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 A NEW FAMILY OF HYDROIDEA. 
 
 BY W. BALDWIN SPENCER, MA. 
 
 REPRINTED FROM THK TRANSACTIONS OF TIIK UOYAI. Sorn.iv or VICTORIA FOK Is'.iu. 
 
* 
 
 BIOLOGY 
 
 LIBRARY 
 
 G 
 
<d far 
 
 ARTICLE IV. A NEW FAMILY OF HYDROIDEA, TOGETHER WITH A DESCRIPTION 
 
 OF THE STRUCTURE OF A NEW SPECIES OF PLUMULARIA, BY W. BALDWIN SPENCER, 
 
 \\ 
 M.A., PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF MELBOURNE. 
 
 (Read Nov. 12th, 1890.) 
 
 The following deals with two hydroid forms obtained by Mr. J. Bracebridge 
 Wilson, M.A., from the neighbourhood of Port Phillip. One of these is so distinct 
 from any hitherto described that it must be taken as the type of a new Family, to 
 which the name of Hydroceratinidse is given, the other is a new and somewhat 
 curious species of the genus Plumularia. The paper is, therefore, divided into two 
 parts (1) On the Hydroceratinidte, a new family of the order Hydroidea, and (2) On 
 Plumularia procumbens, a new species of the genus Plumularia. 
 
 (1) ON THE HYDROCERATINIDOE, A NEW FAMILY OF THE ORDER HYDROIDEA. 
 
 The form in question undoubtedly calls to mind, in the general appearance of 
 the dried specimens, one of the two genera which were placed in a distinct family 
 the Ceratelladae by Dr. Gray,* and, when this paper was read before the Eoyal 
 Society of Victoria, I provisionally referred it to this family. | Dr. Gray had doubtfully 
 
 f NOTE. Up to the time of reading this paper I had only the necessarily 
 somewhat imperfect descriptions (since taken from dried specimens only) of Dr. Gray 
 and Mr. Carter to be guided by together with the figures of the former. Those of 
 Dehitella bore a general resemblance to the specimen in question which was 
 placed provisionally in that genus as a new species. Through the courtesy of Dr. 
 E. P. Ramsay I have since been able to examine specimens of Ceratella and Dehitella 
 from the Australian Museum, Sydney, and have received from there specimens 
 obtained from the New South Wales coast, and from Lord Howe Islands by Mr. 
 ^'liitelegge, to whom I am much indebted for kind assistance and information. The 
 examination of these specimens has shown that the form with which this paper deals 
 differs in such important points not only from the Hydractiniidse but also from 
 the Ceratelladas that it must be placed in a separate family and I desire to here record 
 my indebtedness to the authorities of the Australian Museum, Sydney, for the 
 courteous assistance received from them. 
 
 * Proc. Zool. Soc., Nov., 1868. 
 
122 A NEW FAMILY OF HYDROIDEA. 
 
 placed the family in the group Porifera. Mr. Carter subsequently had the opportunity 
 of examining both Dr. Gray's specimens and others of allied forms from New 
 Zealand and the Cape, and rightly referred them all to the Hydroidea, placing them 
 in the family Hydractiniidae, This classification was adopted by Dr. v. Lendenfeld, and 
 also, at first, by Mr. Bale in his valuable contribution to the literature of Australian 
 zoology the "Catalogue of Australian Hydroid Zoophytes."* Subsequently, Mr. 
 Bale had the opportunity of examining spirit-preserved specimens with the hydranths 
 and soft parts present, and, finding that these differed very considerably from those 
 of the Hydractiniidae in the presence of irregularly distributed capitate tentacles, 
 &c., rightly separated them once more from the Hydractiniidse, and adopted the 
 name of Ceratelladae for the family, t I was unaware of this short paper of Mr. 
 Bale's until my attention was kindly drawn to it by Mr. Whitelegge, of the 
 Australian Museum. 
 
 Whilst Dehitella in its general form (in the dried specimen) is somewhat like 
 the specimen obtained by Mr. Wilson, there can be little doubt that the two are 
 markedly distinct, even so far as the skeleton goes. Dehitella has most clearly what 
 have well been called hydrophoresj;, and agrees closely in this respect- and in the 
 general formation of its skeleton with Ceratella ; in their soft parts we may 
 reasonably suppose that a corresponding agreement exists. Now the soft parts of 
 the Ceratella are known, and they differ very strongly from those of the form with 
 which this paper deals, so that we are probably correct in assuming that a similar 
 difference exists between the latter and Dehitella. There is, on the other hand (as 
 will be shown subsequently), quite as marked a distinction between the new form 
 and any member of the family Hydractiniidas as exists between the former and the 
 Ceratelladas, so that it is necessary to create a new family for its reception. 
 
 I have not so far, though numerous sections have been cut, been able to 
 detect any reproductive elements, and therefore the description is incomplete, but will 
 quite sufficiently serve to distinguish it from other forms. 
 
 Family Hydroceratinidce. 
 
 Hydrophyton, consisting of a mass of entwined hydrorhiza, with a skeleton in 
 the form of anastomosing chitinous tubes ; the surface is studded with tubular 
 hydrothecte, into which the hydranths can be completely retracted. Hydranths 
 sessile and connected with more than one hydrorhizal tube, claviform with a single 
 verticil of filiform tentacles. Defensive zooids present with a solid endodermal axis 
 and nematocysts borne at the distal end. 
 
 * Catalogue of Australian Hydroid Zoophytes. W. M. Bale, 1884. 
 
 t l!ale, Proc. Linn. Soc., N.S.W., Vol. III., pt. 2, p. 748 ; also Brazier, Proo. Linn. Soo., N.S.W., new series, Vol. I. 
 page 575 ; Whitelegge, loc. cit., p. 578. 
 
 * Bale, Proc. Linn. Soc., N.S.W., 1888, p. 749. 
 Bale, loc. cit. 
 
A NEW FAMILY OF HYDROIDEA. 123 
 
 Clathrozoon wilsoni, n. gen. et. n. sp. 
 
 Hydrophyton irregularly sub-dichotomously branched, expanded in one plane fan- 
 shaped. Main stem somewhat flattened and ridged, arising from an expanded base, dark 
 brown in colour, with the surface showing a series of tortuous grooves. Smaller 
 branches cylindrical. The whole composed of a number of branching and 
 anastomosing tubes, with chitinous walls complete, except those of the most external 
 ones. Hydrothecse, which have the form of tubular spaces with chitinous walls, 
 project slightly from the surface of all the branches, irregularly placed on 
 the main stems, spirally on the branches. The whole, except the external 
 openings of the hydrothecas, completely enclosed in a thin transparent chitinous 
 layer, from which arise very numerous cylindrical tubes enclosing the defensive 
 zooids, and which forms also a funnel-shaped collar, projecting beyond the lips of the 
 hydrothecas. 
 
 Locality Near Port Phillip Heads, Victoria. Mr. J. Bracebridge Wilson. 
 
 The description of the species is the same as that of the genus, and I have much 
 pleasure in dedicating it to Mr. Wilson. 
 
 The specimens were obtained from two spots, each within a distance of five 
 miles of Port Phillip Heads, that is, in Bass Straits, close to the Victorian shore. 
 They were dredged in water of from twenty to twenty-two fathoms, and were placed 
 directly into strong alcohol so that, like all received from Mr. Wilson, they are in 
 excellent histological preservation. The polypes are, of course, in a state of retraction, 
 and though very many sections from various parts of the hydrophyton have been cut, 
 nowhere as yet have I been able to detect the presence of reproductive organs, or of 
 individuals modified in connection with these, and though, therefore, unable to give 
 any account of the reproductive organs, I have thought it advisable to describe and 
 figure the animal carefully so far as can yet be done.* 
 
 The largest colony secured by Mr. Wilson measures lOin. in height, by 4in. in 
 greatest width, and at first glance recalls to mind, to a certain extent, one of the 
 dark coloured fan-shaped gorgonid forms. A cursory examination, however, at 
 once shows that it does not belong to this group of animals. 
 
 If the whole colony be secured, it is found to be attached by a broad flattened-out 
 base, attached to some solid structure; from this arises a single large stem, perhaps fin. 
 in thickness, slightly flattened out in one plane, and distinctly ridged (Fig. 1). From 
 the edge at either side of the plane lateral branches are given off, varying much in size; 
 
 * That the animal is not at all common ia shown by the fact that, though during this summer (1890-91), Mr. Wilson 
 has been kind enough to spend a considerable amount of time in dredging, with the special object of procuring more 
 ("pecimens, not a trace of this hydroid has been brought up in the dredge, though the same ground which previously 
 yielded it has been visited. 
 
124 A NEW FAMILY OF HYDROIDEA. 
 
 from these other branches are given off on either side, which again divide in a roughly 
 but not constantly dichotomous manner, the smaller branches being cylindrical in 
 shape. All the branches in the living and spirit-preserved specimens usually lie in the 
 one plane, but when dried they often become slightly twisted, and thus thrown out of 
 the plane. There is thus a considerable resemblance in general appearance between 
 the form Clathrozoon on the one hand, and Dehitella and Ceratella on the other, 
 especially between the two first mentioned, owing to the rounded branches of 
 Dehitella. If, however, we come to examine the structure of the two more minutely, 
 we find that the differences are very marked ; whilst both have the skeleton in the 
 form of a horny network, that of Clathrozoon has more the appearance of being 
 composed of a series of anastomosing tubes, and rather less of the appearance of an 
 open meshwork than is the case in the Ceratelladse ; but, what is of much greater 
 importance, it presents on its surface a large number of circular openings at the 
 extremities of slightly projecting Glutinous tubes, which form true hydrothecae. 
 These are very distinct from what Mr. Carter and Dr. Gray aptly described as scoop- 
 like projections of the chitinous network of the Ceratelladae, though they 
 unfortunately gave to them the name of hydrothecae, that of hydrophore being 
 more correct and suggestive of their real function, as at most they can but afford a 
 support for the proximal part of the hydranth. 
 
 Skeleton (Figs. 4, 5, 6, 7, 8, 9, 11, 12, 13, 16). 
 
 When the branches of spirit-preserved specimens, with the soft parts present, 
 are viewed under the lens, the surface is seen to be covered with a great number of 
 tortuous grooves, filled with light yellowish coloured material the ccenosarcal 
 tubes. The edges of the grooves are formed by the chitinous perisarc of a dark-brown 
 colour (Fig. 6), and arranged in a spiral manner are circular projections, the external 
 opening of chitinous cylinders forming the hydrothecae and containing the retracted 
 polypes s These hydro thecse are encircled by the tortuous grooves, and supported by 
 extensions of the ordinary perisarc. The whole surface is covered over by an 
 extremely thin and delicate colourless layer of perisarc common to the whole branch. 
 This layer is not usually recognised until sections are cut, but the whole surface is 
 seen to be studded with small cylindrical tubes, which are really formed from this 
 thin layer and the spaces in which are continuous with the tortuous grooves below 
 the latter (Figs. 7, 8, 9). 
 
 Sections show also that this thin layer rises up somewhat from the general 
 surface, and is attached to the lips of the hydrothecaB ; beyond the margin of the 
 latter it is continued on so as to form a very thin collar-like extension, acting as an 
 operculum. The arrangement is represented in the figures, especially in Fig. 13, 
 which is meant to be a diagram showing the relationship of a hydrotheca to the 
 
A NEW FAMILY OF HYDROIDEA. 125 
 
 perisarc, as seen in a very thick section cut transversely to the length of a branch. 
 This operculum-like structure is very thin and liable to be torn away, but is present 
 in all well-preserved and cut specimens. Sometimes it projects as a stiff collar, 
 sometimes (Figs. 4, 16) it is thrown into folds, and at others is withdrawn into the 
 hydrotheca. With only specimens in which the polypes are completely retracted to 
 examine, I cannot say how much of the body of the polype is covered by this thin 
 collar when the animal is fully expanded. 
 
