THE ORTHONECTIDA. 225

The ORTHONECTIDAJ a NEW CLASS of the PHYLUM of theWORMS.1 By ALFRED GIARD, Professor in the Facultyof Sciences of Lille. With Plate XXII .

IN 1868, in an interesting memoir2 on the marine Plana-rians of St. Malo, Keferstein figures an animal which hedesignates by the name " problematic parasite." The ex-planation of the plate (plate ii, fig. 8) indicates that thisparasite has been found several times, and often in greatquantity, in the digestive tube of the Tremellaria {Lepto-plana tremellaris). The text gives us no information as tothis curious organism, and it is chiefly by means of therather rough figure of the second plate of Kerferstein's workthat we are led to refer it to the group which we are aboutto study.

Macintosh, in his beautiful monograph of the BritishNcmerteans, published in 1874,s recalls the observation ofKeferstein in reference to a parasite which he met with in.Lineus gesserensis (Nemertes communis of P. «T. VanBeneden), and concerning which he gives the followingdetails, accompanied by a few drawings:

" Another curious parasite is found burrowing in the body-wall of Lineus gesserensis, its presence being readily recog-nised by the perforated and honeycombed appearance of thedorsum of the affected animal, whose textures seem to be theseat of the workings of a microscopic Tomicus typographies.When highly magnified, the affected region appears to becovered with a vast network of pale, minutely granularchannels, which contain numerous opaque, ovoid, granularmasses.

" On rupturing the body of the worm a large number ofthe peculiar structures (plate xviii, fig. 17) slide out of thechannels, and swim through the surrounding water, generally,though not always, with the upper end (in the figure) first.Externally they are coated with long cilia, whose activity inthe free state is of somewhat short duration, for after a timethe animals remain quiet, and they drop off. The body is

1 This memoir has been corrected and new matter, including pre-viously unpublished figures, has been added by Professor Giard to theoriginal memoir, which appeared in ' Robin and Ponchet's Journal,' forthe present publication.

9 Keferstein, 'Beitrage zur Anatomie und Entwiokel. einiger See-planar ien,' Gb'ttingen, 1868.

3 'A Monograph of the British Annelids/ part i, Ray Society, p. 129,and pi. xviii, figs. 17, 18, 19.

2 2 6 ALFRED QIAHD.

distinctly segmented, and tapers slightly towards the pos-terior end ; while the surface is marked by very fine longitu-dinal striae, as in Opalina, though in a much more minutedegree. Anteriorly is a conical portion (a), composed ofthree rather indistinctly marked segments. Two evidentannuli (6) succeed, the posterior part of the last being nar-rowed, so as to cause a constriction of the body-wall. Be-hind are six nearly equal divisions (c), each often appearingdouble, that is, has a broad anterior and a narrow posteriorannulus. The posterior region (d) consists of three indis-tinct segments. The body is minutely granular throughout,and an internal cavity is apparent from the fourth segmentto the last, commencing in the former by a rounded end, andterminating just within the border of the latter. No aper-ture is observed at either end. The opaque, ovoid, granularbodies (plate xviii, fig. 18), scattered profusely throughoutthe infected portions of the Lineus, are evidently earlystages in the development of this species, and they, too, areciliated. On subjecting them to gentle pressure (fig. 19),transverse segmentation is apparent, the number of segmentsvarying according to the degree of advancement. The para-sites are very delicate structures, and in the free slate soonbreak up into cells and granules, after discarding their cilia,as above mentioned. Transverse section of the affectedworms shows that they occur both in the skin and in thewalls of the digestive tract, their ravages in the pigmentarylayer of the former tissue causing the curious appearanceswhich led to their detection. It is a somewhat difficultpoint to determine whether the skin, muscles of the body-•\vall, or digestive canal, constitute the common area of thiscreature's depredations, whether it is piercing the former onits way to the surface, or passing to the alimentary cavity tobe voided per anum. The characteristically segmented con-dition of the mature specimens and their internal structureexhibit a higher type of organisation than the ordinaryOpalina. Professor Keferstein found a very similar parasitein the stomach of Leptoplana tremellaris, but he did notdescribe it further than simply mention, under the explana-tion of the plate, that it is an enigmatical structure. Thecentre of the body is occupied by a double row of large cellsin his figure."

