On the Structure and Functions of the Cerata Or Dorsal Papillae in Some Nudibranchiate Mollusca
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STBUOTURE, ETC., OP OEEATA OF NUDIBRANCHS. 41 On the Structure and Functions of the Cerata or Dorsal Papillae in some Nudibranchiate Mollusca. By W. A. Herdman, B.Sc, F.Ii.S., Professor of Natural History in University College, Liverpool. With Plates VI, VII, VIII, IX, and X. MOST of the Nudibranchiate Mollusca are provided with brightly coloured and sometimes elaborately branched projec- tions from the sides and dorsal surface of the body. These include— 1. The Rhinopbores, or dorsal tentacles. 2. The true Branchiae. 3. The Cerata, or dorsal papillae. The rhinophores are a pair of tentacles placed near the anterior end of the body, on the dorsal surface of the head. They are undoubtedly sense-organs, and are supplied by large nerves arising from the cerebral ganglia. They are present in all the forms discussed in this paper. The branchiae, although they may possibly not be true ctenidia, are specialised organs of respiration. They are not present in all Nudibranchs. The cerata, which were the special subject of my investiga- tion, vary greatly in number, size, and arrangement in the different genera and species, and the characteristic appearance of the animals is in a great measure due to these structures. They are often termed dorsal papillae, or branchial papillae, or even branchiae; and they have been supposed by many zoo- 42 W. A. HERDMAN. legists to be organs of respiration. They are not present in all Nudibranohs, but in many cases they are very large and conspicuous. They may be present along with true branchiae. I find the cerata in the genera which I have examined to be of two kinds: 1. There are those which contain large diverticula of the liver, as in the case of the genera Eolis and Do to. 2. There are those which are essentially processes of the body-wall, and have no connection with the liver, as in the genera Tritonia, Ancula, and Dendronotus. The term " cerata" may, as introduced by Lankester in his article " Mollusca,"x be employed for these processes in general, while those in the first category might be specially denoted as hepato-cerata, and those in the second as parieto- cerata. All the forms which I have examined are either distinctly hepato-cerata or are parieto-cerata. I have found no intermediate conditions. In regard to their morphological nature, if the fold of integument overhanging the foot in Doris is to be regarded not as a mantle edge, but as an epi- podial ridge (see Lankester, loc. cit., p. 655), then the smooth or tuberculated dorso-lateral ridges in the genera Goniodoris, Polycera, and Idalia, the larger row of lateral tubercles of iEgires punctilucens, the lateral clavate processes of Triopa claviger, the palisade-like cerata of Ancula, the branched parieto-cerata of Tritonia and Dendronotus, and probably also the hepato-cerata of Doto, Eolis, Proctonotus, and all other forms, may be considered as epipodia] papillae—outgrowths from a more or less distinct epipodial ridge. The six common British genera, Doris, Ancula, Tritonia, Dendronotus, Doto, and Eolis, show very different con- ditions of the cerata and other dorsal processes, and form an instructive series of types. The general anatomy of all these forms is well known, thanks chiefly to the labours of Alder and Hancock and of Rudolph Bergh, and many points in the detailed structure of particular organs have been worked out by Bergh, Vayssiere, Trinchese, and others; but the method 1 ' Enoy. Brit.,' ninth edition, vol. xvi, p. 655. STRUCTURE, ETC., OF CERATA OF NUDIBRANOHS. 43 of serial sections, giving the exact histological relations of the different parts of the body, has apparently not up to now been made use of by any of these writers on the structure of the Nudibranchiata. My specimens have been collected in the Liverpool Bay district, either in the neighbourhood of the Biological Station on Puffin Island, or at Hilbre Island, in the estuary of the Dee. They were generally killed with Kleinenberg's picric acid, hardened with graduated alcohols, stained in picro-carmine, embedded in paraffin, cut with the Cambridge " rocking" microtome, and mounted in Canada balsam. Some were soaked in gum, cut in the freezing microtome, and examined in water, in glycerine, and in Farrant's solution for comparison with the others. My laboratory assistant, Mr. J. A. Clubb, who is working along with me in the collection and identifi- cation of the Nudibranchs of the district for the Reports upon the Fauna of Liverpool Bay, has given me a great deal of assistance in preparing the specimens and cutting the sections. DORIS. In Doris (PI. VI, fig. 1) there is a pair of short stout laminated rhinophores