 When the soft parts are dissolved away by potash the whole branch appears to 
 consist of a meshwork of tubes with chitinous walls (Fig. 11), which anastomose so 
 freely that the skeleton appears, in surface view, to consist of an irregular chitinous 
 network, which has a considerable superficial resemblance to the skeleton of a horny 
 sponge, though this resemblance is even more strongly marked in the case of the 
 Ceratelladae. On cutting sections, however (Figs. 7, 8, 9), the tubular arrangement can 
 be recognised. The structure is essentially identical in the branches and branchlets, 
 whatever be the size, the only difference consisting in the greater number of tubes 
 entering into the composition of the larger branches as compared with the smaller 
 ones. The tubes (Fig. 7) run roughly parallel to the length of the branch, continually 
 branching and anastomosing. The spaces which they contain vary much in calibre. 
 Towards the exterior the chitinous walls are much stronger and thicker than in the 
 interior, and the most external series are incomplete, forming grooves rather than 
 tubes. The thin external layer of the branch, previously alluded to, touches the 
 chitinous lips of these grooves (Figs. 8, 9), and thus a complete inclosure for the most 
 external-lying coenosarcal tubes is formed. 
 
 Sections also show that the larger circular openings, visible on the outside, lead 
 down into tubular spaces, the walls of which are formed of chitinous material similar 
 to that of the tube walls with which they are connected. These form the hydrothecae, 
 and the spaces within them are continuous with one or more of those within the 
 tubes surrounding them (Figs. 4, 7, 12). The thin external chitinous (?) layer always 
 passes up to be attached to the rini of the hydrothecae. The cylindrical structures 
 (P") on this thin layer are very simple in form, open at both ends, and serve during 
 life to contain the defensive polypes which are directly connected with the external 
 layer of ccenosarcal tubes. (Fig. 5). 
 
 
 
 Measurements of the Skeleton. 
 
 Diameter of branch taken, -7 mm. 
 
 Number of tubes, as seen in longitudinal section, entering into the 
 composition of a branch of this size, 7 9; as seen in transverse 
 section, 4050. 
 
 Thickness of the tube, varying, but averaging about '07 mm. 
 
126 A NEW FAMILY OF HYDROIDEA. 
 
 Thickness of wall of tube, -0525 mm. at the exterior, some of those in the 
 centre being not more than '00525 mm. 
 
 Hydrotheca. Transverse section, % 175 mm. -- -1225 mm. 
 Hydrotheca. Longitudinal section, '525 mm. (avernge). 
 Soft Parts. (Figs. 3, 4, 5, 10, 12, 14, 15). 
 
 The chitinous tiibes are filled by the ccenosarcal tubes which branch and 
 anastomose freely. No one of the latter tubes is more prominent than any 
 other. The polypes are distributed over the whole surface, having apparently no 
 definite arrangement on the larger stems or branches, but, in the smaller ones, 
 a very distinct spiral arrangement (Figs. 6, 11). One corresponds to each of the 
 hydrothecae, seen in the skeleton, and is capable of complete retraction within the 
 latter. When thus retracted the opening is protected by the thin collar-like projection 
 formed of the outer layer of perisarc mentioned above. 
 
 Each polype has the typical hydroid form. It is somewhat tubular, with a 
 conical hypostome, from the base of which arises a single circlet of solid tentacles, 
 which vary in number from six to ten, and are provided with minute nematocysts, 
 the threads of which are short, stiff, and unbarbed. When the tentacles are retracted 
 they curl over towards the mouth, much in the same manner as do those of 
 Pedicellina, amongst the Polyzoa. 
 
 The body of the polype is formed of the typical layers an ectoderm of somewhat 
 columnar cells, an endoderm of larger cells less defined in shape, and between the 
 two a very strongly marked layer of mesogloea (.00875 mm. in thickest part). From 
 the bases of the ectoderm cells pass off long processes (Fig. 14), much more 
 strongly marked than is usual in hydroid forms. In its greatest breadth one of 
 these fibres measures -0058 mm. They give rise to a circular band of what are 
 evidently in function, " muscle fibres." In figure 14 these are represented as seen 
 when an oblique longitudinal section of a polype is cut.* Possibly some of them 
 may have lost their connection with the ectoderm cells from which they have been 
 derived. Each has a very definite outline and contents, which, when stained, have a 
 somewhat granular appearance. They all lie external to the rnesoglcea, and form a 
 very definite band round the body of the polype, the position of which is represented 
 in the diagram (Fig. 12). 
 
 Sections, transverse and longitudinal, show that the basal ends of the polypes 
 are continuous with more than one (sometimes as many as four or five) of the 
 
 * These bear a considerable resemblance to those described by Weismann as occurring in the comosarc of Plumularia 
 ccliimilata. See Entstehung der Sexualzellen bei den Hydromedusen, PI. VIII., Fig. 96. 
 
A NK\V FAMILY OF HYPROIDEA. 1'27 
 
 ccenosarcal tubes. In this they resemble the polypes in the group Hydrocorallina?. 
 The cavities of the polypes nnd of the meshwork of tubes are hence directly 
 continuous. The tubes are formed of the two primary layers with, if present 
 at all, an extremely thin layer of mesogla'a between them. The endoderm is always 
 unilaininar and distinct ; the ectoderm is composed of smaller cells, the nuclei, but 
 not the outlines, of which are clearly marked. It appears to be frequently more than 
 one cell in thickness, and to be irregular in outline, spirit specimens showing 
 projections (Fig. 10) which come in contact with the chitinous walls. Probably 
 during life there is very little space between the hard and soft parts. Within the 
 ccenosarcal tube are frequently seen large globular structures containing darkly 
 staining portions (Fig. 10. x.) 
 
 The whole of the external layer of coenosarcal tubes is studded over 
 with an enormous number of small defensive polypes of very simple structure 
 (Figs. 3, 5, 15). One corresponds to each of the cylindrical structures which rise up 
 from the outer thin peris arc layer, and consists of a stalk and a head. The former 
 is directly continuous with the ccenosarc tube, and consists of an internal axis, the 
 nature of which is difficult to determine. It is solid, and contains no prolongation of 
 the tubular canal of the ccenosarc, but appears to be continuous with the endoderm, 
 though I cannot as yet make out the cellular structure. It is very much shrunken 
 in spirit specimens. The outer layer is thin and continuous with the ectoderm of 
 the coeuosarc (Fig. 15). In spirit-preserved specimens it has the appearance of a 
 thin layer of granular protoplasm with nuclei, the diameter of which is greater than 
 the thickness of the protoplasmic layer, in which the outline of cells cannot be 
 distinguished. No trace of muscle elements can be distinguished. 
 
 The head consists of a little mass of nematocysts, with the remnants of the cells, 
 which have given rise to them, and the nuclei of which can be clearly seen. Each 
 nematocyst is fusiform in shape, and the whole form a little group lying close to the 
 open end of the cylindrical tube, through which the threads can doubtless be ejected. 
 None of the nematocysts have the threads put out, and their exact form cannot be 
 therefore described. 
 
 In relative size and structure these defensive polypes resemble more than 
 anything else the machopolypes of the Plumulariidas, and have no resemblance to the 
 defensive polypes of Hydractinia or Podocoryne, whilst none have as yet been 
 described in the Ceratelladae. 
 
 In figure 3 is represented a diagrammatic restoration of a branch of the colony 
 only the soft parts being drawn. The upper surface is supposed to have been cut 
 away, and the network of cronosarcal tubes is shown branching and anastomosing. 
 The connection of the alimentary polypes with the tubes is drawn, and the small 
 
128 A NEW FAMILY OF HYDIiOIDEA. 
 
 defensive polypes are shown arising from the outermost layer of coenosarc. The hard 
 skeleton parts represented in figure 11 occupy the spaces between the tubes in figure 3, 
 the polypes corresponding in position to the large circular openings leading down 
 into the hydrothecae. The general relationship of the polypes to the skeleton and the 
 coenosarcal tubes is represented diagrammatically in figure 12. 
 
 Affinities of the Hydroceratinidce. 
 
 Clathrozoon differs in important points from any hydroid hitherto described, 
 and it is somewhat difficult to determine its affinities, more especially in the 
 absence, as yet, of any knowledge of its reproductive elements. 
 
 At first glance it would appear to be related to the Ceratelladae, but this is simply 
 in consequence of the superficial resemblance in the skeleton of the two groups ; 
 each consists of a branching mass of entwined perisarcal tubes, and here the 
 resemblance ends, for, whilst the branching network of tubes may be compared with that 
 constituting the skeleton of the Hydractiniidae and the Ceratelladas, there are present two 
 structures entirely wanting in the members of these two families, (1) the hydrothecas, and 
 (2) the external layer of thin perisarc, with its projecting cylindrical tubes. The first 
 of these structures are quite unlike those of any other hydroid in their simple 
 shape, the thickness of their walls, the relation of these to the surrounding 
 perisarc tubes, and the number of openings leading through the wall into the 
 enclosed space. At the same time, if the cavity in the scoop-like projection, formed 
 of a small network of perisarcal tubes in the Ceratelladae, were to become deeper 
 and to penetrate the substance of the branch, and if the branches of the network 
 bounding the cavities thus produced were to " run together " and thus give rise to a 
 tubular structure, in which apertures were left, we should have produced a structure 
 similar to the hydrothecae of Clathrozoon. The second of these structures is, 
 apparently, not present in any other hydroid, and the manner of its origin is difficult 
 to conceive. There is no such external layer of ccenosarc investing the colony as is 
 present in the Hydractiniidae, and which might be supposed to secrete such a layer. 
 On the contrary, the surface of the colony shows a series of much branching tortuous 
 perisarc tubes, each incomplete externally, and the whole covered in by this thin 
 continuous layer, which does not give the idea of having been formed as a separate 
 outer covering for each most external tube, though it must apparently have been pro- 
 duced in this way. The little cylinders arising from it enclose each, it is important to 
 notice, a defensive zooid, which arises directly from the cosuosarcal tube immediately 
 below. In one or two of the sections cut transversely near to the end of a growing 
 branch, the perisarc tubes appear to have a thin innermost lining distinguishable 
 from the rest of the perisarc ; possibly this may be comparable to this outer layer, 
 but at all events the latter, as shown in sections, passes continuously from lip to lip 
 
A NEW FAMILY OF HYDROIDEA. 129 
 
 of the outer grooves, and has nothing to do with any layer lining the latter internally. 
 That it is connected in development with the soft structures in these there can be 
 little doubt if only on account of its relationship to the protective zooids to which 
 they give rise. 
 
 The extension of this layer beyond the mouth of the hydrothecae so as to form, as it 
 were, an operculmn for these, is a curious feature, and one not met with, so far as I am 
 aware, in any other hydroid. This particular part is very flexible, being capable of being 
 thrown into folds (Fig. 16), or even of being pulled back within the outer rim of the 
 hydrotheca by the retreating zooid. Taken altogether the skeleton, though in certain 
 respects showing a resemblance to theHydractiniidae and Ceratelladae, differs essentially 
 from that of these forms in the two important points dealt with above. 
 
 In dealing with the soft structures we find a most curious combination of 
 characters, each one characteristic of hydroid forms, belonging to groups perfectly 
 distinct from one another. 
 
 The network of coenosarcal tubes resembles, to a certain extent, that of the 
 Hydractiuiidse, Ceratelladae and Hydrocorallinas, but even here we have to note the 
 entire absence of an external continuous layer, characteristic certainly of the first 
 and third, and, probably, also of the second group. 
 
 The alimentary polypes are sessile, and distinctly " claviform," that is, have 
 tubular bodies with a conical hypostome and a single circlet of simple solid tentacles, 
 in which points they resemble the genus Clava. Those of the Ceratelladae*, on the 
 other hand, have scattered capitate tentacles, and in the Hydractiniidse they are 
 provided with a strongly developed hydrocope. 
 
 The gastrozoids again resemble those of the Hydrocorallinae, and differ from all 
 others amongst the Hydrozoa in being connected with several of the cceuosarcal 
 tubes. 
 
 The defensive zooids, or dactylozooids, resemble more than anything else, certain 
 individuals characteristic of the Pluniulariidae, to which the name of " machopolype " 
 has been applied. They consist of a solid stalk bearing a number of nematocysts at 
 its free end, each zooid being enclosed in a distinct protective case or nematophore. 
 