Such are the details, certainly rather incomplete, it mustbe conceded, which we have been able to find in earlierwriters touching the curious animals which we havedesignated by the name of ORTKONECTIDA.1

1 We published in the ' Comptus Kendus de 1'Academic des Sciences'

*HE ORTHONECTIDA. 227

II .—SPECIES OBSERVED.

I have had the opportunity of studying the species dis-covered by Macintosh. Lineus gesserensis is very commonat Wimereux, as also its variety Lineus sanguineus, understones, in oozy spots, which are accessible every day evenduring the neap tides.

My attention was drawn to the parasite of this species, arare one, on the whole, by my pupil and friend, J. Barrois,at the time when he was preparing in our laboratory hiswork on the Nemerteans. But I could then, in the absenceof sufficient material, do no more than verify a part of theobservations of Macintosh, and rectify certain errors committedby this eminent zoologist, as I shall point out further on.

In vain I have sought, on several occasions, for the para-site discovered by Refers tern in Leptoplana tremellaris;and yet this Planarian is excessively common at Wimereux,in very nearly the same spots as Lineus gesserensis. PaulHallez, Demonstrator in the Faculty of Sciences at Lille,who has dissected numerous examples of Leptoplana, hashad no better luck than I in this search.

A happy chance made me acquainted, in 1877, with twonew species of Orthonectida, which I have been able sinceto obtain in sufficient abundance to make a nearly completestudy of them, although some gaps still remain to befilled up.

During the autumn of 1877 I had betaken myself toWimeieux in order to study the embryogeny of a species ofOphiura with condensed development. I had a choicebetween two species, Ophioihrix fragilis and Ophiocomaneglecta, both equally common, hermaphrodite, and vivi-parous. Reasons of an entirely technical nature caused meto prefer the second. In opening hundreds of Ophiocoma,to extract from them the embryos, I discovered two speciesof ORTHONECTIDA which have enabled me to undertake thestudy of the group. Both are excessively abundant inthe animal infested by them, but it is quite a rare thingto find an Ophiurid thus infested. According to my notes,each species may be found once among about eighty speci-mens of Ophiurids, so that one has the chance of findingone or other species once in forty specimens dissected.1

(Seance du 29, Octobre, 1870), a short preliminary notice of theseanimals.

1 Besides these two species of Orthonectida, Ophiocoma neglecta presentsat Wimereux a certain number of other parasites, which are interesting.

1. A pretty Vorticella with a very short peduncle [Vortivella ophiocomce,

228 ALFRED GIARD.

In the month of June last I twice found the two speciesof Orthonectida associated in the same specimen of anOphiurid.

It has appeared to me as though the Orthonectida weremore common in the autumn than at any other season. Butthis perhaps depends on the fact that I had been able togive more attention then to my researches than during theacademical session.

III.—ANATOMY OF RHOPALXJRA OPHIOCOMJE.

Nothing is easier than, with aid of fine needles, to separatethe whole dorsal region of Ophiocomma from the ventral discformed by the buccal plates and the arms. The animaldeprived of its dorsal cupola, and consequently of its diges-tive and reproductive organs, lives nevertheless several days,walks, and conducts itself generally as though no injury hadoccurred to it.

In tearing open the dorsal cupola, in order to force out theembryos, the glass slip on which the operation is being con-ducted is sometimes seen to be covered by small whiteflakes, which, on examination with a low power of themicroscope, are seen to be animated by a rapid movement oftranslation in a straight line, which is highly characteristic.

This movement, common to all the species of this group,has led me to give to these organisms the name of Ortho-nectida. These agile little animals resemble large ciliatedinfusoria of a porcellanous aspect and a generally cylindricalform. A certain number, however, have a somewhat shorterform, irregularly ovoid, and move more slowly.

In the one form or the other they are simple Planulce,

nov. sp.), which is found even on the arms of young individuals stillenclosed in the maternal incubatory pouch (very common).

2. An TJrceolaria (Ur. ophiocomte, nov. sp.), also very common.3. A copepod crustacean of the family of the Ergasillidse, or rather

of a distinct family comprising the annelidocolous species. This crus-tacean, which is broad and provided with hooked limbs, attaches itselfto the arm of the Ophiurid between the bristles. It recalls the form ofthe crab-louse. By a very natural association, I name it PhthiriopsisEmilii, and I dedicate this parasite of the star-fishes to M. Emile Blan-chard, who teaches at the Museum of Paris the history of the Arthro-poda. The Phthiriopsis is very rare.

