STBUOTURE, ETC., OP OEEATA OF NUDIBRANCHS. 41

On the Structure and Functions of the Cerataor Dorsal Papillae in some NudibranchiateMollusca.

By

W . A. H e r d m a n , B . S c , 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 withbrightly coloured and sometimes elaborately branched projec-tions from the sides and dorsal surface of the body. Theseinclude—

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 largenerves arising from the cerebral ganglia. They are present inall the forms discussed in this paper.

The branchiae, although they may possibly not be truectenidia, are specialised organs of respiration. They are notpresent in all Nudibranchs.

The cerata, which were the special subject of my investiga-tion, vary greatly in number, size, and arrangement in thedifferent genera and species, and the characteristic appearanceof the animals is in a great measure due to these structures.They are often termed dorsal papillae, or branchial papillae, oreven branchiae; and they have been supposed by many zoo-

42 W. A. HERDMAN.

legists to be organs of respiration. They are not present inall Nudibranohs, but in many cases they are very large andconspicuous. They may be present along with true branchiae.I find the cerata in the genera which I have examined to be oftwo kinds:

1. There are those which contain large diverticula of theliver, as in the case of the genera Eolis and Do to.

2. There are those which are essentially processes of thebody-wall, and have no connection with the liver, as in thegenera Tr i ton ia , Ancula, and Dendronotus .

The term " cerata" may, as introduced by Lankester in hisarticle " Mollusca,"x be employed for these processes ingeneral, while those in the first category might be speciallydenoted as hepato-cerata, and those in the second as parieto-cerata. All the forms which I have examined are eitherdistinctly hepato-cerata or are parieto-cerata. I have foundno intermediate conditions. In regard to their morphologicalnature, if the fold of integument overhanging the foot inDoris is to be regarded not as a mantle edge, but as an epi-podial ridge (see Lankester, loc. cit., p. 655), then the smoothor tuberculated dorso-lateral ridges in the genera Goniodoris,Polycera, and Idalia, the larger row of lateral tubercles ofiEgires punct i lucens , the lateral clavate processes of Triopaclaviger, the palisade-like cerata of Ancula, the branchedparieto-cerata of Tritonia and Dendronotus, and probably alsothe hepato-cerata of Doto, Eolis, Proctonotus, and all otherforms, may be considered as epipodia] papillae—outgrowths froma more or less distinct epipodial ridge.

The six common British genera, Dor i s , Ancula, Tr i tonia ,Dendrono tus , Doto, and Eolis , show very different con-ditions of the cerata and other dorsal processes, and form aninstructive series of types. The general anatomy of all theseforms is well known, thanks chiefly to the labours of Alderand Hancock and of Rudolph Bergh, and many points in thedetailed structure of particular organs have been worked outby 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 thedifferent parts of the body, has apparently not up to now beenmade use of by any of these writers on the structure of theNudibranchiata.

My specimens have been collected in the Liverpool Baydistrict, either in the neighbourhood of the Biological Stationon 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 weresoaked in gum, cut in the freezing microtome, and examinedin water, in glycerine, and in Farrant's solution for comparisonwith 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 uponthe Fauna of Liverpool Bay, has given me a great deal ofassistance in preparing the specimens and cutting the sections.

DORIS.

In Doris (PI. VI, fig. 1) there is a pair of short stoutlaminated rhinophores on the head, and a clump of well-developed branchiae near the posterior end of the dorsalsurface of the body. There are no cerata or other dorsalprocesses. The branchiae are in the form of a number (usually6 to 12) of pinnate plumes arranged in a circle round theanus. In sections the branchiae have the structure shown infig. 2. The branches are subdivided and the surface is veryirregular. The epithelium varies from nearly squatnous tocolumnar, and there are large blood-lacunae forming irregularspaces and passages and coming into close relation with thesurface, being only separated from the ectoderm in someplaces by a very thin layer of structureless connective tissue.

ANCULA.

In Ancula c r i s t a t a (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 positionon the head. Each of them has its upper half stronglylaminated or marked with parallel transverse ridges, whilenear the base of each two simple tapering branches arise, theone directed horizontally forwards and the other rather out-wards to the side. There are three branchial plumes, whichare placed in the centre of the dorsal surface. The largest oneis median and anterior, and the other two form a pair placed alittle further back : they are all much branched.

The cerata form a series of five erect, rod-like processesalong each side of the back. They extend from the centrenearly halfway to each end of the body, and thus form aprotecting palisade along the middle third of the back, ateach side of the branchiae.

In sections through the front of the head (fig. 4) it is seenthat the branches of the rhiuophore are, like the cerata furtherback, prolongations upwards of the body-wall composed ofordinary mesodermal tissue containing only the usual smallblood-lacunse. A few sections behind (fig. 5) we come uponthe 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 branchieeare seen in section. Fig. 6 shows in the centre the basalpart of the first or median branchia cut near its anterior end.It contains a large blood-cavity. On each side is seen one ofthe cerata, that on the right having had several undulations nearits middle. These cerata are seen from this and neighbouringsections to be direct continuations upwards of the ectodermand mesoderm of the body-wall, and to contain no special struc-tures beyond the epithelium and the connective and musculartissues of the integument (fig. 6). There are many small blood-lacunae in the mesoderm, but these are not more numerous norlarger than the corresponding spaces in the body-wall, andnothing approaching the structure of a branchia is seen.

