ON PTtCHODERA FLAVA. 165

On Ptychodera flava, Eschscholtz.

By

Arthur Willcy, D.Sc.

With Plate 5.

PTYCHODERA FLAVA has the distinction of being the oldestas well as one of the least known of the Enteropneusta, havingbeen recorded and figured for the first and only time byEschscholtz in 1825, from material obtained from the Marshallor Rumanzow Islands, in the Pacific, north of the equator.

Eschscholtz, as quoted by Spengel,1 regarded his Ptychoderaas a worm-like animal belonging to the group of the Holo-thuridae. Otherwise, however, his description was very de-fective, and far from being a specific diagnosis. Still, in sofar as Eschschoitz stated that the body of the animal was " derLange nach gespalten," and also from the figure which accom-panied his description, and is reproduced in Spengel's mono-graph, indicating the presence of genital pleurae (see below),Spengel was enabled to retain the species in his amendedgenus Ptychodera.

Since 1825 this species has not been re-discovered in thetrue sense of the term, although Spengel makes the suggestion,which the present contribution provisionally2 supports, thatthe fragments of a Balanoglossus obtained by Dr. Francois in

1 J. W. Spengel, " Die Enteropneusten des Golfes von Neapel, &c,"1 Fauna und Flora des Golfes von Neapel,' Berlin, 1893.

2 I say provisionally because absolute certainty can only be arrived atwhen the form from the first recorded habitat (Marshall Islands) comes to bere-investigated.

166 ARTHUR W1LLEY.

the vicinity of Noumea, New Caledonia, belonged to P. flava.Francois1 simply mentions that his native servant one daybrought him some fragments of a Balanoglossus, and he makesno further reference to it.

I have found a Ptychodera which may probably be identifiedwith P. flava, especially if it may be assumed thatEschscholtz'sfigure represents approximately the natural size of the object,2

in great abundance near the low-tide mark on the small isletof Amedee, upon which stands the lighthouse, some twelvemiles out from Noumea and eight or ten miles inside of thegreat Barrier Reef of New Caledonia.

I t occurred near the surface of the sand, chiefly underneathloose stones, often adhering to the latter, and creeping intothe holes with which the coralline blocks are riddled. On alater occasion I found it to be, if possible, still more abundanton the rocky platform of coral limestone which surrounds agreat part of the Isle of Pines. This platform is, in places,much excavated, and, while it is exposed at low water, thereare numerous rock-pools scattered over it, in some of whichmany different kinds of seaweeds luxuriate. In the shallowerpools P t y c h o d e r a flava occurs in great numbers in the sandat the base of or in the neighbourhood of the tussocks of sea-weed, being often involved in the roots of the latter. Severalspecies of Nemertines occur in the same locality,3 but aremuch rarer than the Ptychodera.

E x t e r n a l F e a t u r e s (cf. Fig. 1).

When first taken from their native habitat the individualsof P. flava average in length approximately from 1^ to 2 oreven 3 inches, and the intestine is often full of sand. But

1 Ph. Francois, " Choses de Noumea," ' Archives de Zool. expeV.' (2), t. ix,1891, p. 232.

3 Spengel (loo. cit., p. 190) suspects that the Ggure given by Eschscholtzrepresents the animal on a reduced scale. Judging by the material obtainedby me in New Caledonia, thi3 need not have been the case.

3 The exact spot on the Isle of Pines where 1 found the Ptychodera wassituated at the point on the opposite side of the harbour to that on whichthe military buildings are placed.

ON PTYOHODERA FLAVA. 167

after being kept for a short time in captivity they dischargethe sand, and the larger specimens may then stretch them-selves out to a length somewhat exceeding 5 inches. Thecollar region may be upwards of a quarter of an inch inlength, while the proboscis is normally somewhat shorter. Therelatively great length of the collar region is characteristic ofthe genus Ptychodera. The gill-region measures from halfto three quarters of an inch in length, being about as long asthe proboscis and collar regions put together. The hepaticregion may measure about one and a half inches.

Eschscholtz correctly described the body as presentingnumerous transverse folds or annulations. When the intestineis free from sand and the body is consequently not swollenout, these annulations are very prominent ridges, and arecharacteristic for the species. They are not continuous allround the body, but are interrupted along the dark yellowishor reddish-yellow coloured lines which mark the^course of thedorsal and ventral nerve-cords. Here and there, especially inthe posterior dorso-lateral region, the annulations are sub-divided into islets. The more faintly marked ridges on theouter surface of the genital pleura are often similarly sub-divided and also branched.