4. A curious little annelid of the genus SphEerodorum, which presentsthe same relations to Ophiocoina which Chlorhema Dujardinii does toPsammechinus miliaris. 1 shall call it Spliarodorum Greefii, dedicating itto the learned Professor of the University of Marburg. About oneOphiurid in fifteen harbours a. Sphcerodorum.

All these parasites of Ophiocoma will be studied in detail in a special

THE ORTHONECTIDA. 229

that is to say, organisms composed fundamentally by twolayers of cells, the one within the other—an ectoderm orexternal layer of cells, which are mostly ciliated, and anendoderm constituted by larger cells,'which are more granular,and form the lining of a central cavity devoid of either mouthor anus.

The first ring terminates in a blunt cone anteriorly, whichcarries a bunch of stiff cilia. It is followed by a cylin-drical ring of the same length, the whole surface of which isbeset with papillae, which are disposed in eight or ten longi-tudinal lines and in four or five transverse series ; this ringis the sole part of the body which does not present vibratilecilia.

The third ring is larger by itself than the two firsttogether; it gradually widens towards its posterior ex-tremity.

The fourth metamere is of the same dimensions as thepapilliferous ring; it is followed by a terminal ring providedwith longer cilia, forming a brush at its posterior extremity.This last ring is conical and subdivided into two metameres,less distinct than those in front.

The last rings form a sort of club, with which the animalstrikes the water, independently of the movement of thecilia, giving sharp strokes, which one is immediately led toattribute to the contraction of muscular elements. Weshall point out below where these elements occur.

Such is the elongated variety. The ovoid variety differsfrom it only by being a third less in length and a greaterbreadth. It seems, at first sight, as though it had beenderived from the former by a contraction along the longaxis ; but it is easy to assure one's self that this is not thecase, and that it is, on the contrary, the elongated formwhich is the ultimate condition of the ovoid form, whichlatter is only the final embryonic phase of the animal.

For a more complete study of the anatomy of Rhopalurait is necessary to employ very high magnifying power, theobjective 6 of Verick, or better, the objectif 9 (immersion) ofHartnack.

The ectodermal cells then appear very distinctly, exceptin the papilliferous zone where it is very difficult to deter-mine their number and form. All the other metameres aremade up by a single transverse row of cells, and the differ-ence in the length of the metameres depends entirely on thedifference in the length of the cellular elements which com-pose it. The terminal rings are formed entirely by fourcells, as in the Dicyemida, the median rings comprise six or

230 ALFRED GIARD.

eight cells; it is very difficult to count with exactitude,since optical sections rarely present themselves, and it isimpossible to make real sections.

The ectodermal cells present very long and very densecilia. By using osmic acid, followed by picrocarmine, it iseasy to preserve the ectoderm with its clothing of cilia.Preparations made a year ago give at this day an excellentidea of the living animal.1

The endoderm is primitively formed of larger cells thanthose of the ectoderm, but they undergo in the adult a verysingular modification. The whole of this layer forms anoval sac, the anterior extremity of which is hidden by thepapilliferous ring, and extends from the penultimate meta-mere until it is inserted, in the form of a sort of pedicle,between the four terminal ectodermal cells (Plate XXII ,figs. 3 and 4). The swollen part of the endodermal sacpresents, when examined with the immersion lens, fine mus-cular bands disposed in a finely granular matrix, and recall-ing the appearance of the endodermic muscular layer ofcertain nematoids.

I do not believe that these muscular elements are derivedfrom the bodily transformation of certain endodermal cells;I consider them rather as a par t only of such cells, which arethus called upon to play a double physiological rdle. Theywould thus be analogous to the epithelio-muscular cellsdescribed by Korotneff in Hydra, but with this difference,that in the present case it is the external part of the cellwhich becomes muscular, the internal part remainingepithelial.

I would draw the attention of histologists, more skilledthan I am in technical methods, to this very delicate point.The question involved is one worthy of their skill.

