d. I" Anat. et Phys., vol. xxi., 1885 ; (28) Uljanin, Fauna, 
 Ac., d. Golfes ton Neapel, voL x., 1884 ; (29) Moseley, " On Deep- 
 Sea Ascid.," in Trans. Linn. Soc., ser. ii., voL L, 1876 ; (30) E. van 
 Beneden and Julin, "Morph. d. Tuniciers," in Arch. d. Biol., voL 
 vi., 1886 ; t?/) Lankester, Degeneration (Nature series), London, 
 1880; (32) Dohrn, "Stud. z. Urgesch. d. Wirbelth.," in MiUh. 
 Zool. Slat. Neapel. 
 
 (W. A. HE.) 
 
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