On either side of the dorsal nerve-cord may be seen a darklongitudinal band corresponding in position to the ciliatedgrooves in the intestinal wall, described by Spengel in otherspecies, and more recently by Hil l 1 in P. a u s t r a l i e n s i s .

But these externally visible lines do not cause any interrup-tion in the annulations or islets of the integument, as they doin P . a u s t r a l i e n s i s , according to Hill's description, and asdoes the single asymmetrical band, present only on the leftside, in P. m i n u t a , as described by Spengel.

As indicated by the specific name, flava, the colour of thisspecies is a nearly uniform dull yellow, somewhat deeper in

1 Jas. P. Hill, "On a New Species of Enteropneusta (Ptychoderaaustraliensis) from the Coast of New South Wales," 'Proc. Linn. Soc.New South Wales/ vol. x (2), 1894. Id., " Preliminary Note on a Balano-glossus from the Coast of New South Wales," ibid., vol. viii (2), 1893.

168 ARTHUR WILLED.

the more opaque regions of the proboscis and collar. Some-times the body has a more brownish tinge. The anterior liversacs, however, offer a relief to the general yellow groundcolour, in that they are of a dark greenish-brown colour, whilethe sacs about the middle of the hepatic region are of a lightbrown, passing posteriorly into the usual yellow colour. Theliver sacculations pass quite gradually behind into the ordinaryannulations of the body-wall, and it is not always easy to saywhich is the last hepatic diverticulum.

In cases where the body has evidently been broken in twoin the hepatic region, and the anterior portion of the body,including the whole of the branchial region has been lost atno very distant period, a new collar and proboscis have beenadded by regeneration immediately in front of the liver-sacs,while the branchial region would no doubt be regenerated later.In such regenerated individuals the collar and proboscis arewhite and unpigmented.

The proboscis in the normal condition is distinctly grooved inthe dorsal middle line, and in this respect P. flava may becompared with Balanoglossus sulcatus , Spengel (cf. fig. 1).

The liver-sacs are not always simple smooth outgrowths, butthe larger ones are distinctly lobed, and sometimes present adigitate appearance. An intensification of this lobed structurewould probably lead to such a diffuse arrangement of theliver-sacs as is met with in P. erythrcea, Spengel.

In only two individuals out of the many that have passedunder my observation have I observed them to be infested with acurious parasite (? Ive balanoglossi , Paul Mayer), originallyremarked by Spengel in P. minuta, and more recently byHill in P. aus t ra l i ens i s . As described by Hill in the latterspecies, the parasite occurs in one of the genital pleurae (in theexample here figured on the right side), where it forms a veryprominent tubular enlargement " (cf. fig. 2). Hill states thatin P. aus t ra l iens is " a large proportion of the individuals"are infested with the parasite.

ON PTYOHODEBA FLAVA. 169

Anal Resp i ra t ion .I should like to direct the attention of those zoologists who

may have future opportunities of observing living Balano-glossus, to the possibility of the occurrence of anal respiration.When the posterior end of the body is protruding from a massof sand and seaweed I have observed the anal orifice in P.flava to open periodically, widely, and slowly for a second ortwo, and then to close up again. This may occur two or threetimes to the minute, and it has apparently no relation whateverto the evacuation of fseces.

In the case of the large Balanoglossus occurring at theIslands of Bimini, in the Bahamas (species not stated), whosedevelopment was studied through theTornaria stage by Morgan,1

the author states that generally the posterior end of the wormprotrudes from the surface of the sand, sometimes as much asan inch. " If," he says, " the spade is thrust rapidly into thesand before the worm has been disturbed, it is easy to cut offfrom six inches to a foot of the hind end of the body, but im-possible to get more of the worm." When it is so deeply em-bedded in the sand, it is conceivable that the branchial respira-tion would not entirely suffice for the needs of the animal, andthat anal respiration may occur as an accessory to the former.

With regard to the tenacity of life exhibited by P. flava,it cannot compete with Balanoglossus Kowalevskii in thisrespect, according to Bateson's account (quoted by Spengel,loc. cit., p. 341). In a dish of P. flava, in which the waterbecame slightly turbid overnight, the Ptychodera were nearlyall dead, those that were not dead being moribund, and theywere outlived by several Annelids.

