NOTES ON HHABDOPIiETJlfA NOItMANf, ATJTJMAN. 2 3

Notes on Rliabdopleura Normani, Allman.

By

O. Herbert Fowler, B.A., IMi.O., F.SK.S., F.I..S.

(With Plate 3.)

THESE notes, written mainly some years ago, did not seemworthy of publication by themselves. But my friend Mr.Havener lately called my attention to some remarkable state-ments made by Messrs. Conte and Vaney' which, seem tojustify tlie publication of tlie present paper, despite the smallquantity and imperfect preservation of ray materials.

These gentlemen state that the peduncle is inserted " enun point d'ou divergent le corps proprement dit, l'epistomeet les deux bras." This point, on the ventral surface, is themouth; but, as a niattei' of fact, the peduncle is insertedconsiderably behind it (compare Professor Lankester's figuresfrom living material8). I can neither confirm nor deny thestatement that the "fibres musculaires de ce pedonele seprolongent dans les bras et dans l'epistome," but I do notthink it probable that they really extend so far; the longi-tudinal muscles of the peduncle are for the retraction of theanimal as a whole in its tube; the graceful movements of armsand epistome, shown so beautifully in Professor Lankester'sfigures, demand an intrinsic musculature, parts of which Ihave already l'ecorded.3 It is stated that I "denied" theexistence of the testis figured and described by Lankester,

1 A. Coute and O. Vaney, ' Comptcs rendus Acart. Soi. Paris/ exxxv, pp.63, 74S.

2 E. B,. Lankester, ' Quart. Journ. Mior. Sot.,' xxiv, pi. 3S.a G. H. Fowler, " Festschrift zum 7Ot,en Gebiirtstage, Rudolf Leuckarls.

Leipzig, 1893j 4to.

24 G. HEBJJIilRT FOWLER.

whereas the original rails that " I have been unable to meetwith any generative organs/' my specimens not being sexu-ally ripe. The account •which the French authors havefurnished leads one to await their figures of the generativeorgans with interest.

To say of the coelom that " les sub-divisions indiquees parFowler n'existent pas" is rather sweepings in the face of thecamera drawings which I furnished in my last paper on thesubject; but as our authors go on to say that they havevainly sought the excretory canals and collar-pores, onebegins fco suspect that either the preservation "of the materialor the technique of the microfcomist was imperfect. Whenwe further learn, of the structure which I regarded as aprobable homologue of the " nofcochord" of Balanoglossusand Cephalodiscus, that " cette pretendue chorde n'etaitautre chose que l'extrernite anterieure du pedonole/J one canonly regret that theso gentlemen have not already figuredthe way in which the latter post-oral and ventral structuregets across, or behind, or beside the mouth, so as to becomecontinuous with the pre-oral " notochord."

I regret that I cannot draw the septa between the body-cavities more clearly than I have already done, but at leastI hope that fig. 19 may convince Messrs. Conte and Taneyof the existence of the collar-canals and pores. This figurelias been dfawu wifcli a camera lncida from five successive

sections; the uppermost exhibiting the external opening,the next two the collar-canal, the last two the internal open-ing; the cell-structure is sufficiently well preserved to allowone to see that the cells are long aud columnar in the canal,with the nuclei near the base of the cell; but, as the histologyas a whole is not good, I prefer to represent the sections as" coupes histologiques scheuiatiques" rather than to drawguesses at cell outlines, which are moreover wholly unim-portant in this connection.

I. THK STALK OF THE ADULT.

la a series of transverse sections the first appearance of

NOTES ON BUABPOPLKUBA NO MONT, AT.LMAN. 25

the insertion of the stalk is indicated by a thin crescenticplate of longitudinal muscle-fibres, which seem to form partof the somatic mosoderm of the body on. the ventral surface.They are first recognisable some little distance above(anterior to) the bend of the alimentary canal. At the levelof the intestinal flexure the muscle-plate has become some-what thicker (fig. 1).