PI. VII , fig. 7, shows a section further back where themedian 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 eachside. This shows well the laminated structure of the branchiaand 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 magnificationthat the numerous lacunae lying in the mesoderm close underthe ectoderm, and containing blood-corpuscles (fig. 8), becomevisible. This is part of a vertical section; while fig. 9 showspart of a transverse section similarly magnified. These twofigures show the deep infoldings of the surface of the branchia,and the former (fig. 8) exhibits well the change in the characterof the ectoderm cells from place to place. The general arrange-ment and structure is the same as in the section of thebranchia 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 sometimesoverlap and become displaced, small pieces of the one arealways distinguishable by their structure from those of theother. The cerata (see figs. 6, 7, 10, and 11) have the ecto-derm very thick, and the infolds are not nearly so deep or soclose as those of the branchiae. A layer of longitudinal muscle-fibres (fig. 10, m) lies under the ectoderm in the cerata, andthere are only a few small lacunae in the mesoderm.

Pig. 11 represents a section further back, in which parts ofall three branchise and of two pairs of cerata are seen. Thelateral branchiae have their inner surfaces much more deeplyfolded than their outer surfaces, and this is especially the casenear their bases. This is shown in fig. 12, a vertical sec-tion of the base of one of the lateral branchiae, where the leftside shows the outer surface next to the cerata, while the rightside is the inner surface nearest to the middle line of the body.Some of the deepest infolds of the ectoderm are seen to end inlittle crypts where the ectoderm cells become suddenly largeand are arranged in a radiating manner around the end of theinfold so as to form a spherical clump (fig. 12, gl). Theseare probably glandular.

46 W. A. HERDMAN.

TRITON IA.

In T r i t on i a (e. g. Tr i ton ia , or Candiel la , plebeia) thebody is long and low, nearly square in transverse section, andtapers rapidly to the posterior end (fig. 13). The rhinophoresare large and complicated, having the base surrounded by asheath and the terminal part divided up into a number ofbranches. There are no true branchiae such as are present inDor is and in Ancula, but placed along each side of the dorsalsurface is found a row of short branched cerata (fig. 13, e).These are seen in sections (figs. 14 and 15) to be merelyprocesses of the body-wall containing no special structures andonly a few small lacunae, such as are present under the integu-ment all over the body. In some transverse sections, wherethe sides of the body are much corrugated the irregular foldsof the surface are almost as much branched as the cerata, andhave very much the same appearance (see k in fig. 14).

It is clear then (1) that true branchiae, such as those ofDoris and Ancula, are not present in Tr i ton ia p lebeia ;(2) that the cerata of the latter are merely processes of thebody-wall like the cerata found along with branchiae in Ancu la ;and (3) that although these cerata may become considerablybranched (see fig. 15) they have not the structure of specialrespiratory organs.

DENDEONOTUS.

In D e n d r o n o t u s a rborescens there is practically thesame condition as in Tr i tonia . Branchiae are absent, but therhinophores are large and complicated, and the branchedcerata arranged along the sides of the back are so greatly de-veloped as to form the most conspicuous part of the animalin the living condition (PI. VIII , fig. 16). There are usuallysix pairs of these cerata, with occasional much smaller onesscattered between. In a specimen 4 cm. in length the largestpair of cerata may be 1 cm. in height, and have stems 3 mm.in diameter at the base. They branch repeatedly so as to forman arborescent structure.

STBUOTUBE, ETC., OF OERATA OF NUDIBRANOHS. 47

The cerata of Dendrono tus have been generally describedas branchiae, and have been universally supposed, until quiterecently, to contain large digestive caeca or diverticula of the" liver." I regard them, however, as being merely excessivedevelopments of the small cerata found in Tr i ton ia , and ashaving no special branchial function; while last summer Mr.Clubb and I showedl that no digestive cseca penetrate into thecerata in D e n d r o n o t u s . Such cseca were described andfigured originally by Alder and Hancock,2 and more recentlyby Dr. R. JBergh/ but these distinguished anatomists workedentirely, I believe, by means of fine dissections, and I canexplain, I think, how it is that a deceptive appearance ofhepatic diverticula is produced which has led to error whennot corrected by the examination of serial sections.

The so-called liver is a very large organ lying underneath(ventral to) the ovo-testis. It consists of a posterior and rightand left anterior lobes, as correctly described by Bergh. Itgives off a few diverticula directed dorsally, but these do notreach to the bases of the cerata, but end blindly in the body-wall. In the specimens examined by Mr. Clubb and myselflast summer we found such prolongations going towards therhinophores and the two first pairs of cerata, and sometimes,but less definitely, towards the smaller succeeding cerata, butin no case, either in dissections or in sections, were they foundto reach the base either of rhinophores or of cerata.

Dissections alone are apt to be misleading, as there are largeblood sinuses (a) in the side walls of the body close to the liverand (b) extending up into the cerata, and these cavities join andopen into the dorso-lateral veins close to where the hepaticdiverticula terminate, so that it is easy to imagine a direct con-tinuity between the slender end of the diverticulum and theblood sinus and so proceed to trace the supposed hepatic csecaonwards into the cerata. In serial sections, however, the pro-

1 'Proc. Liverpool Biol. Soc.,' vol. iii, p. 22.8, 1889.s ' Kay Soc. Monograph/ pt. ii.3 "Bijdragen tot de Dierkunde," 'NaturaArtis Magistra,' Afl. xiii, viii,

p. 25, Amsterdam, 1886.