Geni ta l Pleurae.The genus Ptychodera is distinguished from the other genera

of Enteropneusta, established by Spengel,—above all by thepossession in the anterior region of the body (the branchio-

1 T. H. Morgan, "The Development of Balanoglossus," ' Journ. Morph.,'vol. ix, 1894.

170 ARTHUR WILLEY.

genital region of Spengel) of lateral wing-like expansions,which can be folded over so as to meet one another in thedorsal middle line, and so to completely embrace the branchialregion and the most anterior portion of the hepatic region.These are the genital wings (Genitalfliigel) of Spengel, socalled because they contain the gonads. We may convenientlyrefer to them as the geni ta l pleurae.

In P. flava the genital pleurae have a very low origin,arising from the ventro-lateral margins of the body, and theyconstitute remarkable structures. They are very mobile, andin life can be spread out laterally nearly flat; while, as alreadystated, they can meet over the pharynx in the mid-dorsal line,thus producing a most effective peripharyngeal cavity oratrium, opening to the exterior posteriorly in the neighbour-hood of the anterior hepatic region. The genital pleurse of P.flava attain their maximum development within the branchialregion, and maintain it for some distance into the post-bran-chial region, behind which they gradually decrease in size, andfinally die out on the outer sides of thf iiver-sacs (fig. 1). InP. aus t ra l i ens i s , according to Hi \ , they reach their maxi-mum size somewhat posterior to the gill region.

The Pha rynx (cf. Fig. 3).When, in the living animals kept under observation, the

genital pleurse are spread out laterally, a complete andbeautiful view of the entire pharynx is to be obtained. Thelatter is then seen to stand up, erect and independent, in themiddle of the peripharyngeal area, and the branchial bars arevisible nearly if not quite throughout their whole length.Dorsally, on either side of and adjacent to the dorsal nerve-cord, two whitish pigmented bands extend throughout thelength of the pharynx. These are the bands which in mostEnteropneusta form the inner or median boundary of the lon-gitudinal grooves into which the gill-pores open.

In P. flava, however, the U-shaped gill-slits open freely tothe exterior throughout their whole extent, and their externalopenings are therefore not reduced to minute circular or ellip-

ON PTYOHODEKA FLAVA. 171

tical pores as they are in most other species of Enteropneusta,and indeed in most other species of the genus Ptychoderaitself.

In the possession of this remarkable free pharynx P. flavaexhibits its close affinity with P. erythrsea, Sp., from theRed Sea, and P. bah am en sis, Sp., as described by Spengel,especially, as it would appear, with the latter.

As in all species of Ptychodera, so here, the anterior portionof the alimentary is subdivided by a deep longitudinal con-striction into a dorsal, branchial, and a ventral oesophagealportion.

From what has been said above it is obvious that P. flavais a very favorable species for studying the structure of thepharynx, since the latter can be easily removed and examinedunder the microscope.

As might be supposed, there is not much anatomical detailto be added to the exhaustive account given by Spengel ofthe Enteropneustan pharynx; but there is a point of import-ance in any comparison between the latter and the pharynx ofAmphioxus, which is not emphasised in SpengePs monograph ;in fact, so far as I can ascertain, he makes no reference to itwhatever, and yet it is of prime significance.

On examining the pharynx of P. flava one cannot fail tobe astonished at the relatively enormous size of the tongue-bars as compared with the primary or septal bars (fig. 3).The former are wide, opaque, dark brownish coloured struc-tures, while the former are narrow and semi-transparent.The contrast between the primary and tongue bars in point ofsize and appearance could hardly be much greater than it is inP. flava.

In fact, it may be stated categorically that in the Entero-pneusta in general (as shown by Spengel's figures), and in P.flava in particular, the tongue-bars are much larger than theseptal bars; while in Amphioxus, as is well known, the reversecondition obtains, in that the primary bars are larger (but notso much larger) than the tongue-bars.

This is a most important difference, uot only in an ana-

172 ARTHUR WILLEY.

tomical and physiological sense, but in a morphological sensealso, because while no zoologist would conclude from it thatthe corresponding structures in the respective types were mor-phologically different, yet it serves to explain most of thosedifferences in detail which Spengel so elaborately enumerates.

