The Cephalic Appendages of the GymnosomatousJPteropoda, and especially of Clione.

By

Paul Pelaeneer, ».S©BSBrussels.

With Plate XXXV.

ESCHEICHT fomeriy made known, on the three pairs ofeephaloeoni (ff Kopfkegel") of Clione borealis, some staie-tures which he described and represented as real suckers j atthe same time he drew attention to the analogy of their sifcna-tion with that of the suckers of the Cephalopoda.1

This fact was of great importance, since it accorded with thepresence of suckers on the two buccal appendages of anotherGymnosomate, viz. Pneuniodermoii, and gave great supportto the opinion expressed by B,. Leuckarf, that the sis conicalappendages of the head of Clione correspond with the armsof the Cephalopoda.2

Prom that time the assertion that Clione possesses aeeta-buliferous appendages has been admitted everywhere, and itis found reproduced in the most valuable and most recentworks.

However, the excessive smallness of the structures whichEschricht had described as suckers (he attributes to themO'OOS"' of diameter) permitted one to call in question theirassimilation with the suckers of Pneumodermon and of the

1 Eschricht,' Anatomische Untersuehungen iiber Clione boreal is , ' p. 9.2 R. Leiickart, ' Ueber die Morpliologie und die Verwaudsshaftsferhalt-

nisse der Wirbellose Tiiiere.' . . . .

492 PAUL PELSENEER.

Cephalopoda, the more ao as Hobboll, who has observed Clioneliving, and has often seen their cephaloconi expanded, hasnever remarked that these animals fixed themselves by theseappendages,1 while several naturalists have seen Pneumoder-mon in life, and have noticed that it frequently fixes itselfwith the aid of its acetabuliferous appendages.

Therefore, during the sojourn which I made in the winter of1884-85 at the zoological laboratory of University College(London), Professor Ray Lankester proposed to me to studythe structure of the cephaloconi of Clione.

Having set to work, I immediately saw that in a large numberof well-known and even very recent works great confusion pre-vailed on the cephalic appendages of Clione. The reason ofthis is specially to be found in the imperfection of the originalfigures, which are generally obscure on the subject; and thosewhich are most often reproduced are just the most defective,2

for they give a very bad idea of the cephalic appendages ofClione, or are even absolutely incomprehensible on thispoint.

Those of Eschricht are small, and also difficult to under-stand, so that the reproductions which have been made ofthem are inexact, and, as Keferstein has remarked, " all thoseprocesses which have been named tentacles by Eschricht wanta new description." 3

When I had comparatively examined other Gymnosomata,I remarked that there also a confusion quite as great existedon the cephalic appendages, the more so since the originalauthors do not agree on this question.

In order to dispel the confusion which exists on this sub-ject, I have thought useful to represent on a rather largescale; and under different aspects, the cephalic part of some

1 Eschrichtj loo. oit., p. 9.2 For example, the figure of Clione given by Rang (Rang et Souleyet,

' Histoire naturelle des Mollusques Pteropodes,' pi. vii, fig. 9), reproduced fayKeferstein (Bronn's 'Tliierreich'), by Claus ('Elementary Text-Book ofZoology'), &c

3 Keferstein, Bronn's ' Thierreieh,5 Abth. Ill, p. 613,

CEPHALIC APPEKBAG-HS OP GTHHOSOMATO¥S FEBEOFOBA. 498

Gymnosomata. I thus have been Induced to extend to t i ecephalic appendages of the gymnosomatous Pteropoda a workspecially undertaken to make known the structure of theeepnaloeoni of Ciione.

This paper is divided into three parts, corresponding to thedifferent genera which I have studied: Ciione, Clionopsis,and Pneomodermon.

I. CLIONE.1

Ciione (PL XXXV, figs. 1—4) possesses two kinds ofcephalic appendages t

1. Tentacles properly so called.3. Cepnaloconi or bueeal cones.

I shall examine successively these two orders of appen-dages.

A. Tentacles properly so called,

Ciione possesses two pairs of them—an anterior or lahialpair, and a posterior or nuchal pair.

' Anter ior Pair.-—It is situated on a hood, whose twohalves, right and left, may fall back laterally, or be joined to-gether again on the antero-posterior mesial line, and hide thebuceal opening and the three pairs of "boccal cones. Thesetentacles are long and retractile. They are not absolutelyanterior, as in the figare of Esehrieht, which represents themas equally distant from the dorsal and ventral faces;2 they aresituated nearer the dorsal than to the ventral face, as shown, inHate XXXV, fig. 2.

