K THERMODYNAMICS. By RICHARD WORMELL, M.A. D.Sc. With 41 Diagrams, is. 6d. ZOOLOGY of the INVERTEBRATE ANIMALS. By ALEXANDER MACALISTER, M.D. With 59 Diagrams. is. 6d. ZOOLOGY of the VERTEBRATE ANIMALS. By ALEXANDER MACALISTER, M.D With 77 Diagrams, is. 6d. LONGMANS, GREEN, & CO., LONDON, NEW YORK, & BOMBAY. ZOOLOGY OF THE INVERTEBRATE ANIMALS BY ALEX. MACALISTER, M.D., D.Sc., F.R.S., F.S.A. PROFESSOR OF ANATOMY If THE UNIVERSITY OF CAMBRIDGE SEVENTH EDITION LONGMANS, GREEN, AND CO. LONDON, NEW YORK, & BOMBAY 1896 All rights reserved RICHARD CLAY & SONS, LIMITED, LONDON BUNGAY. EDITORS' PREFACE. OTWiTHSTANDiNG the large number of scientific works which have been published within the last few years, it is very generally acknowledged by those who are practically engaged in Education, whether as Teachers or as Examiners, that there is still a want of Books adapted for school purposes upon several important branches of Science. The present SERIES will aim at supplying this deficiency. The works comprised in the SERIES will all be composed with special reference to their use in school- teach ing ; but, at the same time, particular attention will be given to making the information contained in them trustworthy and accurate, and to presenting it in such a way that it may serve as a basis for more advanced study. In conformity with the special object of the Series, the attempt will be made in all cases to bring out the educational value which properly belongs to the study of any branch of Science, by not merely treating of its M358661 vi Editor J Preface. acquired results, but by explaining as fully as possible the nature of the methods of inquiry and reasoning by which these results have been obtained. Con- sequently, although the treatment of each subject will be strictly elementary, the fundamental facts will be stated and discussed with the fulness needed to place their scientific significance in a clear light, and to show the relation in which they stand to the general conclusions of Science. In order to ensure the efficient carrying-out of the general scheme indicated above, the Editors have endeavoured to obtain the co-operation, as Authors of the several treatises, of men who combine special knowledge of the subjects on which they write with practical experience in Teaching. The volumes of the Series will be published, if possible, at a uniform price of is. 6d. It is intended that eventually each of the chief branches of Science shall be represented by one or more volumes. G. C. R, R M. PREFACE TO THE SIXTH EDITION. THE TEXT of this Edition has been subjected to a careful and thorough revision, and such improvements have been introduced as were consistent with the small space at the Author's disposal. TORRISDALE, CAMBRIDGE. July 20, I 92. PREFACE. THE STUDENT who would acquire a satisfactory knowledge of the principles of Zoology is recom- mended to commence by learning the elementary principles of General Biology ; and having mastered these he should then study the groups of the Inverte- brates as here detailed, coupling his study with a practical examination of such common types as are easily to be obtained. A jellyfish, or a hydra, an earthworm, an oyster, a snail, a cockroach and a lobster, are forms everywhere procurable, and, if examined, will give the student a good general idea of the structure of Invertebrate Animals. It must be borne in mind that without some such practical study, no amount of reading will suffice to convey accurate and adequate ideas of animal organisation. ALEXANDER MACALISTER. CONTENTS. CHAPTER I. PAGE Nature of Animals Processes of Life Tissues and Organs Symmetries of Animals . ..... I CHAPTER II. Classification of Animals Method of naming Resem- blances of Relationship and of Adaptation Mimicry Parasites The Seven Sub-kingdoms of Invertebrate Animals Each Animal has a Life History, not a mere Growth Rudimental Organs Tendency to Individual Variety ......... 6 CHAPTER III. Conditions of Distribution in Time and Space Freshwater, Marine, and Terrestrial Life Methods of Study of Zoology . . . . . . t , . 13 CHAPTER IV. Sub-kingdom I. Protozoa : Rhizopods Amoebae Sun- Animalcules Gregarines and Radiolarians . , . 1 8 Xll Contents. CHAPTER V. I' AGE Infusion- Animals -Luminous Animalcules Summary of the Forms in Sub-kingdom I CHAPTER VI. Sub-kingdom 2. Sponges CHAPTER VII. Sub-kingdom 3. Ccelenterate Animals : Hydras Sea- Firs. Medusae 3 2 'CHAPTER VIII. Sea-Anemones Corals' Dead Men's Toes 'Sea-Pens . 41 CHAPTER IX. Sub-kingdom 4. Echinodermata : S Lone-Lilies Feather- Stars Starfishes * . . . * 47 CHAPTER X. Sea Urchins Sea Cucumbers Summary of Forms in- cluded in Sub-kingdom 4 5 2 CHAPTER XI. Sub-kingdom 5.' Worms :*Turbellarians Tape- Worms- Flukes Round and Thread- Worms . . .57 CHAPTER XII. Wheel-Animalcules Spoon- Worms -Leeches. . . 66 Contents. CHAPTER XIII. PAGE Bristled-Worms Earth- Worms Summary of Normal Worms 7 CHAPTER XIV. Aberrant- Worms Moss- Poly ps and Tunicated Animals . 74 CHAPTER XV. Sub-kingdom 6. Mollusca or Soft-bodied Animals : Arm- footed Molliiscoids ; Bivalves . . . . 78 CHAPTER XVI. Head-bearing Molluscs : Whelks, Snails, c. . . 84 CHAPTER XVII. Cuttle-Fishes Nautili and Squids Summary of Characters of the Classes of Mollusca ...... 90 CHAPTER XVIII. Sub-kingdom 7. Jointed Animals or Arthropoda . . 96 CHAPTER XIX. Crustacea : Crabs, Lobsters, Shrimps, &c. . .98 CHAPTER XX. Spiders, Mites, and Scorpions ...... io3 xiv Contents. CHAPTER XXI. PAGE Myriopoda, Centipedes, and Gally-Worms . . . .112 CHAPTER XXII. Insecta: Insects General Characters and Structure . .115 CHAPTER XXIII. Orders of Insects whose Metamorphoses are imperfect Aphides, Bugs, Straight-winged Insects and Dragon- Flies . ... . I2 3 CHAPTER XXIV. Insects whose Metamorphoses are complete INDEX and GLOSSARY 137 INVERTEBRATA, CHAPTER I. GENERAL CHARACTERS OF ANIMALS. Physical Conditions of Life. An animal, chemi- cally considered, consists of a few elements 1 united with each other into extremely unstable combinations, which are at every moment undergoing chemical change. The constituent materials are constantly becoming grouped into more simple and stable com- pounds, and in that state they are either eliminated cr retained in an inactive condition, while other materials from without are being taken in, and becom- ing so modified that they replace the molecules re- moved by the previous decomposition. As long as life lasts, these conditions of waste and repair continue ; so that the particles of the bodies of all animals are in a state of constant change. The food of animals contains carbon, hydrogen, nitrogen, and oxygen, which must be grouped into complex molecules before the animal can use them for his nutrition. Combinations of the requisite com- plexity can be derived only from previously organised 1 Carbon, hydrogen, oxygen, and nitrogen, with smaller quantities of sulphur and phosphorus. B 2 Invertebrata. materials either animal or vegetable. In most vege- tables the forces concerned in assimilation are suffi- cient to break up originally stable compounds, such as carbonic acid, and to induce the elements to combine into the unstable combinations of which living textures consist. The process of repair in animals has three stages, ist, the taking in of material as food; 2nd, the chang- ing of food into a substance capable of forming part of the living organism, i.e. blood ; and 3rd, the laying down of- this assimilated material in the tissues of the body of which it thus becomes a constituent, repairing the waste sustained by each organ in each discharge of its function. For the life-processes of animals oxygen is neces- sary, and special structures, called respiratory or breath- ing organs, are provided for taking it in. The carbonic acid formed from the waste of the tissues is got rid of by these organs. The material with which the vital properties are connected is of the same nature in all animals and is called protoplasm. The simplest animals are mere masses of this substance, which in them discharges all the functions needful for the maintenance of life ; the more complex are built up of aggregations of particles of the same material, or of substances derived from it in the course of growth. Each of these constituent particles or cells (fig. 9), as they may conveniently be called, usually consists of a mass of protoplasm sur- rounded by an envelope of some material derived by chemical action from protoplasm. Cells continuously grouped make up tissues, and a group of tissues which General Characters of A nimals. 3 performs any special duties in the life of an animal is called an organ. While there are thus varying degrees of complexity among animals, yet the parts of a simple animal have to perform as many essential functions as those of a more complex animal, the in- crease in complexity of an organism being correlated not with an increase in the number of essential functions but with the need for the more perfect fulfilment cf existing duties. Increase in complexity thus results from division of labour, and, with each increase, the sphere of the functional activity of each part becomes narrowed. For example, in jelly-fishes one set of cavities acts as organs of digestion and of circulation, while in higher animals these functions have separate organs, and even subsidiary portions of these great functions have for their accomplishment distinct parts. Functions. Three sets of functions are discharged by organs in the body of an animal : namely, ist, those of Relation ; 2nd, those of Nutrition ; 3rd, those of Reproduction. The organs appropriated to the functions of Rela- tion are those which connect the animal with its environ 1 ng conditions, informing it about its surround- ings, and enabling it to avoid disagreeable or to court agreeable external influences. These organs are of two kinds : (A) those of sensation, such as the skin, or organ of touch, and the special sense organs (eye, ear, nose, tongue), and (B) those of motion, which may be of three kinds, (a) inconstant processes of protoplasm called pseudopodia (fig. 8), (b) minute, constant, hair- like processes having the power of waving to and fro, called cilia^ or (c] contractile cells and fibres in 4 Invertebmta. bundles called muscles. The first kind occur in the lowest animals and in naked protoplasmic particles ; the second in infusorial animals (fig. 14); the third in all but the simplest animals. Connected with the third set of organs of relation we find a system of fine white threads called nerves, whose endings occur in these organs, and whose starting-points are central clusters of nerve-cells, called ganglia. These threads convey the variously received stimuli from the sense organs to the ganglia, and carry the command for motion from the ganglia to the muscles. The function of Nutrition is discharged by four sets of organs : ist, those of feeding, consisting of a cavity or stomach, for the reception of the food, with glands appended thereto, which secrete fluids to assist in assimilation ; 2nd, organs of circulation, which carry FIG. i. Euplotes Charon, a ciliated infusorium showing the stages of division. the assimilated matter, or blood, through the body for the nutrition of the tissues ; 3rd, organs of respiration, by which oxygen is taken in ; and 4th, organs of excretion whereby the waste products are eliminated. m General Characters of Animals. 5 \ There are three stages in the contest between waste and repair which is characteristic of life. In the first, repair is in excess of waste, and individual growth proceeds until a definite limit, constant within certain bounds for each specres, is reached. When this is attained, excess of nutrition still continues but tends to become separate and independent ; by such dis- continuous modes of growth, the third set of functions, or Reproduction, is accomplished. Of this there are ree chief forms : (A) either the whole body of the arent may split into two or more, each becoming a perfect animal like its parent ; this process is named fission (fig. i ). (B) In the second mode of reproduction a small portion of the body of the parent animal enlarges and becomes detached as a bud, which de- velops directly into an organism like its ancestor ; this is called gemmation (fig. 2). (c) In the FlG - 2 - third mode small particles called eggs arise from the tissues of the parent, and, on being ' fertilised by union with organic particles from ' another organism, are capable of developing into newindividuals ; this is called ovulation. The second stage of existence having for a time continued, the organism reaches a third stage, in which waste exceeds repair, and as, by degrees, the assimilated material Gemmation In '.'.-. , the common becomes insufficient to keep up the pro-//Wr^r/^>. cesses of life this stage terminates in death. Summary. Animals consist for the most part of protoplasm, are constantly undergoing waste, and being built up by the assimilation of food. They differ from plants in being usually capable of loco- 6 Invertebrate*. motion (though this has exceptions), in being only capable of assimilating organic matter (except in the case of water and oxygen), and in having their cell- walls composed of nitrogenous matter, while in plants non-nitrogenous matter abounds. Higher animals are strongly differentiated from plants ; the lower forms are often of doubtful position. Animals may be simple or complex, complexity depending on division of labour, and the consequent specialisation of function in organs which become differentiated from each other. The chief functions are Relation, Nutrition and Repro- duction, the last taking place during the stage when individual growth has ceased and while as yet repair exceeds waste. CHAPTER II. ORGANS AND CLASSIFICATION OF ANIMALS. Method of Study. The first branch of zoology ne- cessary to be studied is the anatomy of the organism, and the best method of study is the examination of some of the commoner types of each class. As many of these are small, and optical assistance necessary, the student should provide himself with a good pocket lens. For dissection, the instruments required are, a scalpel, a fine-pointed pair of dissecting forceps, and several sharp-pointed needles fixed in wooden handles and with their extremities ground to sharp edges so as to cut as well as tear. As many small animals can be most easily dissected under fluid it is convenient to have a shallow wooden tray lined with gutta percha or sheet lead for the purpose, while it often facilitates Organs and Classification of Animals. J dissection to have a thin sheet of cork weighted with lead, so as to retain its position at the bottom of the fluid, upon which the various parts may be p nned down. To preserve animal organs the best materials are, spirits of wine, or a weak (2 per cent. ) solution of bichromate of potassium. The study of the forms and nature of organs, and FIG. 3. Common orange Star-fish (Asteracanthion anrantiacus). their relations to each other and to the organism in general, and the laws deduced therefrom, is known as Morphology ; the study of the uses of parts is called Physiology. Morphology. The component organs of an animal's body are disposed around an imaginary axis passing 8 Invertebrate* from the mouth end to the opposite pole. They may be grouped radially in one plane around this, as in the star-fish (fig. 3), or disposed, as in higher forms, on each side of a longitudinal vertical plane in the line of the axis. When such bilateral symmetry exists the surface undermost in progression (ventral) differs from the opposite or dorsal surface, and the organs, instead of lying in one plane, may be arranged in groups dis- posed in successive segments along the axis, as in the worm (fig. 34) or centipede (fig. 62). Different animals live in conditions requiring vary- ing degrees of the action of the several organs, so the development of the organs varies proportionally. In fact animals are so perfectly fitted to their surround- ings that could we know all the conditions under which a given animal existed, we could form a good concep- tion of its structure. The Embryo. We cannot understand the true nature of animal structures without knowing the stages of their formation from their first appearance in the gg until they attain their adult condition. The im- mature germ in the egg is called the embryo, and this is not a simple miniature of the full-grown animal, but first appears as a small mass of protoplasm, a single cell, which undergoes division, thereby increasing into a mulberry-like cluster of cells, which develops still farther into a bilaminar sac. In process of growth the primitive cavity of this sac becomes modified into the characteristic inner cavity of each kind of animal ; the outer layer of the sac becomes the outer layer or cuticle of the animal's body, while the inner becomes the lining of the digestive canal. An intermediate layer developing between these two gives origin to the R Organs and Classification of A nimals. 9 uscular system, the blood-vessels, and the skeleton, ihe embryo thus reaches its perfect state by under- going a series of changes, which follow each other in a definite order, and the study of these is called Em- bryology. Some of the parts which appear during these FIG. 4. Larva of common Acorn-shell (Balanns porcatns), showing an- tennae (a), limbs (b), eye (d). Adult form of Balanns porcatus, showing the absence of eyes and feelers. Compare with fig. 4. processes have only a transient existence, and may dis- appear e.g. the eyes of the free-swimming larva of the common acorn shell (fig. 4), which vanish in the adult (fig. 5) ; or they may leave useless tiaces behind them IO Invertebrata. like the cords into which the embryonic blood-vessels in mammalia degenerate. Such organs are known as provisional organs. In the development of some ani- mals, parts appear which never perform any function and which either vanish or remain permanently as rudimental organs. They are always such as in some kindred form discharge an important duty. Thus em- bryo cuttle-fishes (fig. 53) have a dorsal groove which closes in and forms the cavity wherein the internal shell is secreted; but the octopus or sea- spider, a closely allied form, has a similar groove which vanishes, leaving no trace behind. Instances of the kind might be multiplied, as the majority of the ani- mals of the higher sub-kingdoms exhibit examples of provisional and rudimental organs. Characters Essential and Adaptive. The cha- racters presented by each animal may be divided into such as have been inherited from its ancestors, and such as have been produced in the course of its own life by its surroundings. The former are essential, the latter adaptive. If the environments remain the same the adaptive characters of one generation be- come essential in future races. Such induced cha- racters may so modify the animal as to obscure its affinities ; but these difficulties are removed by .a study of its embryogeny. Thus among the parasitic mites of the genus Pentastomum (p. 109), we could not know the true relations of the worm-like adults if we were not acquainted with the limb-beaiing larvae. Classification and Nomenclature. The animal kingdom is a vast assemblage of individuals, and we require to arrange these in larger categories for Organs and Classification of Animals. 1 1 purposes of study. Those individuals which are so far identical in structure as to lead us to believe that they are descended from common parents we speak of as belonging to the one species. Species is thus our unit in systematic zoology, but as two in- dividuals are seldom absolutely identical in all respects specific distinctions must be more or less arbitrary. A group of allied species embodying the same structural ideas is called a genus. An assemblage of allied genera is & family ; a group of related families make up an order ; while related orders make up &dass, and the several classes included in the animal kingdom are united in certain primary categories called sub kingdoms. Systematic zoologists give a Latin name tt each of these, and for convenience each species is designated by a Latin word to which is prefixed the name of the genus. The specific name is generally an adjective, the generic is a substantive, and should be written with a capital letter. Thus the dog is called by zoologists Canis f ami liaris, Canis being the generic, familiaris the specific name. Canis aureits is the jackal, Canis lupus the wolf. That species in a genus which most strikingly embodies the generic characters is the type of the genus. We also speak of the type of a family, of an order, or of a class, the type being that species which displays most clearly the characters of the group ; and for convenience we attribute certain characters to ideal types to illustrate truths in classifi- cation. The type genus usually gives its name to the family j thus the dog-family is called Canidae. Homology. In comparing animals, the most im- 12 Invcrtebrata. portant resemblances are those which depend on common relationship to the types of the class to which they belong. These likenesses are called resemblances of morphological type. Thus if we compare a dog and a crow, we find in both a skeleton, a brain, a skull, four limbs, a heart &c., and we refer them both to the vertebrate type, inasmuch as they both embody the ideas of structure characteristic of vertebrate animals. Each part in one is said to be homologous with the corre- sponding part in the other, the wing with the fore leg, &c. Homology is thus identity of structure irrespec- tive of function, and parts are homologous which represent the same parts in the ideal type of the class. Such resemblances are the bases of classification. Analogy. Likenesses of parts may also depend on similarity of function ; thus the wings of insects and the wings of birds are used for the same purpose, and have certain resemblances. These similarities are called resem- blances of analogy, and they tell us nothing as to the nature of the organs compared. Organs which are homo- logous consist of homologous parts ; and as this is not the case in organs resembling each Leaf insect (Phylhnm sicci- & folium} and its egg. other only in function, we must be careful to discriminate morphological from physiological likeness. In animals which consist of successive segments in a chain, like centipedes or lobsters, each segment is FIG. 6. Classification and Distribution of Animals. 