 The structure of the gastrozooids thus calls to mind the genus Clava, that of the 
 dactylozooids the family Plumulariidae, and the relationship of the gastrozooids to 
 the crenosarcal tubes, the sub-order Hydrocorallinae. This combination of characters, 
 together with the nature of the skeleton, serves to render the Hydroceratinidae distinct 
 from any family of Hydroidea yet known. 
 
 * Bale, loc. cit. 
 
130 A NEW FAMILY OF HYDROIDEA. 
 
 2. A NEW SPECIES OF PLUMULARIA. 
 
 This form was dredged by Mr. Wilson in Port Phillip, and differs, as far 
 as I can ascertain, from any yet described. In (1) that the nematophores are 
 perfectly independent of the hydrothecse, (2) that no intrathecal ridge is present, 
 (3) that the hydrothecae are cup-shaped, with smooth untoothed margin, and are set 
 at some distance apart from one another, the form in question shows the features 
 characteristic of the group to which Allman gave the name of Eleutheroplea. 
 
 The arrangement of the hydrothecae would prevent it from being placed in any 
 of the three divisions Isocola, Anisocola, and Monopyxis into which Kirchenpauer 
 proposed to divide the genus. 
 
 The species may be described as follows : 
 Plumularia procumbens, n. sp. 
 
 Hydrocaulus upwards of Pin. in length. The whole colony procumbent with 
 large polysiphonic branches running in a roughly horizontal direction. From 
 one side of these principally arise smaller polysiphonic branches, all lying in one plane. 
 From the sides of the branches arise numerous pinnae (at largest Jin. in length) 
 irregularly arranged ; in addition to these, hydrocladia (pinnules) may arise direct 
 from the hydrocaulus. Two pinnules, alternate, arise from each joint of the primary 
 pinnae. Pinnules composed of alternate small and large joints, the latter only 
 bearing hydrothecae and nematophores. Nematophores bithalamic with simple 
 terminal aperture, one beneath each hydrotheca, two at the level of the mouth of the 
 hydrotheca, all independent of the latter. Two nematophores in the angle between 
 the pinnule and main stem of the pinna; nematophores scattered irregularly in great 
 numbers over the stirface of the polysiphonic stem and branches. 
 
 Colour. Light-brown stems. 
 
 Hab. Port Phillip, Mr. J. Bracebridge Wilson. 
 
 The colony reaches a considerable size, and from the fact that the main branches 
 run in one direction (Fig. 20) with the smaller ones arising principally from one side, 
 it is inferred that in all probability it is procumbent in habit. The branches of the 
 hydrocaulus are all polysiphonic and strong, the pinnae and hydrocladia, which arise 
 from them, being very small indeed in comparison. The two latter are given off all 
 round the branches, and reach at most the length of Jin., whilst the main branch 
 may be Gin. in length. 
 
 Skeleton. 
 
 (a) Structure of large branches (Figs. 24, 25, 26, 27, 28). 
 
A NEW FAMILY OF HYDROIDEA. 131 
 
 Under the lens the tubes making up the branches can be clearly seen, the 
 surface being marked by darkish brown lines, due to the edges of the perisarcal 
 tubes seen in optical section (Fig. 25). The tubes on the exterior run parallel 
 to one another along the length of the stem, branching very rarely. From the 
 surface arise irregularly, and on all sides (1) large pinnate shoots, the proximal 
 parts of which are covered with one or more layers of dark-brown perisarc, within 
 which only one tubular cavity is contained ; (2) smaller branches (hydrocladia), 
 corresponding to the pinnules in structure; (3) a great number -of nematophores, 
 or protective cases, for the minute defensive zooids which are characteristic of the 
 group. 
 
 In transverse section (Figs. 27, 28) the branch is seen to be made up of a great 
 number of tubes as many as 40-50 in a branch of average size the number varying 
 with the size of the branch. 
 
 Each tube has a definite and thick perisarcal wall, which shows clear indications 
 of arrangement in layers. In places the cavities within the tubes are seen to be (Figs. 
 24, 27, 28) in communication with one another. A striking feature of both trans- 
 verse and longitudinal sections is the presence of a distinct central tube, always clearly 
 recognisable, both on account of its size and the slightly yellow colour of its walls 
 when compared with the surrounding ones (Fig. 24). This central tube, which, from its 
 relations to the other parts, is probably to be regarded as homologous with the main 
 stem of a monosiphonic form, passes along all the branches of the colony, and from 
 it arise every one of the pinnae and hydrocladia, though, as will be shown shortly, 
 these may be also connected with the other tubes which go to form the polysiphonic 
 stem. Professor Allnian has figured* in Aglaophenia coarctata a connection existing 
 between the various tubes of a polysiphonic stem, and this is most clearly seen in the 
 species under consideration. The connection is, however, somewhat different from 
 that obtaining in A. coarctata, which is thus described by Allman : " Communication 
 is effected by very short processes, which are given off from the component tubes, 
 those of two juxtaposed tubes meeting one another and inosculating in such a way 
 as to suggest the conjugation of a zygnema." In longitudinal section (Fig. 24) the 
 various tubes are seen to be arranged so that they run parallel to the central one, 
 and at intervals their walls are pierced by apertures. Where the pinnae and 
 hydrocladia pass through, the walls of the latter are connected with those of the 
 stem-tubes, and apertures are formed opening into the cavities of the latter. 
 
 (b) Structure of the pinnae, etc. (Figs. 21, 22, 23, 25, 26). 
 
 The origin of these from the stem is shown in figure 25, their structure in 
 figures 21, 22, and 26. 
 
 * "Challenger" Reports, Hydroidea, Part I., Plumulariidte. 
 
132 A NEW FAMILY OF HYDROIDEA. 
 
 The basal portion of each pinna is strengthened by the development of a very 
 strong layer of dark brown coloured perisarc, continuous with that of the tube walls. 
 This thick external layer ends abruptly just before reaching the distal end of either 
 the first or second joint (Fig. 25). Each joint beyond the first one or two carries 
 two pinnules. There appears to be a slight variation in the most proximal joints ; 
 sometimes the first, sometimes the first and second, differ from the rest, in bearing 
 each only one pinnule. 
 
 The pinnules are alternate, and consist of a varying number of joints, which are 
 alternately shorter and longer, the latter only bearing the hydrothecse and 
 nematophores (Figs. 21, 26). This arrangement is constant in all specimens which 
 I have examined. In other Plumulariae the pinnules of which are composed of 
 alternately longer and shorter joints, such, for example, as P. setaceoides, goldsteini, 
 delicatula,* the shorter ones always bear a nernatophore. This is absent in 
 P. procumbens. The hydrothecte are cup-shaped, with a smooth margin, and are 
 placed on the side facing the central stem of the pinna. The most distal pinnules 
 carry one hydrotheca each, the proximal as many as six. The cavity is separated 
 from that of the joint by a septum pierced by a circular opening, which lies 
 near to the external wall. There is no trace of an intrathecal ridge. 
 
 The nematophores are three in number on each of the larger joints ; one is 
 placed below the hydrotheca, two at the level of its upper margin. Each is 
 bithalamic with the terminal chamber cup-shaped, and the proximal one somewhat 
 canaliculate. The walls are thin (Fig. 22), except where the division into the two 
 parts is formed, at which spot they thicken considerably, and give rise to a circular 
 ridge projecting upwards into the distal chamber. The opening is single and terminal, 
 and each nematophore is only attached by its proximal end, where the walls are thin, 
 and may be thrown into slight folds. 
 
 The walls of the pinnule joints show internal ridges, which are always more 
 prominent on the side facing the central stem of the pinna, and thin away towards 
 the opposite surface. They are always arranged thus (1) In the larger joints 
 there is a ridge close to each extremity, with a third one corresponding in position 
 to the septum of the hydrotheca ; this varies much in development, being sometimes 
 scarcely noticeable. (2) In the smaller joints there are uniformly two ridges. (3) 
 In the projection from the joint of the pinna bearing the pinnule there is always one 
 ridge. Taken altogether the result is that each line of division in the pinnule has 
 one ridge immediately on either side of it (Fig. 21). 
 
 In the axil of the pinnules there are present (1) Two nematophores correspond- 
 ing in structure exactly to those described above, (2) between these a curious 
 
 * Bale. Catalogue of Australian Hjdroids, PI. XI. 
 
A NEW FAMILY OF HYDROIDEA. 133 
 
 structure formed of the perisarc, having the shape of a cone with the apex cut off 
 (Fig. 21). The space within the latter communicates by the narrow end with 
 the exterior, and by the broader with the cavity of the pinna joint. Into it 
 cells of the ectoderm may enter to a slight degree, but more usually it appears 
 to he unoccupied (in spirit-preserved specimens), and I am quite unable to 
 attach any meaning to it, though it is a perfectly constant structure. It has 
 nothing to do with the reproductive structures. Possibly it may serve as a means 
 of allowing of the ingress and egress of water to and from the perisarcal tubes. 
 Any space between the ectoderm and the perisarc in the very numerous tubes which 
 compose the colony must presumably be filled by liquid. The openings leading into 
 the hydrothecae and nematophores from the stem are small and narrow, and quite 
 filled up by the soft parts. When sudden contraction takes place part of the soft 
 portions must be withdrawn through these openings and occupy space within 
 this perisarcal tube previously, presumably, occupied by fluid. If there be 
 some means of expelling this fluid then the sudden contraction of the polypes and 
 machopolypea is rendered more easy. It may be that these openings serve this 
 purpose. The openings are guarded, as it were, by two machopolypes. 
 
 Soft Parts (Figs. 17, 18, 19, 24). 
 
 (a) Larger branches. 
 
 These are all polysiphonic. The ccenosarcal tubes of which each is composed 
 may be divided into two divisions (1) a central one, (2) others varying in number 
 and surrounding this. The tubes very rarely branch in such a way that two 
 running longitudinally arise from a common one, though (Figs. 24, 27, 28) they 
 frequently are united with one another by short transverse branches passing through 
 openings in the perisarcal walls. Each consists of ectoderm and endoderm 
 containing a cavity. From the outermost series arise a great number of minute 
 machopolypes or defensive zooids. The central one gives origin to all the branches 
 passing out into the pinnce and hydrocladia, with which the branches are irregularly 
 studded. As they pass from the centre to the external surface they are connected 
 with a varying number of the surrounding coenosarcal tubes, a feature which is 
 especially marked in the case of the pinna? (Figs. 17, 24). This arrangement is 
 clearly seen when sections are cut, and has not, so far as I am aware, been noted before. 
 Bale,* in speaking of the structure of the stem and branches in the Plumulariidse, 
 says that in most polysiphonic species " the primary jointed stem is slender (the 
 requisite strength being given by the compound stem, which is only developed as the 
 zoophyte increases in size), and the branches spring, not from the jointed stem, but 
 from the supplementary tubes which grow up in contact with it. For example, in 
 
 * Genera of the Plumulariidcc, with observations on various Australian Hydroids. Proc. K. S. Victoria, 1886. 
 
134 A NEW FAMILY OF HYDROIDEA. 
 
 Aglaophenia longicornis we find at the back of the original slender-jointed stem a 
 stouter secondary tube, and from this spring, at regular intervals, the alternate 
 
 pinnately arranged branches.* Keeping in mind the 
 
 hydrorhizal origin of the polysiphonic stem we see that in Aglaophenia longicornis, for 
 example, every one of the main pinnae is equivalent to a separate shoot of such 
 species as A. parvida, a fact which is further illustrated by the presence, near the 
 base of the stem in the latter species (and, indeed, in many others), of a long 
 oblique joint similar to that which exists near the base of each pinna in A . longicornis. 
 