Metschnikoff has recently put forward the opinion thatthe striated bands are formed by the tails of spermatozoa,the entire endoderm being nothing but a testicular gland.2

I am quite unable at present to accept the opinion of mylearned opponent. These bands are chiefly visible in youngindividuals; their number is constant; they are alwaysdisposed obliquely, as 1 have figured them, on an ovoidendoderm; and this disposition is not, as I had at firstsupposed, the result of an accidental torsion. On changingthe point of view, the continuation of the spiral is seen

1 I have shown these preparations to various persons, and speciallyto Dr. Macleod, of the University of Ghent, who spent a few days atWimereux during last April.

2 ' Zoologischen Anzeiger,' II, No. 43, p. 619.

THE ORTHONECTIDA. 2 3 1

on the other side of the body, and the clear interspacestake the form of lozenges, just as one sees sometimes inlooking through certain kinds of open basket-work. Finally,it will be seen below, that I have also found these elementsin Intoshia, which Metschinkoff considers as the female formof Rhopalura, and where, consequently, there would be nospermatozoa.

Whatever may be the origin of these muscular elements,they unite, as I have pointed out above, into a sort of fascicleat the terminal part of the adult animals, and by theircontraction give rise to those brusque movements, thosestrokes of the club-like tail, to which the name Rhopalurahas reference.

If the interpretation above given is admitted, therewould not be in the Orthonectida any true middle layer, butonly a splanchno-pleural pseudo-mesoderm, comparable tothe somato-pleural pseudo-mesoderm of the Hydra.

I give to the totality of these elements the name opseudo-mesoderm, because it appears to me desirable toreserve the name of mesoderm, properly so called, for otherstructures which do not exist in the Orthonectida, and thehomology of which in the various groups of Metazoa is verydifficult to establish.

I distinguish :—(1) a solid mesoderm, formed very early atthe expense of the endodermic cells of the embryo (rudi-ment of the notochord of the Tunicates and of the Vertebrates,skeletogenous cells of the embryo of Echinoderms, meso-derrnal cells derived from the four first spheres of the endo-derm of Planarians, of Bonellia, according to the researchesof P. Hallez and of Spengel, &c). (2) Acavitary mesodermformed by the diverticula of the endoderm (enteroccels), andappearing generally at a later epoch (aquiferous system ofEchinoderms, enterocoel of the Tunicates, of the Brachio-pods, of Sagitta, of Amphioxus, &c). The solid mesodermgives rise chiefly to the muscular system ; the cavitary meso-derm forms principally the vascular organs.

The physiological role of a histological element has, be itnoted, only a secondary importance for the determination ofphylogenetic homologies. A muscular element, for example,will always be formed at that point where it is needed,sometimes in a rudiment having an endodermal origin,sometimes at the expense of ectodermic elements (Nemer-teans). It may even be formed by a mere portion of a cell(plastidule), as we find in the Infusoria, in the Ccelentera,and in the Orthonectida.

The interior of the endodermic sac is filled, in certainVOXi. XX. —NEW SER. Q

2 3 2 ALFRED GTARD.

individuals, by cellular elements, which give rise to thegenital products. I am at a loss to understand whyMetschnikoff refuses the name of endoderm to a cell-layerwhich arises absolutely in the same way and plays preciselythe same part as the part called by this name by all embryo-logists in the other Metazoa.

I have seen in some Rhopalurse a cloud of agile corpusclesissue from the sides of the body between the third and thefourth metameres. Are these corpuscles spermatozoa? Itwas not possible for me to get a distinct notion of theirform, and I also am unable to affirm the presence of a naturalopening at this spot on the body. It is possible that thespecimens which presented this appearance of an emission ofspermatozoa were really the victims of lacerations.

IV.—ANATOMY OF INTOSHIA GIGAS.

The second species of Orthoneetida parasitic in Ophiocomaneglecta is much larger than JRhophalura. Its length is, infact, two and a half times that of the latter. I name itIntoshia gigas.

Intoshia presents in swimming alternative movements ofcontraction and expansion in a transverse direction. It doesnot possess a papilliferous ring, but simply a ring devoid ofcilia in its place. The body is of a more regular breadthand less tapering at the two extremities, which are bluntpoints, a little more conical than in the Intoshia parasitic inNemerteans and Planarians.

Further, the anterior part of the body is strongly flattenedin Intoshia gigas, and the non-ciliated segment presents onits inferior face, throughout its breadth, a transverse grooveof some depth, so that the profile of the animal is that of ashoe with a heel to it (Plate XXII , fig. 5).