1 3 composed of parts similar to those of the neighbouring segments. Such parts are said to be serial homologues, for example the fore and hinder limbs of quadrupeds. These homologies may be complete or incomplete. Mimicry. Animals of definite geographical areas often resemble each other in some respects ; thus they may be similar in colour, mostly white, or spotted, or striped, or brightly coloured. Sometimes animals mimic in shape or colour the leaves and twigs on which they live (fig. 6), or the prevalent colour of the herbage. Thus the Kakapo or ground parrot of New Zealand, which can hardly fly, is like in plumage to the mottled green vegetation among which it lives. The ptarmigan and other birds become white in win- ter, and are thus inconspicuous among the snow. Sometimes an insect mimics in appearance another of different nature living in the same district. In such cases the insect imitated is one which, from its dis- agreeable secretions or sting, is not a favourite prey of insect-eaters ; hence the mimicry protects the imi- tator, who is usually rarer than the insect imitated. CHAPTER III. CLASSIFICATION AND DISTRIBUTION OF ANIMALS. Sub-Kingdoms. The animal kingdom includes eight sub-kingdoms. In these we observe a certain pro- gressive increase in complexity, from one end of the series to the other ; but they do not make a linear series, as the highest organism of each is in no degree related to the lowest organism of the next sub- kingdom, being usually much more advanced and specialised, so 14 Invertebrata. that in point of complexity the sub-kingdoms overlap each other. Sub-kingdom i. Protozoa ; the simplest animals, which have neither body cavity nor nervous system, and thereby differ from the other sub-kingdoms, which possess at some time in their life-history a body cavity, and which are collectively called Metazoa. Sub-kingdom 2. Polystomata ; sponges, which have an internal cavity with one outlet and many in- lets, bounded by a bilaminar wall. Sub-kingdom 3. Ccelenterata; jelly-fishes and sea anemones, having a stomach cavity and a body cavity as an outgrowth therefrom, and a radiate symmetry. Month bordered by tentacles armed with thread cells. Sub-kingdom 4. Echinodermata ; star-fishes and sea-urchins, also radiated, with a body cavity separate from the stomach, a nervous system, and a system of water-tubes which are agents in locomotion. Sub-kingdom 5. Vermes ; worms which are bila- terally symmetrical, and composed of successive simi- lar segments, with no jointed limbs, and with a water- vascular system which has no locomotory function. Sub kingdom 6. Mollusca ; oysters, snails, &c., possessing soft bodies enveloped in a leathery mantle, no jointed limbs, a circulatory system, often an external shell and an asymmetrical nervous system. Sub-kingdom 7. Arthropoda; crabs, lobsters, spiders, and insects, which have bodies made up of suc- cessive segments, with a symmetrical nervous system, an external skeleton, chitinous or calcified, and jointed limbs. Cilia are never present. Sub-kingdom 8. Vertebrata, including fishes, Classification and Distribution of Animals. 15 reptiles, birds and quadrupeds, which have an internal skeleton, a brain and vertebral column. This one sub-kingdom includes the most complex of animals whose structure requires more minute examination than does that of the other sub-kingdoms. We will in the present volume consider the seven invertebrate sub-kingdoms. In comparing these sub-kingdoms, we speak of forms as being high or low in organisation according to the degree in which special parts are appropriated for the discharge of special functions. We also notice that no organ appears for the first time in animals in a state of complexity, but on the contrary, there is always in lower forms a prophetic foreshadowing of it in the modification of some part already existing. Distribution. Every species of animal is limited to a definite geographical area. Thus the earth's surface may be divided into regions, each characterised by special inhabitants, and the collected animals of any region we speak of as its fauna. As a rule, life increases in amount in any country with increasing, and dimi- nishes with diminishing temperature. Tims the fauna of a tropical exceeds that of a temperate region. The number of animals is also larger when the difference between the winter and summer temperature is small, than in a country with the same mean temperature but with a greater range between maximum and minimum. Moisture is also favourable to animal life, and the fauna of a moist exceeds that of a dry region, other things being equal. Many animals live in places from which light is excluded, as in caves ; these have rudimental eyes, and 1 6 In vcrtebrata. are white or colourless. Many large caves, like those of Kentucky, Adelsberg c., have thus peculiar blind inhabitants. Sometimes the presence of one animal prevents the diffusion of others ; thus in Africa the tzetze fly renders whole tracts uninhabitable by oxen and horses, which are destroyed by its poisonous bites. The fauna of a limited area of a continent usually exceeds that of an island of equal size in its number of specific forms; and the fauna of an island lying near a continent generally resembles that of its neighbour- ing continent. Oceanic islands or those isolated by very deep straits have often remarkable faunae of their own, e.g. the Galapagos and New Zealand. Tropical species are, as a rule, more limited in range than are those of temperate climates, and simpler animals are usually more widely distributed than are the more complex. Fresh- water inhabitants are the fewest specifically, and as a rule are simpler in organisation than allied forms inhabiting the sea. The fourth sub-kingdom has no fresh- water representatives ; the second has only three, and the third only six species living in this medium ; while the others are not very numerously represented in fresh water. The sea is the home of nine-tenths of the inver- tebrates (if we exclude insects), and there are also definite ranges of extension to be noticed in the cases of marine species. The conditions limiting specific life in the sea are depth, currents, and temperature. Terrestrial animals are the most specialised, and Classification and Distribution of Animals. 7 have organs in a more concentrated condition than in their aquatic allies. Parasitism. Some animals pass their lives within or on the bodies of others, and this condition induces striking alterations in structure. In some cases the intruder collects its own food independently of his nost, being thus only indebted to him for house room ; of this nature are the sponges which live rooted on crabs, or the barnacles on the skin of the whale. The second series of intruders are fellow commoners with their hosts, feeding on the food which their entertainer collects ; while in a third class the parasite is a pen- sioner on the body of his host, feeding on his sub- stance. Such forms are true parasites. In all these conditions there is a diminished necessity for locomotion and for food-capture on the part of the parasite ; so the organs of motion, of sense, and of nutrition retrograde, but as the parasitic condition involves difficulties in the continuance of the species, the organs of multiplication are enor- mously increased in size and complexity. Extension in Time. Species of animals have limited ranges in time as in space, their specific longevity depending on the constancy of the physical conditions to which they are adapted, hence the changes constantly occurring in these tend either to extinguish or alter specific forms, and in the history of life on the earth the old types have been constantly dying out, and new types better adapted to their environments survive. In any locality there is a general resemblance between successive faunae, though each usually shows a progress in specialisation on its Dredecessor. Invcrtebrata, CHAPTER IV. SUB-KINGDOM I. PROTOZOA. General Characters. The constituent animals 01 this sub-kingdom are animals of extreme simplicity, con- sisting for the most part of un differentiated protoplasm. None of them possess a nervous system, sense organs, nor a body cavity, nor do we find differentiated organs present in any of them. These may be grouped around five typical forms into the following classes. FIG. 7. Foraminifer, Globigerina bulloides, magnified seventy diameters. This form is found floating in tropical and temperate seas. Class 1. Rhizopoda. In the fine white sand on the sea-shore or in the mud of the sea-bottom there are Protozoa. 19 to be found minute calcareous shells of varying forms (figs. 7, 8), ranging from -^th to -^th of an inch in diameter. Each shell consists of many separate cham- bers, arranged either one after another in a straight line or in a single or double spiral, or even grouped in more complex fashions. Each chamber is separated from its neighbours by a* partition which is pierced with one or many holes whereby the several chambers communicate with each other. The shell-substance is either white and porcelain-like, or glass-like and more brittle, and pierced not only in the partitions but over its whole surface by numerous holes. On account of these perforations these little shells are called Forami- nifera (hole bearing). The animals which build these wonderful houses are exceedingly simple in their structure. The interior of each chamber in a fresh state is filled with proto- plasm which is jelly-like, highly contractile on being irritated, and not only extends through the holes in the shelly wall but coats the outside of the shell with a glairy external living layer. This layer has no defi- nite uniformity of outline, but is constantly changing its shape by sending into the surrounding water radiating protoplasmic processes which are incon- stant, rapidly retracted, disappearing by being taken into the homogeneous matter of the animal's body, and coalescing when they touch each other. To these the name psetidopodia (false feet) has been given. These little creatures live on any minute organic particles with which they come in contact, and their mode of feeding is simple ; when the ray-like pseudo- podia touch ft particle of which they seem to approve 20 Invertebrata. as a prospective meal they converge around it, and touching each other coalesce, and draw the particle within the body proper, in which it is digested. As these creatures are homogeneous or nearly so, any one spot is as suitable for the protrusion of pseudo- podia or for the taking in of food as another, but FIG. 8. Rotalia Veneta, a Rhi/opod, showing the pseudopodia. usually the processes are most numerous opposite the holes in the shell. As the protoplasm includes its food in the manner described, foreign particles and fine granules become enclosed in it derived from the undigested parts of the food. Sometimes drops of water or of thin fluid may be seen in the protoplasm like little bubbles ; these are called vacuoles (fig, 14), and they with the gran- Protozoa. 21 FIG. 9. ules circulate actively in the body mass ; obscure con- densed points or nuclei also exist, and the name of the class is derived from the root-like spreading of the pseudopodia. A few allied forms are shell-less. Mode of Growth of Rhizopoda. Those Rhizo- pods that separate lime from the sea-water to form shells, begin the process while they are young single masses, and they increase by budding, each bud forming on the newest end of the last bud ; consequently the per- fect animal consists of a rod-like or spiral set of chambers, each chamber being a new, undetached bud. Some buds become quite separate and grow into new individuals. In a few cases each bud becomes detached, so that the animals always remain of one chamber. Shell-forming Rhizopods are occasionally aggre- gated in great masses and sometimes at great depths in the ocean. Such seems to have been their habit in past times, and many of the chalky limestones consist of the accumulated shells of extinct Foraminifera. Monera. Some related forms are even simpler than these Rhizopods, as they not only want the power of house-building but have no nuclei, and are thus the simplest conceivable living beings, mere specks of living jelly (fig. 9). Of these naked forms, some authors make a separate class under the name Monera. Two forms of Protozoa. Protamceba pri- tuitiva, the simplest living animal: Mago- sphcera planula, a compound form. 22 Invertebrate. Fig. 10. Class 2. Protoplasta. In the slowly running waters of ditches, or in bog pools, are found curious creatures in many respects reminding us of the naked Rhizopods. These amoebae as they are called, are little masses of protoplasm, moving and taking food by means of pseudopodia. On close inspection many particulars will be noticed, in which they differ from those simple creatures which we have already ex- amined. Thus their pseudopodia are blunt, and do not freely coalesce, on touching each other ; the granules and vacuoles are not uniformly distributed through the protoplasm, but are for the most part in the central region, while the outer protoplasm is fir.ner. We also notice a denser central spot in the body, to which the name nucleus is given, as can be seen in each component mass of Magosphsera (fig. 9) ; and one or more little clear spaces may be seen occasionally to contract and ex- pand alternately. Thus in the group of organisms of which the" pseudopodia, amoeba is the type, protoplasm has be- come partly differentiated, that is, some parts have assumed characters which the simple protoplasm did not possess. On account of this first trace of the development of tissue we call this group of One of the Sun anima _ nuclei, vacuoles, &c. Protozoa. 23 animals Protoplasta (first tissue). The amount of this differentiation is in some scarcely recognisable, while in others, the sun animalcules or Heliozoa (fig. 10), there are many nuclei, and each of the fine ray-like pseudopodia exhibits distinctly an inner axis of the granule-holding protoplasm and an outer layer of firmer material. These animals multiply by division, not the simple throwing off of pseudopodia, but a division preceded by curious mole- cular changes. In modes of feeding, &c., they resemble the Rhi/opods, with which they are often united. Class 3, Gregarinae. A group of curious parasites, the Gregarince, manifest a similar process of differentia- tion taking place in their life-history. These minute creatures are found in the digestive canals of beetles, earth- worms, &c., and in their mature states they appear as elongated bodies with a firm outer wall which never be- comes protruded, and consequently does not allow of the formation of pseudopodia (fig. i, A, B). This outer stratum may itself consist of A . Gregarine from the two layers; while internally the *JHfc i * i * protoplasm contains a solid nucleus. *-}^s*e from the dragon-fly. Ptxiuia At a certain stage in its existence focuia. P c. Boat-shaped body the adult Gregarine becomes almost or Pseudo-naviceiia. , . - . . , D. Amoebiform body globular and quiescent, loses its set free from Pseudo- nucleus, and its internal material navicella - becomes aggregated into many boat -shaped bodies, contained within the firm outer layer through which they eventually burst ; each of these boat-like bodies (fig. uc) consists of a rigid outer case and an 1 inner Invertebrate*. FIG. 12. particle of protoplasm. The former soon gives way, and the inner portion, freed from external restraints, moves actively by pseudopodia like a Rhizopod (fig. n, D). On reaching a suitable nest this amoeboid particle undergoes further development, and becomes a Gregarine like its parent In size these parasites range from the T V tn to f r< ^ s f an inch. Class 4. Radiolaria. On examining the material brought up from ocean bottoms, there are frequently found small and beautifully sculptured shells, differing from those of the Foraminifera in that they consist of silica, not of lime, and hence they are compara- tively indestructible by maceration in acids, by which process they can be isolated from the mud wherein they are found. In pattern these shells fre- quently consist of symmetrical, radia- ting rods, united by a variously patterned interweaving of threads of silica, the whole making a network often resem- bling flower-baskets, disks, and perfor- ated spheres, hour-glasses, or helmets. The animals which form these exquisitely ornate little shells are found to be comparatively simple, and in many respects allied to the Rhizopods, FHnty Shell of a as they send out fine thread-like pseu- RadHjUmjm EU- d opO( ji a f rO m the surface laver. The cyrtidium lagcna. deeper protoplasm is enclosed in a central membranous capsule, perforated with holes, and it contains fat, cellular masses, pigment and often a central vesicle or sac with striped walls ; curious Protozoa, yellow cells exist in the outer protoplasm in almost all species, considered by many to be parasitic plants. In size these Radiolarians are from ^ to -g^th of an inch in diameter, the larger forms, however, are not single individuals but clusters united into compact colonies, each component individual having its own central capsule. Most of these are found floating on or in the waters of the sea. Some oceanic forms have no skeleton, and are described under the name sea- glue (Thalassicolla). Some allied forms, destitute of central capsule and of yellow cells, are found in fresh-water bog pools in this country. These form connecting links with the Protoplasta. CHAPTER V. SUB-KINGDOM i. PROTOZOA continued. Class 5. Infusoria. If we place under the micro- scope, water in which animal or vegetable matter has been infused for six cr seven days, especially in warm weather, we see that the fluid contains minute, actively moving creatures ranging in size from J-th to the 2^Vo tn of an inch in length. They are mostly oblong in shape and their rapid locomo- tion is due to the action of fine vibratile cilia which clothe, either the whole FIG. 13. blce * s - 26 Invertebrate*. surface or else special areas of it ; sometimes a few of these processes are rigid and act like little feet, or else they are all equal and fine, invisible during their active exercise owing to their rapid rate of motion. The outer layer of their body is a firm cuticle which covers a differentiated protoplasmic lamina con- FIG. 14. Paramtzcium aurelia, an infusorian, showing the contractile vesicles (s\ tndoplast (t\ oesophagus and mouth (o\ cilia and vacuoles. taining one or more clear spaces or contractile vesicles, which when watched appear to expand and contract regularly, pulsating like a heart, aerating the proto- plasm. Within this layer is a more fluid mobile proto- plasm containing granules, vacuoles and two peculiar solid bodies called respectively endoplast or nucleus and endoplastule. Protozoa. 27 Near one end of the body there is usually a funnel- shaped mouth opening into the inner protoplasm, where digestion takes place, as in Rhizopods ; the un- digested particles are ejected at a spot where the outer wall seems deficient, and which sometimes is a distinct opening. These animals multiply either by fission like most of the other Protozoa, or else the nucleus breaks up into egg-like masses which seem to develop into new infusoria. This condition is preceded by the forma- tion of a mucous mass around the animalcule, which becomes quiescent, losing its cilia. The Vorticella, or bell animalcule (Fig. 13, c) is a common form fixed by a slender footstalk, which on FIG. 15. FIG. 16. Noctiluca jniliaris, a marine luminous Acincta mystacina. animacule, showing its flagellum. irritation instantly contracts into a spring-like spiral and the ciliary crown around the mouth of the bell becomes introverted. Another common form, Ophry- dium, has an outer gelatinous envelope, and as division proceeds, this keeps the broods together so that they 28 Invertebrata. sometimes form masses of more than an inch in dia- meter, which are often found floating on standing water. The commonest forms of Infusoria are the slipper animalcule (fig. 14), the boat-like animalcule or u- plotes (fig. i), and the hay-infusion animal or Colpoda, but almost every infusion has its own form of animal. Several groups of microscopic animals are allied to the Infusoria. Some of these are called monads and are mouthless nucleated bodies with one long cilium. Another of these is Noctiluca (fig. 15), a globular creature about -^th of an inch long, with a short obtuse vibrating flagellum or filament and a mouth, but whose interior consists of netted protoplasmic threads whose meshes are filled with water. These organisms are among the commonest of those to which the sea owes its phosphorescence. Other minute forms, called Acinetse (fig. 16), are small, stalked masses whose surface is studded with radiating, retractile tubular suckers, through which they suck the juices of their prey, CHAPTER VI. SUB-KINGDOM II. SPONGES (POLYSTOMATA). Metazoa, All animals above the Protozoa possess an internal body-cavity, the wall around which is made up of two primary layers, often with difficulty discriminate in the lowest forms ; and there is either one terminal mouth into the cavity, or, as in the case of the sponges, many lateral pores communicate there- with. PolystomciTa. 29 Characters of Sponges. The common toilet sponge is a representative of a group of animals whose affinities are not easily understood. On ex- amination with a magnifying glass it will be found to consist of irregularly branching and re-uniting threads of a highly elastic material, so arranged that the inter- spaces between the finer branches appear as pores or canals, which, from the nature of their walls, freely communicate with each other. On examining the surface of a spongy some large holes will be seen, which, on being cut into, are found to be the extremities of wide FIG. 17. spaces or tubes ; these divide within the sponge-mass into smaller canals, which again divide and subdivide until finally they end in the fine canals whose terminations are the minute sur- face pores between the superficial fibres of the mass. The walls of these spaces are themselves full of small pores in the interstices of the fibres which form the sub- stance. This horny mass is really the sponge skeleton, having the same relation to the sponge animal that the SpicilleS Of Radio- Ascetta, a calcareous sponge larians bear to the soft parts of those creatures. We can most easily understand the nature of a sponge animal by examining such simple forms of the group as may be found encrusting sea-weeds or stones on our own shores. These are nearly cylindrical, 3O Invertebrata. attached below (fig. 17), and having a single wide open- ing above, the osculum. The outer wall is a continuous protoplasm with many nuclei, the inner is built up of separate cells, each bearing a single long cilium sur- rounded by a collar-like ridge. The pores traverse these two layers, and when the outer wall thickens they lengthen into canals. These canals may be uniformly lined by flagellate cells, or the ciliated cells may be restricted to dilated areas of the passages. In the wall and around each of the pores are needle- like spicules of carbonate of lime, usually united in threes, and arranged in a radiated manner like the three legs of the Manx coat of arms. Sometimes they are in pairs or in fours. When a cluster of these simple vase-like sponges grows closely pressed together, the outer walls of the several animals partly coalesce with each other ; the pores of each animal communicating with the inter- spaces, which then often appear branched. Thus a complex canal system grows up, according to the degree of thickening of the wall and the amount of coalescence of the separate elements of the clusters. In a living sponge, currents of fluid set in through the minute pores on the surface, setting out in large streams through the osculum. These currents are kept up by the waving of the flagella which bedeck the protoplasm masses that line the canals, and as these currents traverse the canals the small organic particles which they carry in are taken up by the cells of the wall in the same manner as food particles are swallowed by Rhizopods. Thus each cell nourishes itself directly from the surrounding water. Polystomata. 