48 W. A. HERDMAN.

longations of the liver can be followed with exactness untiltheir terminations in the body-wall are found. In one case,for example, amongst our preparations the hepatic csecumgoing towards the left rbinophore can be traced forwardsthrough sixty-six sections, gradually narrowing until it endsblindly, the last section passing through its anterior wall. Atthis point it has not nearly reached the base of the rhinophore.

Dr. Bergh has figured1 the csecal extremities of the hepaticdiverticula in the terminal branches of the cerata as seen intransparent preparations, and I freely admit that such appear-ances are sometimes to be seen and that they look superficiallyvery like granular, dark-coloured liver cseca; moreover, whenone of the cerata is cut off near its base from a living Den-d r o n o t u s the cut surface sometimes (i. e. in darkly colouredindividuals) shows an outer clearer zone, and then a darkchocolate-coloured ring which is very suggestive of the hepaticcaeca, as seen in the cerata of some species of Eolis . Sections,however, show that in both such cases, the terminal twigs andthe freshly cut stumps of the cerata, the appearance is due tobranching masses of pigment-cells lying in the solid mesoderm,always a little way in from the surface and sometimes moredensely aggregated around the blood sinuses. I found that aspecimen killed and hardened rapidly, soaked in syrup andgum, and cut at once in the freezing microtome withoutstaining, showed these pigment-cells much better than did thespecimens carefully hardened and stained and embedded inparaffin. PI. VIII , fig. 19, shows the arrangement of the pig-ment in such a fresh section: it is of a rich reddish-browncolour.

I believe, then, that the appearance of hepatic prolongationsin the cerata, which have been described by various carefulinvestigators, is due to the presence (1) of blood sinuses, and(2) of a good deal of dark pigment in the mesoderm, and thatthe hepatic caeca are not really prolonged into the cerata.

Dr. Bergh has lately suggested to me in conversation thatpossibly my results might be due to the caeca being contractile,

1 Loc. cit., pi. ii, figs. 21, 22.

STRUCTURE, ETC, OF 0GRATA OF NUDIBRANCHS. 49

and having been in some specimens retracted completely intothe body j but that cannot have been the case, because, in thefirst place, it is difficult to understand how a system of csccaextending up into the terminal twigs could be completely with-drawn from a densely branched structure like the cerata; and,in the second place, some of my sections were made from spe-cimens in which the cerata were suddenly cut off from theliving animal with a pair of fine scissors, when fully expandedand healthy, in a dish of sea-water, and these showed the samestructure when sectionised as did the other preserved speci-mens. I mention this, here, to show that this conceivable ex-planation of the absence of the cseca, which might occur toother readers, had been foreseen, and found not to be possible.The argument that as D e n d r o n o t u s belongs to the groupKladohepatica it is very unlikely to be without hepatic caeca inits cerata is worthless, as Bergh has described an Eolid (Bor-nella except a) which has absolutely no prolongations of theliver into the cerata.1

The large cerata of D e n d r o n o t u s are, then, as we wouldexpect from our previous examination of the smaller similarcerata of Tr i ton ia , prolongations upwards of the mesodermand ectoderm of the body-wall, and contain no special structures,such as are found in the cerata of Eol is and Do to.

The upper part of fig. 17 shows a longitudinal section of oneof the cerata, and fig. 18 is a drawing of a transverse section.The ectoderm-cells throughout are of moderate size, of lowcolumnar form, and are not differentiated in any part. Themesoderm, which has the same structure as that of the cerataof Ancula and Tr i ton ia , is penetrated by large, irregularspaces, containing blood-corpuscles. These may be called theceratal sinuses; they are near the centre of the mesoderm, andrun in the main longitudinally (see fig. 17, c.s.); they occasionallybranch, and they open into the numerous minute lacunse whichexist in the mesoderm here as elsewhere. Pig. 18 shows atransverse section where several branches of the ceratal sinus-are present. In fig. 19 also several spaces containing blood-

1 See 'Report upon tlie "Challenger" Nudibranchiata,' p. 41.

VOL. XXXI, PART I. NEW SER. D

50 W.- A. SERDMA.N.

corpuscles are seen. Fig. 20 shows a small portion of themesoderm more highly magnified, to show the networkof connective tissue and the small blood-lacunae in themeshes.

At the bases of the cerata these large ceratal sinuses arecontinued into the body, and their communication can betraced in sections with the anterior and posterior dorso-lateralveins (fig. 17, d. I. v.), which open directly into the auricle.The junction between the ceratal sinus and the dorso-lateralvein is effected by means of a narrow transversely runningbranch, and from this point the ceratal sinus is continuedventrally through the mesoderm of the body-wall outsidethe " liver," and may be called the lateral sinus (fig. 17). Itis with the upper part of these lateral sinuses that the prolonga-tions from the liver come in some places into close proximity,aud so may have given rise iu dissections to the appearance ofa direct continuity between the liver and the blood-spaces inthe cerata.