The metamerisation is less distinct than in Rhopalura.After the cephalic ring and a cervical ring corresponding tothe papilliferous ring there follow three metameres of de-creasing breadth (the third (y) is about half of the first (a),the second (/3) being of intermediate dimensions). To thesefollows a ring (§) of much larger size, which seems some-times to be divided into three, then follow two very smallmetameres (e and £), which have about the same length as y,and, finally, the terminal piece.

The variable dimensions of these metameres is no longerrelated, as in Hhopalura, to the size of the compoundcells.

Each metamere is, in fact, formed by several rows of cells,the cells of each row being regularly placed over the corre-

THE ORTHONKCTIDA. 2 3 3

sponding cell of adjacent rows, so as to form a longitudinalseries. It is to this disposition that the longitudinal striseobserved by Macintosh are due, which led this naturalistto approximate the parasites of Lineus to the Opalinse.

The ring a contains three rows of cells, the ring j3 com-prises two, the ring y only one, and the others only one.

The ectodermal cells oflntoshia gigas are, accordingly, muchsmaller and much more numerous than those of Rhopalura.All the cells, without exception, have long cilia. The headcarries, as in Rhopalura, a bunch of straight cilia directedforwards. This character is, in fact, common to all knownspecies of Orthonectida.

The endoderm forms a regularly ovoid sac, constituted inthe adult animal by beautiful polygonal cells, and enclosing,in its interior, other cells of a rounded shape, more or lessabundant.

The action of acetic acid is such as to detach the ectoderm(PI. XXII , fig. 7), and then brings well to view the endo-dermic sac. The jerking movements of the caudal part aremuch less energetic in Intoshia than in Rhopalura. Weshould, therefore, expect to find the muscular system muchreduced; and this is actually the case, and for a long time Iin vain endeavoured to see the muscular bands at all. Isucceeded, however, last summer by producing a gi'adualand very slight compression of the animal by allowing thewater to evaporate from beneath the cover glass. Thebands were rendered visible at the anterior part of thebody, where the endodermic cells hinder observation theleast. The bands have the same oblique direction as inRhopalura.

V.—GEMMIPAROUS REPRODUCTION OF THE ORTHONECTIDA.The oldest individuals of Rhopalura or of Intoshia pre-

sent a considerable modification of the endodermal cell-layer. The cells of this layer are no longer visible, and theendoderm seems formed by a homogeneous membrane ofgranular aspect, very similar to certain tissues of the Nema-toids. On the other hand, we have seen that the ectodermcan be detached in these animals with the greatest facilityunder the influence of reagents. As a result of the pro-liferation of cellular elements in the interior of the endo-dermal sac, this organ swells, becomes spherical, and burst-ing the ectoderm, which disappears, is transformed into atrue « sporo-cyst" (PI. XXII , figs. 14,15,16 and 17). In theinterior of the sporocyst are seen buds, the cellular natureof which is extremely difficult to demonstrate. The same

234 ALFRED GIARD.

difficulty, it may be noted, is found with regard to the budsof the sporocysts of the Trematoda, and all zoologists whohave occupied themselves with the study of these animalsknow how difficult it is to demonstrate the cells which formthe bud-like embryo of a Bucephalus or of any Distoma.

Occasionally the primary buds produce secondary buds.When these buds have arrived at a certain size, they arecomposed clearly enough by a single layer of cells, whichthen proceed later to form an internal layer by delamination.

It appears, from what has been just stated, that theendoderm alone of the parent animal enters into the forma-tion of the bud-like embryos, but it must not be forgottenthat the papilliferous ring, or that which represents it inIntoshia, is characterised by great opacity, and that it ispossible that at certain points ectodermal cells penetrate tothe interior of the endodermal sac.

With their increasing development, the sporocysts losetheir original form and become very voluminous. Oftenthere are found several sporocysts in the interior of a spe-cimen of an Ophiurid. This very active gemmiparousreproduction of the Orthonectida explains how it is thatthese animals are found in such great abundance in theEchinoderms infested by them. The phenomenon is onevery similar to that occurring in the case of the Dicyemida;but with regard to these latter, it is almost impossible tofind a Cephalopod which does not swarm with them, whilstthe Orthonectida are comparatively rare.