3 1 Spicules. The skeleton of most sponges consists not only of the homy material with which we are familiar in the toilet sponge, but of spicules of silica of various shapes embedded in the horny mass, resembling pins, needles, clubs, crosses, anchors, hooks, wheels, &c. In others, siliceous spicules alone make up the skeleton, which has no horny matter. There is a calcareous skeleton in the simple group just described. Reproduction and Classification. Sponges multiply by division, either natural or artificial. That is, if we cut up a living sponge into many small pieces, each can grow into a perfect sponge. Other modes of growth or reproduction are by contin- uous budding, by the formation of free buds, usually arising in autumn and growing in the ensuing spring, or else by the formation of eggs which have been found in autumn in many forms, and which develop in the following year Sponges are classified according to the material of the skeleton and the shapes of the spicules. Thus there are calcareous, horny, and siliceous sponges. The last class is the largest and includes some remarkable forms, such as the boring sponge (Cliona celata), which pierces holes in old oyster shells on our sea-shore, and is known by its pin- shaped spicules. The remarkable Neptune's cup (Raphiophora) is closely allied, though very dissimilar in shape and size. Hyalonema, the glass rope, from Japan and Portugal, has long twisteii siliceous spicules. Euplectdla, the exquisite Venus's flower-basket, from the Philippines, is now well known as an ornament, and exhibits a most wonderful interweaving of siliceous spicules. Spongilla, the common green fresh- water Invertebrate*. sponge of lakes and rivers, is a familiar form, and each autumn it will be found to display the formation of winter gemmules or free buds. Halichondria is the common leathery sponge found on our sea shores, and Grantia (Sycandra) is the compressed white calcareous sponge found pendulous from rocks, or adhering to sea- weed near low-water mark. CHAPTER VII. SUB-KINGDOM III. CCELENTERATA. CLASS I. HYDROZOA, JELLY-FISHES. General Characters of Hydra. The common Hydra (figs. 2 and 1 8), an inhabitant of our stagnant pools, is the type of the third sub-kingdom of animals. This voracious creature rarely exceeds half an inch in length, and possesses a cy- drical body having the mouth at one end and a sucking disk for voluntary attachment at the other. Around the mouth are six, seven, or even ten slender contractile arms, capable of rapid motion, and about as long as the body, or even longer ; these can be seen actively engaged in seizing prey and dragging it into the central mouth (fig. 1 8 T). The body is com- posed of two membranes, an outer the former, which is called ectoderm (outer skin), making up the whole outer surface, the latter (called endoderm or inner skin) lining the interior Hydra am^antiaca , the orange hydra, showing its tenta- cles and reproduc- tive organs. and an inner Hydra. 33 FIG. 19. of the body, which consists of a simple stomach cavity from which the effete matters are ejected by the mouth. On watching the process of feeding we notice that the arms exercise a power over living prey far greater than we could anticipate from their size ; and on clo&e inspection these tentacles (as the arms are called) are seen to be covered with minute oval sacs, whose outer thin walls (fig. 19 B), are easily burst by pres- sure, and when this occurs a long whiplash-like filament (c) which lay coiled within the cell, is suddenly projected, thus ren- dering the tentacle a formidable organ for seizing prey, their action being either mechanical, or by virtue, possibly, of some poisonous fluid. The cells of the outer layer in Hydra have, projecting inwards or towards the endoderm, slender thread-like contractile processes, acting like muscular fibres. The ectodermal cells resemble nerve A, cells, and hence to this stratum the * name neuro-muscular cell-layer is given. Reproduction. Hydras in early summer send off from near the base of the body small buds (fig. 2), which grow rapidly, each deve- loping a mouth at its free end, together with a crown of tentacles, then, being detached, it assumes a sepa- rate existence. Sometimes a second crop of buds D ?' J r i irea i~ ce u undis " turbed. C, the same v/iih the filament pro- truded. 34 Invtrtebrata. arises from the first bud before it has detached itself from its parent. Later on in the season, eggs form within the Hydra, beginning as modified cells of the outer layer. These FIG. 20. Sertularia cupressina, one of the sea-firs ; a natural size, /; magnified. The figure shov, s the stem, or coenosarc of the colony, with its polypites. burst through the surface (fig. 18, o\ become free, and in spring they shed their outer layer and proceed to develop into new Hydrae, which in course of time give rise to buds. Sub-Class 1 : Hydromedusse. The lower ccelente- rate animals are built on the pattern of the hydra, they Jelly fishes. 35 only differ in the details of their organisation and arrangement. In nearly all the marine forms, the hydra-like animals are united in clusters or colonies on a simple or branched stalk (fig. 20). In each of these colonies every hydra-like organism is called a polypite, and the common stalk of the colony, which is tubular, is called the ccenosarc. These colonies we find on the sea-shore as branch- ing tree-like growths on sea-weeds, often exceeding six inches in length, and some of them are known as ' sea- firs/ Each group begins its existence as a single poly- pite, rooted by that extremity of its body which is oppo- site to the mouth. From this root a stalk of ccenosarc grows upwards, and on this stalk new polypites form, each like a hydra in structure. New polypites arise as outgrowths from, and are structurally continuous with the common stem of the colony ; and so the stomach of each is continuous with the tubular centre of the stalk. On this account there is a community of nutri- tion in the colony, a few actively-feeding polypites being able to make up for the laziness of others, which participate in the nourishment taken in by their more active neighbours. Medusoids. In these colonies some polypites are set apart for the production of eggs for the multiplica- tion of individuals (fig. 21, c, D). Such polypites are much altered in form, and they frequently become detached from the stem and float about freely, often as little jellyfishes about the size of a pea ; they are called medusoids. With a fine muslin surface-net in any of our seas, many forms of these little creatures may be captured, especially in summer or autumn; 3 6 /;/ vertebral a. each appears somewhat umbrella-shaped, and the margin of its disk or swimming bell is adorned with FIG. 2t. Different stages of Meduspids : A, rooted hydroid colony, natural size ; B, polypite magnified, showing tentacles and reproductive polypites in five successive stages of development ; c, D, reproductive medusoids, detached, showing the marginal tentacles, the eyes, and the veil. little, naked, coloured specks supposed to be eye 5 :, and also with marginal tentacles. A central mouth exists on the under side of the swimming bell, opening Growth of Hydroid Colonies. 37 into a stomach, from which four tubes radiate to the margin of the bell, where they are united by a circular FIG 22. Physalia or Portuguese Man of War : a, b, c, three stages of growth : a', air- sac or float ; TT polypites, T tentacles ; d tentacle showing the clustered masses of thread-cells. canal, in the walls of which, or within the wall of the stomach, the eggs are formed. 38 Inverted rata. Unlike as this jellyfish may appear to be to Hydra, it is in reality a polypite modified by a widening and thickening of the body wall at the base of the tentacles, and an elongation of the mouth into a central stalk. These free-swimming jellyfishes are thus not distinct animals, but only detached parts of hydroid colonies, and the eggs produced by them first give origin to small ciliated, infusorium-like but multicellular bodies, called planulae (fig. 19, A) which after a short period of freedom settle on stones or shells at the sea-bottom, and develop into primary polypites and the common stem of a new colony, from which some buds becom- ing specialised and detached form, in turn, medusoids. This life-history is an example of an alternation of gene- rations, the progeny of the egg resembling, not the im- mediate egg-producer, but the form which preceded this. Divisions of Hydromedusae. Of fixed Hydroids there are three orders : i. simple forms, such as Hydra ; 2. compound colonies whose stalks, polypites, and medu- soids are naked (fig. 21) ; 3. compound colonies whose stalks and polypites are covered with a horny casing or perisarc, as in the true sea-firs (Sertularia, fig. 20). Sub-Class 2 : Siphonophora. Floating on the sea there are frequently found colonies of hydroid poly- pites, not unlike those of the sea-firs in structure, but whose common stem instead of being rooted, swims by means of enlarged and altered polypites, whose stomachs are undeveloped, and whose bodies are dilated into swimming bells. From these, the coenosarc extends, supporting the nutritive, protective, and re- productive polypites. Some possess in addition a sac filled with air which acts as a float and aids the Jellyfishes. 39 swimming bell in locomotion. This is seen in the Portuguese Man of War (fig. 22), whose purple crested air sac and long tentacles are well known to sailors, as by their large thread-cells they inflict dangerous stings on those who incautiously touch them. These FIG. 23. a Disk-bearing Jelly-fishes ; a, Rhizostoma ; b, Chrysaora. swimming and floating colonial Hydroids are called Siphon-bearers (Sifhonophora). Sub-Class 3 : Discophora. The large Jellyfishes whose translucent bodies are so often thrown on the shore by the receding tides, are the representatives of a sub-class characterised by possessing single polypites 4O Invertebrata. depending from the centre of a large disk or umbrella varying from an inch to three feet in diameter. The mouth of the polypite is surrounded by lobate tenta- cles (fig. 24), and from the stomach there pass eight or more branching radial canals which are united peri, pherally by a circular canal. The eggs are produced in pouches of the stomach-cavity, and each of these on ripening emits a ciliated germ like an infusorium but multicellular (fig. 24 b\ which after a brief locomotory existence settles down on some solid body at the sea bottom (c}> and develops a hydra-like animal with Medusa turita in different stages of life ; a perfect adult form reduced T \ i 6, c, d hydra-like staye ; *?,_/"nssion stages ;^one of the disks separated from f, and growing into adult form. branching canals which, elongating, forms a little rooted colony (d). Each of these hydra-like forms becomes marked with transverse farrows (e) which, deepening, dip into the interior of the body dividing it into a ver- tical ' pile of saucers/ of which each discoidal segment becomes in process of time free as a new medusa. This is a good example of alternate generations. Jellyfiskes. 41 In the common Aurelia aurita^ whose disk can easily be recognised by the four ring-like, violet ovaries, there are marginal tentacles and also eight little pigment spots and clear vesicles (eyes and ears) symmetrically disposed around the disk, each covered by a little lobe of the umbrella margin. There is no trace of a marginal membrane or veil, such as exists around the mouth of the bell of hydroid medusae (fig. 21, c, D). Thread Cells and Nervous System. The stinging power of these ' sea nettles ' is due to their armature of thread cells, often grouped in batteries on the tentacles. When a hydroid is cut in pieces, each piece may produce lost parts and become a perfect animal if a portion of the margin be preserved, but if not, repro- duction is very uncertain. In these medusoids we meet for the first time with an area of sensitive tissue acting like a nervous system, and connected with the margin of the disk. If this be preserved motion continues to take place, whereas if the margin be cut away motion ceases. Many medusas are phosphorescent, emitting light from the whole surface, especially from their margins. Sub-class 4: Ctenophora, These are more complex pelagic forms, with a degree of bilateral symmetry, and having on the surface eight radial rows of comb -like plates armed with cilia, which act as rowing organs. Summary. The Hydrae and jellyfish es which constitute the class Hydrozoa possess a central stomach cavity into which a single aperture of en- trance leads. They are also armed with thread- cells and possess a body wall of two layers. Invertebrata. FIG. 25. CHAPTER VIII. ACTINOZOA. SUB-CLASS I. ZOANTHARIA : SEA ANEMONES. Struct'ire of a Sea Anemone. In many of the rock-pools around our shores there are to be found the exquisite forms which are the types of this class, and which, like animated flowers, may be seen expanding their sensitive petal-like tentacles in search of the materials that constitute their prey. On the retreat of the tide those that are left un- covered by the water contract, and appear as little, rounded, firm, gelatinous masses, attached to rocks and stones by means of a flat suctorial disk. The body of one of these when expanded is somewhat cylindrical, hav- ing a free extremity which bears the mouth, and an attached end, which is usually capable of volun- tary detachment. This extremity is sometimes called the foot. On mak- ing a transvqrse section the body appears like a double tube ; the outer tube is the body wall, the inner bounds the stomach, body-cavity into which the by a narrow aperture. Vertical section of common Sea ane- mone, Actinia uiesembryauthe- uiiim. in mouth, m' primary mesentery m" secondary mesentery, e ecto- derm, e 1 endoderm, t tentacle, /', ovary, d disc of attachment, f body-cavity. and between them is the stomach-sac opens below Sea Anemones. 43 The body-cavity is lined by endoderm and extends upwards into the hollow tentacles, each of which is in fact a tubular prolongation continuous with this cavity. In life, when the body is expanded, the space between the stomach and the body wall contains sea-water which also distends the tentacles, but on irritation the contraction of the outer wall drives out this fluid, which escapes in minute jets through the terminal pores at tha extremities of the tentacles. The body cavity is divided by vertical partitions which pass inwards to the outside of the stomach-wall, and thus divide the outer chamber into a series of smaller compartments which radially surround the stomach, below which they all communicate with each other. There are five or six such large parti- tions extending the whole length of the body wall, which are called primary mesenteries, and between them are smaller partitions in equal numbers, called secondary mesenteries, between these, there are often still smaller tertiary mesenteries, twice the number of the primaries, and in some related forms other orders of intermediate partitions exist, still farther sub-divid- ing the body cavity, but each set shorter than its pre- decessors. The outer surface of a sea anemone, and especially of the tentacles, is richly covered with thread cells, which, when burst, are sometimes thrown off as a continuous slough. This can be seen when an ane- mone is imprisoned in a bottle of sea-water, and in the same condition we notice that as the water be- comes less able to support the life of the creature from its loss of oxygen and of material for food, that 44 Invcrtcbrata. the body becomes enormously swollen by the inordi- nate amount of water which the anemone takes in. Multiplication. Sea anemones occasionally mul- tiply by division, but in general they increase by the development of eggs, in the thick cord-like ovaries on the surface of the mesenteries. These eggs are emitted by the mouth, and from them arise minute, ciliated embryos which become saccular. Anemones can be multiplied also by artificial divi- sion, and if an anemone be cut horizontally the mouth end still continues to eat, and finally develops FIG. 26. Caryophvllia f.iscwilata, a sclerodermic coral. The left side of the figure shows 'the coral denuded of soft parts; on the right the animal matter is shown, while at the upper part several of the polyps are seen projecting. a foot, while the foot end may (but seldom does) con- tinue to live and may ultimately develop tentacles. One experimenter produced by his sections an ane- mone with a tentacle armed mouth at each end. Corals and Coral Building. Most sea anemones Coral Builders. 45 are solitary ; a few only form colonies by continuous budding, and by the formation of a uniting crenosarc. In sea anemones proper, there is no deposition of an indurating material, but in tropical seas there are closely allied animals which abstract lime from the sea-water, and lay it down in one of two ways, either in the animal's tissues, or else in the centre of the ccenosarc, and around the body, outside the foot of each separate polyp. Hard masses thus formed are called corals ; and all the reef-building corals, the Madrepores and Oculinas, are examples of the former kind, or tissue-depositions. The deposit may be in the body wall, in the mesenteries of the polyps, or in the coenosarc. Growth in these colonial forms takes place either by the formation of buds which remain continuous and may spring from various parts of the original stem, or else by fission, but in this case the new polyps remain connected together. Owing to these different modes of growth there is much variety of shape and structure among the hard parts of different corals. Large masses of coral, which are called reefs, are usually found in the seas of such climates as have a winter average tem- perature over 60 F. and where the water is clear, and not mixed with mud or fresh water. They abound chiefly between the depths of i and 50 fathoms, and vary in form according to the shape and condition of the sea-bottom. A few of the coral-building animals are solitary, like the little Caryophyllia of our own seas. Some of the commonest forms of corals brought to this country from the tropics are the mushroom-shaped 46 In vertebra ta. lamellar simple Fungice, the richly perforated com- pound Madrepores, and the brain-corals, or Alceandrince. Sub-Class 2, Alcyonaria. The sea often casts on our shores large, yellowish, gristly masses, known by the fishermen as l dead men's fingers/ but technically named Alcyonium, which are types of the second sub- class of Actinozoa. On placing this Alcyonium in sea- water the surface sends forth from each pore a little crown of tentacles. These are seen to be in circlets around the mouths of minute polyps, and they differ from the tentacles of sea-anemones in two respects ; first, they are in multiples of four, usually being eight in number, and, secondly, they are pinnately fringed, that is, evenly toothed and lobed around the margin, each little tooth having a hole at its tip : otherwise the organisation is of the same type as that of a sea anemone. Minute calcareous spicules are abundantly scattered through the mass, and in some allied forms, these, together with hard horny matter, make up a con- tinuous, coral-like, foot-secretion in the axis of the coenosarc, as in the fan-corals or Gorgonias. In the precious red coral of the Mediterranean, this axis is of stony hardness ; and in Ists, calcareous and horny joints alternate with each other in the central axis of the stem, thus combining firmness and flexibility. The red organ-pipe coral of the Indian Ocean, with its table-like partition and its green polyps, belongs also to this group. The feather-shaped sea-pens, which are nearly related to the Gorgonise, are not rooted, but have the extremities of their stems buried in sand. Recapitulation. All the animals which make up the sub-kingdom Ccelenterata show a radiated arrange- Echinodermata. 47 ment of parts, the bodies being formed of a series of symmetrical segments around a central axis. In all of them, the body wall of each individual animal is made up of two membranes, an outer ectoderm and an inner endoderm ; all are aquatic, and, with about six exceptions, marine. They all have a central stomach, a mouth at one pole, surrounded by tentacles which are armed with thread-cells, these latter being almost universal in the sub-kingdom. When a ner- vous system exists it is as an obscure ring, and is related to the margin at the base of the tentacles. The two great classes of Ccelenterates, Hydrozoa and Actinozoa may be contrasted thus : the former have, but one internal cavity ; the latter have a central stomach cavity surrounded by a separate, though communicating body cavity, and the egg-producing organs open into this second space. The two great sub-classes of Actinozoa are A, Those with simple tentacles in multiples of 5 or 6 : Sub-class i, Zoantharia. B. Those with pinnate tentacles in multiples of 4 : Sub-class 2, Alcyonaria. CHArTER IX. SUB-KINGDOM IV. ECHINODERMATA : ENCRINITES AND STAR- FISHES. General Characters. Sea-urchins, star-fishes, and sea- cucumbers make up a natural assemblage of animals, called Echinodermata on account of the spiny skins 4 8 Invertebrata. found in most species. They display a radiating sym- metry, but either in the larval or in the adult condition there are traces of a bilateral disposition of parts. A body cavity is always present, containing within it the digestive organs, and the stomach cavity never com- municates with this surrounding space in the perfect or adult animal, as it does in the Coelenterates. The Echinoderms possess a nervous system of radiating FIG. 27. Section of the purple Sea Urchin (Strongylocentrotus lividus) ; a, anus ; a?, oesophagus ; i, intestine ; s, one of the rods of the tooth apparatus ; m, muscles of the jaws ; /, vessels of the sucking feet ; p0, extremity of the water vessel ; ca, water-vascular canal on the ocular plate ; v, ovary. threads, united by a ring of nerve-matter around the mouth, and some of them exhibit pigment masses, which are supposed to be simple eyes containing the ends of nerve fibrils embedded in them. Calcareous matter is deposited in the skin, either in the form of spicules, or of plates which, by being jointed together, build up a shell or outer ' test ' for the body. When one of our common Echinoderms is put into a. vessel of sea-water, locomotion can be seen to take FeatJier Stars and Stone Lilies. 