 / have not hitherto met with any species with branches springing 
 
 both from the jointed stem and the added tubes."* 
 
 It will be seen at once that an important difference exists in this respect between 
 the species now described, and those examined by Mr. Bale, and, so far as I am 
 aware, any other investigator. Not only is there a very distinct connection between 
 the soft and hard parts of all the tubes of the polysiphonic stem, but the central 
 one, though distinguishable from the rest, has the same fundamental structiire as 
 the latter so far as its walls are concerned, and shows no traces of joints. f The 
 question naturally arises What elements are we to consider as entering into the 
 structure of the polysiphonic stem in this form ? 
 
 We may regard it as composed of 
 
 (1.) A central tube equivalent to the hydrocaulus of a monosiphonic form, 
 which is surrounded by a series of added but modified hydrorhizal 
 elements, or 
 
 (2.) A number of hydrorhizal elements forming branch-like structures, and 
 giving off pinna?, or 
 
 (3.) A number of hydrocauli in close apposition to one another. 
 
 The last is the most improbable, since, if it were the case, we might expect pinnse 
 to be given off from all or any of the tubes composing the stern, whilst they all arise 
 primarily from the central one. The same objection applies to the second, and the 
 distinction which undoubtedly obtains between the central and all the other tubes 
 seems to point to the fact that the former is the fundamental portion, the latter being 
 secondary structures. At the same time a hard and fast line of distinction between 
 hydrocaulus and hydrorhiza cannot possibly be drawn. In some forms of 
 Plumulariidse we find the pinnae arising directly from the hydrorhiza, in others they 
 arise from the sides of a hydrocauline structure which grows upwards from the 
 
 * The italics are mine. 
 
 t At the free extremity of each branch it is continued directly on into a jointed stem forming the centre of a pinna, 
 and may probably be regarded as having lost its jointed nature subsequent to its being completely enclosed by the 
 surrounding tubes of the polysiphonic stem. 
 
A NEW FAMILY OF HYDHOIDEA. 
 
 hydrorhiza. Sometimes the structure bearing the pinnae, or the central stem of the 
 pinna) itself, may be strongthenetl by the addition of tubes clearly growing up from 
 the hydrorhiza: These tubes, in some mouosiphonic forms, may be feebly developed, 
 whilst in typical polysiphonic ones apparently homologous structures may be constant 
 and greatly developed. In the former case the distinction between hydrocaulus and 
 hydrorhiza is clear enough, but then if the true polysiphonic form be investigated, we 
 iiud that the supplementary structures grow around the primary jointed stem, and (1) 
 finally (as in Aglaophenia longicornis) give rise to the pinnate branches, just as does the 
 hydrocauline tube in such a form as P.falcata, the jointed stem bearing no branches; 
 or (2) they form (as in P. procumbent) an enclosing mass for the original tube which 
 alone gives off the pinnae, though these may be connected with the hard and soft parts 
 of the enclosing branches. Tims what must be regarded as homologous structures, 
 may either (1) be feebly developed, and retain their original hydrorhizal nature ; or (2) 
 be strongly developed and (a) give rise to pinna?, and, as it were, usurp the function of 
 the original hydrocauline tnbe which they support, or (b) assume an intermediate form, 
 the original tube which they inclose alone giving rise to pinna?, with which, however, 
 ;is well as with the former they are in organic connection. 
 
 (b) Pinna;, etc, 
 
 There is little to say with regard to the structure of the soft parts contained in 
 the pinna? ; the hydranths have the form typical of the genus Plumularia, with the 
 body divided into two parts by a central constriction, the distal part bearing a single 
 circlet of solid tentacles at the base of a broad hypostome, the proximal half being 
 somewhat globular and presenting no special feature. 
 
 The machopolypes of the main branches and the pinnae are of precisely similar 
 structure, corresponding to those of other forms which Lendenfeld has distinguished 
 as "guard animals with corticating capsules."* There is no trace of any with 
 adhesive cells such as are found in the genus Aglaophenia. 
 
 Each machopolype consists of a proximal tubular part lying in the proximal 
 half of the nematophore and a distal swollen part, which contains rounded 
 nematocysts. I have been unable to study the living form, and have only seen 
 spirit specimens, in which the soft parts are of course much contracted ; in these, 
 lines running from the head down the stalk probably indicate muscle fibres, the 
 machopolype being capable of great extension. 
 
 In figure 17 is represented a restoration of the soft parts of a small portion of 
 one of the polysiphonic stem, together with one of the hydrocladia and pinna? which 
 arise from it. 
 
 Ztitsch. I. Wissen. Zool., Vol. XXXVIII., p. 335. 
 
136 A NEW FAMILY OF HYDROIDEA. 
 
 (c) Reproductive structures. 
 
 The only specimen bearing reproductive structures was a male colony. The 
 gonothecas (Figs. 18, 19, 23) are simple and pear-shaped, with a large terminal 
 opening and short stalk springing from the axil of a pinnule. Each contains one 
 gonophore (Fig. 18), which in longitudinal section is seen to arise from the 
 blastostyle and to fill nearly the whole cavity. The blastostyle in the spirit- 
 preserved specimen examined was much shrunken with a terminal swelling (D) below 
 the opening of the gonotheca. The cell-layers could not be distinguished. The two 
 prominent parts of the gouophore are the mass of sperm cells (sp), which stain deeply, 
 and the spadix (Fig. 19, end.) Outside the sperm cells a very thin layer of ectoderm 
 can be distinguished. I failed after long searching to recognise any trace of 
 reproductive elements in the coenosarcal tubes or blastostyle, or any appearance of 
 the formation of more than one gonophore in each gonothecse, though in 
 P. echinulata Weismann states that very often a second gonophore may be found 
 before the contents of the first have passed out.* In P. halecioides also it seems 
 that two may be present at the same time.f 
 
 * Entstehung der Sexualzellen bei den Hydromedusen, p. 180. 
 t Loc. cit., p. 184. 
 
DESCRIPTION OF PLATES. 
 
 Plates 17, 18, 19, and 20 refer to Clathrozoon wilsoni. 
 
 PLATE 17. 
 
 Figure 1. The skeleton of a dried specimen of Clathrozoon wilsoni, life size, 
 from a photograph. 
 
 Figure 2. Skeleton of part of a colony of Clathrozoon wilsoni, drawn from a 
 
 spirit specimen. 
 
 PLATE 18. 
 
 Figure 3. Restoration of the soft parts. The hard skeleton parts are entirely 
 omitted. Only the end of one of the branches of a colony is represented, hut this 
 is typical of the structure of the whole. The upper surface of the branch is 
 supposed to be cut away to show the connection of the gastrozooids with the 
 ccenosarcal tubes. The outermost of the latter are studded with the defensive zooids. 
 The whole is much magnified, the actual size of the branches being represented in 
 figures 1 and 2. 
 
 Figure 4. Represents a longitudinal section of a small portion of a branch to 
 show the connection of a polype (retracted) with the ccenosarcal tubes. In the 
 figure the polype is connected with three of these. (A) The perisarc walls are shown, 
 and the thin external layer forming at the mouth of the hydrotheca an operculum 
 P. Outline drawn with the camera under Zeiss A. oc. 2. 
 
 Figure 5. Transverse section of a small portion of the outermost part of a 
 branch to show a defensive polype, connected with one of the outer crenosarcal tubes, 
 which is connected again with a deeper lying one. The defensive polype is enclosed 
 by the nematophore. E. is one of the most external tubes, the outer wall of which is 
 formed of the thin layer (P.) of perisarc, on which lies an accumulation of foreign 
 substances, spicules, &c. Outline drawn with camera under Zeiss A. oc 2. 
 
 PLATE 19. 
 
 Figure 11. A small branch much enlarged showing the skeleton only, the soft 
 parts having been dissolved in potash. The hydrothecae are shown arranged spirally 
 and projecting with circular margins slightly from the surface. The network of 
 chitinous tubes is seen to represent somewhat in appearance the clathrate horny 
 skeleton of certain sponges. The somewhat prominent lines on the surface corres- 
 
138 A NEW FAMILY OF HYDROIDEA. 
 
 pond to the dark curving ones in figure 6. The thin external layer with the 
 nematophores is only shown at the edges, in reality it covers the whole surface. 
 Letters as before, x 50. 
 
 Figure 12. A diagrammatic drawing to show the relationship of the hard and 
 soft parts as seen at the external part of a longitudinal section of a branch. A 
 gastrozooid is shown partially expanded. Hy. the walls of the hydrotheca. C. 
 coenosarcal tube. Ma. protective zooid arising from the outermost coenosarcal tube. 
 P. the thin external layer of perisarc. P'. the continuation of the latter to form a 
 collar beyond the opening of the hydrotheca. P". the nematophore. T. tentacle. 
 
 Figure 13. A semi-diagrammatic drawing of a hydrotheca as seen in a thick 
 transverse section of a branch very much enlarged. The walls of the hydrotheca are 
 supported by numerous extensions of the perisarcal network, and the openings into 
 the internal-lying end of the hydrotheca are shown. The perisarc tubes have 
 branched and anastamosed to such an extent that their walls form a network, and 
 the tubular structure is not so evident as it is in the more deeply lying parts. 
 Letters as before. 
 
 Figure 14. The basal part of a gastrozooid cut obliquely to show the strong 
 band of muscular fibres derived from the ectoderm together with the thick layer of 
 mesogloea. Ect. ectoderm. End. large granular endoderm cells. M. thick mesog- 
 loea layer. Mus. muscle fibres, some in connection with the ectoderm cells, from the 
 basis of which they arise, others cut at a deeper level, at which their connection with 
 ectoderm cells is not seen. Possibly some have lost their connection with ectoderm 
 cells. Drawn under Zeiss F. oc. 2. 
 
 Figure 15. A defensive zooid highly magnified. It consists of a stalk and head. 
 The former is in connection with a ccenosarcal tube one wall of which, as seen in 
 longitudinal section is shown. The centre of the stalk is solid and continuous with 
 the endoderm, though no cellular structure can be determined. Outside tins is a 
 thin layer of ectoderm in which nuclei are scattered. The head consists of nemato- 
 cysts at the bases of which remnants of the cells in which they have been formed 
 can be seen with their nuclei. Ect. ectoderm. End. endoderm. M. mesoglosa. 
 N. nematocyst. Drawn under Zeiss apo. 4.0 mm. apert. 0.95 oc. 12. 
 
 Figure 16. The collar-like operculum surrounding the margin of a hydrotheca 
 seen from above. The collar is thrown into folds. Drawn under Zeiss F. oc. 2. 
 
 PLATE 20. 
 
 Figure 6. A terminal branch, much enlarged to show the circular openings of 
 the hydrothecae, within which the polypes are withdrawn, and also the general spiral 
 arrangement of the hydrotheca3. The surface is covered with tortuous grooves, 
 
A NK\V FAMILY OF HYDKOIUKA. 
 
 bounded by dark lines, indicating the edges of the perisarc, and is studded with 
 nematophoree seen at the edges of the branch. Hy. Hydrothecae. P. nematophores. 
 
 Figure 7. A longitudinal section of a small branch, showing the skeleton. The 
 tubes of which the branch is composed are seen to be very irregular, but to run, 
 r>i>irially towards the central part, in a direction generally parallel to the length of 
 the branch. E. spaces or grooves, immediately beneath the thin external layer. 
 Hy. hydrothecae. P. thin external layer of perisarc. P'. the continuation of P. 
 to form an operculuin at the mouth of the hydrothecae. P". the iieinatophores. 
 x35. 
 
 Figure 8. A transverse section of a small branch, to show the tubes of which it 
 is composed. The tubes vary in size, and open into one another. The outer ones 
 are incomplete externally, forming grooves, the lips of which are usually touched by 
 the thin external layer of perisarc. Two hydrothecae are cut through. The perisarc 
 is thicker towards the outer than in the inner part of the branch. Letters as in 
 Fig. 7, x 70. 
 
 Figure 9. A small portion of the external part of a branch very much enlarged 
 to show the roughly concentric layers of which the perisarc is formed. Letters 
 as in Fig. 7. 
 