VI.—OVIPAROUS REPRODUCTION.

Side by side with the sporocysts just described, we findin the infested Ophiurids a great number of cellular masses,which must be considered as eggs in various stages of seg-mentation.

In the case of Rhopalura, I have only been able toobserve a small number of these embryonic stages (PI.XXII , figs. 19, 20, 21). The little which 1 have been ableto observe leads me to the conclusion that the segmentationis irregular, and that the planula is formed by epibol£, as inthe Dicyemida. Each division of the cells in the mulberrymass gives rise to stellate figures, so well known at thepresent day by the name of caryolitic figures or amphiasters.The use of osnoic acid enables one to observe them with thegreatest facility.

In the study of the development of Intoshia I have beenmore successful. Here the segmented egg forms at first aperfectly regular blastula, the cells of which are at first

THE ORTHONECTIDA. 2 3 5

very much elongated in a radial direction (PI. XXII, figs.8 to 13). This blastula very soon exhibits a very definiteprocess of delamination; the internal part of each cylin-drical cell gives rise to a spherical cellular mass, whichdetaches itself from the mother-cell, which is shortened bythe process (PI. XXII , fig. 10). This is absolutely thesame mode of formation as that which can be observed inthe production of the mesoderm in the Ophiurids with acondensed embryogeny (Ophiothrix fragilis, Ophiocomaneglecta). In the present case it is the endoderm whichforms at the expense of the ectoderm, but we have seenthat this endoderm contains potentially the mesodermicelements (the muscular bands).

Soon the ectodermal cells become covered with vibratilecilia, and elongate in a definite direction; then the embryo,which was spherical, becomes ovoid, the metameres showthemselves little by little, and by insensible gradations wearrive at the adult condition.

VII.—S YSTEMATIC.

According to what has been stated in the preceding pages,the Orthonectida may be defined in the following manner :—" Metazoa retaining throughout their existence the form ofthe planula, having a ciliated ectoderm (stiff cilia forming abunch on the anterior cephalic region, vibratile cilia on theother regions of the body), presenting metameres, which donot correspond to any internal division of the body, witha sacciform endoderm, which gives rise to a muscularsplanchno-pleural pseudo-mesoderm. Reproduction of twokinds : (1) gemmiparous in the interior of sporocysts, con-stituted by the development of the endoderm ; (2) oviparous,dependent on the combination of male and female elements,formed in all probability in different individuals."

The class comprises at present two genera:I. The genus Rhopalura, characterised by the presence of

a papilliferous ring, by its ectoderm composed of large cellslimited in number, and its definitely muscular endoderm.

Species unique. Rhopalura ophiocomm. Length 0108 mm.II. The genus Intoshia, devoid of papilliferous ring ;

ectoderm formed of small, very numerous cells.Three species:1. Intoshia gigas. — Parasitic in Ophiocoma neglecta.

Length 0270 mm.2. Intoshia linei.—Parasitic in Lineus gesserensis. Length

0-157 mm.

2 3 6 < ALFRED GIARD.

3. I?itoshia leptoplance.—Parasitic in Leptoplana tremel-laris. Length 0135, breadth 0 03 mm.

This last species, like the preceding, is regularly cylin-drical, and rounded at the extremities. According to thefigure of Keferstein, it presents ten metameres, perfectlyregular, in addition to the cephalic and caudal rings. Theendoderm appears to be formed by large spherical cells.

The general form of the body, the regularity of themetatnerisation in the two species parasitic in the Nemerteanand the Planarian, lead to the supposition that there maybe other differential characters present in them, which wouldlead zoologists probably to create a particular genus forthese two types. Such a step in our present state of know-ledge would, it appears to me, be superfluous.

Metschnikoff1 has recently described, under the name ofRhopalura Giardii, a new species of Oi thonectida, which hehas since identified with Rhopalura ophiocoma.

This species, which is parasitic in Amphiura squamata,was found abundantly at Spezzia. It presents itself in twodifferent forms, which Metschinkoff thinks are probablyidentical with Rhopalura ophiocomce and Intoshia gigas,which, according to him, are merely the male and female ofone species.

