ON THE DtPLOOHOIiDA. 715

On the Diplochorda.1

IV. On the Central Complex of Cephalodiscusdodecalophus, Mel.

ByA. T . JUMsleiiiian, M.A., D . S c ,

Lecturer on Zoology, School of Medicine, Edinburgh.

Wiih Plates 32 and 33.

INTRODOCTJON.

IN tlio following description a number of the organs involvedalready possess a plurality of names, arising from the factthat various observers have recognised differing houiologies,and emphasised them in the nomenclature. Thus the"bnccalshield" of Cephalodiscus is also commonly termed the" epistome," "oral disc," and "pre-oral lobe." In the case ofthe two canals opening from the cavity of the buccal shield tothe exterior there is the same difficulty. From homology withBalanoglossus they are termed the " proboscis pores,"although the buccal shield has never been termed the proboscis;and they are more than pores, being of the nature of defiuitocanals. As this work indicates an even closer structuralresemblance to Balanoglossus than lieretofore recognised,it would be well to retain as far as possible the nomenclatureindicating this relationship; hence the terms collar cavitiesand trunk cavities are retained, whilst the term pre-oral canalis used as a synonym of proboscis pore with its attendantcanal. The term subneural gland is retained for the " noto-

1 Head before tlic Royal Society of Edinburgh, May, 1901.

716 A. T. MASTE R.MAN.

chord" of Harmer, with its homology to tlie similarly namedstructure in B a l a n o g l o s s u s . Lastly, the terms pericardialsac, glomerulus, and ectodermal pit are adopted for organsiu G e p h a l o d i s c u s which seem to be homologous with simi-larly named organs in B a l a n o g l o s s u s .

In the region lyiug between the buccal shield and thecollar of C e p h a l o d i s c u s are several important organs, theexact relationships of which have not previously been fullydetermined. This area may be described as the central com-plex, a convenient term already applied to the same region inB a l a n o g l o s s u s . In this region there are externally theectodurinal pit, the pre-oral pores, and the central nerve-mass ;internally are the subneural gland, certain important blood-vessels, the pericardial sac, aiid the meseiiteric walls of thepre-oral ;md collar cavities, together with the glomerulus a.ndmusuulur strands. The general outlines of the subneuralgland and the pre-oral and collar cavities have been indicatedby previous workers (Mclntoshs, Harmers) and by myself,but a detailed examination by carefully orientated and serialsections has brought to light some interesting new facts.

We may describe the organs under the following headings:1. Ectoderm and nervous system, ectodermal pit, pre-oral

pores.2. Subneural gland and pharynx.3. Pericardial sac, pre-oral cavity, pre-oral canals, glome-

rulus, collar cavities, and blood-vascular system.1. Ectoderm.—-Figs. 1—8 all illustrate the condition of

the ectoderm in. this region. Vent rally the ectoderm on thebuccal shield consists of long narrow epithelial cells withnumerous unicellular glands, which form a buccal gland asdescribed by Mclutosh (6). This epithelium is not shownhere. Dorsally it consists of columnar epithelial cells witha very definite cuticle. In the region of the central nerve-mass the inner ends of the cells are seen to pass downwardsas delicate fibres, terminating in peculiar conical ganglion-cells (figs. 1 and 2). At their base these ganglion-cells giveoff other delicate fibres running forwards and backwards.

ON THE D1PLOCHORDA. 717

These fibres make up the main nerve-mass lying over the sub-neural gland; they are seen in transverse section in figs. 6—8, and in longitudinal section in figs. 1—5. Forwards theyrun along the dorsal surface of the buccal shield, and back-wards they branch outwards to form the two lateral cords;hence, in longitudinal sections such as figs. 1—5, the nerve-fibres appear to terminate abruptly backwards against thewall of the pharynx, which here is in contact with the dorsalectoderm. The same remark applies to the dorsal blood-sinus.