Very powerful longitudinal muscles occupy, the whole of theinterior part of these appendages; externally, we "find a thinlayer of annular muscular fibres. The epithelium is like thatof the other parts of the body; their cells are cylindrical andprovided with a large nucleus.

Sections which pass towards the free extremity of these ten-tacles show a rather large number of nervous cells, but I have

1 The species studied is 01. limacina, PMpps, = 01. borealis, Pallas.2 Eschricht, loo. eit, Taf. 2, fig. 10.

494 PAUL PELSENEEB.

not seen any special nervous terminations, such as we shallfind on the buccal cones.

Posterior Pair.—It is situated on the dorsal face of theneck. These tentacles are much shorter than those of the firstpair. Upon the animals preserved in spirit they are alwaysretracted, so that they are difficult to see, their presence beingthen disclosed only by two slight recesses, from which theirextremities are sometimes seen emerging under the form of awhite point.

According to Bschricht, this second pair is oculiferous;1 butthis fact has been called in question by several naturalists,and categorically denied by von Ihering.2

The histological examination which I have made of thenuchal tentacles of Clione, Clionopsis, and Pueumo-dermon, allows me to assert that these appendages do presenteyes at their free extremity.

Unfortunately the specimens which have served me in myresearches had not been specially prepared for the study of thenervous system and of nervous terminations as delicate as theretinal ones. It follows that I can neither draw nor describe ina complete manner the structure of the eyes of the Gymnoso-mata. That is a point which I intend to take up as soon aspossible.

Nevertheless, the profound dissimilitude which I haveobserved between the labial and nuchal tentacles, and thecharacters of the structure of the latter in the three specieswhich I have studied, show that the nuchal tentacles are quitedifferent from those of the first pair, and the presence of arefracting body shows that the sense of which they are the seatis that of sight.

At the free extremity of these tentacles, the epitheliumbecomes much thinner, so as to make a pellucida or cornea.Under this membrane we find a spherical lens, of which thestructure is similar to that of the Pulmonata. As I have

1 Esckricht, loc. cit., taf. 3, fig. 29.2 Von Ihering,' Vergleicliende Anatomie der Nervensystem und Phylogenie

der Mollusken,'p. 240. . . -- . .

CEPHALIC APPENDAGES OF GYMNOSOMATOUS FEEEOPODA. 495

already said, the retinal part is not perfect is any section, batits position and general form recall to mini those of tlte retinaof the Gastropoda. In the specimens of Clione the pigmenthad. disappeared but in those ofPneumoclermoiiit was pre-served. Finally, towards the base of the tentacle the nerretraverses an optic ganglion.

B. Bueeal Cones.They number three pairs, symmetrically situated on the two

sides of the "lips." The two dorsal cones are the longest, thetwo ventral cones the shortest (Ig. 3).

These cones are very extensible, and in the state of expansionthey are much longer than they are generally represented.They are absolutely conical, as my figares show them^ which Imade from specimens1 of the " Challenger/'' preserved with theextended cones and all the appearances of the liYing animal.

These cones are brightly coloured during life. They areinserted on the two sides of the " lips " as shown in fig. 4.- They are hollow in their lower half, and their cavity is con-tinuous with that of the head, which includes the bueealmass and the penis. Examining these cones with a magni-fying glass, one sees that they are covered with, innumerablevery small tubercles, which have been described by Eschrichtas so many groups of suckers.

S t ruc tu re of the Bueeal Cones.On any section of a bueeal cone of Clione we may easily

distinguish three different regions :1. A middle, muscular region formed of two parts: a. an

exterior layer with annular fibres• (fig. 11, a); b. an interiorlayer with longitudinal fibres (fig. 11, of).

2. An internal region formed of glandular ceils (fig. 11, 5).3. An external region, or epithelial clothing (fig. 11, c).

1 Mr. John Hurray, E.R.S.E., Director of the Challenger publications,kindly supplied Professor Lankester with these specimens as soon as he heardthat Clione was undergoing investigation in the laboratory of UniversityCollege. . . . . .