In correlation with the great size of the tongue-bars in thepharynx of the Enteropneusta, it is not surprising to learnthe important fact from Spengel that they, rather than theprimary bars, are hollow, containing a wide prolongation ofthe ccelom. " In Folge dessen," says Spengel himself (loc.cit., p. 725), " kann die Zunge der Amphioxus-Kieme nicht,wie die der Enteropneusten, zwei Zungenzinken enthalten,sondern nur eine, die allerdings aus zwei gleichen Halftenzusammengesetzt erscheint."

Thus, according to Spengel's own assertion, the presence oftwo skeletal rods instead of one only, in the tongue-bars ofthe Enteropneusta, stands in correlation with their hollowcharacter; while the latter, in its turn, is correlated with thegreat size of the tongue-bars. In consequence of the occur-rence of two separated skeletal rods in the tongue-bars, thedorsal arcuate extremities of the branchial skeletal structuresare not continuous as they are in Amphioxus, but are in-terrupted at each tongue-bar (cf. Spengel, loc. cit., Taf. ii,fig. 21).

The fact that the skeletal rod of the tongue-bar is single inAmphioxus and double in the Euteropneusta is an anatomicaldifference of importance, but not necessarily and, it may beconfidently asserted, not in fact a morphological difference.But it accounts, on the principle of correlation, for otherdifferences upon which Spengel lays such stress. It fullyexplains the difference which Spengel has had printed in spacedtype—namely, that " beim Amphioxus gehort jede Skeletgabeleiner einzigen, bei den Enteropneusten aber zwei Kiemen an."

Before coming to the conclusion that " die Kiemen derEnteropneusten und des Amphioxus . . . . wesentlich ver-schiedne, morphologisch einander nicht entsprechende Bild-ungeu siud," Spengel makes a serious attack upon the synap-

ON PTYOHODEBA FLAVA. 173

ticula or cross-bars of the pharynx. He says (loc. cit., p. 726),"Bei den Eateropneusten sind die Synaptikel stabformigeSprossen, welche zwischen einer Zungen- und einer Septal-zinke ausgespannt sind. . . . Anders beitn Amphioxus. Dortsind . . . . die Synaptikel . . . . zwischen zvvei Septalzinkenausgespannt und der dazwischen liegenden Zungezinke nurangelagert."

A glance at fig. 3 accompanying this paper will, I think,show conclusively that the above quotation represents merely asubjective mode of expression. In P. flava the synapticulaon one side of a tongue-bar are approximately often quiteopposite to those on the other side. As the skeleton of thewide tongue-bar is separated into two halves, the synapticulamust necessarily likewise be separated.

By insisting on detailed differences, and more or less neglect-ing the broader distinctions to which they are subordinate,and which would in great measure account for the former,one can really arrive at any conclusion to which the individualmind is inclined.

To see the synapticula in P. flava the pharyngeal wallmust be viewed from the inside, since, as pointed out by Spengel,these structures are placed towards the inner side of the gill-bars, and not on their outer face as in Amphioxus. I have foundit a good method to kill in dilute formol, and having removedand opened out the pharynx to mount it in the same liquid.It is well to cut away portions of the genital pleura beforepreservation in formol, as they are otherwise liable to becomeglued together in the dorsal middle line.

The number of synapticula in a vertical or dorso-ventralseries in P. flava is from ten to thirteen. In this species, asin the majority of Enteropneusta, the gill-bars are not straightas they are in Amphioxus approximately, but are markedlybowed, the convexity facing outwards.

Gonads of P. flava.

Another remarkable feature of the species under considera-tion, which it presents in common with P. erythrcea, P.

174 AETHUE WILLEY.

bahamensis , and the post-branchial region of P. au ran t i aca ,is the diffuse arrangement of the gonads. They are not in theremotest degree arranged one after the other, in a mannerresembling a paired metameric series, as they are more orless in most other Enteropneusta, but they are scattered in themost irregular way in the substance of the genital pleura(cf. figs. 4 and 5). In correspondence with this multiplicationof the gonads, Spengel has shown in the above-named threespecies of Ptychodera, which he had at his disposal to examineby sections, that several gonaducts may be involved in a singletransverse section, each gonad having its own duct, opening tothe exterior on the inner surface of the genital pleura. Thisis also the case in Balanoglossus canadensis, Spengel,in which, however, there is a multiple series of gonads, bothmedial and lateral, of the gill-pores (cf. Spengel, loc. cit.,Taf. 17, fig. 22). Spengel states that he has never foundgonads mediad of the gill-pores, either in Ptychodera orSchizocardium.