49 place by means of numerous little tubular processes which project through holes in the surface of the shell, each of them ending in a little sucking disk. There are five pairs of rows of these feet in most species, and these, by attaching themselves and then contracting, draw the body of the animal along. Each of these pedicelli, or tube feet, as they are called, is hollow, and contains water, and there are five long tubes in each Echinoderm, which pass meridionally, and by fine vessels convey the fluid m into the pedicelli from a tubular ring which surrounds the mouth. To the tubular system which supplies the little feet, the name ambulacral system is given. This arrangement of water-vessels connected with locomotion is peculiar to Echinoderms. The sub-kingdom consists of four chief classes. CLASS I. Encrinites (Crinoidea). These animals abounded in former times in the seas of our globe, but they are now for the most part extinct. The name is derived from the resemblance of many of the fossil forms to the flower of a lily, the infolded arms having a petal-like appearance. With a few exceptions, they are not, in their adult state, free and capable of loco- motion, as are all other Echinoderms, but are fixed on a jointed calcareous stalk. Though when first hatched from the egg all Crinoids are free-swimming ciliated bodies, yet very soon they settle down, develop a stalk, and become rooted (fig. 28). There is a cen- tral mouth, surrounded by a circle of movable arms, which are often branched, and between these arms are little plates pierced by holes through which the ducts from the egg-producing organs open. In general E Invertebrata. appearance these Crinoids look like star-fishes fixed on central jointed stalks, and they are furnished with feathery ambulacral feet on their upper surface, which as they cannot serve for locomotion seem to act as gills for breathing. The commonest of the living Crinoids FIG. 29. Embryo of the Feather Star, showing its stalked, encFi- nite-like stage. Rosy Feather Star (Antedon fosaceus), adult or free con- dition. becomes free in its adult stage, and has a singular his- tory. Beginning its life as a free- swimming embryo, it soon becomes fixed, and appears as a stalked orga- nism (fig. 28), but, after existing in this state for a short period it loses its attachment, becomes free, and forms the exquisitely tinted rosy feather star, with ten to forty arms, found in our seas. CLASS II. Starfishes (Stellerida), (fig. 3, p. 7). The form of the animals of this class is expressed by Starfishes. FIG. the name. They all possess a central disk from which five to twenty arms radiate. The surface is generally roughened with stiff ridges and spiny points (paxillcz), and under this integument is a layer of calcareous plates ; each arm has on its under surface a groove in which lie the ambulacral vessels, and the nerve-cord, which is tubular; and many starfishes have imperfect red eyes at the end of this ambulacral groove. On our sea-shores there are two kinds of star- fishes to be found; the first kind or Brittle-stars have a rounded, or five-sided flat disk at the centre, and slender, jointed, snake-like arms, which, as they do not contain processes of the viscera, the animal can break off when irrita-- ted. These brittle-stars are chiefly found on sea- weeds. The other and commoner kind of star- fishes have thick, flat- tened triangular arms which are continuous with, and not jointed to, the disk. These have a mouth in the middle of the under surface of the body, and around it on the skin are curi- ous little spines whose T ^jS^J). pluteus of the Bn ' ttte - star extremities are movable, ', mouth; *, stomach; .9, calcareous skeleton. and two- or three- bladed, like little pincers. To these little grasping or- gans, which assist in seizing the prey, the name E 2 t(2 Invertebrata. pedicellaricz (' little feet ') is given. From the central stomach, which communicates directly with the mouth, long blind pouches extend into the arms (usually two pouches into each arm) thus increasing the size of the digestive sac. There is an ambulacral ring around the mouth, and radial vessels extend from it into the arms to supply the tube feet ; and to convey sea water into the central ring there is a canal, usually rilled with sand, which starts on the dorsal surface of the disk from a spot where there is a wart-like, finely perforated plate, which from its likeness to a piece of coral is called the madreporiform plate. Through it, as through a sieve, the sea-water niters into the sand- canal, and thence into the ring around the mouth. Besides the ambulacral vessels, there exist in star- fishes fine vessels on the surface of the digestive cavity, which unite to form a second vascular ring around the mouth ; this second system is one of blood- vessels directly concerned in the nutrition of the body. The larvae of starfishes, on leaving the egg, appear very dissimilar from the adults, looking like little easels, and are hence called ////&/ (fig. 30) ; those of the brittle-stars have a delicate calcareous skeleton, which is wanting in those of the common five-fingered starfishes, which in many respects seem to resemble the comb-bearing Hydrozoa. In their adult states these starfishes move mouth-downwards and are extremely voracious, attacking molluscs, dead fishes, and other kinds of animal matter in the sea. Sea Urchins. 53 CHAPTER X. SEA URCHINS AND SEA CUCUMBERS. CLASS III. Sea Urchins (Echinoidea). The globulin or heart-shaped sea-eggs, found along our sea coasts, are representatives of the next group of Echinoderms. In these the surface is covered with movably jointed spines, each of which shows on section a beautifully reticulated structure which varies in each species and the attached end of each spine is hollowed to fit on a tubercle (fig. 27, p. 48) on the hard shell or corona beneath, with which it thus forms a ball-and-socket joint. On removing the spines FIG. 31. the shell is found to consist of numerous flat angular plates, arranged in 20 meridional rows. This corona has the mouth at one pole, which in the living animal is undermost and the excretory orifice at the opposite extremity : and from mouth to The a P ical end of the she11 J } ... . of Echinus esculentus. apex the Shell IS divided intO ambulacral plates, Winter- ."..', ,, ambulacral plates, opposite ten meridional segments, five of i the dark space is the i i . / T . j mad reporiform plate, ^anal Which COnSlSt Of plates pierced opening, c ocular plktes, d with holes for the ambulacral ovarian plates. feet, and five of imperforate plates ; these are placed alternately, and each segment, perforated or imper- forate, consists of two rows of plates (fig. 31, a, b). The mouth is surrounded by a soft area of skin bearing modified spines, modified tube- feet and pedi- 54 Invertebrate. cellariae, somewhat like those of starfishes ; the oppo- site or aboral pole is surrounded by five plates, each placed at the end of one of the imperforate meridians, and each pierced by the end of the duct from one of the five large ovaries or egg-secreting glands which lie within the shell between the ambulacra. Between these ovarian plates and at the end of the ambulacra are five smaller plates, each pierced by the end of the radial nerve threads, and bearing a little eye-speck ; these are called ' ocular plates, 3 One of the former set of plates is always unsyminetrical, swollen, and finely pierced with holes, being in fact the combination of an ovarian plate with the madreporiform tubercle at the end of the sand canal as in starfishes. Sea Urchins have ambulacral vessels just like those of starfishes, with a tubular ring around the mouth and five branches, one along the inner sur- face of each set of perforated plates, through the holes in which the tube feet project. Most Sea Urchins have the mouth armed with a com. plex system of teeth, five of which are placed around the orifice with their points directed to- wards it, each being situated in the centre of a wedge- FIG. 32. Tooth apparatus of the Sea Urchin, showing the arrangement of the muscles. Sea Urchins. 55 shaped jaw (fig. 33), which itself consists of two sym- metrical halves. Twenty-five accessory pieces are ap- pended to these parts, and the whole apparatus is moved by thirty muscles (fig. 32). This apparatus is fixed by FIG. 33. Jaws of the Sea Urchin, A, two jaws seen laterally ; B, exterior view of a single jaw ; /$, rough surface of jaw ; t, teeth ; , radius ; k 1 , lateral piece of alveolus ;/, edge of jaw ; z, lotula. The epiphysis is shown below /. muscles and fibrous bands to calcareous loops which project inwards at the mouth end of the shell, and :t can easily be dissected in the common sea urchin. The larvse of sea urchins are pluteus-like, containing a calcareous skeleton. Two types of sea urchins are found in British seas. One, like the common Echinus esculmtus, is globular or slightly flattened, with ambulacral areas extending from pole to pole, and with the mouth and anus at op- posite poles ; the other type, exemplified by the heart- urchin (Echinocardium cordatum)iowsM& on sandbanks, has the anus not opposite but approximated to the mouth, which in our native forms is toothless, and the ambulacral rows not extending from pole to pole, but in petal-like areas on one surface of the shell alone. 56 Invertebrata. CLASS IV. Sea Cucumbers (Holothuroidea). Singular, elongated or cylindroidal animals, closely allied to sea urchins, but without either spines or hard test, make up this class. They are found among the tangles or in moderately deep water along our shores. The body-wall is muscular, and contains in its surface- layer calcareous spicules, resembling anchors, wheels, &c. The mouth is surrounded by plumose ten- tacles, and, on the surface of the body are rows of tube-feet, more irregularly disposed than in sea urchins. The intestine is moderate in length, and ends in a small aboral sac or cloaca, into which also open (in most species) two very remarkable tree-like organs which lie in the body cavity, and which are adjuncts to the water- vascular system for purposes of Fro breathing. In organisation they resemble sea urchins, and often reach very large sizes. In tropical seas some of these Holothurians are inhabited by little parasitic fishes, and even in temper- ate regions small shelled mollusks are found in or I -on their bodies as parasites. I One species of Holothurian, the Trepang, is imported in large quantities from NE. Gwwwwtr^^^^ One of the Australia into China, where it is regarded as a luxury of diet. Recapitulation. Echinoderms are characterised by the presence of highly differentiated tissues, a dis- Echinoderms, Worms. 57 tinct nervous system, a digestive canal which in the adult is separate from the peritoneal or body cavity (though in the embryo, the body cavity arises as an outgrowth from the primary intestine), a radiating symmetry tending to become bilateral, and a water- vascular system whose little tubular feet or offsets act as locomotory organs. The chief sub-types may be tabulated thus A. Body stalked at some period of life, ambulacra! feet not locomotory = Class I. Crinoidea (mostly fossil). B. Never stalked, star-like, with ambulacra as organs of locomotion. = Class II. Stellerida. a. Arms jointed to the disk, not containing viscera = Order I. Ophiuroidea (Brittle Stars). b. Arms not jointed, containing viscera Order II. Asteroidea (common Star- fishes,). C. Never stalked, globular, disk-like or heart-shaped, with a continuous test Class III. Echinoidea. D. Never striked, elongated, with a soft integument containing spicules = Class IV. Holothuroidea. The Echinodermata are all marine, and are never united together into colonies. CHAPTER XI. SUB-KINGDOM V. VERMES. WORMS, though often mean and uninteresting in external appearance, are yet in many respects among 58 Invcrtcbrata. the most curious forms in the animal kingdom. They are elongated, soft-bodied creatures, which have their organs arranged in a bilaterally symmetrical manner. In many of them the body consists of successive seg- ments, arranged in a chain, each intermediate segment being like its preceding and succeeding neighbours. There is a nervous system in most forms, consisting of two or more nerve knots about the pharynx and a cord prolonged backwards along the under side of the body, beneath the digestive canal. A water- vascular system exists in some form or other in all worms, but it has never any connexion with the function of locomotion. It usually consists of a sys- tem of tubes, one pair in each of the successive segments of the body, opening by one extremity on the surface and by the other end communicating with the body cavity. These sets of tubes are commonly known among the higher worms by the name of segmental organs. Blood-vessels often exist, and sometimes con- tain coloured blood, but there is no true heart, and the colour does not depend on the existence of minute floating coloured corpuscles in a colourless fluid, as is the case with the blood of vertebrates. The com- mon earth-worm and leech may be taken as well- marked examples of the sub-kingdom. Many worms are parasitic and live within the bodies of higher animals ; among these the circulatory, water-vascular, and digestive systems become rudi- mental, the nervous system remains undeveloped, the body cavity often vanishes, and the reproductive organs alone are fully represented. This sub-kingdom includes the following classes : Planar ians. 59 FIG. 35. CLASS I. Turbellaria. The simplest worms with which we are acquainted are found on the sea- shore, under or adhe- rent to stones, or else in fresh-water pools, as small, ciliated, flat- tened soft bodies, which glide with a slug-like motion over wet surfaces, or swim by the vibrations of their cilia. These Turbellarians (so call- ed from the commo- tion produced by their cilia in the water around them) have a mouth placed gene- rally beneath, not at the anterior extremity* and the part of the digestive canal im- mediately within the mouth is protrusible as a kind of proboscis. This COn tains, in SOme Polycelis Iczvigatus, a common Turbellarian. 7 a. mouth ; b, buccal cavity ; Proglottis, or perfect joint of Tapeworm (Ttznia- .Ibling So!ium ) x 2 ^ rt> water-vascular tube ; b, vulva ; Of distinct Se2f- ^> uterus ; d, spermatic gland ; e, vesicula semi- & nalis \f y vas deferens : each Of B, Head of Teenia inedio-canellata x 20. which contains a complete egg-producing apparatus. When we remember that these worms may attain the length of 25 feet, and that there are at least twenty perfect joints in a foot, and that each joint can produce many scores if not hundreds of ova, we can form some idea of the amazing fecundity of these parasites. The growth of the individual takes place from the head, so that the oldest segments are those which are most remote from it and the newest are the fine joints close thereto. The life history of the tapeworm is curious. The eggs are protected by a very firm horny capsule and thus they can maintain their vitality for long periods of time, and can resist maceration and even short 62 Invertcbrata. exposures to high temperature. Being set free from the capsule on entering the digestive organs of some animal with its food or drink, the embryo, a little oval body armed in front with weak hook-like or boring spines, travels through the tissues of its host. On reach- ing a suitable site it anchors, and the body dilates into a sac full of water. In this cystic condition the animal may remain stationary for a length of time, and by budding the number of cysts is capable of a rapid in- crease. When the flesh of an animal containing such cysts is eaten by another, the liberated saccular worm loses its outer wall, and its inner portion lengthens and in a short time becomes a true tapeworm. In most cases the life cycle requires two kinds of animals as hosts, one in which the larva or scolex state is pro- duced and one for the perfected worm. Thus the human tapeworm has its cystic stage in the flesh of the pig, the condition of pork called * measly ' being due to these cysts in the muscles of the pig. Similarly, the tapeworm of the dog develops from cysts found in the hare ; that of the cat from cystic worms in the mouse, that of the fox from cysts in the field-mouse, &c. In Ireland, the commonest human tapeworm has four suckers but no hooks on its head (fig. 36 B), and is known as Tania mediocanellata ; its larva inhabits the muscles of the ox. In Russia and Switzerland, the human tapeworm is quite a distinct species, with very flat body, no hooks, and two long grooves on its head in place of suckers ; its larvae live in the waters of certain lakes, and their ciliated embryos develop within the bodies of fresh-water fishes, and thence they gain entrance into the human body, Flukes. Trematodes or Flukes. The second order of sucker-bearing parasitic worms consists of the c flukes/ FIG. 37- FIG. Distouia lanceo'atuni) the liver fluke. a, anterior sucker ; b, posterior sucker; c, digestive canal ; d, yolk-gland ; e, ovary ; /, posterior terminal pore of the water-vascular system ; .g, duct of yolk-gland to oviduct ; h, lateral tube of water-vascular system; z, /, sper- matic gland ; k, spiculum. Oxyuris vermicnlaris, the common threadworm of children. The central tube is the digestive canal, the coiled tube is the ovary. met with in the liver of the sheep, and of allied forms ( n g- 37) ; these are not united in chains as in the 64 Invertebrata. tapeworms, but each consists of a single segment bear- ing one or two suckers (a, b}. In many respects they resemble the Turbellarian worms, but are not ciliated and often present formidable armatures of recurved hooks. They are like tapeworms in the development and complexity of their ovaries, and many of them show in their history alternations of generations as curious as those of their relatives the tapeworms ; for example, the larvae of some liver flukes live for a time free, in water, and develop within their bodies little cylindrical worms, which are set free on the bursting of the wall of the parent, and in turn enjoy an inde- pendent life. Within these worms again there may form another brood of internal buds, which also grow, burst their envelope, and become for a time free, but soon attach themselves to some soft aquatic animal in whose body they become encysted, to develop finally into the mature forms when their first host is eaten by some larger animal. Thus the flukes found in water-fowl have their larvae in water molluscs, &c. To these flat sucker-bearing parasites the name Trematoda is given. CLASS TIL Nematelmia. These, the commonest forms of parasitic worms, are cylindrical, tapering to each end, and possessing a body cavity (fig. 38.) They are never divided into successive joints, although their surface may be finely ringed, and there is almost always a digestive canal with an outlet, as well as a mouth. The round worm, often found in the small intes- tines of children, is a good example of the order. It is about seven or eight inches long, ringed on its surface, with the mouth at its anterior end, surrounded by three little lobes ; from this, a tube, the osstiphagus, passes T/ireadzuorms 65 to the stomach, which is a small suctorial muscular cavity, communicating by a straight intestine with an anus which is not terminal. Beside this common round Ascaris lumbricoides, the human digestive canal is the occasional dwelling-place of two other worms, one of which, Qxyuris vermicularis, is a small thread-like worm (fig. 38), the other Trichocephalus dispar, much more common in the human caecum, has a very slen- der neck and a thicker body. A species closely allied to the last named is the Trichina spiralis, a minute worm found in the flesh of pigs, calves, &c., which when introduced into the human body, often multiplies rapidly in the voluntary muscles of the system, causing dangerous and even fatal symptoms. These worms are as prolific as their fellow parasites, and the early stages of many live for a time in water, from whence they enter into the bodies of their hosts, and in those whose life-history we know, the free and parasitic conditions appear very dissimilar. It has been supposed and with reason that many of the free Ne- rnatelmians found in stagnant pools are early stages of parasitic species. Gordiacea. The horsehair-like thread - worm which is found in rainwater pools is an example of a second order of round worms. This remarkable animal begins life as a little larva living in mud or in water pools ; it is armed with a boring spine, whereby it pierces into the body of a beetle or other aquatic or terrestrial insect ; here it becomes encysted, and, having grown in this condition to a considerable length, often ten times as long as its host, it becomes free and aquatic and produces its eggs. So rapidly do some F 66 Invertebrata. of these multiply, as, for example, the common Mermis albicans, that they have given rise to the belief that they have fallen as ' worm-rains.' These worms are called Gordiaceae and are distinguished from the other round- worms by the rudimentary condition of their digestive canal. They are also remarkable .for their extreme tenacity of life, as they can be dried into hard brittle threads and yet appear lively and active on being moistened. CLASS IV. Acanthocephala. The * thorn-headed ' worms are rounded, or cylindrical, each with a pro- trusible proboscis armed with many recurved hooks. They are remarkable for the total absence of the mouth and intestine in their adult condition. The common- est species are found in the intestines of swine, &c., with their heads buried in the substance of the wall of the digestive tube. CHAPTER XII. NON-PARASITIC WORMS. CLASS V. Wheel- Animals, Rotatoria. On tracing th? development of the more complex free worms we find that the larva, after emerging from the egg, ap- pears as a free-moving creature, with circlets of vibrat- ing cilia around its extremities, these ciliary lobes being in some forms large and rounded (fig. 47, A). In rain pools and ditches, small creatures are fre- quently met with which resemble the larvae of worms, Spoon-worms and Lecclies. 