496 PAUL PELSENEEB.

I shall examine these different parts successively.1. Muscles of the Buccal Cones.—The muscular cells

are unstriped, elongated, and contain a nucleus of a prismaticform (fig. 13, V), The muscular external layer of annular fibresis much less developed than the internal layer. This latter isvery powerful, which explains the great extensibility of thecones. The longitudinal fibres are united in distinct groups(fig. IS, b).

2. Glandular I n t e r n a l Cells.—A transversal section ofone of the cones shows that their interior is filled with cellsunited in groups (fig. 11, b). In the lower half of the conethe centre of the sections is empty, and the cells are onlyfound against the longitudinal muscular layer.

A longitudinal section will make this disposition betterunderstood (fig. 12). The cells in question are united inelongated groups, having the form of follicles. Each cellpossesses a proper prolongation, which is continued to theepithelial covering of the cone. Each of these groups possessesa basement membrane of connective nature, but the differentgroups are pressed one against the other without one beingable to see between them any free connective tissue^ underthe form of cells or fibres. The spaces which are seen inseveral places, on the figure 10 of the plate, proceed from dis-placements which occur during the preparation of the sections.Among the groups which have been displaced I have not seenany traces of connective tissue.

These groups of secreting cells do not constitute a gland,for nowhere on any section, longitudinal (fig. 16) or transversal(fig. 17), can a lumen be seen, nor efferent duct. Each.cellis an independent unicellular gland. The contents of thesecells is a slightly granular substance. The nucleus is largeand spherical; it gives indications of its reticulated structure,but not clearly enough to make drawings of them showing thisstructure.

The cells situated at the interior extremity of the groupshave excessively long prolongations (fig. 16, d); the cellssituated near the muscular layer have, on the contrary, much

CEPHALIC APPENDAGES OF GYMNOSOMATOTJS PT1EOPODA. 49?

shorter ones. When these prolongations arrive at the longitu-dinal muscular layer their contents change their appearanceand present themselves under the form of a fibroid secretion(fig. 18, c) which absorbs much hsematoxylin. These fibroidprolongations pass between, the gronps of longitudinal muscularfibres {d} fig. 13), traverse the layer of circular muscles, andpass into the retieulum of subepithelial connective tissue;afterwards each prolongation, penetrates into an epithelial cell,•which it traverses by passing between the membrane and thecellular contents.

3. Ep i the l i a l Investment.—This is the moat character-istic part of the buceal cones of Clione. The epithelial cellsare united in a variable number, so as to form an infinity ofsmall circular groups on the surface of the cones. It is thesesmall groups which give to the cones their rugose or wrinkledaspect.

A transversal or longitudinal section shows that thesegroups, pressed one against the other, cover the whole surfaceof the cone, and that each group is formed of a little elevationupon which the epithelial cells are found.

Examining one of these groups with an ordinary magnifyingpower (Veriek, obj. 6) we see that the space between theannular muscular fibres and the epithelial cells is occupied bya reticulum of connective tissue which unites the two above-named elements.

Theepithelialcells(fig. 13,/) are elongated, nearly cylindrical,but wider towards their lower part. They are separated fromone another at their higher part, and end at their free extre-mity in a button-like enlargement.

The ceEular contents (h) have nearly the form of the cell.At the lower part the contents have the form of a club, thebig end of which would be turned towards the bottom; atthe higher part the contents fill exactly the terminal enlarge-ment.

This cellular substance is finely granular, but does not com-prise any nucleus.

The cellular membrane is rather thick and presents, in its

498 • PAUL PELSENEEE.

thickness, longitudinal striae which are strongly coloured by the-hsematoxylin.

Between the cellular substance and the membrane we findthe fibroid secretion (d) of the glandular cells which occupythe interior of the cone.

Each of the epithelial cells so constituted has been taken byEschricht for a sucker.

Under each epithelial group, at the surface of the annularmuscular layer, or even between the fibres of the latter, isfound a large sensorial cell (i), which sends a prolongation (k)across the subepithelial connective tissue. This prolongationcontinues between the epithelial cells and terminates freely atthe surface of the cone.

These sensorial cells possess a large refracting nucleus (j),with a strongly colourable nucleolus.