It has been quite impossible for me, under the circumstances,to prepare a series of sections, and I have had to make thebest of hand preparations and dissection. But the diffuse andirregular arrangement of the gonads in P. flava can perhapsbe even better realised in in toto preparations than in sections.

Figs. 4 and 5 represent a few of the gonads in male andfemale individuals respectively, as seen under a low power insmall detached portions of the genital pleura. The gonads, asshown in the figures, have in both sexes the most variableoutline. Their appearance naturally varies somewhat with thestate of contraction or extension of the animal or portion ofthe animal. Detached fragments of the genital pleura willcreep from under the cover-glass like a Planarian.

The integument over the testes on the inner face of thegenital pleura in P. flava is characterised by patches of darkbrown pigment, and on this account it is possible to distinguishthe males from the females (fig. 4a).

The female gonads (fig. 5) contain a variable number ofova, which do not take up the whole volume of the gouads, but

ON PTYOHODERA FLAVA. 175

are surrounded by a mass of small refringent globules. As theova in the individual from which the preparation representedby fig. 5 was taken appeared to be sub-mature, it seems notimpossible that these globules are of the nature of mucousgranules. Spengel says that in P. minuta , when the forma-tion of sexual cells commences the fat-like substance begins todisappear, and is finally quite replaced by ova and spermatozoa.The conditions in P. flava appear to differ from this.

The ova (fig. 6), when obtained free by artificial rupture ofthe gonads, are seen to be surrounded at an interval by a hyalinedouble-contoured membrane, the follicular membrane. Theyare opaque, being filled with fine yolk granules. They measure,apart from the membrane, "006 mm. in diameter.

With regard to the shape of the gonads in the branchial andpost-branchial regions of the genital pleura, there is no differencewhatever in P. flava (cf. fig. 4a). But in P. minuta andP. Sarniensis , Spengel states (loc. cit., p. 653) that in thebranchial region the gonads are almost always simple un-branched sacs, while in the post-branchial region their formbecomes more complicated.

In one instance of a male individual of P. flava I observeda much elongated gonad, as long as four or five of those oneither side of it taken together.

Sys temat ic Posit ion of P. flava, Eschschol tz (char,emend, mihi).

As might be expected, the short description given by Esch-scholtz, beyond indicating by the presence of the genital pleurathat his species belonged to the genus Ptychodera in Spengel'ssystem, fell far short of being a satisfactory specific diagnosis.

In consequence of this, Spengel has wrongly placed thisspecies in his genus or sub-genus Tauroglossus. He does not,however, finally assume this, but puts a mark of interrogationagainst it.

Spengel has, as it seems with justice, subdivided the genusPtychodera, suggesting the formation of the family Ptycho-deridse, with the three genera, Ptychodera, Tauroglossus, and

176 ARTHUR WILLEY.

Chlamydothorax. But, to avoid confusion, it is more con-venient at present to regard these as sub-genera of the genusPtychodera.

Ptychodera (sensu str icto) has rudimentary genital pleura;to it belong P. minuta , Kowalevsky, and P. Sarniensis ,Koehler.

Tauroglossus is distinguished by the dorsal origin of thegenital pleura; and to it belong T. aper tus , Spengel, T.claviger, Delle Chiaje, T. gigas, Fr. Muller,T. aura n t iacus ,Girard, and T. aus t ra l iens is , Hill.

Finally, Chlamydothorax is characterised by the ventralorigin of the genital pleura; and to it are assigned Ch.ery thrseus, Spengel, Ch. bahamensis , Spengel, and probablyCh. ceylonica, Spengel, although the last-named species isonly referred to in Spengel's monograph, and not fullydiagnosed.

From the account given above of P. flava, it is at onceevident that its place is under the sub-genus Chlamydo-thorax.

The fact of its close affinity to P. bahamensis instead ofto its neighbour, P. aus t ra l iens is , of New South "Wales, isinteresting in connection with the fact that the Amphioxus(Asymmetron caudatum) which I obtained from the Loui-siade Archipelago, and described in a previous communication,is likewise much more closely related to the Bahama species(A. lucayanum, Andrews) than to the Australian forms.