67 but which remain permanently in this ciliated condition. In these the ciliary lobes are prominent and rounded, acting as locomotory organs, and from the rapid vibra- tion of the cilia which clothe them they FlG - 39- seem like rotating wheels, hence these little creatures are called Rotatoria. They are microscopic in size, varying from 7 ^th to \\\\ of an inch in length, but from the exquisite transparency of their bodies the details of their organisation can be seen by the aid of the microscope. The male rotifers are few and small and have no digestive canal ; the females have a com- plex nutritive system, and many species are provided with an organ of mastication like an anvil acted on by two hammers. These animals can bear much ill usage, and are capable of reviving again on being moistened, after having been almost com- pletely dried up. On irritation the trochal disks (fig. 39, c) can be retracted into the cavity of the body, from which they are gradually * Rotifer vulgar- protruded again on the cessation of the is. a, mouth- lobe ; b, eye- ochal stimulus. Some rotifers are rooted ; others possess a forceps posteriorly, whereby they can hold on to foreign bodies ; others again are contained in a vase-like sheath, into which they can re- tract themselves, on being irritated. CLASS VI. Spoon-worms or Squirt- worms, Gephyrea, These are interesting marine worms whose spots ;c, troch disks ; d, chew- ing organ with calcar in front; , malleus ; f, alimentary ca- nal ; g, gastric caeca ; k, de- veloping em- bryos ; /, anus. 68 Invertebrate, elongated or sac-like bodies contain a long tortuous intestine, ciliated inside and outside. They exhibit no division into segments, nor have they locomotory pro- cesses of any kind, and they never have any calcareous FIG. 40. FIG. 41. Tooth and Muscles of Leech. Embryo Leech. Adult Leech. Mouth of Leech, showing the anterior sucker and three horny jaws. or siliceous spicules in their skin, although sometimes there are a few bristles scattered on the surface. The mouth is at the anterior end, and it is provided with a protrusible proboscis, sometimes of great length. CLASS VII. Leeches, Hirudinea. This group of worms is exemplified by the common horse-leech or by the medicinal leech. They are soft-bodied annulated worms which live parasitically on the outside of verte- brated animals, from which they draw their nourish- ment. Their bodies are composed of segments, which are indistinctly or not at all marked from each other Leeches. 6 9 on the surface, but can easily be distinguished within, as the organs of the body are arranged in successive groups. Leeches have at their front end a sucker, and some have a second suctorial disk at the hinder ex- tremity, and several species are even pro- vided with lateral suckers. The mouth is generally situated in the front sucker, and it is armed with three horny jaws or plates (fig. 41) with serrated edges. These plates act as teeth, enabling the leech to make incisions in the skin of its host through which to suck the blood. The digestive canal is straight and consists of a central tube with a row of eleven blind pouches along each side (fig. 42, B) which can become distended, hence the body can take in a great quantity of blood. There is a pair of nerve-ganglia in each segment of the body ; the first (fig. 42, c) of these is compara- tively large and made up of several smaller ganglia grouped together ; the successive ganglia are united into a chain by fine filaments, and they lie on the ventral or under side of the digestive organs. Leeches possess proper blood-vessels in which their own nutritive fluid circulates. Their water- vascular system takes the form of a series of scgmental Reproductive organs of leech. B Di- gestive canal of leech. C Nervous system of Malacobdella. The two cords are more separate than in other species. 70 Invertebrata. organs or tubes opening laterally, one on each segment. The egg-producing organs are very complex (fig. 42 A). Locomotion takes place by the suckers : the hinder one being fixed, the animal elongates itself and, fixing its front sucker, sets free the hinder one, then shortening its body it proceeds in a similar manner. Leeches can also swim, and when so progressing the body becomes flattened by the contraction of vertical muscular fibres which run from the dorsal to the ven- tral surface, and then by undulating movements it advances like a wavy ribbon. Medicinal leeches are principally imported from Hunsrarv and Sardinia. CHAPTER XIII. NON-PARASITIC WORMS. CLASS VIII. Bristle-footed Worms (Chaetopoda). The worms of this class re- semble the leeches somewhat in internal structure, but are not parasitic, and all possess a body cavity (absent in most of the leeches) which con- tains a fluid rich in minute floating corpuscles. The class includes not only the D &Sn^l f earth. worms and their fresh- S^df layJ" ''cj S Water allies With their SCantV plates; rf, segmental organ; A, J arterial, and z, venous blood- armament Of bristles, but also vessel ; g, intestine ; , ovary. the numerous species of bristle-bearing worms, of Worms. FIG. 44. which the lug-bait or the hairy-bait of fishermen may be taken as representatives. These worms have bodies made up of a succession of similar segments, each of which bears on each side a lateral pro- cess or parapodium, divided at its extremity into a dorsal and a ventral bristle-bearing eminence. These are the organs of locomo- tion. The bristles vary much in form, being sometimes shield-like and always are developed in pouches of the skin and capable of being moved by dermal mus- cles : these processes are known as dorsal and ventral oars. The mouth is on the second segment, and is often armed with sharp teeth. The intestine is usually straight and very often has lateral glandular pouches ap- pended to it. There is a vas- cular system consisting of long tubes, dorsal, ventral, and late- ral, generally joined by cross branches in each joint, and containing red, green or white blood. The gills, when present, are usually arranged along the dorsal surface of the body, close to the root of the dorsal oar, Arenicola piscatorum. Lug-bait worm. Dorsal sur- face showing the dorsal oars (notopodia) and gills. and in these the blood is purified by being exposed to the oxygen 72 Invenebrata* held in solution in the sea water. There are also seg- mental tubes opening one on each side of each seg- ment, and sometimes the eggs, which are produced within the body, escape through these canals. The chain of nervous ganglia is also well developed. Some worms secrete a glutinous material from their surface, which cements together sand-grains and other foreign bodies into a tube wherein the animal lives. Other worms secrete from their surface calcareous matter which makes up a tube as a dwelling-house, in which ihe animal is permanently contained. Such forms have the gills developed only on the foremost seg- ments of the body, and have the dorsal and ventral oars of all the other joints rudimentary ; but they possess tentacle-like, branching processes about the head. Of these the common Serpula, whose white calcareous snake- like concretions are so common on the stones and shells on the sea shore, and the Spirorbis, whose minute white whorled shells dot the surface of the shore-tangles, are examples. A few worms are phosphorescent ; many others, like the sea-mouse, are clad with iridescent scales and bristles. The common earthworm has only eight short re- curved bristles on each segment, arranged two in the place of each oar, and not elevated on processes of the surface. The body is closely ringed and tapers from the middle forwards to an acute point in front. The armature of each ring can easily be felt by draw- ing the body of a worm between the fingers from tail to head, although they are scarcely to be detected when we feel the body in the reverse direction, In Worms. 73 beginning to burrow, the worrh lengthens its body and pushes its sharply pointed head into the mass of soil which it is about to perforate, then having insinuated the few foremost rings of its body into the mould, the whole animal contracts in length, its muscular pharynx is driven forward, thus swelling the front of the body in thickness and forcibly dilating the opening made by its fore part, the worm being prevented by its bristles from slipping out of the opening; then it again lengthens its body in front, its setae giving it a fixed point from which to act, and by a succession of such elongations and thickenings it can ' worm ' its way through even a hard gravel walk. The mouth is placed on the second segment, the first being a cephalic lobe or lip, and from it the wide digestive canal extends as a straight tube through the body. This tube is always found full of earth, which is devoured in large quantities for the sake of the organic particles contained in it, the remaining part being passed out and heaped by the worms at the outlet of their burrows, as ' worm casts/ For the better division of the material swal- lowed, the digestive canal is provided with a muscular gizzard about eighteen rings behind its mouth. The eggs in earthworms are produced in two small ovaries in the thirteenth ring and opening on the fourteenth ring. The twenty-sixth to the thirty-second ring in adult worms are white and swollen from the lar^e development of cutaneous glands. Worms are propagated exclusively by eggs, the common belief, that, when cut in pieces, each part is capable of inde- pendent life ; not being strictly true. If we divide an 74 Invcrtebrata. earthworm about its middle, the hinder segment dies after a short time ; the fore segment will probably live and its wound heal. Similarly, if we cut the anterior four or five segments away the small fore fragment will soon die, while the large hind mass will recover. CHAPTER XIV. MOSS POLYPS AND TUNICARIES. CLASS IX. Moss Polyps (Bryozoa). The broad leathery fronds of the tangles along our shores are often encrusted with beautiful lace-like patches of regular and minute patterns. If we put a fresh, living piece of this into a vessel of sea- water, we find that each of the cell-like spots is the home of an elegant little organism which may be seen to protrude through the mouth of its cell a delicate little crown of tentacles. Each colony of these animals con- sists of a common stock, bearing numerous little cham- bers, each of which contains its delicately organised inhabitant. Some of the little creatures become modified into bird's-beak-like graspers with two horny jaws, for the protection of the colony (fig. 45, B) ; others become altered into globular pouches for the reception of the eggs after their extrusion. Each of the dwellers in these little cells consists of a saccular body containing a looped digestive canal, in the bend of which a nerve ganglion is placed, and it is provided with a crown of hollow tentacles guarding the mouth. Most of these moss-polyps are marine and have Tunic at a. 75 a circular protrusible basis, supporting the tentacles ; some few are inhabitants of fresh water, and these have the tentacles on a horse-shoe-shaped basis ; these also have a little lobe or epistome overlapping the mouth, present in only two of the marine forms. Each of the little constituent animals of one of these colonies has its own digestive canal, its own nervous system, FIG. 45. A. Natural size of Acamarchis avicularia, one of the Moss Polypes ; B. Magnified view of one Polype, showing its ' bird's head ' and its crown of tentacles, seated on the bases or lophophore around the mouth. and its own egg-producing apparatus, and these are essentially like the corresponding organs in worms. CLASS X. Tunicata. These also are marine soft- bodied animals, met with in abundance attached to shells and stones among the tangles on our sea shores. They are often called sea-squirts, on account of their ejecting little jets of water from their terminal open- ings when irritated. They appear as irregular or oval 7 6 Invert cbrata. FIG. 46- *,&*'* JsswSWj masses of semi-transparent, often gristly material, and of a whitish, pink, or brownish colour. They vary in length from one to six inches. In each tunicary there are two apertures on the surface; one of these (c, fig. 46) opens into a large chamber whose wall (e) is a vascular mem- brane, and at the bottom of which is the mouth (/). The digestive canal ends at the bottom of a second chamber (;z), of which the lower or hinder opening (/) is the outlet. Be- tween these two chambers, which thus lie over the digestive canal, there is a partition wall which is pierced by many small holes whereby the water which enters into one can pass into the other, thus bathing the surface of the lining membrane, and enabling the blood contained in the spaces in its texture to become aerated. The first chamber (fig. 46) is called the branchial chamber, the second is called the atrial. Between the opening of the branchial chamber and the atrial ori- fice there is a nerve ganglion send- ing a fine loop of branches around Amonroncium argits. a tunicated worm. A, Pharynx, or respiratory portion of the body ; B, stomach ; c, egg-producing organ ; a, tunic ; b, branchial chamber ; d, tentacles, non-protrusible ; h, atrial chamber ; /, ganglion beside which are two eye-spots ; /, liver ; in, digestive canal ; o, heart ; / ' > P" > e gg- s in various stages ; q, ovary ; r, vas deferens. Summary. 77 the mouth. The heart (o) lies at the lowest part of the body and from it the vessels (, air- chambers ; ', body whorl of shell ; c, siphuncle or tube communicating between the air-chambers ; d, lateral mus- cles attached to shell ; e, hol- low arms or tentacles ;_/", ten- tacles ; g, membranous disk at head ; h, eyes ; z, lower wall of funnel. been given. Other species, like the Octopus or sea- spider, have no shell either internal or external. Anatomy. On account of the fore- shortening of A n at o my of Cu tt lefts /ies. g 3 the body the mouth is brought into the middle of the foot in the adult, and hence the name ' head- footed' {Cephalopoda} given to this class. The lobes into which the foot is divided are usually eight or ten in number and are long tapering muscular pro- cesses which in the common species in our own seas vary from a few inches to two feet in length, but in one rare form they attain very much greater size. During the year 1875, a specimen was captured on the Irish coast, with arms thirty feet long. The mouth is furnished with a strong beak, like that of a parrot, with two formidable horny jaws. By reason of the numerous suckers on the arms (a com- mon octopus possessing about 60 on each arm), which seize hold of their prey with a cupping-glass- like tenacity, these cuttlefishes are among the most terrible of marine monsters, and those of large size would probably prove to be more than a match even for man himself. Each of the sucking disks is singularly perfect in its structure. There is a muscular adhesive disk of a circular shape, around whose edge is a hard crown of horny consistence, and in the centre there is a mus- cular retractile piston, whose contraction produces a vacuum, and thus causes a close adhesion of the sucker. There is a large mantle cavity with a strong mus- cular wall. When the animal is swimming it moves with its arms directed backwards and the upper or pointed end of the body forwards; in this position the opening of the mantle cavity or funnel is directed backwards, and the propulsive force which drives the body forwards is the sudden and often repeated con- 94 Invertebrata. traction of the wall of this cavity, which by driving a column of water in the opposite direction with great force, propels the creature by rhythmical jerks, and at a rapid rate. On slitting open the mantle two large gills are seen, one on each side, above and between which is the heart-ventricle. The blood enters the gills by large veins which form dilatations at the base of these breathing organs, then after being aerated, it is col- lected in two cavities called systemic auricles, from whence it passes into the muscular ventricle which drives it through the arteries into the lacunae (or tissue interspaces of the body). Along the hinder edge of the long digestive canal there is a slender tube, whose opening is also at or near the mouth of the funnel, and whose upper end expands into a large spongy- walled sac lying close beside the liver. This sac secretes a brown or black inky material which is poured out in enormous quan- tities when the animal is pursued, and which by ren- dering the water opaque covers the flight of the cuttlefish. At the mouth of the mantle-cavity but not actually connected with it there is a funnel, which when the margin of the mantle contracts forms a narrow tubular outlet for the fluid of the cavity. Cuttlefishes possess a brain, made up of large confluent ganglia around the pharynx, and over this there is a cartilaginous cover, interesting as being one of the first signs of an internal skeleton, like that of vertebrates in the animal kingdom. There is also a large and complex eye, more like that of a vertebrate Anatomy of Cuttlefishes. 95 animal in appearance though not in structure than is the eye of any other group of invertebrates Cephalopods were abundant in former ages, but there are now not more than 230 species living ; of these the Nautili possess a chambered shell, four gills, and many tentacles, while all others have only two gills, and eight or ten sticker-bearing arms. Recapitulation. Having thus very briefly re- viewed the sub-kingdom Mollusca, we may, by way of recapitulation, place in a tabular form the dis- tinctive characters of the group and its divisions. They are all soft-bodied, never distinctly divided into segments nor provided with limbs ; enveloped more or less in a dermal mantle, which often secretes a shell, and their larval stage is usually ciliated, worm-like. The divisions are : A. Having no distinct head, bivalve shells with the valves right and left, heart composed of auricle and ventricle, gills lamellar = Class I. Lamelli- branchiata or Acephala. B. Having a distinct head, univalve shells (at some period of existence) = Class* 5 !!. Cephalo- phora. a. Entirely enclosed in a mantle, secreting a tubular shell = Sub-class I. Scapho- poda (Dentalium). b. Not entirely enclosed in a mantle, swim- ming by finlike processes on the upper side of the foot = Sub-class II. Pteropoda. 96 Invertcbrata. c. Creeping by the foot or swimming, but not by fin-like processes = Sub-class III. Gasteropoda. C. Head-bearing, with the fore-shortened foot around the head, and modified either into tentacles or sucker-bearing arms = Class III. Cephalo- poda, CHAPTER XVIII. STTB-KINGDOM VII. ARTHROPODA (jOINTED ANIMALS). General Characters. This sub-kingdom includes those animals whose bodies are furnished with an ex- FIG ternal hard protective layer, and which bear jointed limbs appended to each segment of the body (fig. 55). The armour-plating of the body is known by the name exoskeleton, to distinguish it from the bones, which form the axis of support for vertebrate animals, to San TLiUms. r which the name endoskeleton is given, The exo-skeleton consists of chitin, a horny substance, which is capable of resisting all re- agents except the most powerful corrosives. This layer is usually coloured, laminated, and to the micro- scope shows very little structure except the numerous fine canals which pierce it from within, to which the name pore-canals is given. The body of an arthropod consists of a chain ot segments, all built on a common pattern, and each Jointed A nimals. 97 one strengthened by the possession of a ring of exo- skeleton consisting of two parts, a dorsal, and a ventral half arch. The limbs are articulated on each side, between the half arches, each segment possessing one pair. These generally remain distinct, even when, as often happens, the segments fuse together, so that the number of constituent segments can often be detected from the limbs, even when the body-rings are united into a continuous shield. The bodies of arthropods are bilaterally symme- trical. They are also remarkable for the absence of cilia at all periods of life. Each limb consists of several joints, each having an external chitinous exoskeleton, containing the muscles which move it. In the simplest arthropods, the limb consists of a basal segment (protopodite) bearing two appendages, an outer and an inner. In the higher forms the limbs are divided into five, seven, or more joints. These limbs are used for various purposes, becoming modified into feelers, water-bailers, jaws, swimmers, pincers, or walking feet. All arthropods have a circulatory system and most of them possess a heart which is a dorsal tube, divided by valves into successive chambers, but there are rarely fine blood vessels. There is a distinct respira- tory system, and a complex digestive canal, except in parasitic forms, and they have all a symmetrical ner- vous system, consisting of a ring of nerve matter around the pharynx, followed by a chain of ganglia in the ventral portion of the body. They are divided into four great classes, i. Cnistacea, including all those that breathe by gills, as crabs, lobsters, &c. ; H OS Invertebrata. 2. Arachnoidea, spiders, mites, and scorpions ; 3. Myriopoda, centipedes, &c.; 4. Insesta. Many arthropods are parasitic, and these are at first not unlike allied non-parasitic species, but shortly after hatching they retrogress, such parts as are not necessary disappear and hence the adult parasites are in their organisation much simpler even than they themselves were in their embryonic states ; but as has been already noticed, the egg-producing organs are much increased in development. The two, four or six foremost segments of the body in arthropods become united to make up a head, which carries sentient organs, such as the eyes, ears, and antennae or feelers, with the mouth. In the head likewise is the large supra-cesophageal nerve-ganglion or brain, which sometimes is large and complex, as in ants. CHAPTER XIX. CRABS AND LOBSTERS. CLASS T. Crustacea, The animals of this class are all water-breathers, usually with a calcified integument, hence the name, either provided with gills, or else with a thin surface layer through which the blood becomes directly aerated. The structure of a crustacean can be most easily learned by examining a lobster or fresh- water crayfish. In either of these the body is divided into two regions, an anterior, covered by a dorsal shield of two pieces one in front called ce> halostegite, and a hinder called omostegitesm& a posterior, consisting of a series of rings, ending in the fanlike tail. There are Segments of a Lobster. 