The cerata contain also bands of muscle-fibres, mostly lon-gitudinal in direction, nerves, pigmented connective tissue,forming branched masses and ramifying threads of a richbrownish colour, and finally masses of large distinctly nucleatedcells, lying in meshes of fibrous connective tissue (see fig. 21).These occur chiefly in the smaller branches of the cerata, andare possibly mucus-secreting glands j they resemble the smallgroups of gland-cells seen under the ectoderm in the cerata ofsome species of Eo l i s (see fig. 37). The contrast in structurebetween transverse sections of the cerata of Dendrono tus andof Eol i s is seen by comparing figs. 18 and 34 or 35.

DOTO.

In the genus Doto there are no true branchise; the rhino-phores are large with simple filiform distal ends, but havingtheir bases surrounded by large funnel-shaped sheaths. Thecerata form a row along each side of the back (fig. 22) ; theyare very large and complicated, being swollen, tuberculated,usually brightly coloured, and forming the most conspicuous

STBT70TURE, ETC., OF OJSBATA OF NUDIBRANOHS. 51

part of the body. They contain large branched hepaticdiverticula, and are therefore hepato-cerata.

Transverse sections of Doto co rona ta show the relativelyvery large size of the hepato-cerata (fig. 23), and the mannerin which they are occupied by numerous branches of the largehepatic caeca; ten or a dozen branches may often be foundlying in one section. In fact, in this form, there is far more ofthe liver in the cerata than in the body proper. The medianportion of the liver is reduced to a small tube flattened dorso-ventrally, which lies along the under surface of the large ovo-testis (see figs. 23 and 27, m. I.), and gives off at intervals lateralbranches, which run up the sides of the ovo-testis (figs. 25 and26, h. c!) to enter the cerata, and there expand into the largebranched cseca. The difference, t'hen, between this state ofaffairs, where the part of the liver in the body is little morethan a duct leading from the hepato-cerata to the stomach, andthat seen in Ancula, Tr i ton ia , and D e n d r o n o t u s , wherethe liver is wholly in the body, and the parieto-cerata aremerely processes of the mesoderm and ectoderm of the integu-ment, is very great, and affords sufficient ground, I think, forthe separation of the cerata into two categories.

EOLIS.

For my present purpose it is convenient to use the termEolis in its older, wide sense, as employed for example byAlder and Hancock, and as including the modern generaFacel ina, F labe l l ina , Coryphel la , Galvina, &c. Inthese forms we have much the same condition as in Do to.There are no true branchiae, rhinophores are present, and thereare also large coloured hepato-cerata arranged along the back(PI. X, fig. 29), and constituting the most conspicuous part ofthe animal.

The hepatic diverticula in the cerata are either simple or notso much branched as in Do to, and are not csecal, but com-municate indirectly with the exterior at their apices. Thehepato-cerata also contain at their apices cnidophorous sacs

52 W. A. HEEDMAN.

which open at the upper end to the external world, and at thelower into the extremity of the hepatic diverticulum.

This state of affairs was long ago pointed out by Alder andHancock1 as seen in transparent specimens, and it has morerecently been demonstrated by Bergh in Phid iana Selencse,Facel ina Jan i i , Chlamylla boreal is and Gonieolistypica ; but the communication has often been denied ordoubted, and Ray Lankester probably expressed the mentalattitude of most zoologists towards the matter when he wrotein 1883 that the supposed communication of the hepatic csecain the dorsal papillae or cerata of some of the Ceratonota withthe exterior by means of apertures in the apices of the papillae"requires confirmation."2 Last year Mr. Clubb and I de-scribed and figured3 sections showing the exact manner inwhich the communication takes place in specimens of an Eolisfrom the Puffin Island Biological Station, and I now givesome more detailed figures here (PI. X, figs. 32, 36, and 37).

The upper end of each of the hepato-cerata is occupied by asac containing a number of large cells (the cnidocysts) filledwith cnida or thread-cells. This cnidophorous sac is evidentlyan invagination of the ectoderm, the cnidocysts being modifiedectoderm cells (figs. 32, 36), and it communicates with theexterior by a small but perfectly distinct and clearly-definedaperture at its apex, through which the thread-cells are some-times found protruding (fig. 36).

The size and shape of the cnidophorous sac varies in differentspecies. Figs. 28A to 28c represent the upper ends of hepato-cerata from Facelina drummondi where the sac is greatlyelongated, may become irregularly shaped, and overlap theupper end of the hepatic caecum (fig. 30). The cnida areovate or nearly spherical in shape (figs. 30, 38, and 39), with asmall terminal projection, and the everted threads bear somelarge spines arranged in a spiral round the base (fig. 39), andsmaller ones projecting alternately from opposite sides all the

1 ' Ray Soc. Monograph,' part iii.3 Article " Mollusca," ' Ency. Brit.,' ninth edition, vol. xvi, p. 859.3 ' Proc. Biol. Soc. Liverpool,' vol. iii, p. 233, and pi. xii.

STRUCTURE, ETC., OF OERATA OF MJDIBKANOHS. 53

way along. In (?) Cuthona nana (figs. 36, 37) the sac isshort and rounded, and the cnida are much smaller than in thelast species, but still spherical in form; while in Gralvinapicta (figs. 31, 32) the sac is more elongated, the cnidocystsare very distinct (fig. 40), and the cnida are narrow rod-likebodies (fig. 33).