The prolongation, rather narrow in the connective tissue,enlarges a little between the epithelial cells and constitutes arod in the form of an elongated cone (Z), with the base turnedtowards the surface. The part of the prolongation, containedin the connective tissue, presents in some series of sectionsstrongly coloured, a very special aspect; it appears to be reti-culated (fig. 14, a). The conical part, situated between theepithelial cells, presents numerous longitudinal strise. Therod is terminated by a kind of small horizontal rather thin disc(fig. 13, TO), which is more strongly coloured than the sub-jacent part. This disc does not bear cilia such as we see uponthe extremity of some sensorial cells. I do not think that inthe specimens I have studied, the sensorial cells have lost theirciliary covering, for the other parts of the body which bearcilias have their ciliary investment intact.

At the prolongation of the sensorial cell, towards the base ofthe epithelial cells, we find a spherical or ovoid refracting body(re) joined to it, whose membrane seems rather thick, and inthe interior of which we find a corpuscle deeply coloured bythe hsamatosylin.

The complicated structure of these epithelial groups is veryclearly shown by a series of transversal sections of these groups.

CEPHALIC APPENDAGES OF GYMJTOSOlf ATOKJS PTBEOPODA. 4 9 9

that is to say, by sections made tangentially on. the siirface ofthe cone. Such sections are difficult to obtain, exactly; butwhen they are in. the direction wished for̂ they are veryiastraetiYe.

I represent four of these sections^ which I stall describesuccessively, passing from the base to the free extremity.

1. Section passing atove the annular muscular layer (fig. 19).We see the sensorial eel (a) in. t ie middle of the retiealatedconnective tissue (5), in which we find also fibroid prolonga-tions (c). of the internal glandular cells.

2. Section passing through the base of the epithelial group(fig. 20). Here we see the prolongation (a) of the sensorialceil, the continuation of the fibroid prolongations (c), and thesurrounding connective tissne (5).

3. Section passing through the extreme base of the epithe-lial cells (fig. 21). We find the rod (a) at the centre, with therefracting body (b), which is joined to it. All around are thebases of the central epethelial cells (e), in the interior of whichwe see the fibroid secretion (d) of the internal glandular cells.A.t the external part one sees eonneetiYe tissne (e).

4. Section passing through the epithelial cells (fig. 22). Theexternal cells are already separated from their neighbours.We see in the middle the section of the rod (a), of which wedistinguish the striated structure. In the membrane (b) of theepithelial cells we find some coloured points (e), indicating thesections of the longitudinal strias in this membrane,, which Ihave already described.

Summary.—What Eschricht has taken for suckers areepithelial cells terminated by a button-like enlargement. I tis noticeable that there is no nucleus to be observed in thesecells. Besides, they are penetrated by the secretion of theglandular cells which occupy the interior part of the cone..The latter are so numerous (apparently as numerous as thecells of the epithelial groups) that they doubtless fulfil im-portant functions. I think that their secretion is spread out-_side of the cone across the button-like extremity of theepithelial cells, for on some specimens of which the epithelium

500 - ' 'PAUL PELSENEER.

has remained quite intact tMs secretion had spread outwardly,had become coagulated under the influence of the spirit, andhad formed a stratified deposit, which absorbs much colouringmatter on the surface of the cone.

Each group of epithelial cells is provided at its centre with asensorial cell with a rod-like prolongation. TMs fact deter-mines the buceal cones of Clione as organs of special sensi-bility. A refracting body is invariably found towards thecentral part of the epithelial groups at the base of the cells;it is also probably, on account of its situation in contact withthe rod. an integral part of the sensorial apparatus.

With respect to the special nature of these organs, I maymake a remark with reference to the sense of smell in aquaticanimals. Is smelling possible in water, such as exists amongthe superior Vertebrata ? I do not think so. There must bea special sense of which Mammalia cannot be conscious; and atruly aquatic animal cannot have an idea of the smelling ofaerial animals. This sense must lie a peculiar one, interme-diate to that of smell and that of taste. The buceal cones ofClione are probably the seat of such a sense.

II.—CLIONOPSIS.1

Clionopsis only possesses one kind of cephalic appendages—tentacles properly so called.

As with Clione, there are two pairs—a labial pair and anuchal pair (fig. 5).

The labial tentacles are less elongated than with Clione.As in the latter they are inserted more dorsally than ven-trally. Their structure is that of the corresponding appendagesof Clione.