99 FIG. 56. four openings on the under sidethe mouth, the anus in the middle line of the last joint of the tail on the under surface, and the paired openings of the egg- producing organs at the base of the third pair of walk- ing limbs. Through these the eggs are extruded, and are carried in clusters under and around the bases of the hind series of feet. In the large anterior mass of the body, sheltered by the dorsal shield, there are fourteen segments united, comprising the head, thorax, and abdomen. The head segments bear their six pairs of appended limbs, the first pair of which are modi- fied into stalks for the eyes, which are remarkable organs, each consisting of a large number of rods of a crystal- line appearance, each placed at the end of a nerve fibril or thread, and surrounded by a mass of pigment. The of the optic nerve pass in and each fibril ends in its crystal rod, the mass of rods being arranged in a cluster, slightly divergent so as to exhibit a rounded outer surface, over which the chitinous skin extends as a fine, perfectly trans- parent covering. The eye stalk is two- jointed. The second pair of limbs are feelers called the antennules or lesser antennae, consisting of three basal joints, terminated by a pair of slender processes each made up of many little rings ; these are followed Vertical section through the eye of an Insect, showing the stalk or optic nerve, the white radiat- ing lines or secondary optic nerves and the crystal cones. numerous united fibrils the centre of the stalk, IOO Invertebmta. by the third pair of limbs, or the large antennae, con- sisting of five basal joints succeeded by a long feeler. The bases of the fourth pair of limbs are modified into biting-jaws or mandibles, and they bear an internal appendage named the mandibular palp. The fifth and sixth pairs of limbs are also jaws, and are called maxillae ; they also bear rudimental appendages. The three segments that follow the head segments, and are united thereto, make up the thorax, and' their limbs are also in the lobster modified into organs of mastication, and hence are known by the name of foot-jaws; each of these except the first bears outer and inner appendages, but the third pair is of very little use as a chewing organ, but bears a gill as does also its anterior neighbour, the second pair of foot jaws. Following the limbs of the cephalo-thorax, for so the united head and thorax is often called, we come to five pairs of walking legs, each pair being the limbs borne by a segment of the abdomen. These five abdominal segments are also in the lobster covered over dorsally by the dorsal shell, but on the under or ventral surface, their separateness can be very well recognised. The first pair of limbs consist of the pincers or chela. These formidable organs in the lobster are made up of seven joints, the last but one of these is very large, and its outer angle is prolonged into a finger-like process capable of being opposed to the last joint, thus making a grasping organ of great power. In the lobster the two pincers are not quite symmetrical ; one is armed along the edges of the blades of the pincers with rough tubercles, the other with small serratures ; the former claw is prob- Lobster's and Crabs. 101 ably used as an anchoring apparatus, the latter for seizing articles of food. The two succeeding pairs of abdominal limbs are also pincer-like at the extremity; the two following are simply pointed, but still exhibit seven joints. All the abdominal limbs, except the last, carry gills ap- pended to the basal joint, and placed under cover of the dorsal shell. The six movable rings which form the ' tail ' of the lobster, bear laterally limbs adapted for swimming, each made up of a basal part, and two flattened appendages external and internal; the last of these segments not only carries the widely expanded swimmer or tail fins, but bears at its hinder extremity also a single median flap or i telson/ sometimes regarded as a separate segment. These movable rings make up the post-abdomen. The ear in the lobster is a sac on the upper surface of the first joint of the antennule; the gills lie under the hinder and lateral parts of the dorsal shield. The stomach is a gizzard-like cavity with calcareous masses lining its walls, followed by a narrow soft-walled digestive stomach and intestine, below which lies the nerve-cord, and above it is the heart. The crab differs from the lobster not only in shape but in the comparative immobility and small size of its post-abdomen, which is turned in and sunk into a groove. While the young lobster only differs from the adult in the possession of small outer appendages on the walking limbs, and the smaller size of the tail, the crab emerges from the egg in a form utterly unlike the adult, as a little swimming creature with IC2 Invertebrata. a dorsal shield armed with a strong median spine (fig. 57, A) and followed by a jointed post-abdomen which bears no appended limbs. A closely allied animal common on our shores is the hermit crab, which protects its soft, almost limb- less post-abdomen by inserting it into the deserted shell of a whelk, or other univalve mollusc. In these FIG. 57. Stages in the development of the common Shore Crab (Carcinus matnis). A, First or zoea stage ; B, Megal stages of growth. All magnified. . A, First or zoea stage ; B, Megalopa stage with tail ; c, D, advanced ll the pincers are usually unequal, so that on the animal being molested, one can be retracted while the larger one blocks up the passage. The soft abdomen acts as a sucker, whereby the hermit crab retains its hold on its habitation. There are many varieties of form among Crustacea, and those above described are among the most highly organised, all having stalked eyes and ten walking feet. The mantis shrimps have their thoracic limbs fitted for walking, as well as the abdominal legs, so that instead of ten, there are fourteen or sixteen legs. Small Crustaceans. 103 All these forms make up a sub-class of Crustacea named Podophthalmia on account of their stalked eyes. The sand-hoppers (fig. 55), wood- lice, and fresh- water shrimps, make up a second sub-clcss, charac- terised by possessing sessile eyes. Thei-e also have bodies made up of twenty segments, each of \\hich, except those of the head and thorax, has ivs cv,n in- dependent chitinous ring, and the two hinder pairs of foot-jaws are used for locomotion. Some, like sand- hoppers and freshwater shrimps, are laterally com- pressed and have the three hindmost pairs of abdomi- nal feet arranged so that their joints bend forwards while all the other limbs bend with their joints concave back- wards. These are called Amphipoda, to distinguish them from depressed forms like wood-lice, and slaters, whose legs are all directed one way, which are called Isopoda. The king crabs of the Mollucca Islands and of North America form the types of a third sub-class. They resemble the lobster in having the head, thorax, and abdomen covered by a great dorsal buckler, but differ in that there are six walking limbs around the mouth, whose bases are spiny, and compressed, acting as jaws. The eyes are not stalked, and there is a long bayonet-shaped tail, behind the post- abdomen, corresponding to the telson of the lobster. The segments of the post-abdomen have a single dorsal shield over them. In stagnant pools of fresh water little creatures called water fleas can be seen, by the aid of the micro- scope, actively darting about ; these are representatives of a fourth sub-class. They are all minute at the pre- sent day, but in the past ages of the world, larger allied 1O4 Invertebrata. forms existed abundantly ; these fossil forms had three- lobed bodies, and hence are known as Trilobites, and they are only found in FlG. 5 8. . i rrii palaeozoic rocks. Those now living bear gill processes appended to their feet, and hence are known as ' gill-footed ' or Branchiopods. Many closely allied species have the dorsal wall extended in the form yclops quadricornis carrying its egg sacs. o f nn ^irp>lr'nrr Ki' f he small figure is the Nauplius or larva. OI E " ( valve shell, just like the gill covering laminae in the lobster. Those crustaceans which are parasitic are closely related to the water fleas, and undergo retrogression until they become reduced to little sacs with bristles for jaws, with sucker-like fore feet, and often with no trace of segmentation (fig. 59. A), Some live on the bodies of larger crustaceans such as lobsters, others on tunicates, but they are mostly found attached about the gills of fishes. The early stages of these are little, free, marine Nauplii or larvae, with developed jaws and a moderate post-abdomen (fig. 58). Many non- parasitic species remain for their whole life in a state like that of the larvae of these parasites. In all these lower crustaceans the earliest stage of existence after emission from the egg is in the form of a minute oval body with three pairs of limbs and one central eye. This is known as a Nauplius, and it assumes its adult form by the growth of new segments and new limbs. The nauplius stage of Metamorphoses of Crustaceans. 105 higher crustaceans is transitory, and sometimes is passed over before the embryo leaves the egg, and in crabs the form assumed by the newly hatched young is that of a small shield-covered body with two eyes and long jointed post-abdomen (fig. 57, A) ; this curi- ous larva is called Zoea, and by the shortening of its FIG. 59. Pennella, parasitic on the Sunfish. A. Entire animal, half real size ; B, head. tail and the vanishing of its dorsal spine, it becomes changed into its adult form. All crustaceans undergo successive moultings or changings of shell, and during these changes lost parts become restored and the several changes in meta- morphosis can be seen at these periods. Crustaceans part with their limbs easily under circumstances of fright or seizure ; thus if a limb be taken hold of forcibly the animal will probably break it off between the first and second joints in its efforts after freedom. At the next moult, there appears a new limb budding on the soft uncovered body, and when the new shell forms and hardens, a small limb is seen in place of the lost one. Io6 Invertebrata. Some of the metamorphoses of crustaceans are strange ; in none more so than in the barnacles and acorn-shells (figs. 4 and 5, pp. 9 and 10), the lowest subclass of the series. Acorn-shells or Balani, are the little limpet-like shells which encrust the rocks along all our coasts and which can be at once recognised by the opening at the top of the conical shell, which is closed by the lateral valve-like or beak-like plates. Barnacles are commonly found adhering to logs of wood or to ships' bottoms. These begin life as active nauplius-like larvae, which every autumn are to be found swimming along our coasts. This larva at its early moults develops a lateral mantle-fold. At its fourth change in shell, the front of its head becomes fixed by the flattening of one of the joints of the antennae and by the secretion poured out by a gland which, though placed in the body, has its duct open- ing in the altered joint of the antennae. At the fifth moult the eyes and antennae vanish, the head becomes fixed by a broad base of attachment, the mantle-like fold of integument surrounds the body and becomes calcified into a shell of many valves, within which the hinder parts of the body are enclosed together with their six pairs of limbs. These limbs remain free and capable of slight protusion, while the mouth with its mandibles lies at the bottom of the mantle cavity. Some Balani select curious places of residence. Coronula lives on the skin of the whale ; Anelasma often is adherent to fishes, and many others to corals. One closely allied group of degraded forms are para- sites on the abdomen of crabs. To the sub-class Classes of Crustaceans. 1 07 comprising all these forms the name Cirripedia, or bristle-footed, has been given. Recapitulation. We have thus seen that the seven classes of water-breathing, many-jointed forms which make up the class Crustacea are very dissimilar in details. They may be arranged in a tabular series as follows : A. Sessile Crustaceans, often pseudo-parasitic, usually enclosed in a many valved shell = Sub-class I. Cirripedia. B. Free, with a cephalo-thorax and two pairs of tho- racic limbs, none of the feet bearing gills = Sub-class II. Copepoda (fig. 58). C. Free, with the body enclosed in a bivalve shell made of the extended dorsal integument ~ Sub-class III. Ostracoda. D. Free, with no enclosing shell, feet gill-bearing, segments less or more than twenty = Sub-class IV. Branchiopoda. E. Free, with a large cephalo-thorax, small walking limbs, six pairs of which are arranged around the mouth = Sub-class V. Pcecilopoda. King Crabs. F. Free, with a cephalo-thorax, stalked eyes and a body of twenty segments = Sub-class VI. Podophthalmia. A. Thoracic limbs masticatory, ten abdominal limbs alone fitted for walking = Order I. Decapoda. a. With a long post-abdomen : Lobsters = Sub-order I. Macrura. b. With a soft limbless post-abdomen : Hermit Crabs = Sub-order II. Ano- mura. I o 8 /;/ vertebra ta. c. With a short up-turned post-abdomen : Crabs = Sub-order III. Brachyura. B. Some thoracic limbs ambulatory, thus mak- ing twelve, fourteen, or sixteen walking limbs=Order II. Stomapoda. G. Free, with a cephalo-thorax, twenty segments and sessile eyes = Sub-class VII. Edriophthalmia. CHAPTER XX. SPIDERS AND MITES. CLASS II. Arachnoidea. These are terrestrial air- breathing creatures in which the segments that com- pose the head and thorax are united to form a single cephalo-thorax, but their articulated limbs are to some extent represented, and of these, four pairs are usually used in walking. There is an abdomen and very rarely a post-abdomen. Whenever eyes are present they are not compound bundles of crystal rods covered by a common cornea, as in crustaceans, but they consist of separate transparent cones surrounded with pigment and always few in number. There are never any mandibles developed as such, but the mandibular palps are often present, and sometimes, as in scor- ' pions, they form pincers or claws like those of a crab. The antennae are modified into jaws, called chelicera ; the two pairs of maxillary palps form the first and second pairs of walking limbs, while the first and second pairs of thoracic limbs are developed as the third and fourth pairs of legs, and the third pair of thoracic limbs is absent, unless a curious pair of Spiders and Mites 109 comb-like organs in the scorpion may be such. The parts of each limb are like those in Crustacea; the body and its organs are however much shorter than in that class. The nervous system is concentrated, the digestive canal often has blind pouches appended to it. There is an abdominal heart in all, except in a few mites, and there is usually some kind of breath- ing organ. Organs of touch, smell and hearing seem to be deficient. Many of these animals are parasites, either ex- ternal, or internal ; but except in these, there are few in which the young undergo much metamorphosis after hatching. The outer surface is often hard and chitinous, but never calcified. The dorsal surface layer is seldom extended over any of the neighbouring segments or appendages, or when extended it is immovable. Mites. The three chief orders in the class consist of mites, spiders and scorpions. Of these the mites are the simplest and are exemplified by the cheese- FIG. 60. mite, found in mouldy cheese, or the sugar-mite often met with in brown sugar. In these the abdomen is unsegmented, and usually indistinctly separate from the cephalo-thorax. The breathing organs are fine tubes named tracheae, which open cheese-mite on the surface, and break up within the (Acams). body into branches, which admit air into the tissues. The mouth in mites is often proboscis-like or armed with a spiny beak. Most of them are parasites either upon animals or plants. One curious group inhabits the body cavities of vertebrate animals, wherein their worm-like bodies may be mistaken for tape- or thorn- no Invertebrata. headed- worms. They are however easily distinguished by their embryos bearing true jointed limbs, although these are lost in the adults. One form has been found in the contents of the small fat glands on the human face, and another is the cause of the disgusting skin disease known as ' itch/ Other larger forms are the 'ticks' found so commonly on sheep, dogs, bats, camels, &c. Of non-parasitic forms, the little ' red- spider ' so often seen on the sea-shore under stones between tide- marks, and the ' glass- ' and ' garden-mites ' found in damp moss and among vegetables are examples. Spiders. In spiders the cephalo- thorax is joined to the sac-like abdomen by a narrow stalk, and the latter portion is unsegmented and never bears any limb pro- cesses. The tracheae, instead of being bundles of branch- ing tubes, are condensed and flattened, and included in definite spaces, in which the compressed tubes look like FIG. 61. the leaves of a book, the whole laminated organ, on account of its being circum- scribed and lung-like, is called a trachea I lung, and the spiders are often called pul- monary arachnoids on account of their possessing two or four of these organs. Spiders have little clusters of simple eyes on their foreheads, bright small specks usually eight in number and generally arranged in two rows. The antennary jaws have at their inner side the duct of a poison-gland whose secretion they instil into the insects which constitute their prey. The stomach is like a hollow ring from Spiders. 1 1 1 which radiating blind pouches pass off, and the diges- tive tube is short. Near the hinder end of the abdomen in spiders, there is a flattish l spinning area ' upon which open the glands which secrete the web. On this area there are usually three pairs of little wart-like spinnerets ; and numerous small pores, from each of which a minute thread of web-material issues, open on the surface of each spinneret. Sometimes one pair of the little knobs consists of a palp-like process. In the common house-spider (Tegenaria domestica) there are 400 such holes on each wart, hence each thread of the web consists of several hundred strands ; the material is at first fluid, but rapidly becomes hard and chit noid. In commencing to spin, the spider applies the spin- nerets to the surface of some fixed body, and then as it moves away, the material is drawn out. The hind feet press the several strands of the web-thread to- gether, their comb-like claws appearing to be im- portant instruments for this purpose. In the web of the garden spider, whose geometrical nets are frequently seen on old fences and palings, there are three kinds of threads to be noticed, ist. The marginal and stoutest radial threads. 2nd. The intermediate radial threads, both of which are uniform, though differing in size and in elasticity, wherein the secondary exceed the primary. 3rd. The concentric threads which are bedecked at regular intervals with little viscid globules. t Other spiders excavate cavities in the ground ; these ey line with a silky web, and over the mouth of em they make a trap-door lid of alternate layers of 112 Invertebrate*. earth and web united together and hinged by a silken hinge. These trap-door spiders are found along the shores of the Mediterranean, California and Jamaica. Some spin little cocoons or silken cases for their eggs, which they carry about with them, and in protecting which they exhibit great activity. The mandibular palps are never pincer-bearing or used for walking although sometimes long. Scorpions. The scorpions and their allies are characterised by the possession of a long segmented abdomen, followed by a post-abdomen. The mandi- bular palps form pincers, like crabs' claws, and breath- ing takes place by pulmonary sacs like those of spiders. The last joint of the post-abdomen bears in scorpions a sharp claw at its end, perforated by the duct of a poison-gland, and thereby it inflicts painful wounds. A little creature named Chelifer, somewhat allied to scorpions, but with no post-abdo- men nor sting, is often found in old books. CHAPTER XXL CENTIPEDES. GALLYWORMS. CLASS III. Myriopoda. This comparatively small class includes the centipedes, whose long jointed bodies are to be seen rapidly crawling under old rotten sticks and stones and shunning the light. In this country they rarely exceed two inches in length, but in the tropics they reach from six to twelve inches or even more, and their bites are poisonous and severe. One Centipedes. FIG. 62. blus) ' British form is phosphorescent, and another is described as capable of giving electric shocks. The body consists of many segments which, with the exception of the head and the last joint, are similar in appearance ; the head bears the eyes, which are usually simple like those of spiders, and generally in two rows. Near these there are the sensitive, slender, thread- like antennae, consisting rarely of seven, usually of fourteen joints or more. These animals are usually carnivorous and have a strong pair of mandibles with a little jointed palp on each, and two pairs of maxilla, either or both of which are sometimes united together in the middle line, forming a lower lip for the mouth, and neither of these have jointed palps. The hindermost segment of the body has often a pair of long limbs directed backwards. There is a straight digestive canal with a number of tortuous glands appended to it, and a long tubular heart made up of a chain of chambers one in each segment separated from each other by valves. On each side of the body open the mouths of the tracheae or tubes for breathing ; there may be one on each segment or one on every second joint. Each opening is the beginning of an air-tube, which on entering the body branches irregularly, the fine branches freely communicating with each other. To keep these tubes open there is a spirally coiled thread of chitin in their lining membrane, which like a spring prevents them from collapsing, and to keep the mouth from being choked there is usually a raised margin H4 Invertebrata. sometimes provided with little processes. One curious group has no tracheae. Many of these centipedes have minute pear-shaped glands placed along the sides, which secrete a brown irritating fluid, emitting a disagreeable odour. There are more than twenty segments in the body (except in one little species), and each bears one or two pairs of legs, all with six or seven joints like those of a spider or crustacean ; each limb terminates in one or two claws. Subdivisions. There are three orders of these animals, millepedes, centipedes, and pauropods. Millepedes ( Chilognathd) possess two pairs of limbs on most of their segments, a condition due to the union of the true segments in pairs. They have also small antennae of seven joints and tracheal openings in front of the articulation of each leg. They are found in this country in the rotten wood of decaying trees, and when disturbed roll themselves up into balls. Centipedes (Chilopoda) are found under stones in damp outhouses, or in rotten palings. They have but one pair of limbs on each joint of the body, and never more than one pair of stigmata. In these the four segments which follow the head are united into a 4 basilar segment/ the second pair of whose appendages are strong curved poison claws. The native forms are small, but the foreign Scolopendrae are of very large size and their bites are exceedingly severe. The one species of Pauropus is a minute white creature found among decaying leaves, with no tracheae, ten segments and five-jointed antennae. Structure of an Insect. CHAPTER XXII. INSECTS. CLASS IV. Insecta. Insects, the most numerous and the most highly organised of invertebrates are found FIG. 62. Generalised insect. Grasshopper^ showing the structure and composition of an insect's body, a, head ; b, eye ; c, antenna ; d, thorax, foremost segment ; e, foremost pair of legs ; f, middle segment of thorax ; g-, foremost pair of wings ; k, second pair of legs ; i, hindmost seg- ment of thorax ; /, posterior pair of wings ; k, femur of third pair of legs ; /, tibia ; m, tarsus ; ab, abdomen. in almost every conceivable locality on the earth's 1 1 6 Invertebrate. surface. Scarcely a plant exists but it harbours some species of the tribe, and many animals, living or dead, supply food for other species. Insects are usually of small size, and have the four, five or six foremost seg- ments united to form a head. The three succeeding segments form a thorax, which alone bears the legs, one pair on each of its rings, and when wings are present they are borne by the middle and hindmost of these thoracic rings. The abdomen consists of eight segments not bearing any limbs, and followed by one, two or three post-abdominal rings, continuous with and not easily distinguished from the hindmost abdominal segments. A cockroach, a bluebottle fly, and a butterfly may be taken as types of the class. Organs of Sense. The head of an insect bears a pair of compound eyes, and often several simple eyes in a cluster. The former have a cornea or transparent surface divided into many facets, each of the nerve rods having its own pigment mass and its own cornea. In the common house-fly there are 2000 such facets in each eye, and in the dragon-fly there are 28,000 (fig. 56). The head of an insect also bears one pair of antenna or feelers, jointed organs which vary much in shape and structure, being sometimes simple, filiform, comb-like, or lamellar. These are organs of touch and hearing, possibly of smell, and also of communi- cation between one insect and its fellow. Month. The mouth is on the fore and under part of the head, and varies in shape according to the method whereby the insect is accustomed to feed. In beetles, dragon-flies, &c. the mouth is armed with chew- ing jaws. There are two lips, an upper or labrum (fig. Mouths of Insects. 