on the head, and a clump of well- developed branchiae near the posterior end of the dorsal surface of the body. There are no cerata or other dorsal processes. The branchiae are in the form of a number (usually 6 to 12) of pinnate plumes arranged in a circle round the anus. In sections the branchiae have the structure shown in fig. 2. The branches are subdivided and the surface is very irregular. The epithelium varies from nearly squatnous to columnar, and there are large blood-lacunae forming irregular spaces and passages and coming into close relation with the surface, being only separated from the ectoderm in some places by a very thin layer of structureless connective tissue. ANCULA. In Ancula cristata (fig. 3) there are rhinophores, well- developed branchiae, and large but simple unbranched cerata. 44 W. A. HBRDMAN. The rhinophores are large, and are placed in the usual position on the head. Each of them has its upper half strongly laminated or marked with parallel transverse ridges, while near the base of each two simple tapering branches arise, the one directed horizontally forwards and the other rather out- wards to the side. There are three branchial plumes, which are placed in the centre of the dorsal surface. The largest one is median and anterior, and the other two form a pair placed a little further back : they are all much branched. The cerata form a series of five erect, rod-like processes along each side of the back. They extend from the centre nearly halfway to each end of the body, and thus form a protecting palisade along the middle third of the back, at each side of the branchiae. In sections through the front of the head (fig. 4) it is seen that the branches of the rhiuophore are, like the cerata further back, prolongations upwards of the body-wall composed of ordinary mesodermal tissue containing only the usual small blood-lacunse. A few sections behind (fig. 5) we come upon the rhinophore proper, showing the broad lateral laminae, while the stem contains a large bundle of nerve-fibres and, further up, a ganglionic mass of nerve-cells (fig. 5, g). Some way further back in the body the cerata and branchiee are seen in section. Fig. 6 shows in the centre the basal part of the first or median branchia cut near its anterior end. It contains a large blood-cavity. On each side is seen one of the cerata, that on the right having had several undulations near its middle. These cerata are seen from this and neighbouring sections to be direct continuations upwards of the ectoderm and mesoderm of the body-wall, and to contain no special struc- tures beyond the epithelium and the connective and muscular tissues of the integument (fig. 6). There are many small blood- lacunae in the mesoderm, but these are not more numerous nor larger than the corresponding spaces in the body-wall, and nothing approaching the structure of a branchia is seen. PI. VII, fig. 7, shows a section further back where the median branchia is cut through longitudinally about its central STRUCTURE, ETC., OP OERATA OF NUDIBRANCHS. 45 part, while the second pair of cerata are seen one on each side. This shows well the laminated structure of the branchia and the bundle of muscle-fibres branching through its interior. A few spaces are visible in the branchia, but they are com- paratively small, and it is only under a higher magnification that the numerous lacunae lying in the mesoderm close under the ectoderm, and containing blood-corpuscles (fig. 8), become visible. This is part of a vertical section; while fig. 9 shows part of a transverse section similarly magnified. These two figures show the deep infoldings of the surface of the branchia, and the former (fig. 8) exhibits well the change in the character of the ectoderm cells from place to place. The general arrange- ment and structure is the same as in the section of the branchia of Doris (fig. 2), and is very different from the struc- ture of the cerata when similarly sectionised and magnified (see fig. 10). So that, although in sections such as are repre- sented in figs. 7 and 11 the cerata and the branchiae sometimes overlap and become displaced, small pieces of the one are always distinguishable by their structure from those of the other. The cerata (see figs. 6, 7, 10, and 11) have the ecto- derm very thick, and the infolds are not nearly so deep or so close as those of the branchiae. A layer of longitudinal muscle- fibres (fig. 10, m) lies under the ectoderm in the cerata, and there are only a few small lacunae in the mesoderm. Pig. 11 represents a section further back, in which parts of all three branchise and of two pairs of cerata are seen. The lateral branchiae have their inner surfaces much more deeply folded than their outer surfaces, and this is especially the case near their bases.