CONCLUSIONS.

My investigation of P. flava, necessarily somewhat super-ficial, has nevertheless sufficed to establish its systematic posi-tion, but would hardly allow me to engage in an extendedmorphological discussion. Still there are a few points uponwhich one might venture an opinion, especially since it isimpossible to have once seen the free, erect, upstandingpharynx of P. flava without being deeply impressed.

Moreover the account, admirable enough, which Spengel

ON PTYOHODEEA FLAVA. 177

has given of the other two described species of the sub-genusChlamydothorax was based in each case on a single specimenpreserved in spirit, so that an Enteropneust with an eminentlyfree pharynx has never been studied in the living conditionbefore. And this makes a difference.

The conclusions arrived at by Spengel, based as they wereupon such laborious and prolonged researches, are entitled tothe profoundest respect. Still, with the best will in the world,I cannot follow him in his adverse criticism of the theory asto the relationship of the Euteropneusta to the Chordata. Onemight conceivably be able to relinquish the idea of the existenceof a notochord or its representative in the Enteropneusta, butthe gill-clefts are a perpetual fact, and it seems little less thanperverse not to recognise it.

Indeed, in his remarks directed against the assumed Chor-date affinities of Balanoglossus, it would almost appear thatSpengel has carried the analytical method of argumentationto an extreme, and that he is unable to see a general corre-spondence or homology through the veil of differences in detail.The other extreme is to imagine correspondences where noneexist. But it is certainly not necessary to force matters inany way in order to clearly recognise an affinity between theEnteropneusta and the Chordata.

Unfortunately we are here in the presence of one of thosedistressing instances, so common in the realm of morphology,in which two entirely opposed views can be more or less equallysupported. This is due, as Spengel himself points out (loc.cit., p. 722), to the lack of a definite method or criterion inattempting to answer morphological questions.

There is, however, a principle which should be of service inthis connection, namely, the principle of correlation betweenstructural and topographical features on the one hand, andphysiological or functional peculiarities on the other.

Spengel lays great stress upon the dorsal position of thegill-pores in the Enteropneusta and their ventral (sic) positionin Amphioxus, this difference in position being especially in-dicated by the relations of the vascular system, the propelling

VOL. 4 0 , PART^l.—NEW SER. M

178 ARTHUR WILLBT.

vessel being dorsal in the former and ventral in the latter.1

This fact, according to Spengel, is in itself almost enough toprove that the gills of the Enteropneusta and of Amphioxus areessentially different structures, and that they do not correspondwith one another morphologically. It may, indeed, be said tobe Spengel's strongest point in his objection to the supposedChordate affinities of the Enteropneusta. But, by applyingthe above-mentioned principle of correlation to the elucidationof this problem, these and other differences may be viewedfrom quite another aspect.

The question, with what other characteristic in the organi-sation and mode of life of the Enteropneusta the dorsal posi-tion of the gills and gill-pores may be correlated, is not con-sidered by Spengel.

Balanoglossus (employing the name in the wide sense) is acreeping animal,2 and the ventral surface, as in all creepinganimals, is the locomotor surface. Some animals may swimon their backs and others on one side, but all who creep doso on their ventral surface. I t is inconceivable that gills orgill-pores could occur on the locomotor surface.

On the contrary, Amphioxus, when active, is essentially aswimmer, and it can no more creep than Balanoglossus canswim. There is, therefore, no such locomotor surface inAmphioxus, and the dorsal region of the primitive alimentarycanal is converted into a skeletal support for the body, viz.the notochord.

The gill-slits and gill-pores of the Enteropneusta are placeddorsally, therefore, in correlation with the locomotor functionof the ventral surface, the latter not having such a function inAmphioxus; and the general homology between the pharyn-geal apparatus in the two types is not thereby prejudiced.

1 That such a difference in the direction of flow of the blood should not beoverrated in the Protochordata is shown by that very well-known faculty ofthe Tunicate heart of reversing its action and consequently the direction ofpropulsion.

s The kind of burrowing undertaken by Balanoglossus is a variety ofcreeping, but it creeps too, apart from its burrowing propensities.