117 65, e) and lower or labium. the second pair of max- illse in the lobster and crayfish, which in most insects are united toge- ther, but sometimes as in cockroaches (fig. 64) and locusts remaining separate. The labium bears a pair of feelers called labial palps (k). Between the labrum and the labium are two pairs of jaws placed vertically, The lower (/) represents Fig. 64. Under side of mouth of Cockroach, a, notch for cervical sclerite ; g, outer edge ofgalea;/^, maxillary palp; t\ ligula ; *, paraglossae ; k, labial palp; /, mentum ; in, submentum ; i, cardo or hinge ; 2, stipes ; 5, lacinia of maxilla ; 6, galea, or sensitive process of max-illa. FIG. 66. 1 Upper side of head of Cockroach. , fenestra or rudimental eye ; ft, antennary fossa ; b, epicranium, or top of the head ; c, compound eyes ; d, clypeus ; e, labrum ; f. mandibles ; g, h, i, k, 5, 6, as in fig. 64 ; A, antennae. Mouth of Flea. Showing the slender style- like labrum between the long mandibles medially, pnu th-^ labial and max- illary palps laterally. 1 1 8 Invertcbrata. so that in acting they move in a horizontal plane. The upper pair are named mandibles or biting jaws, the lower pair maxillae or chewing jaws. The last- named have usually appended to them on each side a pair of small jointed feelers or maxillary palps. In the bluebottle and house-fly the lower lip is lengthened into an elongated gutter-like sheath in which are con- tained the maxillae and mandibles, which are reduced to mere bristle-like processes. In the bee (fig. 77, p. 132) the upper lip and mandible are strong and fitted for chewing, while FlG . 6 7 . the maxillae and lower lip are long and channelled, so that when placed in apposition they make a tube through which the insect sucks in honey. In these crea- tures the lower lip consists of two parts, an upper or tongue and a hinder part or nientum. In the butterfly, the mouth has lost all trace of its chewing func- tion and the maxillae form two half tubes, and when opposed as and the labial palps on each they always are they make up a si'^e of the antennae and J J between the eyes. canal, and being very long and curved, this is sometimes called the proboscis. Each of these maxillae has within it also a fine tube, and thus a transverse section through the proboscis shows three tubes, one medial between the maxillae and one lateral on each side within each maxilla. Behind this proboscis lies the labium, which has usually large palps between which the proboscis lies when retracted ; for, unlike the tube in the bee, this proboscis is freely retractile. Nervous System of Insects. i \ 9 Body. The head is joined to the thorax by a nar- row neck, and this region is generally strong, and the limbs are attached to the under part of the side of each of its three rings. Each limb is composed of five joints : hip (coxa\ a ring segment (trochanter\ thigh (femur, fig. 63, k\ a shin (tibia), and a tarsus of seve- ral joints ending in the claws to which sucking cushions or pads may be appended. The wings are jointed to the middle and hinder rings of the thorax ; these are modified lateral flaps of the body wall, such as exist in some crustaceans ; the thin skin folds of which they consist are supported by chitinous ribs (costa) con- taining branches of the tracheae. Internal Structure. On the sides of each abdo- minal ring are the apertures of the long, finely branch- ing tracheae, which sink into the body and are distri- buted widely among the tissues. Each tracheal tube has a membranous wall strengthened by a coiled spiral chitinous thread which keeps it open for con- veying air from the surface through the body. Each motion of the body by altering the tension of the vessels promotes this method of respiration. The dorsal tubular heart placed in the abdomen, consists of a chain of chambers separated the one from the other by valves. This receives the impure blood together with the new blood from the intestines, and propels it by the chief blood-vessels into lacunae or interspaces between the tissues which are thus nour- ished. The blood is colourless, or green, rarely red. Insects have a large and complex nerve ganglion in the head, and ganglia in all the segments from the head backwards. The head ganglion sends branches I2O Invertibrata. FIG. 68. -to the eyes and antennae, while the thoracic ganglia supply the limbs. Some cave-dwelling insects have no eyes, others have these organs rudimental. The digestive canal of insects consists of a stomach to which the long oesophagus or gullet leads from the mouth ; to this a thin walled sac or sucking stomach may be ap- pended as in butterflies, in others there is a gizzard with hard horny tooth-like pro- cesses, and this is followed by the glandular thin- walled true digestive stomach which ends in. an intestine, whose length depends on the nature of the food, being longer in Nervous system of Beetle, TIT showing central double nerve those that feed on solid than cord and chain of ganglia. matter, and longer in the herbivorous than in the predaceous forms. Glandular tubes opening into the end of the intestine exist in many insects, and from their first describer are known as Malpighian. Some insects are luminous. In the glow-worm (Lampyris noctihicd) there is a large fatty body in FIG. 6 9 . the abdomen richly supplied with tracheae and nerves from which a bright light is emitted. The fire-fly (Elater noctilucus) sends out light from two oval spots on the thorax. Grasshoppers and crickets emit Glow-worm, female i 11* r ^i and male. sounds by rubbing one part of the Metamorphoses of Insects. 1 2 1 body against another, and such have usually a special hearing organ which in crickets and locusts is placed under the knee on the outside of the foremost pair of limbs. Development and Metamorphoses. Insects' eggs have often a sculptured shell (fig. 6), and are laid in such places as are suitable for the supply of food to the newly hatched larvae. For this egg-laying the parent has often an organ formed of the modified appendages of the abdomen or of the post-abdomen. These organs are in the form of bristles, pincers, or saws, and by these the insect prepares the place for and de- posits its eggs ; hence, the organ is called an ovipositor. The young of most insects emerge from the eggs as worm-like animals called caterpillars or larvae. Each of these is a little jointed creature, having a head which bears eyes and a pair of antennae. Its mouth is armed with strong jaws, and the surface is often covered with bristles. Each of the three anterior segments of the body of a caterpillar is usually provided with a pair of little stumpy feet, and sometimes, as in the larvae of butterflies and saw-flies, the hindmost joints have also foot-like processes. Caterpillars are very voracious in their habits and grow rapidly, frequently moulting or shedding their skin. On reaching the limit of size, many caterpillars begin to spin for themselves a case or cocoon. The glands from which this pro- ceeds are two long tubes placed in the abdomen, but opening on the lip, and the material of the cocoon is silk. Those that have no such labial glands also give up eating, and their skin thickening they become fixed and rigid and are known as pupae, or from 122 Invertebrata. their occasional metallic lustre, chrysalides. In this pupa stage the animal lies for a considerable time; this skin then bursts and the perfect insect emerges, at first soft and moist but soon becoming firm and fit for independent life. Caterpillars often differ in structure from the adult insect; thus the digestive canal of the cater- pillars of butterflies is fitted for the digestion of solid food, while that of the imago or perfect insect is only fitted for sucking the juices of plants. The antennae likewise of caterpillars are attached to the front eoge of the forehead shield, and outside the arti- culation of the mandible, whereas ihe antennae of the imago or perfect insect are articulated further forward, and on a plane with the joint at the base of the man- dible ; the antennae of the caterpillar may therefore represent the long antennae of the crab and lobster, while those of the perfect insect represent the anten- nules of crustaceans Insects display an amount of intelligence far superior to that of the lower verte- brates; ants, wasps, and bees, the most highly or- ganised as well as the most intelligent of the class, exhibit a wonderful power in the mode of ordering and governing their communities, and the skill shown in the construction of their habitations is scarcely inferior to that of man himself. There are at least thirteen orders of insects known to the naturalist, a few of the commoner and more interesting representatives of which are shortly de- scribed in the next two chapters. Bugs. Springtails. Earwigs. 123 CHAPTER XXIII. ORDERS OF INSECTS. ORDER I. Rhynchota, This group consists of those insects which either undergo no metamorphoses or a very slight change in FIG. 7 o. the process of growth. Almost all have suc- torial mouths (fig. 70), consisting of a long tubu- lar labium, whose base is open or covered by the labrum like a little lid. The mandibles and maxillae are altered into piercers or bristles which work within the tube. A few, however, like the bird-lice, have hook-like mandibles and chewing mouths. Many of the insects of this order are parasites on animals and Mouth of Bug. Showing the median elongated labium, the four bristle-like styles on the mandibles and maxillae, also, at the sides, the antennae and eyes. wingless, as lice and bugs ; others, the aphides, the small green insects which are so abundant on roses, geraniums, &c., are plant parasites, and often winged. These aphides are marvellously prolific, a single pair being capable in one year of producing a progeny of twenty thousand millions or even more. Some aphides have glandular tubes on the abdomen which secrete a sweet honey- like fluid. This fluid is used 124 Invertcbrata. as food by some species of ants, especially in this country by the red and yellow ants, which can be seen to 'milk' the honey tubes with their antennse and swallow the fluid. Several species of aphides appear to be kept as ' milch kine ' by these ants, and are fed by them apparently for this secretion. Other representatives of this order are the cochineal and lac insects, the * water boatmen ; and ' water scorpions/ as well as the numerous and often brightly coloured field bugs. ORDER II. Thysanura. Fringe-tails, an unim- portant group, consisting mostly of very small crea- tures, sugar-lice, and spring-tails, which live in moss or under stones, in cellars, or sugar stores, and can be seen hopping or springing about, shunning the light. They scarcely undergo metamorphoses, being the most generalised of all insects, and their mouths ar6 suited for chewing. The extremity of the abdomen is prolonged into a forked tail or a pair of bristles, whereby the animal is enabled to progress by leaping. The scales of the bodies of some of these Podurse 01 spring-tails are marked with very minute furrows. ORDER III. Euplexoptera. This order includes the earwigs, which are remarkable for their curiously folded hind wings, that lie folded like a fan under cover of the hard-shielded forewings. They have a masticating mouth, and posteriorly there is a pincer- like post-abdominal appendage in both male and female. The earwig is remarkable for sitting on her eggs to hatch them, and for the maternal protection which the female exercises over her young which resemble her except in the absence of wings. Thrips. Cockroaches. Locusts. 125 ORDER IV. Thysanoptera, or fringe -winged in- sects, including a not uncommon little fly, named Thrips. whose contact with the face in warm weather is often a source of considerable itching from the titilla- tion caused by its plumose wings and bristled body. One species of this order by piercing the immature wheat grain with its bristle-like mandibles causes the seed to shrivel, and occasionally destroys even the corn stalks. ORDER V. Orthoptera, straight-winged insects, includes cockroaches, grasshoppers and locusts. These have four wings, of which the often parchment- like front pair are the smaller ; the second pair are usually large, and when at rest are folded like a fan. The mouth is masticatory and both pairs of maxillae are free. Some of the tropical forms of this order are wingless and assume extraordinary forms, the walking leaves, mantis, and w r alking stick (Bacterium} sometimes resembling dry twigs or bits of branches. In the common cockroach (Periplaneta orientalis] which is a native of the East, the legs are fitted for running and have spiny tibiae, the head is over- lapped by the front segments of the thorax and bears long antennae, and the parts of the mouth are distinct (figs. 64, 65). The wings are very small, especially in the females. The Drummer Cockroach of the West Indies adds to its other disagreeable qualities that of making a knocking noise, which is sometimes sufficiently loud to keep awake the inha- bitants of houses infected with these insects. Troctes pulsatorius, a minute insect found in books, old pic- .tures, &c., also produces a sound which has earned for it the name ' death-watch ' or * death-tick.' 126 /// vertebrata. Locusts are terrible scourges in tropical countries, devouring all vegetation and leaving bare the regions over which they pass. Their body is long and laterally compressed and the long hind legs act as leaping organs. They produce a chirping sound by rubbing the elevated ribs of their wings against the thighs. In the grasshopper and cricket a similar sound is produced by the rubbing together of spots on the wings provided with raised ridges. The white ants or Termites of tropical regions also belong to this order, and build ant-hills of extra- ordinary size and hardness. Their colonies are very complex, and consist of several kinds of inhabitants, females, males, workers and soldiers. The dragonflies, which also belong to this order, have aquatic larvae, breathing by means of tracheal FIG. 71. Larva of Dragonfly, showing the ' Mask.' gills or tuft-like processes of their body-wall contain- ing tracheae, but with no openings. These processes are lost in the perfect insect ; in one American genus, however, these appendages are retained during life. The larva of the common dragon-fly has a long and jointed upper lip, which is folded over the face when at rest and is called the mask, but when the animal is feeding it becomes extended as a formidable tongs- like weapon for the grasping of prey (hg. 71). Fleas. Flics. 1 27 CHAPTER XXIV. INSECTS WHICH UNDERGO PERFECT METAMORPHOSES. THE four orders of insects which follow are small, but contain some interesting forms which deserve a passing notice. FlG ?2 ORDER VI. Neuroptera, nerve-winged insects, including the scorpion-flies (fig. 7 2), snake- flies, and ant-lions. These in their perfect stage possess a mouth fitted for chewing, and four equal membranous wings, Panor pa, or Scorpion-fly. of which the hinder pair are never folded. Few of these insects are natives of this country. ORDER VII. Trichoptera, including the caddis- flies which have hair-clad or scaly unequal wings, the hinder of which are folded. Their larvae agglutinate small shells, stones, straws &c. by silken threads secreted by a small spinning gland placed on the lower lip, and of these they make cases in which they live. Having attained its full size, the pupa fixes its case under water and spins a silken network or grating over each end of it, thus shutting itself in for its pupa sleep, while it does not exclude the water which it requires for breathing. After this stage of rest the pupa by its strong jaws bites through its prison, and after moulting assumes its adult form. ORDER VIII. Strepsiptera includes the curious 1 2S Invertebrate parasites (Sty lops) which live on the abdomen of bees and wasps. In these the males have four wings, two in front, small and twisted, from which the order is named, two behind, large and fan- like ; the females never lose their last pupa skin, and are wingless, with a worm-like abdomen and are viviparous. ORDER IX. Aphaniptera. Includes fleas which have laterally compressed bodies and exceedingly rudimental wings, the scale-like traces of which are with difficulty noticeable. The suctorial mouth (fig. 66), with style -like upper lip, has long slender saw- like mandibles, which are sheathed by the four-jointed labial palps at their base. The antennae are very small and lie in a groove, but the maxillary palps are large and prominent. The hindmost pair of limbs are long, muscular, and well- fitted for leaping. The larvae are white footless grubs which feed on animal matter for about twelve days, spin for themselves a cocoon, and pass to the pupa stage. After about fourteen days' quiescence in this stage the perfect insect emerges. In many respects the flea is closely allied to the next order. ORDER X. Diptera, two winged flies, including flies, gnats, mosquitoes, &c. In this order the hind pair of wings is rudimental and represented by scale- like or pin-like processes under the developed pair of wings, and the mouth is a proboscis. The larvae are footless, often headless maggots, such as are found on putrid meat. Some forms of Diptera, like the gnats and mosquitoes, are provided each with a long pro- boscis enclosing six long sharp bristles. The larvae of the gnats are aquatic and breathe air by means of a Moths. 1 29 tube with which they are provided which opens at the surface of the water. Some of these insects are very destructive to vegetation ; the larvae of the common daddy-longlegs for instance, feeds on the roots of grass and will thus sometimes destroy large patches of meadow. The Hessian fly is still more formidable, often destroying whole fields of wheat by attacking the plants when in flower. ORDER XI. Lepidoptera, is also a large order, and includes those most beautiful of all insects the but- terflies, characterised by possessing four wings covered with fine coloured scales. These microscopic scales overlap each other on the surface of the wings, and are of different shapes in different species. Butterflies have suctorial mouths (fig. 67), the proboscis-like sucker being rolled up when not in use. The larvae or cater- pillars consist of thirteen joints and are very unlike in mouth, structure, and general appearance to the perfect forms which emerge from them. On the lower lip in the larvae of most moths there is the duct of two tubular spinning glands, which when the larva has reached its full size secretes a silken cocoon within which it is enclosed in the pupa state. These insects vary in size ; some, as the clothes moths and fur moths, are very small. The larvae of the leaf-rollers, a form nearly allied to the clothes-moth, roll up the leaves of plants on which they feed~ into tubes, within which they live and pass their pupa sleep, and whence they emerge in due time as little broad-winged moths. Another related form often found on elder trees is the looper or swallow-tail moth, named from the pecu- K 130 Invertebrata. liar looped attitude which the larva assumes in pro- gression. The silkworm moth, a native of North China, secretes by its labial glands the silk of com- merce. The sphinx moths, called so from their FIG. 73. FIG. 74. Chrysalis. Deilelphila Elpenor, Hawk Moth. attitude in which the caterpillars are often found, with their heads and fore parts raised, are known by their prismatic antennae and by the long horn on the tail end of the caterpillar. One of these, the death's head moth (whose large green larva feeds on potato leaves) bears on the back of its thorax a rude coloured figure like a skull, hence the ominous name given to it. While nearly all moths are nocturnal, the true butterflies, recognised by their brighter colours and their club-shaped antennae, are diurnal in their habits. The best known examples are the white cabbage butterfly, the nettle tortoiseshell, and the thistle painted Beetles. 