The hepatic ceeca occupying the greater part of the interiorof the cerata (see figs. 34 and 35, which show transverse sectionsof two species) reach nearly or quite to the lower end of thecnidophorous sac, and communicate with it by means of alonger or shorter slender tube with thin walls strengthenedby a few muscle-fibres. In Facelina drummondi (figs.28 and 30) the connecting-tube is very long, and may be bentupon itself. In the small species of Eolis shown in figs. 36and 37 (probably Cuthona nana) the cnidophorous sac isnearly spherical, and the connecting-tube is short and has adistinct muscular thickening, forming a sphincter around thesmall opening into the hepatic caecum. This condition suggeststhat possibly in all cases the communication between thehepatic caecum and the exterior through the cnidophorous sacmay not be permanently open, but be kept closed when requiredby the contraction of the sphincter muscle.

FUNCTIONS OF THE CERATA.

In regard to the functions of these various kinds of ceratain the Nudibranchiata, in the first place I do not think that inany case they are specially branchial. In Ancula, as I haveshown above, there are parieto-cerata existing along with truebranchiae, and the two have a distinct structure, so that,although in sections pieces of the cerata and of the branchiaemay become displaced, they can be distinguished by theirstructure from one another. Then in Tritonia and inDendronotus I have shown that the parieto-cerata agree instructure with those of Ancula, and not with the truebranchiae of Doris and Ancula. Prom a recent conversationwith Dr. Bergh I learn that he regards the cerata as having abranchial function, and even in Ancula, where there are

54 W. A. HER DM AN.

large true branchiae present, he thinks that the cerata aresupplementary respiratory organs. I am still of opinion, how-ever, that, considering the relatively large size of the branchiaeand the perfection of their adaptation to their function andthe absence of any such adaptation in the cerata, the actionof the latter in effecting respiration must be so feeble, com-pared with the action of the branchiae, that it may be neglected.

Dendronotus arborescens is the form in which it mightbe most readily supposed that the parieto-cerata have ac-quired, secondarily, a branchial function, but a close com-parison of sections shows that these processes do not containmore blood-cavities than the general body-wall, and have noteven so many small lacunae close to the surface as some partsof the dorsal and lateral integument. Hence, although theymay by their extended surface aid somewhat in respira-tion, still they cannot be regarded as in any way specialisedbranchiae.

Then, again, in Eolis and Do to, although from theirrelatively very large size the hepato-cerata may be of someimportance in respiration, it is merely as being an extension ofthe general integument, and not as being special respiratoryorgans. Nearly the whole of the space in the hepato-ceratain these two genera is occupied by the large hepatic caeca, andthere are only a few small blood-lacunae to be seen scatteredhere and there in sections. Specimens of both Eolis andDoto continue to live after being deprived of most of theircerata; so, both from their constitution and as the result ofexperiment, it may be inferred that these structures cannot beof primary importance as respiratory organs. One function,of course, of the cerata in these genera is to contain the greaterpart of the liver; and no doubt this has led to an increasedsize and some modification of structure. In Eolis, finally, theapices of the hepato-cerata accommodate the cnidophoroussacs, which act, doubtless, as important organs of offence.

But I believe that, in addition to these minor functions, thecerata of the Nudibranchiata are of primary importance ingiving to the animals, by their varied shapes and colours,

STRUCTURE, ETC., OF OBRATA OF NUDIBBANCHS. 55

appearances which are in some cases protective and in othersconspicuous and warning ; and in this, it seems to me, we havean explanation of the extraordinary development and variety ofthese otherwise mysterious processes of the dorsal body-wall.

For several years past I have been paying some attention tothe colours of Nudibranchs and their variations in connectionwith their habits and natural surroundings. In October, 1888,I described1 a peculiarly coloured Dor is (Archidoris) tuber-cula ta which was especially well protected from observation,and since then I have found the same species repeatedly lyingin hollows in the surface of large sponges, generally Hal i -chondr ia panicea , and simulating the colours of its sur-roundings so closely as to be quite inconspicuous.2 Giard hasrecently noticed this same point on the coast of Normandy,and has also recorded a few other cases of protective colouringamongst common Nudibranchs.3

The view which I have given above in regard to the primaryfunction of the cerata occurred to me early last summer, whenobserving some of the Nudibranchs in their natural conditionson the shore at Puffin Island, and I have since brought thetheory briefly before the notice of the Liverpool BiologicalSociety and before Section D of the British Association at therecent Newcastle-on-Tyne meeting.4 Since then Mr. Garstanghas independently arrived at practically the same conclusionsin regard to the function of the cerata from his observa-tion of the colouring and habits of the Nudibranchs atPlymouth.

I shall now give a few instances from my own observationsin support of my views.

T r i t o n i a (or Candie l la ) p lebeia is fairly abundant atPuffin Island and at Hilbre Island, near Liverpool, and isalways found (so far as I have noticed) in these localities

1 'Proo. Bird. Soc. Liverpool,' vol. iii, p. 13.2 I see that Air. Walter Garstang, in his recently published " Report upon

the Nudibranchs of Plymouth Sound," has noticed this same instance ofprotective colouring. ' Journ. M. B. A.,' vol. i, No. 2, p. 174.

3 ' Bulletin Scientifique de la France et de la Belgique,' t. xix, 1888, p. 492.4 See Abstract in forthcoming volume of Reports.