ON PTTOHODEEA FLAVA. 179

To return t o P t y c h o d e r a flava, the formation of a tem-porary atrial cavity round the branchial sac by the mutualapproximation of the genital pleura is a most striking fact.Spengel calls attention to this, and rightly urges that theperipharyngeal cavity so formed is more readily comparableto the atrium of Amphioxus than anything else that has beensuggested. He then goes pn to add, however, that in hisopinion it is nothing but an entirely superficial resemblance.

Nevertheless it is a real resemblance. The branchial sacbeing dorsally placed in accordance with the principle abovereferred to, the peribranchial cavity must be also dorsal inPtychodera. Presumably there can be no doubt that there isa general homology between the atrium of the Ascidians andthat of Amphioxus; and yet in the former it is a dorsalstructure (except in the free-swimming Appendicularise), andin the latter ventral.

With regard to the synapticula or cross-bars in the branchialskeleton, Spengel (loc. cit., p. 725) draws attention to the factthat they are not present in the genera Glandiceps and Balano-glossus, but are present in Ptychodera and Schizocardium,which, he says, are probably younger phylogenetically. Butit is very much open to doubt whether Ptychodera is phylo-genetically younger than the other genera of the Entero-pneusta.

On page 357 of his beautiful monograph Spengel gives alist of what he regards as signs of a primitive organisation inthe group. These are open to the criticism that they are,without exception, all negative properties,—the lack of this,that, and the other.

Then with regard to Ptychodera he says (p. 358), " Als diehochste Form erweist sich endlich Ptychodera." For my part,I deny this, and oppose the view on the following grounds.

In the first place, the positive fact of the diffuse arrange-ment of the gonads, which is a characteristic of P. flava andof the other species belonging to the sub-genus Chlamydo-thorax, bears all the marks of an archaic type.

Secondly, it seems only reasonable to suppose that the

180 ARTHUR WILLEY.

elements which compose the branchial skeleton, namely, pri-mary or septal rods, secondary or tongue-rods, and synapticulaor cross-rods, were developed at a time and in a type in whichtheir presence was absolutely necessary to prevent the collapseof the branchial sac. It is not so easy to see that theirpresence is directly necessary to those forms in which thesepta between adjacent gill-slits are fused with the thickparenchymatous tissue of the body-wall, and the slits onlyopen to the exterior by minute pores. But they are there not-withstanding, namely, because they are derived from forms inwhich the presence of skeletal supports for the much-perforatedpharyngeal wall was a sine qua non. Such a form is P.flava, with its free and otherwise unsupported pharynx.

If so much is admitted, then the presence of the genitalpleura, covering over the unprotected pharynx, needs nospecial comment.

Thirdly, the fact that in Schizocardium and in GlandicepsHacksi the anterior region of the alimentary canal is notsubdivided into branchial and oesophageal portions militatesstrongly against these genera being regarded as more primitivethan Ptychodera.

Finally, the habitat of Ptychodera in the littoral zone,often between the tide-marks, is another positive indication ofthe primitive character of the genus. The greatest depthrecorded by Spengel for a species of Ptychodera is 20 feetfor P. minuta in the Bay of Rio de Janeiro. Schizocardium(S. brasi l iense, Spengel) descends to 18 to 20 fathoms;Glandiceps (G-. ta labot i , Marion) descends to 450 metres,while another species (G-. abyssicola, Spengel) was obtainedby the "Challenger" from 2500 fathoms; BalanoglossuaKupfferi , von Willemoes-Suhm, was obtained from 12 to 16fathoms.

It is very possible that the forms which have migrated intodeeper water may have retained some primitive features whichare lost to the littoral or tidal forms, just as many of theElasipoda among the Holothuroidea have retained theprimitive connection of the stone-canal with the exterior,

ON PTYOHODERA FLAVA. 181

which has been lost by all other recent Holothurids.1 But itseems clear enough that the occurrence of diffuse gonads, andthe free, open pharynx in the sub-genus Chlamydothorax, andparticularly in P. flava, are facts which point conclusively tothe archaic character of Ptychodera.

SUMMARY OF PRINCIPAL RESULTS.

1. This is the first time that an Enteropneust with a freepharynx has been studied in the living condition.

2. The P t y c h o d e r a f lava of Eschscholtz (char, emend,mihi) is rightly assigned by Spengel to his amended genusPtychodera, as shown by the presence of the genital pleura,of external liver saccules, and by the length of the collarregion.