Immediately in front of the central nerve-mass is seen apit or depression, the ectodermal pit (fig. 1). This pit liesexactly ovei* the apex of the subneural gland, and extendstransversely in a slightly crescentic form. At the outer endsof the crescent open the pre-oral pores, which are situatedat the posterior termination of the pre-oral cavity, at thelevel of the central complex. The ectodermal pit, therefore,represents the line of division dorsally between the head orbuccal shield and the collar, as do the "epidermistache"of Balanoglossus and the epiblastic pit of Actinotrocha(formerly termed the neuropore).

It is well to notice in dealing with this part that thenerve-mass is co-extensive with the collar, as this part isconsiderably narrower in the dorsal region; and further, thatthe subneural gland lies entirely in the collar area. At firstsight one is inclined to suppose, reasoning from prior know-ledge of Balanoglossus, that the two collar cavities areproduced forwards into the pre-oral cavity, but such is notthe case. The buccal shield is produced backwards ventrally,but the subneural gland lies in its primitive position in thecollar, and is in no way produced into the pre-oral cavity.It is a backward ventral extension of the buccal shieldwhich makes the subneural gland lie in front of the mouth,—not, as in Balanoglossus, a forward median extension ofthe subneural gland into the pre-oral cavity.

2. The Subneural Gland and Pharynx.—The sub-neural gland is an elongated csecal tube or prolongation of

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718 A. T. MASTERMAN.

the anterior wall of the pharynx. Its total length is usuallyabout "14 mm., and its breadth about "02 mm. It usuallyhas for about four fifths of its length a central lumen, whichopens posteriorly into the pharynx, and terminates anteriorlyin a variety of ways. Fig. 1 shows the subneurol gland cutthroughout nearly its whole length. The lumen usually, as inthis case, contains a rod of glandular secretion of the natureof mucus. A t its apex the gland is bent dorsally. Throughoutthe greater part of its extent its wall is composed of a simpleglandular epithelium, but at its distal extremity the cellsshow a chordoid modification. The cells become vacuolatedand reticular, producing the well-known chordoid structure ofthe "notochord" of Balanoglossus, Actinotrocha, and theVcrtebrata. This is well seen in figs. 8 and 9. The extentof this chordoid modification varies immensely, and it is onlythe largest (oldest) individuals which show such a completechordoid apex as in figs. 8 and 9. This specimen also showsa not uncommon feature in the complication of the centrallumen. In the apex it forks out into two lateral canals aswell as the median central canal (fig. 7)—a character alsofound in some Ente ropneus ta .

The relationships of the subneural gland to the phai'ynxhave been already described elsewhere (10), and its connectionwibh dorsal pharyngeal and peripharyngeal grooves has beendemonstrated. In fig. 3 the commencement of the dorsaland peripharyngeal grooves is shown with their numerousunicellular glands. The commencement of the pleurochordis seen in fig. 5. It is important to notice the relationshipsof the subneural gland to the pre-oral and collar cavities.It is bounded laterally throughout its extent, except at theapex, by the walls of the two collar cavities, and ventrally bythe wall of the pre-oral cavity. Above it the two collarwalls form a median dorsal mesentery (fig. 2), and thendiverge under the ectoderm to form the dorsal blood-sinus.At its distal end or apex the subneural gland reaches justbeyond the collar walls, and plugs up the mouth of the heart,as described below (figs. 1 and 2).