 Figure 10. Small portion of a ccenosarcal tube with the perisarc walls from 
 the interior of a branch. Ect. the ectoderm in which the outline of the cells cannot 
 be clearly distinguished, and which varies in thickness in various parts coming in 
 contact, in certain places, with the perisarc walls. End. the unilaminar endoderni, 
 the cells of which are larger than those of the ectoderm, x. globular structures of 
 unknown significance containing darkly staining parts which lie apparently within 
 the cavity of the tube. Drawn under Zeiss F. oc. 2. 
 
 Plates 21, 22, 23 refer to Plumularia procumbens. 
 
 PLATE 21. 
 
 Figure 17. Restoration of the soft parts only of Plumularia procumbens. The 
 polysiphonic stem is shown in section with the transverse connections between the 
 various tubes which compose it. Down the centre runs the main tube from which 
 arise all the lateral branches pinnae and hydrocladia which are in connection with 
 a varying number of the surrounding tubes. The most external ones give off 
 numerous machopolypes, and on the pinnae are shown the groups of polypes, each 
 consisting of one gastrozooid and three machopolypes. One blastostyle is shown 
 with a gonophore, arising from the angle between a pinnule and the main stem of the 
 pinna, x 30. 
 
 Figure 18. Longitudinal section of a male gonangium with its contained 
 blastostyle and gonophore. In the centre of the gonophore lies the mauubrium. Bl. 
 
140 A NEW FAMILY OF HYDROIDEA. 
 
 blastostyle. D. swollen distal, end of blastostyle. Sp. sperm. G. walls of 
 gonotheca. 
 
 Figure 19. Transverse section of a male gonangium and gonophore. On the 
 left side between the gonophore and the wall are seen traces of the much compressed 
 blastostyle. End. endoderm of manubrium. Ect. ectoderm outside sperm cells (sp.) 
 
 PLATE 22. 
 
 Figure 20. Portion of a colony of Plumularia procumbent showing the poly- 
 siphonic stem and the pinnae arising irregularly from this, x \\. 
 
 Figure 21. Much enlarged portion of a pinnule or hydrocladium to show the 
 alternate longer and shorter joints. The latter bears no hydrotheca or 
 nematophores ; the former bears a hydrotheca with the nematophores at 
 the level of the mouth of the former, and one in the median line 
 below. The thickenings in the walls of the pinnule are shown in their positions 
 and the two nematophores in the axil between the pinnule and the main stem of the 
 pinna, and at A, the conical structure opening to the exterior. N 1 . upper pair of 
 nematophores. N 2 . lower median nematophores. N 3 . nematophores in the axil of 
 the pinnule and pinna stem. A. conical process in the axil leading from the interior 
 of the pinna to the exterior. 
 
 Figure 22. Much enlarged view of a nematophore seen in optical section, and 
 showing the two chambers. 
 
 Figure 23. Much enlarged view of a male gonangium. 
 
 Figure 24. Semi-diagrammatic drawing of a longitudinal section of the poly- 
 siphonic stem of the same to show the relationship of the hard and soft parts and the 
 distinctness of the central tube with its walls slightly yellower than those of the 
 surrounding tubes. C. central tube. H. central stem of pinna. H'. hydrocladia. 
 L. lateral tubes. N. nematophores. T. transverse connections uniting the various 
 tubes, x 30. 
 
 PLATE 23. 
 
 Figure 25. Much enlarged view of the termination of a polysiphonic stem, 
 showing the numerous nematophores on the stem and the pinnae &c., arising 
 irregularly. The basal parts of the pinnae are strengthened by the formation of a 
 thick perisarc wall continuous with that of the tubes forming the stem, x 20. 
 
 Figure 26. A highly magnified portion of a pinna. 
 
 Figures 27 and 28. Transverse sections of the polysiphonic stem to show the 
 component tubes skeleton only with the large central one, from which in figure 28 
 a pinna is arising. The external tubes are studded with nematophores. 
 
T*ans.R. Victoria Plate I! 
 
 
 CLATHROZOON WILSON 
 
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ON THE 
 
 STRUCTURE OF CEKATELLA FUSCA 
 
 (GRAY). 
 
 By W. BALDWIN SPENCER, M.A., 
 
 Professor of Biology in the University of KIcllii'in n, . 
 
 *{iri;ivni. iiin.M -nil. THAN- ... ilu: Kov.M, Som/iv <n \'ici(H;i\ 101; 1861-. 
 
ON THE STRUCTURE OF CERATELLA FDSCA (GRAY), 
 
 BY 
 
 W. BALDWIN SPENCER, M.A., 
 
 PROFESSOR OF BIOLOGY, IN THE UNIVERSITY OF MELBOURNE. 
 
ARTICLE II. ON THE STRUCTURE OP CERATELLA FUSCA (GRAY), BY W. BALDWIN 
 SPENCER, M.A., PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF MELBOURNE. 
 (With Plates 2, 3, and 3a.) 
 
 (Read Thursday, June llth, 1891.) 
 
 I have to thank Dr. Ramsay and the Trustees of the Australian Museum, 
 Sydney, for the opportunity of examining the structure of this interesting hydroid 
 form. The specimens examined came from Bondi on the New South Wales coast 
 and from Lord Howe Island, where they were collected by Mr. Whitelegge of the 
 Australian Museum, whom I have to thank for kind assistance. The Lord Howe 
 Island specimen had the zooids beautifully expanded. 
 
 Dr. Gray* was the first to describe and figure Ceratella and the closely allied 
 genus Dehitella from specimens in the British Museum. He had dried specimens 
 only at his disposal and his description necessarily refers only to the hard parts 
 and these having the form of a horny network somewhat resembling in general 
 appearance the skeleton of a horny sponge, led him to place the two forms 
 provisionally amongst the sponges. For their reception he constituted the family 
 Ceratelladcz. Five years later Mr. H. J. Carterf published a paper entitled 
 " Transformation of an Entire Shell into Chitinous Structure by the Polype 
 Hydractinia, with short Descriptions of the Polypodoms of five other Species." In 
 this he refers to Ceratella fusca and Dehitella atrorubeus and describes two new forms 
 belonging to the former genus under the names of C. procumbens and C. spinosa. 
 Both of these came from South Africa the former from the Cape of Good Hope and Natal 
 the latter from Natal. In the same paper he describes a new genus Chitina with a 
 single species C. ericopsis which came from New Zealand. Reference to these will 
 again be made after the anatomy of Ceratella fusca has been described, meanwhile 
 it is sufficient to note that Mr. Carter's investigations of a species of Hydractinia 
 (H. levispina) with a skeleton composed of a horny network which incrusts and eats 
 its way into univalve shells led him to re-examine the two forms placed by Dr. Gray 
 in the family Ceratelladae and in consequence of his finding undoubted traces of 
 thread-cells in the dried specimens both of these and of the two new species 
 of Ceratella mentioned above, he rightly recognised them as belonging to the 
 
 * Proo. Zool. Soc., Nov., 1868. 
 
 t An. and Mag. Nat. Hist., Jan., 1873. 
 
ON TUK STRUCTURE OF CEKATELLA FUSCA (QUAY). 9 
 
 llyitrouk'H and not to the sponges in which group Dr. Gray had provisionally placed 
 them. At the same time it may perhaps be well to note that the finding of 
 thread-cells in dried specimens with a skeleton in the form of a horny network 
 is not in itself absolute proof of their "belonging to the Hydroidea inasmuch as at the 
 present time sponges are known to exist the substance of which is pierced by hydroid 
 growths so that it would be possible to find thread-cells in the dried skeletons of 
 sponges. At the same time Mr. Carter was perfectly right, as events have proved, in 
 removing the Ceratelladac from the Porifera though the subsequent discovery by 
 Mr. Bale of the nature of their soft parts has shown that they cannot be placed, 
 where Mr. Carter put them, in the family Hydractiniida). 
 
 The next notice of these forms occurs in the Proceedings of the Liuuean Society 
 of New South Wales for 1886 (p. 575) when Mr. Brazier recorded the occurrence of 
 Ceratella fnsca from various localities near Sydney such as Boudi Bay and Coogee. 
 In the Proceedings of the same Society for the year 1888 (p. 745) Mr. Bale for the 
 first time gave some description of the soft parts and showed that the zooids were 
 formed on a very different type from those of the Hydractiniida;. The latter have a 
 single circlet of filiform tentacles surrounding the hypostome whilst those of Ceratella 
 are irregularly distributed over the body and are capitate. Mr. Bale accordingly 
 removed Ceratella from its position amongst the Hydractiniida; assigned to it by 
 Mr. Carter and placed it in a distinct family to which he gave the name Ceratellidu;. 
 He apparently overlooked the fact that Dr. Gray had already adopted the name of 
 Ceratelladiu for the family including his two genera Ceratella and Dehitella, so that 
 this name given in 1868 must now be retained. With a more abundant supply of 
 material I have been enabled to work out the structure in greater detail than was 
 possible to Mr. Bale to whom we owe the first description of the soft parts and 
 the determination of the fact that Ceratella belongs to a family distinct from the 
 Hydractiniida). 
 
 DESCRIPTION OF THE STRUCTURE OF CERATELLA FUSCA. 
 
 I have endeavoured so far as possible to give a complete account of the anatomy 
 of both soft and hard parts the only figures yet published being those of the external 
 form given by Dr. Gray* both of which suffice to clearly identify the genera. 
 
 Skeleton. 
 
 The colonies of Ceratella procured on the New South Wales coast measure from 
 l 5 inches in height and are of a rich brown colour. The largest specimen which 
 I have seen is the one procured by Mr. Gabriel which came from Flinders Island 
 in Bass Straits where it had been washed ashore. The figure given by Dr. Gray 
 
 * Loc. cit. 
 
10 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 
 
 represents fairly well the macroscopic characters of the skeleton but conveys the idea 
 of one which has been much water-worn and has lost many of its smaller branches. 
 The branching is much richer and closer than is represented by him and the 
 projecting hydrophores are more distinct and regular. At the same time there is a 
 certain amount of difference in the growth of various colonies some of which are 
 more bushy in appearance than others. That secured by Mr. Gabriel has the 
 branches more distinctly arranged in one place than is the case with the others. 
 The whole colony may be described as follows : The colony arises from a much- 
 branching root-like base encrusting foreign objects. The root-branches unite to 
 form a strong stem common to the colony which is flattened in the same plane as 
 that in which the branches of the colony, generally, expand. The common stem 
 may have the appearance of being formed of intertwined branches and from it arise 
 irregularly larger or smaller branches the former being more or less flattened in the 
 same plane with itself. 
 
 From the larger arise irregularly on either side smaller branches which again 
 branch until a somewhat bush-like or fan-shaped colony is formed the whole being 
 more or less flattened in the plane of the main stem. Except the main stem close 
 to the root branches and the latter all the branches may bear bracket-like 
 projections the hydrophores which are arranged in a roughly spiral manner and 
 are especially abundant on the smaller branches except the growing ends which are 
 somewhat swollen and flattened in a plane at right angles to that in which the 
 branching of the colony takes place. 
 
 In addition to considerable variations in the general form of the colony some 
 appear to have the hydrophores so arranged that they lie along the two opposite sides 
 only of the branches whilst in others they are arranged all around. The more 
 bush-like the growth is, to the greater extent does the latter obtain and vice versa. 
 Usually there is only one main stem arising from the root-branches but at times one 
 or more smaller ones may arise independently. (Fig. 1.) 
 
 The skeleton when examined by the lens has much the same appearance as that 
 of Hydractinia and differs considerably from that of Clathrozoon and more still from 
 that of the polysiphonic stem characteristic of certain species of Plumularia. It forms 
 in the larger branches a meshwork of chitinous tissues so similar at first sight to 
 that of certain homy sponges that as Mr. Bale says " a portion broken off and 
 examined separately might well be mistaken for sponge tissues." This however is 
 only true with certain limitations for, if the soft parts are present, the distinction 
 between the two is readily seen as is also the case when the hydrophores are present. 
 Apart from this also nothing comparable to the layer of cells secreting the sponge 
 skeleton is present nor are the chitin fibres of the two similar in their minute 
 structure. 
 