56 W. A. HERDMAN.

creeping over the surface of colonies ofAlcyoniura digi-t a t um. The specimens of T r i t o n i a p lebeia are markedwith many colours (none of them bright) including tints ofyellow, brown, blue, grey, black, and opaque white; and whenexamined in a vessel by themselves considerable differencesbetween individuals are noticed, but when in their natural con-dition on the Alcyon ium colony they are nearly all equallyinconspicuous. The colonies of Alcyonium differ consider-ably amongst themselves in tint, some being whiter, othersgreyer, and others yellower than the rest. Different parts ofthe same colony also vary in appearance on account of thedifferent states of expansion of the polypes, and on account ofirregularities of the surface and of adhering sand and mud, sothat the varieties of colouring found in T r i t o n i a plebeia donot render it conspicuous, but are suited to the varying con-ditions of the Alcyon ium colonies. The small branchedcerata along the back of the T r i t o n i a aid the protective re-semblance not only by contributing to the general colouring,but also by their similarity in appearance to the crown oftentacles of the partially expanded polypes. They are placedat just about the right disbance apart, and have the necessarytufted appearance.

Then, again, Doto co rona t a when isolated is a very con-spicuous and brightly coloured animal, but I find it at HilbreIsland invariably creeping on the under surfaces of ledges andstones on which are large colonies of the zoophyte Clavamul t i co rn i s , and in that position the Doto is not readily seen.The gay appearance of this Nudibranch is mainly due to thelarge and brightly coloured cerata, and these agree so closelyin their general effect with the upper ends of the zooids ofClava, covered with the numerous tentacles and the clusters ofsporo-sacs, that when the Doto remains still it is hidden to avery remarkable extent.1

D e n d r o n o t u s , again, with its large branched cerata and1 Mr. Garstang tells me, in a letter just received (October), that at Ply-

mouth the specimens of Doto are not so highly coloured, and are found uponCalyptoblasts, Clava being rare there,

STRUCTURE, ETC., OF CBRATA OF NUDIBRANCHS. 57

its rich reddish-brown and yellow markings, is a handsome andmost conspicuous object, but I have frequently found itamongst masses of brown and yellow zoophytes (coarser formssuch as Se r tu l a r i a ab ie t ina and Hydra l lmania falcata)and on purplish-red seaweeds, where it was very completelyprotected from observation and I did not for several secondsrecognise what I was looking at.1

Now, these are all cases where the colouring is protective,and I have no doubt there are many other similar instances tobe found amongst the Nudibranchiata,2 but the species ofEolis appear to belong to a different category. They arenoted for the very brilliant hues of their cerata and they arealways conspicuous, so far as I have noticed, even in theirnatural conditions.

Then, again, the species of Eolis are rarely found hiding inor on other animals; they are not shy, and they are active intheir habits—altogether they seem rather to court observationthan to shun it. When we remember that the species ofEolis are protected by the numerous stinging-cells in thecnidophorous sacs placed on the tips of all the cerata, and thatthey do not seem to be eaten by other animals, we have atonce an explanation of their fearless habits and of their con-spicuous appearance. The brilliant colours are in this case of awarning nature for the purpose of rendering the animal pro-vided with the stinging cells noticeable and easily recognisable.It is, of course, important for the soft-bodied Nudibranch thatit should be not only disagreeable to taste but also as con-spicuous as possible, in order that it may not run the risk ofbeing tried by voracious animals. An experimental snap froma fish might cause the death of the Nudibranch even though itwas immediately rejected as food.

These, then, are the grounds upon which I base my viewthat the cerata from their structure cannot be important respira-

1 Professor Giard finds it at Wimereux amongst red seaweeds of the genusCallithamnion.

3 Such as the interesting cases of Hermsea bifida and H. dendritica,described by Garstang, loc. cit., p. 191.

58 W. A. HERDMAN.

tory organs and that their chief function is by their variedshapes and colours to enable the animals to assume protectiveor warning appearances as may be found best suited to theirsurroundings and mode of life.

I t is still necessary for the satisfactory establishment of thistheory that I should have some more definite experimentalgrounds for my opinion that such forms as Doto and Dendro-notus are edible, while Eol i s is distasteful to (say) fishes, andI have lately arranged a series of experiments which will beconducted in the fish-tanks of the aquarium here, with thekind assistance of Mr. T. J. Moore, the curator of the Liver-pool Museum. We have just commenced observations, andhave got satisfactory results so far with eight species of shorefishes, but at this season it is almost impossible in this neigh-bourhood to get Nudibranchs in any quantity. As soon asmore material can be obtained the experiments will be resumed,and I shall give a detailed account of the results when suffi-cient evidence has been accumulated.

Summary .

1. In Dor i s there are true branchiae and no cerata. InAncula both branchiae and cerata are present. In T r i t o n i aand D e n d r o n o t u s there are cerata, but no true branchiee.In Ancula , T r i ton ia , and D e n d r o n o t u s the cerata, whethersimple or branched, large or small, are merely processes of thebody-wall (parieto-cerata) and contain no special organs orstructures.

2. In Doto and Eol is there are no true branchiae. Thecerata (hepato-cerata) are large, and contain extensive hepaticdiverticula.

3. In Eol is the hepato-cerata contain also cnidophoroussacs which communicate on the one hand with the distal endof the hepatic caecum, and on the other with the exterior at theapex of the ceras.