3. P. flava belongs to Spengel's sub-genus Chlamydo-thorax, as shown by the ventral origin of the genital pleura,the diffuse gonads, and the free pharynx.

4. In the fact of the gill-slits being open directly to theexterior throughout their entire length, P. flava is moreclosely related to P. b a h a m e n s i s than to any other describedspecies. This is also indicated by the simple rows of pairedliver saccules as opposed to the irregular multiple arrangementmet with in P. erythrsea.

5. The genus Ptychodera. (referring more especially to thesub-genus Chlamydothorax) probably represents an archaictype, as shown by the diffuse arrangement of the gonads, thefree pharynx, and its littoral habitat; and it is probably not ,as Spengel supposes it to be, phylogenetically younger thanthe other genera of Enteropneusta.

6. The gill-slits, branchial skeleton, and the temporary atriumformed by the apposition of the genital pleura in Ptychodera,offer a general homology to the corresponding structures inAmphioxus and the Ascidians, while presenting many differ-ences in the details of their structure and relations.

7. Some of these differences are comparatively unimportant,1 Cf. Hjalmar Th6el, "Report on the Holothuroidea," part ii, 'Chall.

Hep. Zool.,' vol. xiv, 1886.

182 ARTHUR WILLEY.

and such as might well be expected to occur in distantly re-lated forms with such totally different habits of existence,while others are to be accounted for by a wide interpretationof the principle of correlation between structure and function.

8. Many differences of detailed structure in the pharyngealwall and its skeletal supports between the Enteropneusta andAmphioxus are to be correlated with the fact that, in theformer, the tongue-bars are larger (often, as in P. flava, verymuch larger) than the primary bars, while in the latter thereverse condition obtains.

Addi t iona l No te .

As the Marshall Islands are distant about two thousandmiles from New Caledonia, and as the species figured byEschscholtz renders possible the interpretation that its genitalpleura had a more dorsal origin than in the species abovedescribed from New Caledonia, it is advisable provisionallyto name the latter P. flava-caledoniensis, or simply P.caledoniensis , until the form from the Marshall Islandscomes to be re-examined.

ISLE OP PINES;

August 2nd, 1896.

ON PTYCHODBEA FLAVA. 183

EXPLANATION OF PLATE 5,

Illustrating Mr. Arthur Willey's paper "On Ptychoderaflava, Eschscholtz."

FIG. 1.—Dorsal view of Ptychodera flava. The simulations of thegenital pleura are indicated anteriorly on the left side; those of the bodyproper behind the hepatic region, pr. Proboscis, c. Collar region, bBranchial region ; the genital pleura are slightly divaricated, and the pharynxis visible, p. b. Post-branchial region, h. Hepatic region, l.s. Dorsolateral streaks marking the course of the ciliated grooves in the intestinalwall. d. n. Line marking the course of the dorsal nerve-cord, a. A.nuc.Drawn from the living object.

FIG. 2.—Anterior portion of another individual from the dorsal side, showing the tuberosity caused by a Copepod parasite. The cross-lines over theinfested region represent continuations of the annulations of the genitaplcuron.

PIG. 3.—Portion of the pharyngeal wall of P . flava, including the lowerportions of three tongue-bars, to show the larger size of the latter as com-pared with the primary bars. The lower free extremities of the tongue-barsare differentiated from the rest of the bar, being marked off in each case by apigmented line of demarcation, t.b. Tongue-bars, p.b. Primary or septalbars. sy. Synapticula. g. s. Gill-clefts, ce.s. CEsophageal ridge ( =Grenzwulst of Spengel), forming the boundary between the branchial andcesophageal portions of the alimentary canal. Drawn from a preparation informol. Zeiss cam. luc, oc. 3, obj. A.

FIG. 4.—Groups of male gonads of P. flava sketched from a detachedpiece of the genital pleura.

FIG. ia.—A single testis from the branchial region with the pigmentpatches over its surface.

FIG. 5.—Groups of female gonads of P . flava with ova. The portions ofthe gonads not occupied by the ova are filled up with coarse mucous (?)granules.

FIG. 6.—An artificially liberated ovum of P . flava surrounded by thedouble-contoured follicular membrane. Zeiss cam. luc, oc. 3, obj. D.

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