ON THE STRUCTURE OP CERATELLA FU8CA (GRAY). 11 
 
 The meshwork is extremely irregular in the larger branches as is shown in the 
 sections represented in figs. 9 and 11. There seem to be two not clearly 
 distinguishable portions present (1) large and strong fibres (2) smaller ones which 
 form connecting bars and often have the appearance of thin web-like plates which 
 are especially well developed on the hydrophores. Fig. 5 represents a medium-sized 
 branch, from which the soft parts have been removed by potash, seen by reflected 
 light. At times the fibres seem to run for some distance parallel to the long axis of 
 the branch though this is much more strongly marked in some branches than in the 
 one figured. There are no definite hydrothecne present within which the zooids can 
 be completely retracted but the branches are studded sometimes only at the sides 
 sometimes all over with little bracket-like projections which give a characteristic 
 serrate appearance to the branches, very different however from that which is 
 produced by the hydrothecac of the Sertularians. Mr. Bale has aptly suggested that 
 the term " hydrophores " (originally applied by Allman to the calyces of Halecium) 
 should be used to describe these structures which form merely supports for the 
 hydroid zooids. Each one may perhaps be best likened to a very concave scallop 
 shell with ribs formed by the strong fibres continuous with those of the branch whilst 
 the spaces between them are filled up by a thin fenestrated web of chitin. In some 
 hydrophores the ribs are more strongly marked than in others and project as small 
 points around the margin. (Figs. 5 and 6.) The growing ends of the smaller 
 branches usually contain two or three longitudinal fibres connected by transverse 
 bars, often somewhat web-like, and each of these growing ends is clearly 
 distinguishable (1) by its being flattened in a plane at right angles to that in which 
 general growth takes place and (2) by the entire absence of hydrophores and zooids. 
 It may be added that the longitudinal arrangement of the fibres in the branches is 
 more clearly noticeable in specimens with the soft parts present than in those in 
 which the latter are absent since the soft parts conceal from view largely the 
 connecting and the deeper-lying fibres and thus prevent to a large extent the network 
 structure from being seen. The whole skeleton also in living specimens is 
 completely enclosed by the soft parts though only a thin layer of tissue is present 
 over the external surface. (Figs. 9, 10 and 11 E.) There is no protective covering 
 for the reproductive structures. 
 
 Soft Parts. 
 
 The structure of these has, as yet, been only briefly described once and that by 
 Mr. Bale, whose short account refers only to the external form of the hydroid zooids. 
 He pointed out, as stated before, that these possessed irregularly scattered capitate 
 tentacles and that hence they differed considerably from those of Hydractiuia. 
 
 Amongst the specimens from the Australian Museum is one collected by 
 Mr. Whitelegge on Lord Howe Island with the zooids fully expanded and others 
 with the soft parts well preserved. 
 
12 ON THE STHUCTURE OF CEIIATELLA. FUSCA (GRAY). 
 
 (1). Hydroid zooids (Figs. 2, 4, 6, 7, 9, 13). These may be found on all the 
 branches, large and small except at the growing ends. Often also they are absent on 
 the larger main branches and are always fewer in number on these than on the 
 smaller ones (Fig. 4). Each zooid, in a well preserved spirit specimen, reaches a 
 length of about 1'4 m.m. and in the general form of the body resembles, as Mr. Bale 
 says, those of Coryne. The body (Figs. 4 and (3) is somewhat elongate with a 
 terminal almost conical portion, at the apex of which lies the mouth opening. The 
 basal portion which is seated upon the hydrophore is broad, this is followed 
 by a slightly contracted portion, then comes a slightly swollen part which gradually 
 diminishes in size towards the mouth end. Over the surface are scattered irregularly 
 the capitate tentacles from 10-14 or perhaps even more in number (Figs. 4 and 0), 
 and one or two of these are frequently placed close to or upon the basal region. 
 
 The minute structure is generally that which is typical of hydroid forms and is 
 represented in Figs. 2 and 7. The ectoderm (Eel*) is unilaminar over the general 
 surface the cells being in close apposition each with a large nucleus and except in 
 the capitate ends of the tentacles there are no thread cells present. At the ends of 
 the tentacles the ectoderm is swollen out and apparently forms a mass of cells in 
 which large nematocysts are present with barbed threads (Fig. 7). The nematocysts 
 here and elsewhere seem to be all of the one size. The ectoderm lies on a thin layer 
 of mesogloea scarcely visible in extended zooids but more prominent in retracted 
 ones. There cannot be detected any fibrous musciilar elements such as form so 
 distinct a feature around the basal region of the zooids in, for example, Clathrozoon. 
 The endoderm consists of large vacuolate cells each one subtending the base of, as a 
 general rule, at least three ectoderm cells. The protoplasm appears to be always 
 concentrated at the inner ends where the nuclei are placed and where in preserved 
 specimens the cell outlines are completely lost. In zooids which are feeding 
 (Fig. 7) this inner end of the cell is filled with minute food particles the digestion 
 being evidently, in part at least, intracellular. 
 
 The ectoderm is continuous at the base with the layer which covers externally 
 the whole colony whilst the endoderm is continuous with that of two or more of the 
 ccenosarc tubes. 
 
 (2.) Gonophores. I have only been able so far to find the male gouophores 
 which are present on three colonies secured at Coogee on the New South Wales 
 coast. Each of these colonies carries numbers of minute somewhat pear-shaped 
 structures which are only from one-third to one-quarter of the length of the hydroid 
 zooids and which are seen when rendered transparent or cut into sections to be 
 medusoid in nature. 
 
ON THE STRUCTURE OF CERATELLA Ft'SCA ((ilt\V). 13 
 
 i^onophores arise directly from the ccenosarc and arc not carried by modified 
 :: / n lids or blastostylcs. 
 
 Tlioy may be very numerous indeed especially on the medium-sized branches 
 win-re their number in one specimen far exceeds that of the hydroid xooids. Two 
 ;ire represented as seen by reflected light in figure and in all specimens examined 
 the gonophores are at the same stage of development. I have been unable to detect 
 any indication of the formation of reproductive elements in the hydrophyton. 
 
 The gonophores much resemble in structure those figured by Weismann* in 
 Pennaria Carolina or Cladocoryne floccosa. In essential structure the transverse section 
 of the gonophore of the former is identical with that of a similar section of a 
 Ceratella gonophore as shown in figure 8. The longitudinal section again of the 
 gonophore of Cladocoryne or of Pennaria as figured by Weismaun agrees almost 
 precisely with that of Ceratella represented in figure 12. In the latter the 
 manubrium is well developed and surrounding this lie the reproductive elements 
 which in longitudinal section (Fig. 12) form a horseshoe-shaped mass and in transverse 
 section a.ring. External to the latter is a thin layer of ectoderm (ect.') which comes 
 in contact with the external layer of ectoderm (ect.) at the part corresponding to the 
 mouth of the medusa bell where the former layer dips inwards. (Fig. 12, M .) This layer 
 must correspond to the sub-umbrella ectoderm of the medusa and the special point 
 mentioned indicates also the position at which, in development, the " glockenkern " 
 of Weismann grew in by proliferation of the ectoderm cells. I have been unable to 
 find any gonophore younger than the stage figured though many have been cut in 
 section, the manubrium being in every case well developed. 
 
 Between the two layers of ectoderm lies the endoderm with four radial canals 
 seen clearly in transverse sections (Fig. 8) whilst in longitudinal sections (Fig. 12) 
 the indication of a ring-shaped space around the distal end can always be detected. 
 
 Each gonophore may be connected with more than one of the coenosa-rc tubes 
 and its ectoderm is continuous with that which covers the colony externally. 
 
 (3.) The Hydrophyton. This may be divided into two parts (1) the external 
 layer common to the whole colony and (2) the branching network of tubes. 
 
 The external layer is formed entirely of ectoderm cells (Figs. 3 and 12) and is 
 only one cell thick though it may come into contact with the ectoderm of the tubes 
 lying immediately beneath it. It is especially well marked in the younger branches 
 and may be worn away to a greater or less extent in older ones though typically it 
 forms a covering for the whole of the colony (Figs. 9 and 13). It is directly 
 
 * Die EntsteliunR <ler sexualzellen bri den Hydromedusen. PI. XVII., Figs. !-."> and Fig. 7 ; PI. XVIII.,- Fig. 1. 
 
14 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 
 
 continuous with the ectoderm of both the hydroid zooids and the gonophores, and 
 has in many respects a close resemblance to the superficial layer of ectoderm as 
 described and figured by Professor Moseley in Millepora.* In the latter the exact 
 relationship of the superficial ectoderm to the zooids could not be ascertained but 
 in Ceratella where the latter are not retracted into spaces within the skeleton its 
 direct connection with the ectoderm of the zooid can easily be seen in sections. 
 Figure 3 represents a small portion of the layer as seen under a high power. The 
 outlines of cells cannot be definitely distinguished in the specimen in question 
 though a somewhat light space with a fairly distinct outline surrounds the thread 
 cells. The inner ends of the cells are in contact with the ectoderm of the tube 
 beneath and the layer thins out just where it passes over the projecting point of a 
 portion of the skeleton. In younger branches (Fig. 14) the cells of the layer are much 
 more definite in form and outline being each cubical with a distinct nucleus whilst 
 comparatively very few thread-cells are present. 
 
 This superficial layer is only known to exist in the Hydrocorallinse, the 
 
 Hydractiniidae and, now, in the Ceratelladas. In the last mentioned the soft parts of 
 
 only Ceratella fusca are known as yet but the skeleton of Dehitella is so closely 
 
 similar to that of the former that we may with much probability infer that a close 
 
 agreement exists in the nature of their soft parts. The coenosarc tubes form a 
 
 richly branching network of tubes occupying all the spaces in the chitinous 
 
 meshwork which forms the skeleton. In Figures 9 and 10 this is represented 
 
 diagrammatically by the grey colour the former being a longitudinal and the latter 
 
 a transverse section of a branch. Throughout the whole system the endoderm is 
 
 never more than one cell thick whilst the ectoderm is very irregular. Very often the 
 
 endoderm cannot be recognised or else it forms an indistinct layer which stains more 
 
 darkly than the ectoderm and contains no space, a result probably due to the action 
 
 of reagents. At other times (Figs. 3, 11, 12, and 14) a distinct tubular space 
 
 can be distinguished. The number of tubes varies naturally according to the size of 
 
 the branch. Figure 13 represents a longitudinal section through a portion of a small 
 
 branch with a smaller offshoot which formed part of a specimen brought by 
 
 Mr. Whitelegge from Lord Howe Island. The general appearance of a part of the 
 
 same specimen viewed as a solid object is represented in Figure 4. This particular 
 
 specimen has the branches much finer than those of the others and the skeleton and 
 
 ccenosarc tubes somewhat more regular in arrangement, whilst the hydrophores 
 
 are not very strongly developed. Up the small offshoot pass the main skeletal ribs 
 
 united by cross bars which are thin and almost web-like and up the centre runs a 
 
 single coenosarc tube (B) which is connected with at least three of those in the larger 
 
 branch. This tube consists of an internal layer of endoderm (Fig. 13 a, end.) and 
 
 * On the Structure of a Species of Millepora occurring at Tahiti, Society Islands. Phil. Trans. B.S. London, 
 1870, Vol. CLXVII., p. 117. 
 
ON THE STRUCTURE OF CEHATELLA FU8CA (GRAY). 15 
 
 an irregnkur external layer of ectoderm (ect.) the former being continuous with that of 
 the xooids. The whole is covered by a uuilaminar ectoderm (E). A few thread cells 
 are present. Up the layer branch the tubes run more regularly than usual and three 
 or four may be traced for a considerable distance running parallel to its length but 
 giving off lateral branches. In sections both transverse and longitudinal of Ceratella 
 these connecting bars or webs crossed by ccenosarc tubes form a very characteristic 
 feature (Figs. 11 and 13. C). 
 