4. Morphologically, all the forms of cerata are probablyepipodial processes.

STRUCTURE, ETC., OP CERATA OF NUDIBBANOHS. 59

5. The large, elaborately branched parieto-cerata of D en d r o-notus are merely a further development of the small tuftedparieto-cerata of Tritonia, and although they may on accountof their extended surfaces have secondarily acquired to a certainextent a respiratory function, they cannot be regarded asspecialised branchiae.

6. The cerata, whether they are large branched parieto-cerata as in Dendronotus, or hepato-cerata containing thegreater part of the liver as in Doto, or having cnidophoroussacs in addition as in Eolis, are not of primary importanceeither in respiration or in digestion, but give to the animals,by their varied shapes and colours, appearances which are insome cases protective and mimetic, and in others conspicuousand warning, as may be found best suited to their individualsurroundings and mode of life.

EXPLANATION OF PLATES VI, VII, VIII, IX, X,

Illustrating Prof. W. A. Herdman's paper " On the Structureand Functions of the Cerata or Dorsal Papillae in someNudibranchiate Mollusca."

a. Artery, ap. Aperture of cnidophorous sac. b. c. Blood-corpuscles.br., br1., br1. Branchiae, b. s. Blood.space, c, c1., c1. Cerata. en. s. Cnido-phorous sac. c. t. Connecting tube between hepatic caecum and cnidoplioroussac. c. Us. Connective tissue, d. I. v. Dorso-lateral vein. ec. Ectoderm.ed. Invaginated ectoderm-cells (cnidocysts) which produce cnida. ec". Youngcnidocysts or cells of cnidophorous sac. f. Toot. / . gl. Toot-glands, g. Gang-lion, gl. Gland-cells, h. c. Hepatic caecum, h. c'. Narrow part of hepaticcsecum in the body of Doto . j . Junction of Cerata with body in Doto .k. Folds of the integument. I. Liver. I. m. Longitudinal muscles, m.Muscle-bands, m'. Longitudinal muscle-bands in body of Dendronotus.m. I. Median part of liver in body of Doto. mes. Mesodermal tissues. *.

60 W. A. HEBDMAN.

Nerves, o. t. Ovo-testis. p. Pigment, r. c. Renal cavity, rh. Hhinophoresph. Sphincter muscle, t. m. Transverse muscles, in. Oral tentacles.

Diameters.

S. 1 = Swift's 1 in. obj.,oc. 2 . . . . magnifying about 45S. i = „ i „ . . . . „ „ 230S. £ = „ i „ . . . . „ „ 330Z. J j = Zeiss's Jg- „ (oil immersion), oc. 2 . „ „ 505

oc. 4 . „ 950 to 1363

Where not otherwise stated, the drawings were made from specimenshardened in Kleinenberg's picric acid and graduated alcohols, stained in picro-carmine, embedded in paraffin, and cut with the " rocking " microtome.

PLATE VI.

FIG. 1.—Outline of a Doris seen from the left side, showing the rhino-phores (rh.) and the branchiae (6r.). About natural size.

FIG. 2.—Part of a longitudinal section through the branchia of Doris(Acanthodoris) pilosa. b.s. Large blood-space. S. ^.

TIG. 3.—Outline of Ancula cr is ta ta seen from the right side, showingthe rhinophores, the branchire, and the cerata (c). x 3.

FIG. 4.—"Upper part of a transverse section through the head of Ancula(middle of odontophore), showing the tentacle-like branches of the rhinophores(rh.) cut in longitudinal section. The mesoderm contains only a few smallblood-spaces (b. s.). S. 1.

FIG. 5.—Transverse section through the front of the body of Ancula,showing the rhinophores cut longitudinally, m. Muscles, n. Nerves, g.Ganglion. S. 1.

FIG. 6.—Transverse section through Ancula at the anterior end of themedian branchia (ir1.), showing the first pair of cerata (c1,), and the largeblood-space in the branchia (6. s.). S. 1.

PLATE VII.

FIG. 7.—Transverse section through Ancula in the middle of the medianbranchia, and first pair of cerata. S. 1.

FIG. 8.—Small part of median branchia in longitudinal section, showingthe blood-spaces in the mesoderm. S. £.

FIG. 9.—Small part of same branchia in tranverse section. S. £.FIG. 10.—Part of the outer edge of one of the cerata shown in Fig. 7, near

base. S. £.

STRUCTURE, ETC., OF OEBATA OP NUDIBRANOHS. 61

FIG. 11.—Transverse section of Ancula through the lateral paired branchiae(br-.) S. 1.

FIG. 12.—Vertical section of part of base of lateral brauchia. S. -J-.FIG. 13.—Tritonia (Candiella) plebeia, from right side, x 3.FIG. 14.—Outline of transverse section of Tritonia plebeia through

rhinophores, showing processes of the body-wall (&.). S. 1.FIG. 15.—Outline of transverse section of Tritonia plebeia through

middle of body, showing cerata. S. 1.

PLATE VIII.

FIG. 16.—Dendronotus arborescens from the left side. Natural size.FIG. 17.—Transverse section of Dendronotus near the middle of the

body, showing one of the second pair of cerata cut in longitudinal section(tf3.). The relative positions of the liver (L), short hepatic caecum (A, c),ceratal blood-sinus (c. s.), and dorso-lateral veins (d. 1. v.) are shown. S. 1(reduced).