When clear of the body of the polyp, the soft part of thestalk ("gymnocaidns" of Lankester) shows the relations re-presented diagramtnatieally in fig1. 2. It is presumablycovered entirely by ectoderm; this ectoderm is certainlythick and glandular on the upper side, that turned towardsthe. polyp. Beneath this lies the longitudinal muscle as twoJ-shaped bands separated from one another by a septum,which bisects the cavity of the stalk. At the ventral borderof this septum bhe ectoderm is thickened into a triangle, thecells of which are not piginented, as is the rest of the ecto-derm, and stain very faintly; they have very much theappearance of a superficial nerve (figs. 2, 3, a). Abutting onthis triangle a fine canal is excavated in the substance of themesentery, recognisable in many sections and several speci-mens, but not in all; it may perhaps be an artificialstructure (fig. 2, b). In the central part of the stalk anothercavity is always visible, generally completely filled with agranular mass, but in the section figured this mass hadshrunk away from the walls, which are thus rendered moreconspicuous (figs. 2, 3, end?).

At the junction of the soft stalk with the body the rela-tions are extremely difficult to determine, owing to theobliquity of the structures concerned and to a rotation of thestalk. The coelom is comparatively broad at the point ofinsertion, and I believe that I can trace the paired cavities ofthe stalk into the coelom, and the central cavity of themesentery into continuity with the enrloderni. In palliationof this uncertainty, I have drawn the outline of a human redblood-corpuscle on the same scale (fig. 2, r. c), from which itmay be gathered readily that the difficulty of study of such

VOL. 4 8 , l'AKT 1. NEW SKRIES. 3

26 G. HHRBRRT FOWF.KR.

minute objects in imperfectly preserved aud limited materialis considerable.

Afc the transformation of soft stalk (gymnocaulus) intohard stalk (peetoeaulus) the high ectoderm spreads ronndthree-quarters of the circumference, aud presumably secretesthe dark brown caulotheca, or stalk-pipe (fig. -3). Stillfurther posteriorly the caulotheca invests the peetoeauluscompletely, the muscles disappear, and the soft tissues nowconsist of a central core, apparently contiiinous with thecentral (? endodermal) core of the gyranocaulus, and sur-rounded by a membrane; it is certainly flanked, andprobably entirely surrounded, by pigmented ectodei-m-cells.

As figs. 1 to 4 are all drawn in the same position asregards the polyp, it will be noticed that there is a rotationof the stalk through about 90°; the mesentery, whichoriginally lay in the oro-anal plane of the polyp, finallycomes to lie right and left as regards the polyp-axis,although dorso-ventral as regards the colony. This may beaccidental {as Mr. Havnier suggests), but is at Rny ratenot unusual.

IT. THE ANATOMY OF A BTJD.

The specimen which I select for description was apparentlyat a stage intermediate between Nos. 6 and 7 of ProfessorLaukester's fig. 3, pi. 39, in that the lophophoral arms werelonger t.Tiim in No. 6, but liad not yefc begun to developfilaments. It lias been drawn as fig. 18 of this paper. Theproboscis or epistome is large, the collar region small aud onlyslightly Jiirger than the trunk, the trunk indistinguishableexternally from the gymnocaulus. At this stage, therefore,the long axis of the body is a continuation of that of thegymnocanlus—a condition unlike that of the adult (of.Lank ester, op. cit., pi. 37, fig. 1).

As to the lophophoral arms and upper part of the proboscis,there is nothing of special developmental interest to sny; thearms simply grow out from the collar region, and containoff-sets of the collar body-cavity from the beginning.

NOTKS ON lUtABDOPl.EUnA NOHWANT, ALT.MAN. 27

Figs . 5 to 14 are from a continuous series of successivesections, all of which, are d r a w n ; it is therefore possible tofollow the smafcomy minutely. The sections arc slightlyoblique. Starbing with fig. 8, there seems to be a well-marked stomodteum, which, owing to the obliquity of the sec-tions, appears erroneously to open on the r ight side only. Thisstomodseum is sharply separated from the upper (rectal) pa r tof the al imentary canal by a stout membrane ; the canalitself ab this level appeared to be a vacnolated masa, in whichno epithelial-cell outlines were recognised. A.11 three sub-divisions of the coelom were represented in this section—asmall par t of the proboscis-cavity (6c.1), t he left coliar-cavity (5c.2), and the trunk-cavity, apparently divided intotwo par ts by the alimentary canal dorsally arid ventral ly(be.3). On the animal's r ight side the section passed nearlyulong the septum between the collar- and trunk-cavities.