The nuchal pair has already been described by Troschel.2

Gegenbaur says that it is absent.3 Upon the specimen which1 The species studied is Clionopsis Krohni, Troschel, = Clio, medi-

terranea, Gegenbaur.a Troscbel, " Beitrage zur Kenntniss der Pteropoden," 'Arch, fur Naturg.,'

1854, p. 229.3 Gegenbaur, ' Untersuchungen iiber Pteropoden und Heteropoden,' p. 70,

Taf. iv, fig. U.

CEPHALIC APlEHDA&Eg OF G-YifflGSGMAfGUS PTEEGPGBA. 501

I have sstadied the nuchal tentacles were extended and vetjeasily visible. Their structure is that of the nuchal tentaclesof Clione. The eyes are not situated near the tentacles, asTroeehel says,1 but on their top.

Cliomopsis does not possess any otter cephalic appendages.The lips open into a narrow buccal cavity, and do not form ahood which can fall back, as with Clione.

III . PNEUMODESMOH.2

Pnenmodermon is provided with two kinds of cephalicappendages:

1. Tentacles properly so ealledj corresponding to those ofClione and Clionopsis.

2. Two acetabnliferous buecal appendages, characteristic ofthe genus (figs. 8, 9).

A. Tentacles properly so called.Like Clione and Clionopsis, Pneumoiermon pos-

sesses two pairs of tentacles, a labial pair and a nuchal pair.As with the two preceding genera, the labial tentacles are

farad situated on the two sides of the month, rather dor-sally.3

The nuchal pair has already been described by Souleyet.*Gegenbaur asserts that the first pair is the only one whichexists.5 The second pair is, indeed, Tery difficult to see,being retracted in the preserved specimens, the recesses whichresult from it being imperceptible; howefer, this place is lesscoloured than the adjacent parts. By making some trans-versal sections in the anterior part of Pneumodermon, I

1 Troschel, loc. cit., pi. x, fig. 9, o.The species which. I studied are : Pneumodermoa mediterraHeumj

van Ben., and P . Peronii , Lam. #s Mgure 7 maj be applied to Pneumodermon as well as to Olioaopsis.

Besides Soulejet ('Zoologie du voyage de la Bonite,' pi. XY, fig. 15) hasalready very well represented this aspect of Pnenmodermoa.

4 Loo. cit., vol. ii, p. 256, and pi. xv, fig. 145 GegeEbaur, loo. cit., p. 24.

VOL. XXV. NEW SER. K K

502 PAUL PELSENEER.

have been able to assure myself with certitude that the secondpair of tentacles positively exist in this genus, as with Clioneand Clionopsis. But I have not remarked that they hadthe bifid form indicated by Souleyet.1 These tentacles bear attheir free extremity an eye, which has the same structure asthose of the two preceding genera.

B. Acetabuliferous Buccal Appendages.

These appendages are inserted on the internal -wall of thebuccal cavity, on the ventral side. Two figures of Souleyetshow this disposition perfectly (loc. cit., pi. 15, figs. 17 and30); fig. 17 has been badly understood by Fischer, who givesthe two groups of suckers for the jaws.2

The acetabuliferous appendages of Pneumodermon havethe form of a flattened cylinder, upon which are inserted thepeduncles of the suckers. These vary in number according tothe species. Pneumodermon Peroni i possesses a largenumber of them, about thirty on each, appendage; P. viola-ceum, ten to fourteen on each appendage, and P. medi ter-raneum five or sis. However, in this last species I havesometimes found seven suckers, but then one or two were verysmall.

In the state of inactivity the suckers have the form of avery flat porringer. It is in P. medi te r raneum that Isaw the largest j they were one line in diameter.

S t ruc ture of the Acetabuliferous BuccalAppendages.

It does not at all resemble that of the buccal cones ofClione. The whole of its mass is formed by longitudinalmuscular fibres. Externally we find a uniform epithelium;that is to say, it is not provided with sensorial cells like theepithelium of the cones of Clione. The buccal appendages ofPneumodermon are not, then, sensorial organs, as some

1 Loc. cit., vol. ii, p. 356.2 Eischer, ' Manuel de Concbyliologie,' fig. 42, p. 44.

CEPHALIC APPENDAGES OF GTMKOSOMATOUS PTEBQPOBA. 503

authors say.1 Moreover, there is nothing in these appendageswhich recalls to miad the internal glandalar mass of the conesof Clione.

S t ruc ture of the Suckers.

The only histologies! knowledge of these organs which weformerly possessed proceeds from Gegenbaur's researches,who attributes a very simple structure to them.2 But thisknowledge was not very extensive.