 As stated above there is a strongly-marked difference between the endoderm and 
 ectoderm. The former (Fig. 3) is regular and takes stain somewhat more readily 
 than the latter, which is often very irregular and several cells thick, though most 
 often the outlines of cells cannot be recognised, and a structure resembling a 
 syncytium is formed. In the latter are found (1) nuclei, (2) thread cells, (3) bodies 
 surrounded by a clear space and staining evenly and deeply (Fig. 3A). The 
 thread cells are apparently confined to the ectoderm, though of this I cannot feel 
 absolutely sure, and are found in great abundance in the inner parts of the branch 
 whence they must migrate to the surface if they are to be of service to the colony. 
 It is a somewhat curious fact that they are as a rule present in far greater numbers 
 iu the ectoderm of the cosnosarc tubes than in the most external layer. Figure 3 
 represents a small portion of the latter on a part of a colony where the 
 gonophores were numerous and here thread cells were present in greater numbers 
 than elsewhere. Of the nature of the third-named structures it is difficult 
 to be certain but it is probable that they are ectoderm cells in which thread 
 cells are being formed. There is at all events a curious agreement in appearance 
 between them and the structures which Professor Moseley has described as developing 
 thread cells in Millepora.* He says " the thread cell appears to be developed out of 
 the nucleus of the ectodermal cell, the ectodermal cell becoming much enlarged and 
 forming a wide chamber, in which the process of development takes place. The 
 ovoid nucleus becomes enlarged together with the cell, but not at all in the same 
 proportion the cell always appearing as a wide cavity around it. The nucleus as it 
 enlarges has a rounded nucleolus developed at one end of it." The nucleolus has 
 large granules developed within it, whilst the nucleus becomes finely granular. In 
 the next stage one large coil of the thread appears in the nucleus." 
 
 The earlier stage seen in Millepora when the nucleus with nucleolus at one 
 end of it lies in the cell which forms a clear cavity around it, corresponds exactly to 
 that represented in figure 3A.c. in the case of Ceratella. Though a complete series of 
 stages could not be obtained still those drawn in figure 3A. will serve to show that in 
 all probability Ceratella resembles Millepora closely in the formation of thread cells. 
 In figure 3A.rt. the cell is small and the nucleus but little larger than that of an 
 
 * Loc. cit., p. 129. 
 
16 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 
 
 ordinary ectoderm cell though stained very deeply and having a homogeneous 
 appearance ; in b. the cell has increased in size, the nucleus is much larger, and has 
 a clear space all around it between it and the cell wall ; this is clearly marked in c. 
 and d. where, in the former, a nucleolus is present and in the latter two darker thread- 
 like lines possibly indications of the commencing formation of the thread ; in c. what 
 is evidently a very young thread cell is seen it is somewhat darkly stained without 
 a definite thick wall such as is seen clearly in later stages, and down the centre is a 
 lighter line corresponding to the thicker attached part of the thread. It has also the 
 shape of the thread cell but there is no trace of the clear space present in earlier 
 stages, a certain amount of stained protoplasm being attached to it. In/. and#. two 
 later stages are shown in which the thick cell wall is present and the coiled thread 
 can be clearly seen. These thread cells evidently resemble closely in structure the 
 three-barbed ones described by Professor Moseley in Millepora. 
 
 The only other point to notice in regard to the coonosarc is the structure of the finer 
 growing branches which are somewhat flattened out. A longitudinal section of one 
 of these is represented in figure 14. Up the centre runs a coenosarc tube with a large 
 cavity and clearly-marked endoderm the ectoderm being as usual irregular. From 
 this central tube short branches are given off (D) which run outwards towards the 
 external layer with which, as at the point x, they may come into direct contact. At 
 this point the cells of the two layers are well marked, and in all probability this 
 shows us the earliest stage in the formation of a zooid. It has already been noted 
 that the ectoderm of the latter is in direct connection not with that of the coBiiosarc 
 tubes but with the common external layer and this method of formation would 
 explain this otherwise somewhat curious fact. The endodermal process grows out 
 into a bud the early stage of a zooid carrying with it the external ectoderm layer 
 which thus, as further growth takes place, naturally gives rise to that of the zooid 
 itself. At the same time the ccenosarc tube branches as the stem increases in size 
 and thus the zooid, if the branching be near the base of the latter, will become 
 connected with two or more tubes. 
 
 AFFINITIES OF THE CERATELLADTE. 
 
 When Dr. Gray first described these forms there were only two specimens 
 available neither of which possessed the soft parts. The hard parts whilst agreeing 
 in important points differed from each other sufficiently to be regarded by him as 
 species of two distinct genera Ceratella and Dehitella. 
 
 Mr. Carter, with more material at his disposal, recognised the fact that they 
 were hydroids and owing to similarities in their skeleton and that of Hydractinia 
 placed them in the family Hydractiniidae, thus abandoning Dr. Gray's family 
 Ceratelladao which had been created under the assumption that the two forms were 
 allied to the sponges. 
 
ON THE STRUCTURE OF CERATELLA FFSCA (GRAY). 17 
 
 I'ndoubtedly in many rospocts Ceratella and Dehitella call to mind the 
 Hydraetiniidir, but it is doubtful if even our knowledge were confined to that of the 
 structure of the hard parts whether Mr. Carter's classification could be upheld. The 
 one point of resemblance and at first sight it is the most striking feature is that 
 the skeleton of both consists of a very irregular branching chitinous network. In the 
 Ilydractiniido) however this has the form of an encrusting network with at most very 
 feebly developed branches arising from it ; these may more correctly be described as 
 spines and they do not appear to carry any zooids. In the CeratelladiO the whole 
 colony consists of a much-branching structure arising from a compiratively small 
 encrusting root-portion which may itself be made up of branches more or less 
 entwined. In addition to this all the branches bear hydrophores or special 
 developments of the network to support the hydroid zooids. These are never 
 present in the Hydractiniidne but always in the Ceratelladze. Now that the soft parts 
 are known there can be no doubt about separating the two families. The hydroid 
 zooids are quite different those of Ceratella being provided as arc those of Coryne 
 with scattered capitate tentacles whilst there is no trace of protective zooids such as 
 are present in Hydractinia and Podocoryne. In addition to this the gonophores 
 arise directly from the coenosarc and not from modified zooids. The most important 
 points of agreement lie in (1) the existence in Hydractinia and Ceratella of a 
 common external layer of coanosarc which covers over the whole skeleton mass 
 whether this be encrusting or branching in nature ; (2) the presence in both of a 
 network of coenosarc tubes forming the hydrophyton. It may however be noted 
 that in both these points we find a similar agreement to exist between Ceratella and, 
 for example, Millepora amongst the Hydrocoralliuae as between the first named and 
 Hydractinia. 
 
 The presence of this external layer which, in the Hydrocorallinffl and Ceratella 
 at all events, consists simply of a layer of ectosarc is very difficult to explain. 
 Professor Moseley* lias represented it in Millepora as if it formed the outer layer of 
 the surface cconosarc tubes though even in this case it passes over all the 
 parts (occupied by the calcareous skeleton) which on the surface lie between the 
 tubes, and is very different in appearance and in the relative size of its cells from 
 the ectoderm which elsewhere forms the outer wall of the tubes. In Ceratella it is 
 perfectly independent of the tubes all of which have their own ectoderm covering 
 though at the surface this comes in direct contact with the outer layer. I am not 
 aware of any determination in Hydractinia of the exact relationship of this outer 
 layer though very probably it will be found to agree with that of Ceratella. 
 
 It is not apparently connected in any special way with the formation of the 
 chitinous network as this lies deep within the structure of the branch, and the only 
 
 * Loc. cit., PI. 3., Figs. 10 and Hi. 
 
18 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 
 
 purpose which it can apparently serve is that of a covering layer which prevents 
 foreign objects from passing in between the meshes of the network and interfering 
 with the general welfare of the colony. It is strange however to note, if this be its 
 function, that most usually the internal ccenosarc contain a far greater number of 
 thread cells than this external layer does in Ceratella. 
 
 In Millepora Professor Moseley was unable to determine its exact relationship to 
 the zooids but in Ceratella by its means all the ectodermal structures lying on the 
 external surface are brought into direct continuation with one another. 
 
 It may be noted in passing that though in the genus Clathrozoon* the branches 
 of the colony are formed of a somewhat similar network of soft parts there is nothing 
 present resembling this external layer the whole branch being in this instance covered 
 with a thin protective perisarc. 
 
 Taking both the hard and soft parts we find the following points of agreement 
 to exist between the Hydractiniidae on the one hand and the Ceratelladae on the 
 other though it must be borne in mind that we only know as yet the structure of the 
 soft parts in one member of the latter family. 
 
 (1.) The skeleton has the form of a branching chitinous network. 
 
 (2.) The hydrophyton consists of a network of freely branching and 
 anastomosing ccenosarcal tubes. 
 
 (3.) The zooids arise directly from this network and no true hydrothecae 
 or gonothecas are formed. 
 
 (4.) A common external layer is present enclosing the whole colony. 
 The two differ from one another in the following points : 
 
 (1.) Hydractiniiclae form encrusting masses with at most spinulose branches 
 arising from the surface which do not bear zooids. The Ceratelladae 
 always form freely branching masses either erect or procumbent : the 
 basal part which serves to attach the colony being alone of an 
 encrusting nature whilst even this has the form of intertwined 
 branches. 
 
 (2.) The Ceratelladae always possess hydrophores or special developments of 
 the skeleton which serve as a support for the basis of the hydroid- 
 zooids and nothing similar to which is found in the Hydractiniidae. 
 
 (3.) The hydroid zooids Ceratelladae possess scattered capitate tentacles 
 those of the Hydractiniidae being filiform and arranged in a single 
 circle beneath the mouth. 
 
 * Tiaus. B.S., Victoria, 1890, p. 121. PI. 18, Fig. 3; PI. 19, Fig. 12. 
 
ON THE STRUCTURE OF CEUATELLA FUSCA (GRAY). 19 
 
 (4.) Tlic gonophores of the Ceratelladae arise directly from the ccenosarc 
 and are not developed on special zooids as in the case of the 
 Hydractiniidte. 
 
 Whilst the points of agreement detailed above serve to show a general 
 resemblance between the members of the two groups those of difference are of 
 sufliricnt importance to justify their separation into two distinct families. 
 
 As stated previously Dr. Gray's name Ceratelladre will be retained and the 
 following gives the characters of the family (modified from Dr. Gray's and Messrs. 
 Carter and Bale's descriptions) and the list and characters of the genera and species 
 yet kno\\n. 
 
 Family Ccratclladcc (Gray, Proc. Zool. Soc., 1868, p. 575). 
 
 Forming branching colonies. Skeleton in the form of a chitinous network with 
 slight bracket-like or tubular projections (hydrophores) serving as a support for the 
 bases of the gastrozooids. 
 
 Hydrophyton a network of branching anastomosing tubes the whole enclosed by 
 a common ectoderm layer. 
 
 Gastrozooids naked. 
 
 Gonophores medusoid : fixed and arising directing from the hydrophyton. 
 
 Genus. Dehitella. (Gray.) 
 
 Colony dichotomously branched, expanded growing in a large tuft from a broad 
 creeping base. Stem cylindrical, smooth ; branches tapering and cylindrical. 
 Hydrophores slightly tubular and on the smaller branches divergent nearly at right 
 angles from the stem. 
 
 (1.) Dehitella atrornbcns. (Gray.) 
 
 The description of the species is the same as that of the genus. It is known at 
 present simply from that given by Dr. Gray* who states that the genus " is 
 distinguishable from Ceratella bythe greater thickness and cylindrical form of the stem, 
 by the more tufted and irregular manner of growth and by the tufts of spicules (oscules 
 or cells) being more abundant and equally dispersed on all sides of the branches and 
 branchlets." The " oscules or cells " of Dr. Gray must be the structures which, 
 following Mr. Bale, have been described above as " hydrophores." 
 
 Locality. Delagoa Bay, Africa. 
 
 * Proc. Zool. Soc. 18U8, p. 679. Fig. 1. 
 
20 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 
 
 Genus. Ceratella. (Gray.) 
 
 Colony irregularly branching ; more or less expanded in one plane ; growing 
 from a creeping base. Main stem flattened, branches rounded and beset with 
 bracket-like hydrophores. 
 