FIG. 18.—Transverse section of one of the third cerata of Dendronotus,showing the solid mesoderm (mes.) containing muscles (m.) and ceratal blood-sinuses (e. s.). S. £.

FIG. 19.—Transverse section near the base of one of the cerata cut offthe living animal, rapidly hardened, cut in the freezing microtome, and ex-amined unstained in water, to show the distribution of the dark-brownpigment. S. 1.

FIG. 20.—A small piece of the inesodermal tissue as seen in some sectionsof the cerata, to show the lacunte containing blood-corpuscles. S. i .

FIG. 21.—A section near the top of one of the cerata, from a specimenkilled with corrosive sublimate and glacial acetic acid, then hardened graduallywith alcohols of different strengths, soaked in gum, out with the freezingmicrotome, and stained with picro-carmine, to show the masses of gland-cellslying in meshes of connective tissue. S. £.

PLATE IX.

FIG. 22.—Outline of Doto coronata, from the left side. X 3.FIG. 23.—Transverse section of Doto coronata through the posterior

part of the body, showing the large cerata and their contained hepatic caeca(«.«.). S.I .

FIG. 24.—Part of a similar transverse section more highly magnified, showingthe junction of one of the cerata with the body. S. A.

FIG. 25.—Part of another similar section, showing the continuation of thehepatic ctecum (h. c1.) into the body alongside the ovo-testis (p. (.). 8. £.

62 W. A. HBRDMAN.

FIG. 26.—Part of another similar section, showing the narrow continuationof the hepatic csecum (h. cl.) sinking into the body and moving to a moreventral position, so as to reach the median tube lying under the ovo-tcstis.S. i.

FIG. 27.—Part of another section, showing the median tubular part of theliver (m. I.) lying in the body below the ovo-testis (o. (.). S. £.

FIG. 28, A, B, C.—The extremities of three cerata of Eolis (Facelina)drummondi preserved in glycerine and then treated with potassic hydrate,and slightly squeezed to show the long recurved tube connecting the tip ofthe hepatic caecum with the cnidophorous sac. S. 1.

PLATE X.

FIG. 29.—Eolis (Galvina) picta from the left side. X 6.

FIG. 30.—Part of the tip of one of the cerata of Eolis (Facelina)drummondi from Hilbve Island, preserved in glycerine and treated withpotassic hydrate, showing the connecting tube (c. t.) between the cnidophoroussac (en. s.) and the hepatic csecum (h. c). sph. Sphincter muscle, ec. Ecto-derm, covered with fine cilia. I. m. and t. m. Longitudinal and transversemuscle-bands. S. £.

FIG. 31.—Transverse section near the tip of one of the cerata of Eblis(Galvina) picta, showing the cnidophorous sac (en. s.) containing largeinvaginated ectoderm-cells (ec'.), in which are placed the cnida. S. £.

FIG. 32.—Longitudinal section through the tip of one of the cerata ofEolis (Galvina) picta, showing the ectoderm (ec.) turning in at theterminal aperture (ap.) to form the large cells or cnidocysts (ec'.) in thecnidophorous sac (en. s.). S. £.

FIG. 33.—Three of the cnida of Eolis (Galvina) picta. Z. ^ .FIG. 34.—Transverse section (with freezing microtome) of one of the

cerata of Eolis (Acanthopsole) coronata from Puffin Island, ec.Ectoderm, mes. Connective tissue and muscle-fibres, b. s. Blood-spaces inthe mesoderm. h. c. Hepatic caecum. S. •§•.

FIG. 35.—Transverse section of one of the cerata of Eolis (Cuthona)nan a (P), showing in addition to the hepatic ctecum (h. c.) groups of largegland-cells (gl.) embedded in the mesodermal tissues. S. %.

FIG. 36.—Longitudinal sectioD through the tip of one of the cerata ofEolis (Cuthona) nana (?), showing the opening (ap.) of the cnidophoroussac (en. s.) to the exterior, the continuation inwards of the ectoderm-cells(ec.) to form the cnidocysts (ec'.) which contain the cnida, the sphinctermuscle (sph.) round the opening of the cnidophorous sac into the hepaticcsecum, and the clumps of gland-cells (gl.) lying in the mesodermal tissues.

STRUCTURE!j ETC., OP CERAtfA OP NUDlBRANCIfS. 63

FIG. 37.—A neighbouring section to the preceding one, showing the con-necting tube (c. t.) between the cnidophorous sac and the hepatic crecum, cutopen in the greater part of its length, being only crossed by a few of thefibres of the sphincter muscle at its lower end. The other parts are before. S. A.

FIG. 38.—Group of cnida of Eolis (Pacelina) drummondi in variouspositions ; the upper four are unexploded, the lower one has the thread everted.

FIG. 39.—Onida of Eolis (Facelina) drummondi more highly mag-nified (x about 1360), showing the arrangement of the spines on the basalpart of the thread. Z. -Jj, oc. 4, tube.

FIG. 40.—One of the large cells or cnidocysts (<?e'.) from the cnidophoroussac of Eolis (Galvina) picta, in which the cnida are formed, showing thenucleus and nucleolus and the numerous elongated cnida embedded in theprotoplasm. Two younger cells {ec".) are seen at the base. Z. J j .

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