During the month of January, 1885, I stndied the structureof these suckers by means of a series of transversal sections.I had scarcely finished this study when I received the thesis ofNiemiee," Recherches morphologiques sur les ventouses dans leregne animal.5's In this work the structure of the suckers ofPnenmodermon is carefully described and represented. Myown observations agreeing almost entirely with those of theSwiss zoologist, I believe it useless to explain them at length.Hence I refer to the memoir of Niemiee, limiting myself tothe principal points relative to the structure of the sucker andto indicating a few points of detail in which I do not entirelyagree with the Swiss zoologist.

The body of the sucker is formed of a layer of large, pris-matic, muscular cells (figs. 24 and 25), the contents of whichare formed of different separated fasciculi of fibres (a). Thesefasciculi have the form of very flat, three-sided prisms, ofwhich the most acute angle is turned towards the centre ofthe cell. In this place we find the nucleus (b), which has ashape analogous to that presented by the nucleus of the mus-cular cells of the cones of Clione. All around, towards theupper side of the disc formed by this layer of prismatic cells,we find a sphincter formed of not very numerous annular mus-cular fibres. The upper face of the muscular disc is coveredwith- a pavimentous epithelium (fig. 23, c) with excessively flat-tened cells, whose nucleus is consequently excessively flat.

1 Glaus, ' Handbuoh der Zoologie/ French translations p. 1056.3 Gegenbaw, loo. oit., p. 77.s ' ReeueE zoologique suisse,' t. ii, 1885.

504 PAUL PBLSBNBBE.

Between this epithelium and the prismatic muscular cells wesee absolutely no connective tissue. On the upper side ofthe disc, outside the sphincter, the epithelial lining thickensvery much, which is explained by the fact that it is by thispart that the adherence of the sucker to foreign bodies isproduced.

I have not remarked the constant presence of the cuticnlarpads ("bourrelets") which Niemiec described.1 Perhaps theseparts are specially visible in suckers of large size.

Beneath the epithelial thickening of which I speak, we find,all around the disc, glandular cells in the form of a flask witha very narrow neck. The efferent duct of these cells traversesthe epithelial tMckening and passes out at the upper face ofthe exterior ring of the sucker, that is to say, on the pointwhere the adherence is produced. The secretion of these cellsprobably makes this adherence more perfect.

On the lower face of the sucker the epithelium is less flat-tened than on the upper face, and it is united to the prismaticmuscular cells by connective tissue.

The peduncle is covered with an epithelium continuous withthat of the upper face of the sucker, and quite analogous to it.The peduncle itself is formed by the continuation of the longi-tudinal muscular fibres of the buceal appendage. These fibresturn to the lower part of the acetabular disc.

According to Niemiec, some fibres of the peduncle go to theside of the sucker. Such fibres I have not observed. In myopinion the longitudinal muscular fibres of the peduncle (fig.23, a) only go to the central part of the disc, and are insertedon it, between two prismatic muscular cells, by their extremity,which ends in a point (6). A retracting muscle inserted onthe side of the sucker would be far from having a usefuleffect. Since it is at this point that the adherence is pro-duced, the action of such muscles could only tend to combatit, while the muscles inserted on the centre of the sucker, byremoving this point from the body to which the circumference

1 Loc. oit., pi. iii, fig. 2, c.

CEPHALIC APPENDAGES OF GTHNO8OMATOTTS FTEBGPOBA. 505

of the disc adheres, augment the vacuum un fa this disc, andconsequently the adherence.

Except this point of detail, I quite agree with Niemiec onthe physio!ogieal mechanism of the suckers of P n e i m o -dermon. I am specially persuaded that the prismatic mus-cular cells fulfil an important part during the first moments offixation. These cells are especially worthy of the attentionof those who are interested in the comparative study of dif-ferent forms of muscular tissue.

ClKRIFEE.

In 187% G-. Pfeffer described by this name a gymnosoiaatoiisPteropod, which much resembles Pnenm.oderm.oiij whosecaudal and lateral gills it possesses.l

This Pteropod bears two buecal appendages like Paeumo-derma, but instead of being provided with suckers these twoappendages are terminated by two small branches Tjent roundin the form of a sickle. According to the drawing of Pfeffer,these appen d ages differ farther fram those of P n e am. o d e r m on,in that they are not inserted separately on the buecal wall, butreunite in a common stem before arriving at that wall.