ON THE DJPLOCHOBDA. 719

3. The Pericardial Sac and Heart, Pre-oral Cavity,Pre-oral CaDals, G-lomerulus, Collar Cavities, andBlood-vascular System.—The pericardial sac lies ante-riorly to the distal extremity of the subneural gland.In most specimens it is nearly square in cross-section,but may be compressed at its base as in fig. 6. Roughlyits cross-section is about *05 mm., and its length about•08 mm. It appears to be a closed sac formed of verydelicate endothelium; its posterior wall is invaginated toform the heart. This inner wall is thickened, and hasnumerous muscular fibres stretching across the cavity of thesac to its onter wall (figs. 6 and 2). It is doubtless con-tractile, and the shape of the pericardial sac varies greatlyaccording to its state of contraction. On its ventral wallthere is a fairly constant transverse groove (fig. 1). The saclies in the blood-space or cavity between the walls of thepre-oral and collar cavities, and its walls do not differ exceptin their extreme delicacy from those of these cavities. Intransverse sections it is seen that the pericardial sac is bentover the apex of the subneural gland dorsally and ventrally(figs. 7 and 8). Laterally it is bounded by the wall of thepre-oral cavity, which is thickened into an epithelial lining ofthe pre-oral canal. In fig. 6 both pre-oral canals are clearlyseen, and the right pre-oral canal is cut throughout its lengthfrom the pore at the base of the ectodermal pit to the inneropening on the wall of the pericardial sac. The canal islined by a delicate columnar epithelium, apparently ciliated.In this connection we may note the statement of Ehlers (2)that the "proboscis canals" of Cephalodiscus end in blindsacs. There can be no doubt whatever that Mclntosh andHariner were perfectly correct in stating that they openfreely into the pre-oral cavity, though in a specimen examinedas a transparent object the pre-oral canal might appear toterminate in the pericardial sac.

I have elsewhere (7) described the blood-vascular systemof Cephalodiscus, and we have here to notice that, asindicated by Harmer (4), the organ I first took to be the

720 A. T. MASTEBMAN.

heart now proves to be a pericardial sac, containing the trueheart1 within it. The dorsal sinus can be seen running alongimmediately under the ectoderm and above the dorsal collarmesentery (figs. 1—4). Anteriorly it terminates against theposterior wall of the pericardial sac (which in a large numberof specimens is ruptured). Here it is also joined on eachside by a branchial vessel coming from the branchial plumes(fig. 7). Further, the anterior end of the dorsal sinus iscontinued into the cavity of the heart by paired lateralcanals, the relationships of which are not easy to find nor todescribe. If we could pull the apex of the subneural glandbackwards from the mouth of the heart it is clear that thedorsal sinus would communicate directly with the heart. Inthe normal condition, however, this wide aperture of theheart is almost completely plugged up by the apex of thesubneural gland. Dorsally and ventrally (figs. 7 and 8) thisorgan rests closely up against the pericardial wall, butlaterally a small canal remains running downwards fromdorsal sinus to heart (fig. 4). This canal is bounded pos-teriorly and laterally by the wall of the collar cavity, andanteriorly by the wall of the pre-oral canal (pre-oral cavity).It is doubtless through this paired canal that the blood findsits way from the dorsal sinus to the heart.

Below the subneural gland is a well-defined ventral sinus,which, passes backwards to the level of the mouth and roundit on either side. It is wide and large posteriorly, but passesforwards, getting narrower and narrower till it is lost in theglomerulus (tigs. 7 and 8). Ventrally it is bounded by the wallof the pre-oral cavity, which also extends ventrally, laterally,and anteriorly to the pericardial sac. Various parts of thiswall (pre-oral cavity) are thrown out into cascal prolongationsinto the cavity, with thickened protoplasmic walls. Thecavities of these cseca are in direct communication with theblood-sinuses. They produce an appearance closely similarto that of the glomerulus or pericardial gland of Balano-glossus, with the exception that the walls are simple and

1 The " pre-oral sac " of my previous work.

ON THK D1PLOCHOKDA. 721

not of a definite cellular structure. There is usually a pairedpatch of this glomerular tissue on the antero-lateral surfacesof the pericardial sac (figs. 1 and 5) in close proximity to theinternal apertures of the pre-oral canals. Fur ther , the wallof the vent ra l sinus shows a similar s t ructure (figs. 3, 4,and 7). In many cases the glomerular tissue of the ventralsinus is also paired, and the ventral sinus is then almostconskicted into two paired sinuses.

There is li t t le doubt tha t this glomerular tissue is homolo-gous with the pericardial gland or gloraerulus of B a l a n o -g l o s s u s . Antero-dorsally to the pericardial sac we maynotice a pre-oral sinus b r ing ing blood back from the buccalshield (fig. 2) to the glomerulus.