Tu the section iibove this (fig1. 7) the collar-cavity of ther ight side appeared, and the tvnnk-cavifcy of t h a t side hadalmost vanished. The next sectiou upwards (fig. 6) was un-fortunately folded between proboscis and coelom, so tha t notmore than has been drawn could be recognised ; it wasobvious, however, that the stomodreal groove of the previoussection had been folded off as a rod, which contained (I think)a. cavity. In the highest section figured (fig. 5) the al imentarycaual wus no longer met with ; the rod of the previoussection was in the position of the notochord.

Passing downwards from fig. 8, the next section (alsofolded at the at tachment of the proboscis) showed a thickmuscle-band on the outer wall of the right-Tinnd half of thetruuk body-cavity, other structures remaining much as before(fig. 9). I n fig. 10 the stomodaaum had entered the alimentarycanal (ce), and the lower lip had been reached. In fig. 11the r ight t runk body-cavity had increased considerably insize, and the at tachment of the proboscis had been passed.The left collar-cavity had all but disappeared in fig. 1 2 ; theleft t runk-cavity showed its longitudinal muscle, and a septumseparated the two trunk-cavities ventrally. In fig-. 13 the

28 G. HBHBNii'T

alimentary canal began to diminish, the mesentery to elon-gate i and in fig. 14> the alimentary canal appeared to Tjerepresented by fclio central core of the mesentery of thegymnocaulus, the two trunk-cavities becoming- the pairedcavities of the stalk,

I have endeavoured to express my interpretation of thesesections by an imaginary longitudinal section in fig. 15. If myviews are correct, two things Follow—that t h e notoc l iordin the bud is of ectodermal origin, and that thegymnocaiiliis contains all th ree embryonic layers,the proliferation and growth of which give rise to equivalentstructures in the adnlt.

As regards the notoehord, I have long suspected that itwas a stomodosal structure in Balanoglossns and Cephalo-disens, and there can be little hesitation in assigning it tothe ectoderm in buds of Rhabdopleura on the strengthof these sections. Figs. 7, 8, aud 9 show an epithelial iu-vagiuation below the proboscis-stalk, which, from the cha-l'acter of the cells, is fairly cei'tainly ectoclormal, and iscontinuous with the so-called notocliord; the alimentarycanal, on the other hand, appeals, so far as I can see, to besyiicytial aud vacuolated rather than epithelial; this is shut offby a basement membrane from the stomodienm at the pianoof these sections, aud is presumably the future endoderm.

As regards the structure of the adult gyranocaulus, I haveno personal doubt of the view given above, that the contentsof the central cavity in the scpturn are continuous with thealimentary canal of the adult, and give rise to the ali-mentary canal of the bud; they are presumably of eudo-derrnal origin. Similarly the paired cavities of the gym-nocaulus are traceable fairly unmistakably into the trunk-cavities of the bud, less certainly into those of the adult.At the same time, the structures in question are so minntcthat these views have only the value of a personal conviction,and require confirmation from other sources.

These notes and drawings of tlie structure of the stalk andbud, such as they are, were made before the publication of

NOTES ON RHABDOPLKURA HORMANI, ALLMAN. 29

Dr. Masterman's paper1 on the budding of Cephalodiscus,butI am unable to bring the two sets of observations intoaccord. There is no doubt that Masterrnan's picture of thestalk in Oeplialodiscus is correct in exhibiting two cavitiesbounded by a thickish membrane (as in his pi. i, fig. 18).,whatever may be the correct interpretation of these struc-tures. There is equally no doubt that my fig. 2 is alsocorrect (interpretations excepted) in showing the coelom ofthe stalk divided completely by a septum. But Masteriuaninterprets the cavities in Oephalodiscus as " blood-" sinuses,whereas my specimeus lead me to believe that the centralcore of the Rlmbdopleura septum is continuous with tlie lining

of the alimentary canal. Unfortunately buds smaller thanthat described in detail above proved to be too uiiuute toallow of definite conclusions being drawn,2 and the prelimi-nary remarks of MM. Conte and Vaney are too brief andvague to settle the matter (op. eit., p. 749).

Cophalodiscus and Rhabdoplenra agree in the precociousformation of the epistome, in the continuity of the stalk-ccclom with that of the bud, and in the presence of a nerve-like stripe of ectoderm on the stalk.

EXPLANATION OF PLATE 3,

Illustrating Dr. G. Herbert Fowler's "Notes on lthabdo-pleura Normani, Allnian."