Pfeffer describes two superior or labial tentacles j it is veryprobable that the nuchal tentacles, Yery small and retracted,have escaped him, as those ofPneumodermon have escapedGegenbaur.

Summary.

The different authors agree but little on the cephalic appen-dages of the gymnosomatous Pteropoda, and many of themconsider all these appendages as tentacles. In short theirhomologies are very obscure.3

We have stated that in the Gymnosomata there exists in aconstant manner two pairs of tentacles properly so called. I

1 'Monatsbericite der Akad. der Wissenseii.,5 Berlin, 1879, p. 249, fig. 20.2 " The connections befrweea these conformations and tie tentacles of the

Gastropoda are not yet very clear," Gegenbam, ' Gnmdziige der Tergieiohen-den Anatomie,' Irench translation, p. 481.

506 PAUL PELSENEER.

do not think it will be rash to identify these appendages -withthe two pairs which the Gastropoda Euthyneura (Gpistho-branchia and Pulmoaata) possess, and which occupy the sameposition among these animals as with the Gymnosomata.

la the Thecosomata we find a pair of rudimentary ten-tacles, for example, in Hyalaea, Cleodora, and Creseis,1

Cuvieria and Spirialis/5 Tiedemannia3 and Cymbulia.4

With several of these animals the tentacles present rudi-mentary eyes, Creseis for example.5

If they do not present eyes in the adult state they possessthem in some stage of the development, as in Tiedemanniaand Spirialis.6

This pair of tentacles is, in my opinion, equivalent to theoculiferous nuchal pair of the Gymnosomata. A3 to theanterior pair, its disappearance is explained by the displace-ment of the swimming lobes, which encircle the head andbetween which the mouth opens. The development of the finsin this position has caused the disappearance of the anteriortentacles.

Besides the two pairs of tentacles properly so called, wehave seen that most of the Gymnsomata possess buccalappendages; such are Clione, Pneumodermon, Cirrifer.

Without prejudging anything as to the morphological valueof these appendages, I believe that they have the same origin,however varied their aspect may be in the three genera abovenamed.

Apparently it seems that with Clione they are insertedaround the mouth, while with Pneumodermon and Cirr i ferthey are inserted on the internal wall of the buccal cavity.But it should be noted that in Clio n e there exists a hood whichcan fall back. This hood covers the buccal cones, and itsopening corresponds to the buccal opening of Clionopsis,

1 Gegenbaur, ' Untersuckungen iiber Pteropoden,' p. 8.3 Souleyet, loc. eit., vol. ii, pp. 199 and 209, pi. xii, fig. 32, and pi. xi, fig. 15.3 Gegenbaur, loc. cit., p. 60.4 Gegenbaur, loc. cit., p. 45.

CEPHALIC APPENDAGES OP GYKNOSOKATOUS PTEEGBOBA* 50?

Pneumodermon, and Cirrifer (compare figs. 23 4, and 7)The " l ips" situated between the cones of Ciione are notthen equivalent to the lips of the other Gymnosomataj inwhich there is no deTeloped hood like that of Ciione.They would be a differentiation of the internal wall of thebnccal cantys which is prodnced behind the buceal appendages.

In this manner we see that with Olione, as with. Pneu-modermon and Cirrifer, the buceal appendages are insertedon the internal wall of fche buccal cavity.

At the same time it must be remembered that this frontportion of the buceal eaYity may be regarded as not part of thetrue oral ca¥ity3 but as only an " introTert" like that of pro-bossidiferons Gastropods.

EXPLANATION OF PLATE XXXV.

Illustrating Dr. Paul Pelseneer's Paper on " The CephalicAppendages of the Gymnosomatous Pteropodaj and espe-cially of Ciione/'

FIG. 1.—Head of Ciione. Dorsal aspect, a. Labial tentacles. 5. Nuchalteatacles. c. Buceal cones, d. Ens. e. "Lips." / . Idge of the hood.

EIG. 2.—Head of Glione. Oral view, the hood being nearly closed, a.Labial teatacles. e. Biiccal cones, / . Edges of the hood.

I IG. 3.—Head of Glione. Lateral view. a. Labial tentacle, b. Nuchaltentacle, c. Buceal cones, d. fin. e. loot f. Edge of the hood. g.Orifice of the penis.