W e may now briefly run over the figures given here, notingthe special points of each. F igs . 1 to 5 are selected from aseries of very nearly sagittal orientation. In fig. 1 the sub-neural g land is cut almost throughout its length, its openinginto the pharynx being more to the r ight . The r igh t collarcavity is cut just at the apex of the gland, so tha t the sinusis rather more to the r ight anteriorly than posteriorly. Ther ight glomerulus is also seen, whilst the cavity of the hear t isspacious, al though not a t i ts largest (in the median l ine) .The dorsal sinus is cut throughout its length, and two oralgrooves may be recognised. In fig. 2 the collar mesentery iscut for some portion of its extent, and the glomerulus of theventral sinus is coming into view; the left dorsal groove isalso just appearing. In fig. 3 the pericardial sac is cut inthe median line ; the peculiar shape of the hear t is noticeable.Fur the r back the left collar cavity is alone seen, the dorsalsinus is restricted, and the subneural gland is interrupted.The left peripharyngeal and dorsal grooves are differentiated.I n fig. 4 the heart is no longer visible, bu t the left canal fromdorsal sinus into heart is seen. The posterior portion is stillmore to the left, showing the grooves as before. F ig . 5 iseight sections further to the left. I n following the sinus onenotices the left pre-oral canal becoming gradually more pro-minent, first laterally and then dorsal ly; the left glomerulus

722 A. T. MASTERMAN.

appears, and the left collar cavity increases greatly iu size.Posteriorly the first trace of the left pleurochord is seen,lying laterally to the dorsal groove. In this section theganglion-cells are no longer seen, and the nerve is inter-rupted at the pre-oral canal.

In figs. 6 to 8 we have selected sections from a transverseseries. The right side of the figures is slightly posterior tothe left. Thus in fig. 6 the right pre-oral canal (on the left)is cut throughout its length, but the left only in part. Inthis section the pericardial sac is cut transversely, and theheart is seen in its greatest size. Fig- 7 is a few sectionsfurther back. Here the apex of the subneural gland is cutthrough, and shows three internal canals and a chordoidstructure. Dorsally the pericardial sac is still cut, and belowthe ventral part is the glomerulus of the ventral vessel. Onthe right is seen the left branchial sinus leading out from thedorsal sinus, and a wider right branchial sinus opposite. Thetwo horns of the ectodermal pit are also seen. Fig. 8 is stillfurther back. The pericardial sac is no longer seen dorsally,but is still cut ventrally. The walls of the two collar cavitiesare approximately in the middle line, and behind the sub-neural gland will form a dorsal mesentery. The lateral nervesof the post-oral ring are seen in this section.

In fig. 9 the chief features here described are reproducedin a semi-diagrammatic median section of the entire animal.I have also shown the pharyngeal structure formerly described,i. e. the pleurochords, the dorsal and ventral grooves, and theoral grooves.

The facts above described must inevitably tend to bringCephalodiscusinto even closer union with Balanoglossusthan heretofore. Not only is every organ in the centralcomplex of the former to be directly compared to its homo-logue in the latter, but the latter has no organ in this regionwhich does not occur in the former. The only essentialdifference is one which several years ago appeared to me tobe of fundamental importance, but which must now he regardedas of secondary value. In Balanoglossus the pericardial

ON THE DIPLOCHORDA. 723

sac, glomerulus, and subneural gland protrude forwardsinto the pre-oral cavity, and hence are covered dorsally aswell as ventrally; but in Cephalodiscus they protrudeupwards between pre-oral cavity and collar cavities,and they are therefore dorsal and posterior to the former.Iu this way the pericardial sac lies in contact with the dorsalectoderm, and the subneural gland is only separated there-from by the dorsal sinus. This difference cannot be l'egardedas fundamental in view of the anatomical resemblance, andwe have seen above that it is due to the forward protrusioninto the pre-oral cavity of the subneural gland in Balauo-glossus, whereas in Cephalodiscus it remains in thecollar.