 (2.) Ceratella fnscn. (Gray.) 
 
 Colony branching and fan-shaped ; expanded in the one plane ; erect. Skeleton 
 consisting of a light or dark-brown chitinous network ; the main stem broad and 
 flattened ; brandies numerous with the bracket-shaped hydrophores nrranged on 
 them in a roughly spiral manner and formed of ribs continuous with the fibres of the 
 stem and united by thin perforated lamina; the ribs projecting at the outer margin. 
 All the spaces within the chitinous network filled by a much branching hydrophyton 
 and the whole enclosed by an external layer of ectoderm. Gastrozooids seated on 
 the hydrophores, erect, with capitate tentacles irregularly scattered (10-14). 
 Gonophores medusoid, fixed. 
 
 Localities. Coogee, Bondi (N.S.W.), Broughton Island, Flinders Island, Lord 
 Howe Island. 
 
 (3.) Ceratella procnmbcns. (Carter*). 
 
 Colony procumbent, thickly branched on the same plane ; the larger stems 
 chiefly on one (the lower) side, hard, flexible, of an ochre-brown colour, tinged here 
 and there with purple. Trunk short, solid, compact, compressed vertically, soon 
 dividing irregularly or subdichotomously into round branches which are confined to 
 the lower surface, ending in branchlets with sub-clavate ends, that appear on the 
 upper or opposite side, not reuniting or anastomosing. Hydrophores consisting of 
 a little semitubular plate, extending outwards and forwards from the side of the stem 
 on the proximal border of an aperture in the latter; scattered thickly over all the 
 branches, but most prominent on the branchlets ; frequently represented by the little 
 hole alone in the stem where the projecting portion lias been worn off; scanty on the 
 lower side of the main stems. Minute structure ; composed of clathrate chitinous 
 fibre throughout, whose meshes are subrectangular ; hydrophore formed of the 
 semitubular scoop-like plate mentioned supported on its proximal side by an extension 
 of the clathrate structure of the stem and bordering the little hole also above 
 mentioned, which extends into the centre of the stem ; surface of the larger stems 
 bluntly microspined. Size of largest specimens 11 inches long by 5 inches broad, 
 and about 1 inch thick or vertically. 
 
 Locality. Cape of Good Hope, Natal. 
 
 * Ann. and Mag. Nat. Hist., 1873. Transformation of an entire shell into chitinous structure by the Polype 
 Hydractinin, with short descriptions of the Polypidoms of five other species (PI. 1). The descriptions of G. procumbens, 
 C. spinosa and Clatina ericopsis are taken with only slight alterations from this paper. 
 
ON THE STUI i llltE OF CEUATELLA FUbCA (ciUAY). 21 
 
 (4) Ccratclla spinusa. (Carter.) 
 
 Colony procumbent; thickly branched hard flexible of a dark rich red-purple colour. 
 Main branches round, brownish, covered with small, smooth, often subspatulate erect 
 spines. Stem dividing subdichotomously into purple branchlets, which terminate in 
 a Inaptly pointed extremities. Hydrophores the same as in the foregoing species; 
 most prominent in the round branchlets to which they give, en profil, a serrate 
 somewhat sertularian appearance, the teeth of which are inclined forward. Minute 
 structure: Main stems composed of clathrate chitinous fibre, of which the ineshes are 
 more or less oblong, passing into prominent longitudinal lines on the branchlets 
 where they terminate on the backs of the semitubular plates which respectively form 
 the floors of the hydrophores, to which they thus give support. Size of specimen, 
 which is merely a branch 4J inches long by 2 broad. 
 
 Locality. Port Natal. 
 
 Mr. Carter adds that " the spines on the surface distinguish this from the 
 foregoing species, add to which its longer and more pointed branches, longitudinally 
 ridged clathrate fibre and rich red-purple colour." 
 
 Genus. Chitina. (Carter.) 
 
 Colony erect, bushy, fragili flexible, fawn coloured. Trunk long, hard, 
 irregularly round, composed of many stems united clathrately and obliquely into a 
 cord-like bundle, which divides and subdivides irregularly into branches which again 
 unite with each other in substance (anastomose) when in contact and finally form a 
 straggling bushy head. Hydrophores long clathrate tubular, terminating the ends 
 of the branchlets, or prolonged from some of the proliferous' tubercles which beset 
 the surface of the trunk and larger stems. Minute structure : Composed of clathrate 
 chitinous fibre throughout, whose network is subrectangular and massive in the stems, 
 where there is no difference between the centre and circumference, with the exception 
 that the fibre is stouter in the former or oldest part; hydrophores composed of several 
 longitudinal fibres or ridges lattice-worked together transversely into a tubular form, 
 somewhat contracted at the extremity, in the centre of which is an aperture of the 
 meshwork a little larger than the rest. Height of specimens about 14 inches, trunk 
 about 1 inch in diameter ; hydrophores averaging l-3rd of an inch long by 
 l-60th of an inch in its broadest part and an aperture 1- 90th of an inch in 
 diameter. 
 
 (5) Chitina ericopsis. (Carter.) 
 
 The description of the species is the same as that of the genus. 
 Locality. New Zealand. 
 
22 ON THE STRUCTURE OF CERATELLA FUSCA (GRAY). 
 
 DESCRIPTION OF PLATES. 
 
 PLATE II. 
 
 Fig. 1. Ceratella fusca. Life size. The specimen of which this is a drawing 
 was washed up on Flinders Island, Bass Straits. The main stem of the colony 
 springs from a root-like structure made up of intertwined branches. On the left side 
 arises from the roots a very small independent stem. 
 
 Fig. 2. Transverse section through a portion of the wall of an expanded 
 gastrozooid. Externally is the layer of ectoderm consisting of ciibical cells. The 
 mesoglcea is an extremely thin layer and the endoderm consists of large vacuolate 
 cells with granular protoplasm aggregated at their inner ends which face into the 
 gastral cavity. The nuclei are conspicuous and placed at the same end. Camera. 
 Zeiss E, oc. 2. 
 
 Ed. ectoderm. End. endoderm. M. mesogloea. 
 
 Fig. 3. A small portion of the external surface of a branch in the region in 
 which gonophores are numerous to show the external ectoderm containing thread 
 cells and a portion of one of the ccenosarc tubes. In the latter the endoderm forms 
 a definite layer of darkly-stained, small cubical cells. Surrounding this is the 
 ectoderm in contact with the external layer and containing developing thread cells. 
 
 E. external layer of ectoderm. Eel. e'ctoderm of coenosarc tube. End. 
 endoderm. A. developing thread cells. Sk. skeleton. Camera. Zeiss E, oc. 2. 
 
 Fig. 3a. Developing thread cells found in the ectoderm within the branches, 
 a. an ectoderm cell in which the micleus is slightly larger than usual and appears 
 homogeneous, b. the nucleus has increased in size, stains very darkly, lies at one end 
 and is surrounded by a clear space. c. the nucleus begins to show a darker spot 
 within it. d. two darkly- staining thread-like structures are present, e. the nucleus (?) 
 does not stain so deeply, and is not siirrounded by a clear space but has a small 
 amount of protoplasm clinging to it; within it at the somewhat pointed extremity can 
 be seen a light line indicating the larger terminal part of the thread, f. the wall 
 of the thread-cell and the thread itself are clear. 9. The fully developed thread-cell. 
 
 PLATE III. 
 
 Fig. 4. Portion of a small branch with the zooids expanded. The skeleton is 
 covered by the soft parts but the dark lines indicate the external parts of the chitinous 
 network which show through the soft structures. 
 
ON TUB STRUCTURE OF CERATELLA FUSCA (GRAY). 23 
 
 Fig. 5. Portion of the skeleton of a somewhat larger branch showing the 
 chitinous network of which it is composed and the little bracket-like hydrophores. 
 (Hy.) x 12. 
 
 Fig. 6. Highly magnified small portion of a branch seen partly by direct and 
 partly by transmitted light. Two gastrozooids are shown each of which is placed upon 
 a hydrophore. The latter shows prominent ribs which project beyond the margin 
 and arc connected by a thin fenestrated web of chitin. Two male gonophores arise 
 from the cccnosarc and they and the gastrozooids are quite naked. The whole branch 
 is covered by a thin external layer of ectoderm, x 20. 
 
 G. gastrozooids. Gon. gonophore. E. external layer of ectoderm. Hy. hydro- 
 phore. Hy. 1 web connecting the ribs of the latter. 
 
 Fig. 7. Transverse section across the body of a gastrozooid. The tentacles 
 are solid with swollen ectoderm ends filled with thread cells. The zooid has been 
 feeding and the endoderm cells are full of little food particles. F. Remnant of 
 small crustacean on which the zooid is feeding. It lies close against the endoderm 
 on one side and the food particles are passing into the interior of the cells. Outline 
 drawn with Camera, Zeiss E, oc. 2. 
 
 Fig. 8. Transverse section across a gouophore. Mn. endoderm of the 
 manubrium. Sp. sperm cells in the ectoderm of the latter. Ect. ectoderm of the 
 medusa. R. radial canals. Ect. ectoderm of the sub-umbrella surface. Outline 
 drawn with Camera, Zeiss E, oc. 2. 
 
 PLATE IIIA. 
 
 Fig. 9. Semi-diagrammatic drawing of a longitudinal section through a 
 small branch. The skeleton is coloured brown the soft parts grey. E. external 
 layer of ectoderm. Gon. gonophore. G. gastrozooid. Hy. hydrophore. Sk. 
 general network of skeleton cut across in various directions. Outline drawn with 
 Camera, Zeiss A*, oc. 2. 
 
 Fig. 10. Semi-diagrammatic drawing of a transverse section across a good-sized 
 branch. Letters as in figure 9. Ect. ectoderm of endosarc tubes. End. endoderm. 
 Outline drawn with Camera, Zeiss A*, oc. 2. 
 
 Fig. 11. More highly magnified portion of a branch cut in transverse section. 
 The ectoderm of the gastrozooid is directly continuous with the external layer of the 
 branch and the base of the zooid is continuous with two of the coenosarc tubes. 
 At C. is represented the characteristic feature of the latter crossing over live 
 connecting strands of the chitinous network. C. points at which the soft parts 
 
24 ON THE STKUCTUUE OF CEKATELLA l-'UBCA (GHAY). 
 
 cross thin connecting strands of the skeleton. E. external layer of ectoderm. 
 Ect. general ectoderm. G. gastrqzooid. Ply. hydrophore. Sk. general skeleton. 
 Outline drawn with Camera, Zeiss A*, oc. 2. 
 
 Fig. 12. Portion of a longitudinal section of a branch from which arise two 
 gonophores. The latter are cut in longitudinal section. E. external laver 
 continuous with the ectoderm of the gonophore. End. endoderm of the manubrium. 
 Ect. ectoderm. Ect'. ectoderm of sub-umbrella layer of the medusa. G. 
 gastrozooid. M. point at which the ectoderm dips in corresponding to the mouth 
 of the medusa, ft. radial canals. Outline drawn with Camera, Zeiss C, oc. 2. 
 
 Fig. 13. Longitudinal section of a branch from which another small one arises. 
 In this specimen the skeleton has more than usual a more or less definite 
 arrangement into ribs which run parallel to the length of the branch and are 
 connected by transverse bands. Outline drawn with Camera, Zeiss A*, oc. 2. 
 
 Fig. 13A. A small portion of the small branch in figure 13 more highly 
 magnified to show the single ccenosarc tube passing along the centre. The ectoderm 
 is irregular. Letters as before. Drawn under Zeiss F, oc. 2, 
 
 Fig. 14. Portion of a terminal branch devoid of zooids. Up the centre runs 
 a single tube with an internal unicellular layer of endoderm and an irregular ectoderm. 
 The endoderm gives oft' hollow processes which at certain parts (x) come into direct 
 contact with the external ectoderm (E). Further growth of these will probably give 
 rise to the gastrozooids the external layer of the colony thus forming their ectoderm. 
 Drawn under Zeiss E, oc. 2. 
 
Trans RS Victoria 1891 Plate L 
 
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 Fig. 3 
 
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