Pie, 4.—Head of Olione. Oral view, the hood being open. a. Labialtentacles, o. Buceal coaes. d. "Lips." / . Edge of the hood.

FIG. 5.—Head of Clionopsis. Dorsal aspect.Ym. 6.—-Head of Clionopsis. Lateral Yiew. a. Labial tentacles, b. Nuchal

tentacles, c. Foot. d. lias. e. Orifice of the penis.j?£G. 7, Head of Clionopsis. Oral view. a. Labial tentacles, b. Month.

e. Lips.

508 PAUL PELSEN1EE.

FIG. 8.—Head of Pneumodermon. Dorsal aspect.FIG. 9.—Head of Pneumodermon. Lateral view. a. Labial tentacles.

b. Nuchal tentacles, c. Acetabuliferous buceal appendages, d. Mas. e. Lips.f. Foot. g. Orifice of the penis.

FIG. 10.—Head of Creseis. Dorsal aspect. (After Gegenbaiir.) a. Mouth.b. Tentacles with eyes. e. Foot. d. Fins. e. Mantle.

FIGS. 11—22.—Glione.

Fig. 11. Transverse section of a bnceal cone. a. Annular muscularlayer. «'. Longitudinal muscular layer, b. Internal glandular cells.c. Epithelial covering.

Fig. 12. Longitudinal section of a buceal cone. Letters as in Jig. 11.Fig. 13. An epithelial group of the section Fig. 11, more magnified.

a. Annular muscular layer, b. Longitudinal muscular layer. V. Nu-cleus, o. Internal glandular cells, d. Their fibroid secretion, e. B.e-ticulated connective tissue. / . Epithelial cells, g. Its button-liketermination, h. Cellular contents, i. Seasonal cell. j . Its nucleus.k. Its rod-like prolongation. I. The striated conic part of the rod.m. The terminal disc of the rod. n. Refracting body.

Fig. 14.—An epithelial group, showing the reticulated aspect of the rodof the sensorial cell, a. Prolongation of the sensorial cell. b. Striatedpart of the rod. c. Refracting body. d. Epithelial cells, e. Fibroidsecretion of the internal glandular cells.

Fig. 15.—An epithelial cell, showing the stris of the membrane.a. Striffi of the cellular membrane, b. Cellular contents, c. Mbroidsecretion.

Fig. 16.—Longitudinal section of a group of glandular cells of the in-terior of a buceal cone. a. Internal glandular cells, b. Its nucleus.e. Basement membrane, d. Prolongation of the glandular cell.

FIG. 17.—Transversal section of several groups of internal glandularcells, a. Glandular cells, b. Nucleus, e. Basement membrane.

FIG. 18.—A glandular cell adjacent to the muscular part of the cone,showing the passage of the cellular substance to the fibroid secretion.a. Nucleus, b. Cellular contents, c. Fibroid secretion.

FIG. 19.—Transverse section of an epithelial group, passing a littleabove the annular muscular layer, a. Sensorial cell and its nucleusb. Reticulated connective tissue, c. Fibroid secretion, of the internalglandular cells.

Fig. 20. Ditto, through the base of the group, a. Prolongation of thesensorial cell. b. Reticulated connective tissue, c. Fibroid secretionof the internal glandular cells.

Kg. 21. Ditto, through the extreme base of the epithelial cells, a. Pro-longation of the sensorial cell. b. Refracting body. c. Base of thecentral epithelial cells, d. Fibroid secretion, e. Connective tissue,

CEPHALIC APPENDAGES OF GYXJTOSOMATOTJS PTEROPGBA. 509

E g . 22. Ditto, through the body of the epithelial cells, m. Rod or pro-iongatioa of the sensoriai ceil. h. Membrane of the epitlielial ceils,e. Striaj of this membrane, d. Ilbrold secretion.

FIGS. 23—25.—Fnenmodermon.

E g . 23. Part of a transverse section of a sucker, showing how thelongitudinal muscles of the peduncle are inserted on the muscular disc.a. Longitudinal muscular ibre of the peduncle, b. Its pointed ex-tremity, c. Epithelium of the upper face of the sucker, d. Prismaticcells of the muscular disc.

Fig. 24. Transverse section of a prismatic muscular cell. a. Prism ofmuscular fibres, b. Nucleus.

¥ig. 25. Longitudinal section of a prismatic muscular cell. a. Nucleus.