Of other points the homology of the subneural gland is amost important question. It appears desirable to adhere tothis term, firstly, because it is unquestionably glandular infunction; secondly, because it has precisely the same re-lationship to a system of dorsal and ventral grooves inthe pharynx as is the case with the similarly-named orgauin Tunicata (10); and thirdly, because its anatomical positionis exactly under the main nerve-mass. These and otherfacts led me to doubt its homology with the " Eicheldarm"of Balanoglossus, but its relationships to pericardial sacand glomerulus and the chordoid structure of its apexappear to me to be conclusive in favour of accepting Har-mer's original comparison. I would extend the appellationof subneural gland to the organ in Balanoglossus, for, asin so many other features, Cephalodiscus would appear toshow us a more primitive condition of the organ than Balano-glossus. Iu making this comparison it appears to me to bequestionable how far the subneural gland is at all comparableto the notochord of the Vertebrata. As indicated elsewhere(11, p. 412), a chordoid histological structure by itselfcannot be regarded as an absolute criterion of homology, andthe occurrence of chordoid organs of the same nature as, butnot homologous with, the Vertebrate notochord is to beexpected in these low chordates. The view of Willey that

724 A. T. MASTERMAN.

Cephalodiscus is to be regarded as a degenerate ally ofBalanoglosaus has not much to commend itself; the conse-quent assumption that the former has lost numerous gill-slits perforating its anatomy in all directions, not to mentionnumerous other organs, has no justification in fact. We maywith Lang (5) suppose that Cephalodiscus has undergonecertain important modifications due to a semi-sedentary habit,but the assumption that its proximate ancestors had manypharyngeal clefts and gonads has nothing to recommend itbut its necessity for Willey's theory. I would prefer toregard Cephalodiscus as the more primitive form, as itswant of metatneric segmentation and its primitive methodof feeding would imply (9). On this basis the " Eichel-darm" of the Enteropneusta must be regarded as a glan-dular specialisation of the anterior end of the pharynx, tobe termed the subneuial gland, owing to its functions andstructural relationships.

In Cephalodiscus its distal end often exhibits a com-mencing degeneration into chordoid tissue (which, by itsdevelopment in Actinotrocha, is clearly an arrested form ofglandular epithelium), whilst it is still functionally active asa gland. In Balanoglossus, with a specialised burrow-ing habit, the original function has been largely lost (thoughthe " Eicheldarm" of Balanoglossus is unquestionablyglandular), and the chordoid tissue with supporting functionbecomes still more in evidence. The organ to which thename of subneural gland was given in Actinotrochaoccupies exactly the same position as in Cephalodiscus,but as it is only embryonic its walls would hardly be expectedto be of a definitely glandular nature.

The pericardial sac of Cephalodiscus and its containedheart ai'e so similar to the pericardium (Herzblase) andheart respectively of Balanoglossus, and so different fromany structures found elsewhere, that the homology need notbe insisted upon. In a similar manner the mutual relationsof the ectodermal pit, the pre-oral canals, the pericardial sac,and the surrounding blood-siuuses speak for themselves.

ON THE DIPLOOHOUDA. 725

Lastly, there can be little question that we have iu theglomerulus a homologue of the proboscis gland of B a l a n o -glossus . Each is a proliferation of the pre-oral coelomicendothelium in the neighbourhood of the pericardia! sac andpore canals, consisting of ctecal vascular processes.

It is evident that in the study of the budding processes(10) the origin of the pericardial sac must have been over-looked; but as we do not yet know how this organ arises inthe demersal larva of B a l a n o g l o s s u s , nor even withcertainty in To in ar ia , this is not surprising. From certainindications it appears that in Cepha lod i scus it is a portionof the pre-oral cavity constricted off from its posterior end,and therefore ccelomic in origin.