NOTE.—As in my previous paper (op. cit. supra), (he tiuuk-eccloni has beendrawn all round the alimentary canal OK the authority of Prof. Lankestui'sobservations on living specimen!), although in my shrunken specimens it is

1 A. T. Masterman, ' Trans. Koy. Soo. Ediu./ xsxix, p. 507.2 At the same time, the structures are large enough to allow of accurate

determination in material specially preserved; mine had been roughly pre-served (apparently merely in strong alcohol), for the sake of the Lophopboliaon which it grew; as it was "Challenger" material, thirty years' preservationhas not improved it.

30 0. HEEBBET FOWLER.

only visible here and there; this has necessitated a slight readjustment of thecomparative thicknesses of the body-layers in the figures. The ectoderm hasin many figures been drawn thicker than it actually appears. In my depig-mented specimens it is invisible over a large part of the body and stalk. Withthe exception of fig. 15, all outlines have been drawn with the Abbe" cameralucida. Fig. 15 is ba«ed on a plotting of the actual section.drawings onscaled paper, free-hand curves being drawn through the points thus oblainerl;the horizontal scale is therefore nearly correct, the vertical scale arbitrary,but estimated roughly on the thickness of the sections.

REFERENCE LMTEBS.

a. Streak of unpigmeiited ectoderm in the gymnocaulus (? nervous), al.Alimentary canal, asc. Ascending half of the alimentary canal, h. Space inLite mesentery (? blood-vessel or artificial). 4c1. Ccslom of the proboscis orepislome. 6c2. Ccelom of the collar region. 4c3. Ccelom of the trunk or bodyregion, caul. Caulotheca, or stalk-pipe, c. c. Collar-canal, d. mes. Dorsalmesentery, desc. Descending half of the alimentary canal, eel. Ectoderm.end. Endoderm of the adult, end. ?. Core of the mesentery, probably endo-cteimal. mes. Mesentery or septum of the ^yniuocuulus. muse. Longitudinalretractor muscle, n. Dorsal thickening of ectoderm {? nerve-plate), nch.Stomodasal diverticulura (so-called nolochord). m. (Esophagus, pr. Pro-boscis, r.c. Outline of a human red blood-corpuscle, for scale, s. Septumbetween the body-cavities of the proboscis and collar, si. Stontodseiirn.tub. Tubarium. v.mes. Ventral mesentery.

PLATE 3.

FIGS. 1—4 relate to the stalk of the adult.T?ig. 1.—Section of the posterior end of the adult, at the point of flexure

of the intestine, showing the continuation of the longitudinal muscle ofDie stalk on to the body, x 430.

Fig. 2.—The gymnocaulus, below the body of the animal, x 820.Fig. 3.—The gymnocaulus, at the commencement of the pectocaulus.

X 820.Fig. 4.—The pectocaulus. x 820.

FIGS. 5—14 arc successive seel ions of the bud drawn as fig. 18. The planeof section is somewhat oblique and the epistome twisted, x 520.

Fig. 5.—Below the attachment of the lophophoral arms.Fig. 6.—Through the highest point of the alimentary canal, dorsally.

No anus was visible.Fig. 7.—The stomodftsuin, open on the right side.

NOTBS ON RIIABDOPLEUEA NORMANI, ALLMAN. 3 1

Fig. 9.—-Tlie right longitudinal muscle of the stalk appears.Fig, 10.—'l'lie oesophagus separated from the atomodtcuiu by the lower

lip.Tig. 11.—Below the proboscis-stalk.Tig. 12.—The left longitudinal muscle of the stalk appears.Fig. 14.—Tli e gy m n ocaulus.

F I G . 1 5.—Diagrammatic reconstruction of the foregoing sections as a longi-tudinal section beginning just below (In; insertion of the lophophoral arms, tlieoutline of the trunk body-cavity, which of course is not cut in a median dorso-ventral seclion, being, marked by dashes. The numbered arrows indicate thecoi responding figures of the transverse sections.

FIGS . 16, 17, IS.—Buds at the end of a lerminul branch, a short length ofpeolocaulus intervening between Llie successive figures. Of these fig. 1G is thegrowing end of (he branch, and fig. 18 the oldest bud drawn. X 140. Tlielowest bud in Fig. 1G is viewed from the right side, and gives a good idea ofthe way in which the lophophoral anus spring from the cad of the bodyproper, und the proboscis stands out on the vonlral side.

F I G . ly.—Successive sections of the collar-pore and canal of tlie light sideof an adult animal. X ubout 520.