1 3 1 lady butterflies. The larvae of the true butterflies do not spin a cocoon. ORDER XII. Coleoptera. Beetles form numeri- cally the largest sub-division of the animal kingdom, there being over seventy thou- sand species. In these the fore wings are converted into a hard thick pair of wing- covers or elytra overlapping the hinder pair, which are membranous, folded, and usually capable of flight. Beetles are found in almost every condition and feed on almost every kind of material ; cayenne pepper, can- tharides, medicinal rhubarb, animal and P u ? effete matter, putrid flesh and decaying vegetables are the favourite nourishment of some forms. There are forty-eight families included in this ' polymorphic ' order ; one of these contains the little ladybird or Cocdnella, whose spotted bodies are often seen on nettles in pursuit of the aphides on which they feed. It has only three large joints in the tarsus of each foot. The destructive Colorado or potato beetle (Doryphord) somewhat resembles the ladybird but is ten-striped and not spotted. Many beetles are ex- tremely destructive to vegetation, both in their larval and perfect states, the strong mandibulate mouths being able to cut even hard woods. Of these, the turnip-fly, the wire-worm (which is the larva of the beetle called Agriotes\ the pine-beetle, the typographic beetle, Scolytus the elm-beetle, Lymexylon the oak beetle, are illustrations. K 2 132 Invertcbrata. Other beetles are found in articles of food, such as Tenebrio, the meal-worm often found in ships' biscuits, FlG 76i Dermestes, or the bacon grub; others are the pests of museums, like the little Anthrenus or Ptinus the herbarium beetle, and The blistering \K*\Z (C antharis PtUimiS, the bookworm. A few are temporary para- sites ; thus the larva of Rhipidius lives in the abdomen of the cockroach. Some beetles are luminous, such as the glowworm and the firefly. Some beetles emit an ammoniacal smell when irri- tated; others, like Meloe, secrete a drop of acrid oil under the same circumstances. This secretion renders the bodies of some of them useful in medicine for blistering purposes ; thus the bodies of Cantharis vesi- catoria (fig. 76) are the Spanish or blistering flies of commerce. Some species of beetles inhabit caves and are eyeless; others are aquatic and fitted for swimming. The sizes of beetles are also exceedingly variable ; some, like the large Hercules beetle, being nearly six inches long, while others are of microscopic dimension. The antennae are of very variable shapes and sizes, being in some much longer than the body, in others very short and inconspicuous ; in some, like the common cockchafer, lamellar, in others stag-horn- shaped, &c. ORDER XIII. Hymenoptera (membrane-winged) includes bees, wasps and ants, and in these the com- plexity and intelligence of the class culminates. They are characterised by having four naked membranous Bees. Wasps. 133 wings, with few veins, and a mouth with strong man- dibles but with suctorial labium and maxillae. The abdomen is often joined to the thorax by a narrow foot-stalk, and the abdominal limbs are modified into a sting, the representative of the ovipositor, which coa- FIG. 77- b i. Mouth of the Bee. d, clypeus ; e, labrum ; f t mandible ; g, maxilla, h, maxillary palp ; i, epipharynx ; k, labial palp; *, lingua; **, para- glossae ; b.i is over the ocelli ; A, antennae ; c, eye. 2. Tongue, more highly magnified. sists of two poison glands opening into a common vesicle whose duct is elongated into a tube provided r ith a piercer barbed at the point. In bees the wings are not folded, and the basal joint of the hindermost tarsus is flattened and often iristle-clad to collect the pollen for the food of larvae ; 134 F' 1 hereby many of them lay the vegetable kingdom under great obligation, as they convey the pollen from flower to flower and thereby fertilise the seeds of many plants. Many bees secrete wax by means of a wax gland placed on the abdomen, and with this material they build their hexagonal cells for the shelter of their eggs and larvae. The common humble bee makes a nest of moss; others use clay or wood ; and some, like cuckoos, lay their eggs in the nests of other species. In the hives of the common honey bee the inha- bitants are of three kinds, the queen or perfect female, the drones or males, and the workers or imperfect females. Wasps have no special organ for the collection of the pollen, and have their wings longitudinally folded; they also have a more slender petiole or stalk joining the abdomen and thorax. Many of these also live in colonies, making nests of paper formed of vegetable matter chewed by their jaws into a pulp and moulded into hexagonal cells with rounded bases. Other examples are the saw-flies which have a saw-like organ for the deposition of their eggs, and the Ichneumons, which have the singular instinct of laying their eggs in the bodies of the larvae of other insects, so that the young are hatched in the midst of abundant food, for they feast upon the tissues of their host and barely leave him enough of organ to prolong existence until they are ready for emergence. Other Hymenopteious insects lay their eggs under the cuticle of plants; and thereby form small tumours or galls. One such species infests the oak and pro- Recapitulation. 135 duces the nut-galls so important in the manufacture of ink. Another species attacks the rose, arid a sin- gular gall-fly has a cuckoo-like habit of laying its eggs in the galls formed by other insects. The ants are probably the most intelligent of insects, having the most complex social organisation and pos- sessing the most complex nervous system in proportion to their size of any invertebrate. The males and females are winged, the workers are wingless, and their sting gland secretes formic acid, the material whereby they irritate or sting. No group of animals are better worthy of study, and their house-building and polity, slave-holding, aphis-cow-keeping, and other habits have long been favourite subjects of observation with entomologists. The ants form a fitting termination to the Inverte- brata, as in intelligence and in interest they may be looked on as bearing to the other invertebrates some- thing of the relation which man has to his neighbour- ing vertebrates. Recapitulation. The sub-divisions of insects are by some looked upon as deserving of a higher than ordinal rank, but as the nature of a group depends on the nature of the range of organic structure in the forms comprehended therein, and not on the number of in- cluded individuals, we cannot but see that, in each order of insects, the component species are constructed so much on one type as to forbid us from making of them more than ordinal groups. The orders of insects which we have briefly noticed may be summarised as follows : A. Insects with imperfect or no metamorphoses. 136 Invertebrata. a. With suctorial mouths, wings absent, or when present having the fore pair thickened. Order Rhynchota. Wings four equal, membranous. Order Thysanoptera. b. With masticatory mouths, abdomen with a terminal appendix of bristles or a bifid tail. No wings. Order Thysanura. Abdomen with a terminal two-bladed forceps. Order Euplexoptera. Abdomen ^syith no appendages, wings with reticulated costse. Order Orthoptera. B. Insects which pass through a quiescent pupa stage. a. Mouth masticatory; wings membranous, equal. Order Neuroptera. Wings hair-clad or scaly, unequal. Order Trichoptera. Fore- wings converted into hard wing- covers, Order Coleoptera. b. Mouth suctorial; with rudimental wings and compressed bodies. Order Aphan- iptera. With two wings. Order Diptera. With four wings, the anterior short, twisted. Order Strepsiptera. Wings large and scale-covered. Order Lepidoptera. Wings naked, membranous, few-veined Order Hymenoptera. INDEX AND GLOSSARY. ABD A BDOMEN, the group of seg- ** ments of the body which con- tains the digestive organs.. Aboral, at the extremity opposite to the mouth. Acamarchis, a moss-polype, 75 Acanthocephala, 66 Acarus, a mite, 109 Acephala, headless molluscs, 95 Acineta. 27 Actinia, a sea anemone, 42 Adaptive characters, TO Adductor muscles, 83 Agriotes, 131 AIcYcnaria. 46 Alcyonium, 46 Alternation of generations, 40 Ambulacra, the tube-feet on which sea-urchins move, 49 Amoeba, 22 Amoebiform, composed of protoplasm like amoebae. Amphipoda, 103 Analogy, similarity in function, 12 Anelasma, a barnacle which lives on living fishes, 106 Anemone, sea, 42 Animalcule, a minute animal, Anomura, 107 Antennae, jointed feelers, as in in- sects. Antennules, 99 Anthrenus, 132 Ant-lion, 127 Ants, 127, 135 Aphaniptera, fleas, 128 Aphides, 123 Arachnoidea, 98, 108 CAL Arenicola, the lug bait, 71 Arthropoda, 14, 96 Ascnris, round worms, 65 Assimilation, the process whereby food is converted into blood. Asteroidea, 57 Auricles, 94 OACTERIUM, the walking-stick *-* insect, also the most minute protoplasmic particles, 125 Palani, acorn shells, 106 Barnacles, 106 Fees, 134 Beetles, 131 Bilateral symmetry, equality and proportion of the two correspond- ing sides of an animal. Bivalves, 80 Blood, 2 Borlasia, a large sea worm, 60 Brachiopoda, 80 Brachyura, 108 Branchiae, gills. Branchiopoda, 104 Breathing, the process whereby oxygen is taken in to aerate the blood. Bryozoa, 74 Bugs, 123 Butterflies, 129 Byssus, a fibrous material whereby mussels and other molluscs anchor themselves, 84 f ADDIS FLIES, 127 v -' Calcareous, consisting ig of lime. 138 Index and Glossary. Calcified, being impregnated with salts of lime Cantharis, 132 Capsule, 24 Cardo, part of the mouth of an in- sect, 117 Carnivorous, flesh-eating. Caryophylla, 44 Caterpillar, 121 Cells, 2 Centipedes, 112 Cephalophora, 85 Cephalopoda, 90 Cephaiothorax, 100 Chaelopoda, bristle-footed worms. 90 Chalk, 21 Chamber, atrial, 76 Chelae, the pincers or first pair of abdominal feet of a crab or lobster, 100 Chelicerae, the antennary jaws of a scorpion, 108 Chelifer, 112 Chitin, a hard material which forms the outer layer of insects, 96 Chiton, amultivalve shell, 89 Chrysalis, 122 Chrysaora, 39 Cilia, minute vibratile hairs made of protoplasm, 3 Cirripedia, barnacles and acorn shells, 106 Classification, 10 Cliona, a boring sponge, 31 Cloaca, the excretory chamber of animals, 56 Clypeus, 117 Cnidae, the thread cells or stinging cells of jellyfishes, 33 Coccinella, the ladybird, 131 Cochineal insect, 124 Cockle, 85 Cockroaches, 125 Cocoon, 121 Coelenterata, 14, 32 Coenosarc, 35, 45 Coleoptera, 131 Colonies, clusters of animals united on a common stalk. Colorado beetle, 131 Colpoda, 28 Commensals, 17 Contractile vesicle, 26 Copepoda, 107 Corals, 44 Cornea, ioS Coronula, 106 EPI Costae, 119 Cotylidea, 60 Coxa, 119 Crayfish, 98 Crinoidea, 49 Crustacea, 97, 98 Ctenophora, 41 Cucumaria, 56 Cuttlefishes, 90 Cyclops, 104 Cyst, a membranous sac. Cystic, possessing a sac-like mem- branous envelope. p)ADDY-LONG-LEGS, 129 *-^ Daphnia, 103 Death's head moth, 130 Dacapoda, 107 . Deilelphila, 130 Dentalium, 86, 88 Dermestes, 132 Differentiation, the setting apart of separate tissues for different pur- poses. Diptera, 128 Dlstoma, 63 Distribution, 15 Dorsal, 71 Doryphora, 132 Dragonflies, 115, 126 , 124 Echinocardium, 55 Echinodermata, 14, 47 Echinoidea, 53 Echinus, 53 Ectoderm, 321 Edriophthalmia, an order of crus- taceans, 108 Elater, the glow-worm, 120 Embryo, the immature condition of an animal, as developing from the *- eg ?'- 8 kncrimtes, 49 Encystation, the condition of being enclosed in a cyst or enveloping layer, 23 Endoderm, the inner layer of jelly- fishes, sea anemones, &c., 32 Endoskeleton, an internal firm framework of bones or gristles for the support of the organism, 96 Epeira, the garden or geometrical spider, in Epicranium, part of an insect's head, 117 Index and Glossary. 139 EUP Euplectella, a sponge called Venus, flower-basket, 31 Euplexoptera, an order of insects, including earwigs, 124 Euplotes, a minute animalcule, 28 Exoskeleton, an external firm frame- work for the purpose of protection or support, 96 T7AMILY, a group of genera, n A Fauna, the collective name for all the animals of a country, 15 Feather stars, 50 Femur, the thigh bone in verte- brates, or the third joint of an in- sect's leg, 119 Fission, the process of multiplication in animals by splitting, 5 Flagellata, a group of microscopic animals, 28 Flagellum, a minute vibratile hair- like filament which is the chief organ of locomotion of the Flagel- lata. Fleas, 128 Flies, 128 Flukes, parasitic worms, 63 Food, i Foraminifera, a group of microscopic shells, 19 Function, the office performed by any organ or part of the body, 3 Fungia, a mushroom-like coral, 46 C* ALATHEA, a genus of bivalve ^ shells, 83 Galea, part of an insect's head, 117 (fig.) Gall-flies, the flies which produce nut-galls on oaks, &c. Gallyworms, 112 Ganglion, a swelling on a nerve which acts as a centre for the evolution of nerve-force. Gasteropoda, a class of molluscs, 89 i Gemmation, multiplication by the ! production of buds, 5 Genus, a group of closely allied species united under a common name, n Gephyrea, a group of marine worms, 67 Gills, vascular organs which are fitted for aerating the blood which KOI they contain, by means of the air dissolved in the water which bathes them. Gizzard, a stomach with thick mus- cular walls. Globigerina, a minute shell, 18 Gordiaceae, threadworms, 68 Gorgonia, a horny coral, 46 Grantia, a sponge, 32 Gregarinae, microscopic parasites, LJ AIRY-BAIT, 71 1 A Halichondria, a sponge, 32 Heliophrys, 22 Heliozoa, minute animals found in bog pools, 22 Hessian fly, 129 Hinge of shells, 83 Hirudinea, leeches, 68 Holothuroidea, sea cucumbers, 56 Host, an animal inhabited by a parasite. Hyalonema, a sponge, 31 Hydra, a minute fresh water polype, 3 2 Hydroida, animals like the hydra, Hymenoptera, an order of insects, including ants and bees, 132 JCHNEUMON, a group of flies, Imago, the perfect or adult state of an insect, 122 Infusoria, animals found in stagnant waters, 25 Insecta, 98 Integument, the skin of an animal. Iridescent, producing a play of colours by decomposing incident rays of light Isis, a genus of corals, 46 Isopoda, an order of crustaceans, including woodlice and their allies, 103 TELLY-FISHES, 3 IT'OINOSITES, animals which 1V live on, and feed with, their hosts, 17 140 Index and Glossary. LAB LABIUM, the lower lip of an insect's mouth, 116 Labrum, the upper lip of an insect's mouth, 116 Lac insect, 124 Lacunae, interspaces between tis- sues. Ladybird, 131 Lameilibranchs, bivalve molluscs, such as the oyster, 81 Lampyris, the firefly, 120 Larva, the first active stage of an animal while as yet immature, T2I Leaf- roller moths, 129 Leeches, 68 Lepidoptera, 129 Life, i Ligament, a fibrous band uniting two parts. Ligula, part of the so-called tongue of an insect, 117 Limulus, the king crab, 103 Lingula, the duck-bill shell, 81 Lithobius, the common centipede, 113 Locusts, 125 Lug-bait, 71 Lymexylon, a wood-boring beetle, g TV/TACRURA, lobsters, 107 *** Madrepores, reef - buildin corals, 45 Madreporiform plate, a rough plate on the surface of star-fishes, 52 Magosphsera, 21 Malacobdella, a leech, 69 Malpighian glands, glands in insects, named after their first describer, Malpighi, 120 Mandibles, the second pair of jaws in insects, too Mantle, the leathery outer la) r er in molluscs, 14, 79 Mason spiders, in Maxilla, 100 Meandrina, brain coral, 46 Medusa, a jelly-fish, 40 Medusoids, detached portions of hydroids which resemble medusae, Meloe, 132 Mentum, part of an insect's mouth, 118 Mermis, 66 ORT Mesenteries, folds of membrane GUS pending the digestive sac, 43 Metamorphoses, changes undergone by an animal in its development from its larval to its perfect state. Metazoa, animals with an internal cavity, 28 Millepedes, 114 Mimicry in animals, 12 Mites, 109 Mollusca, 14, 78 Monads, small flagellate animals, 28 Monera, the simplest known ani- mals, 21 Monocystis, a gregarine, 23 Morphology, the science which treats of the forms of animal organisms, 7 Moss polypes, 74 Moths, 130 Muscles, 4 Mussels, 85 Myriopoda, 98, 112 TVFAUPLIUS, the larval stage of ^ crustaceans, 104 Nautilus, the most complex of molluscs, 92 Nematelmia, 64 Nemerteans, marine worms, 60 Nerve, 4 Neuromuscular cells, 33 Neuroptera, an order of insects, 127 Noctiluca, a luminous marine ani- malcule, 27 Nucleolus, 26 Nucleus, 21 Nutrition, 4 /"}CULAR plates, plates bearing !~ .eyes, 54 Ocuhna, a coral, 45 GEsophagus, the tube which conveys food to the stomach, 65 Oikosites, parasites which live with, but do not feed on, their host, 120 Operculum, the lid which closes the mouth of an univalve shell, 87 Ophiolepis, a star-fish, 51 Ophiuroidea, star-fishes, 75 Ophrydium, 27 Optic nerve, the nerve which con- nects the eye and the brain, 99 Organism, an animal made up of separate organs or parts, 13 Orthoptera, an order of insects, 125 Index and Glossary. 141 osc Osculum, the mouths in sponges, 3 Ostracoda, minute crustaceans, 107 Ovipositor, the organ whereby in- sects deposit their eggs, 121 Ovulation, the mode of reproduction by the development of eggs, 5 Oxytricha, 25 Oxyuris, a worm, 63 Oyster, 85 pAL/EOZOIC, the age of the world in which the oldest fossil- bearing rocks were formed, Pallial line, the line on a shell indi- cating the margin of the mantle, 83 Palp, a feeler or jointed appendage on the jaw of an arthropod. Paraglossae, part of an insect's mouth, 117, 133 Paramcecium, a common infusory animalcule, 26 Parasites, 17 Pauropods, 114 Pedicellariae, jointed pincer-like ap- pendages to the mouth in Echino- dermaia, 51 Pedicelli, small sucking feet in star- fishes, 49 Pennatula, a sea-pen, 46 Pennella, a parasitic crustacean, 105 Pentastomum, a parasitic mite, 10 Pericardium, the space of the body cavity around the heart, 81 Periplaneta, the cockroach, 125 Pharynx, the upper part of the di- gestive tube near the mouth. Physalia, 37 Physiology, the science which treats ^of the functions of organs, 7 Pixinia, a gregarine, 23 Planula, the ciliated embryo of a jelly-fish, 33 Pluteus, the larval stage of a star- fish, 51 Podophthalmia, crabs, &c., whose eyes are on stalks, 107 Podurse, 124 Poecilopoda, king-crabs, 103 Polycelis, a tubellarian worm, 59 Polypites, hydra-like animals when in colonies, 35 Polystomata, 28 SEG Pores, the fine openings in sponges, 3 Postabdomen, that part of the ab- domen behind the openings of the reproductive organs, 101 Proglottis, one of the mature joints of a tape-worm, 61 Protamoeba, one of the simplest known animals, 21 Protoplasm, 2 Protoplasta, amcebae, 22 Protozoa, 14, 1 8 Provisional organs, those organs that fulfil a temporary function, and then disappear or waste, 8 Pseudonavicellse, 23 Pseudopodia, 3, 19 Pteropoda, 89 Ptinus, 132 Pupa, the quiescent stage in the life of a butterfly before the per- fect imago condition is reached, 121 Pyrosoma, a luminous marine mol- lusc, 77 D ADIOLARIA, 24 *^- Raphiophora, Neptune's Cup, a sponge, 31 Reproduction, 5 Rhizopods, 18 Rhizostoma, a jelly-fish, 39 Rhynchota, an order of insects, in- cluding bugs, 123 Rotatoria, wheel animalcules, 66 Rotifer, a common wheel animalcule, 6 7. Rudimental organ, an imperfect, functionless structure, 9 CALPA, a pelagic mollusc, 77 *** Sand hoppers, small crusta- ceans, 105 Scallops, 84 Scaphopoda an order of molluscs,, 95 Scolopendra a centipede, 114 Scolytus, a wood-boring insect, 131 Scorpion-flies LZJ Scorpions, 112 Sea-cucumber.*), 56 Sea-glue, 25 Sea-urchins, 47, 53 Segment, one of the successional 14-2 Index and Glossary. SEG morphological units of the body of a jointed animal, 13 Segmental organs, excretory tubes in the segments of worms, 58 Serpula, 72 Sertularia, 34 Setae (bristles), 73 Shell, 90 Silica, flint. Silkworms, 130 Siphon, 84 Siphonophora, 38 Siphuncle, 90 Snail, 87 Solen, the razor-shell, 84 Specialisation, setting apart of an organ for a special function, and for it alone, 13 Species, a group of identical indi- viduals under a common name, n Spicules, siliceous or calcareous masses embedded in animal tissues, Spiders, no [31 Spinnerets, in Spirorbis, 72 Sponges, 29 Spongilla, a freshwater sponge, 31 Spoon-worms, 67 Star-fishes, 7, 47 Stellerida, 50 Stipes, part of an insect's head, 117 Stock, the common stem of a colony. Strepsiptera, a small order of in- sects, 127 Strongylocentrotus, a common sea- urchin, 32 (fig.) Symmetry, 8 , the tapeworm, 60 Tarsus, the last joints of an insect's leg, 115 Teeth of sea urchin, 54 Tegenaria, the house spider, in Telson, the middle flap of a lobster's tail, ioi Tenebrio, the meal-worm, 132 Tentacles, feelers, 32, 33 Terebratula, 80 '1'ermites, 126 Test, a shell or exoskeleton. Thalassicolla, sea-glue, a group of marine protozoa, 25 Thorax, the chest, or the region of the body of an insect which bears the legs, 1 16 VOR Thread-cells, stinging cells of jelly- fishes. 33 Thrips, 125 Thysanoptera, an order of insects, 125 Thysanura, 124 Tibia, part of an insect's leg, 115 Ticks, no Tracheae, air-tubes for breathing, 109, 113 Tracheal lungs, groups of tracheae compressed together, no Trematoda, an order of parasitic worms, 63 Trepang, an edible sea-cucumber, 56 f ' Trichina, a parasitic worm, 65 Trichocephalus, 65 Trichoptera, an order of insects, 127 Trilobites, 104 Trochal disks, the ciliated lobes on the heads of some minute worms, 79 Trochanter, the second joint in the leg of an insect, 119 Troctes, the death-tick insect, 125 Tubipora, the organ-pipe coral, 46 Tunicates, 75 Tunic, 75 Turbellaria, 59 Types, n T J NDIFFERENTIATED, not ^ separated into specialised parts. "\7ACUOLES, clear spaces in v masses of protoplasm, 20 Valves of shells, 80 Ventral, the under side of the body. Ventricle, the cavity of the heart which by its contraction drives on the blood in the circulation, 94 Venus' Flower Basket, a sponge, 31 Vermes, worms, 14, 57 Vertebrata, 14 Viscera, the digestive and other internal organs of the body of an animal. Vorticella, 25, 27 Index and Glossary. WAS \\TASPS, 134 vv Water- boat men, 127 Water-fleas, 103 Water-scorpions, 124 Wax-glands, 134 Whelk, 87 White ants, 126 Wire worms, 131 Woodlice, 103 VOEA, the larval stage of the " common crab, 102 Richard Clay & Sens, Limited^ London &* THE LONDON SCIENCE CLASS-BOOKS. KDXT1II> BY G. CAREY FOSTER, F,R,S, S Sir PHILIP MAGNUS, B.Sc., BJL ASTRONOMY. By Sir ROBERT STAWELL BALI, LL.D., F.K.S. With 41 Diagrams U id. BOTANY OutlSmes of Morphology and Physiology. By W. R. McNAB, M.D. With 42 Diagrams .. e U Kd. BOTANY, Outlines of the Classification of Plants. By W. R. 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