During the progress of my work on C e p h a l o d i s c u s Cole(1) has published a short paper upon the bulbous termina-tions of the twelve branchial plumes. His results appear toindicate that the migration of oval lens-like bodies out of theepithelial cells to the exterior is to be regarded as a normalprocess, and that Mclntosh's view (6) that these organs aremasses of unicellular glauds is correct. Assuming that themigration might be an abnormality, I had suggested that " itseems most reasonable to regard them tentatively as primitiveeyes," a view I had already abandoned before the uuexpectedappearance of Cole's work. Cole further finds that theglandular bodies break up to form rhabdites, which I thinkquite probable, especially as I had already found and describedindications of " one or more areas iu the centre (of the bodies)staining moi'e deeply than the rest (7)." I cannot agree withCole's description of the epithelium in these terminal knobsas normally correct, as such a vacuolated swollen mass withlittle or no cuticle occurs commonly in other parts of thebody, and seems to be an abnormality; the vacuolated con-dition of the bulbs is undoubtedly present, especially inolder specimens. Cole denies the existence of a cuticle, ofpigment, and of nerve-endings in the cells. In respect to thecuticle I am hardly prepared to inaugurate a discussion uponthe line of distinction between a " peripheral deeply-staining

726 A. T. MASTERMAN.

membrane and a cuticle. There is little doubt that the in-tracellular bodies under discussion arise in the young formin close contact with this limiting membrane, but it ispossible that they do not actually arise from it. I am still ofthe opinion that fine brown pigment granules are scatteredthroughout the cells (Mclntosh [6] previously remarked uponthe " deep yellowish t int" of this region); and I still believethat the inner end. of each cell " tapers to a fibre-like thread,which I believe to have in some cases traced iDto the mainnerve of the plume" (7,p. 344). Indeed, it is rather difficultto understand otherwise in what region the very evident nervedown each plume terminates. None of these features areopposed to the " battery" function as suggested by Cole,though I have not as yet seen the rhabdites, which appear torequire special staining. If their presence is corroboratedit would form by no means the least interesting feature ofCepha lod iscus . Cole, as a critic of the work of his prede-cessors, might perhaps make a somewhat more sharp distinc-tion between a tentative suggestion and a definite statementof fact; but leaving this apart we may regard his work asconfirming Mclntosh's previous interpretation of the bulbousendings as masses of unicellular glands, the glandular se-cretion being extended to the exterior through the surfaceof the cells. Further, there is every reason to believe that,according to Cole, some at least of the glandular massesbreak up into rods.

LITERATURE.

1. COLE, F. J. Mourn, Limiean Soc.,' vol. xxvii, No. 175.2. EHUSKS. 'Abli. k. Gesellsch. Wissenscli. Gottingen,' Bd.xxxvi (1890).3. HARILEB, S. P.—Appendix to "Challenger" Report, vol. xx.4. „ 'Zool. Anzeiger,1 No. 545.5. LANG, A.—' Jenaische Zeitsclirift,' xxv (1890).6. MCINTOSH, W. C.—"On Cephalodiscus dodecalophus," '"Chal-

lenger " Report,' vol. xx.

ON THE DIPLOOHOBDA. 727

7. MASTEKMAN, A. T.—"On the Diplochorda," part ii, 'Q. J. M. S.,' Aug.,1897.

8. „ "On the Notochord of Cephalodiscus," 'Zool.Anz.,' No. 545, 1897.

9. „ " On the Origin of Vertebrate Notochord and Pha-rjngeal Clefts," ' Rep. Brit. Assoc.,' Sept., 1898.

10. „ "On Further Anat. and Budding Processes ofCephalodiscus," 'Trans. Roy. Soc. Edinb./vol. xxxix, pt. iii, No. 17.

11. „ "On the Diplochorda," part iii, ' Q. J. M. S.,'vol. xliii, pt. ii.

DESCRIPTION OP PLATES 32 & 33,

Illustrating Mr. A. T. Masterman's paper " On theDiplochorda."

FIGS. 1—5.—Selected sections (1, 3, 5, 7, 15) from a longitudinal sagittalseries through Cephalodiscus dodecalophus (Zeiss, obj. 7, eyep. 1).

FIGS. 6—8.—Selected sections (1, &, 6) from a transverse series throughCephalodiscus dodecalophus (Zeiss, obj. 7, cyep. 1).

FIG. 9.—A semi-diagrammatic right half of a polyp.