I must confess that this idea has often occurred to meduring my researches, and I do not entirely give it up evenat this moment. The strongest argument in its favour isthat the two forms, Intoshia and Rhopalura, exist in aboutthe same quantity in Ophiocoma neglecta ; and that it wouldbe curious to find in this little Ophiurid two different repre-sentatives of a group so rare as are the Orthonectida. Atthe same time, the difference between the two forms isgreater than I had at first supposed. Further, we have notelsewhere any example of an animal in which some femalesproduce eggs, giving birth to males exclusively, and otherseggs from which only females issue. We might, perhaps,try to remove the difficulty by supposing that, in one or theother case, there was parthenogenesis (arrenotoky or thely-toky); but this would be, at present, pure hypothesis.However this may be, I have no fundamental objection tosuch a mode of explaining the facts, but I shall wait tomake up my mind for the time when I shall have found inanother species of Intoshia (Intoshia linei, for instance) anaccompanying form analogous to Rhopalura.

Without doubt sexual differences, such as exist in Bonelliaviridis, and in other worms, such as Bilharzia hcematobia,

1 ' Zoologischen Anzeiger,' II , No. 40, and No. 4'3.

THE ORTHONECTIDA. 237

are of a nature to make one very cautious on this subject;but, in addition to the special characters of the histologicalconstitution of the ectoderm, I have recently observed newfacts which still further separate Inioshia gigas from fflwpa-lura ophiocomce.

It appears, as the result of an examination of manyhundreds of adult individuals, that Intoshia gigas never hasits non-ciliated segment provided with papillae, nor evenwith refringent corpuscles. The " refringent corpuscles" ofM. Metschnikoff form actual projections on the ring, whichI call the " papilliferous r ing" in Rhopalura. It must,then, be admitted that if such corpuscles exist in the sup-posed females of the parasite of Amphiura squamata, thisparasite belongs to a new species (as is probable enough apriori), and that the sexual dimorphism is less accentuatedin this species than in the other.

VIII.—PHYLOGENY.With reference to the position of the group of the Ortho-

nectida on the genealogical tree of animals, it is very difficultto make a definite statement. But there is no doubt thatthese parasites ought to be attached to the phylum of theVermes, and take their place at the base of this phylum sideby side with the Dicyemida. The phylum of the wormswould then, according to my idea, be represented by the fol-lowing diagram :

CESTODA,

TUBBELLAEIA, TREMATODA,

GASTEROTMCHA, PROTHEI.MIHTHA, DICYEMIDA,

ORTHOHECTIDA,

Gastrseada.

The Orthonectida must occupy in this scheme an inferiorposition to that of the Dicyemida. These latter are evidentlymuch degenerated by parasitism. Their organisation musthave been formerly much higher than it is to-day. Theepiderm contains very clearly {Dicyema of Sepia) the rod-like bodies characteristic of the skin of the Turbellaria, andthe embryo presents a very complex organ, the urna: nothingof this sort is seen in the Orthonectida.1 One of the most

1 See on " Dycyemida," the beautiful memoir of Edouard Van

2 3 8 ALFRED GIARD.

characteristic features of the group of the Vermes thuslimited is the existence, in all the animals of this group, ofa gemmiparous reproduction (sporocysts, echinococci, &c).This peculiarity only disappears in the higher worms, theTurbellarians, which are related to the rest of the group byso numerous and so important morphological features thatno serious zoologist will entertain the idea of separatingthem from the Trematoda and Cestoda, in order to approxi-mate them to the Annelids, as has sometimes been done.

Amongst the animals formeily classified with the preceding,some (Bryozoa, Annelida, and satellite-groups) are intimatelyrelated to the true Molluscs, with which I unite them to con-stitute the group of the Gymnotoca, whilst others form anassemblage which we may call the Nematelmia, includingtherein the JVematoidea, the Echinorhyncha, the Desmoscole-cida, the Gasterotricha, &c. The Tunicata must be placedat the base of the phylum of Vertebrata. Budding from theinterior of sporocysts is found, it is true, in other classes ofthe animal kingdom. Leaving aside the somewhat aberrantcases found among Arthropoda, we see in certain Rotifers(Callidina) an internal gemmiparity very similar to that ofthe Vermes. Further, the Gasterotricha, the parent-stockof the Rotifers and perhaps of all the Gymnotoka, appearto me to be readily connected with the Vermes by means ofthe Orthonectida.

We must not forget that the external resemblances betweenthe Ortnonectida, Gasterotricha, parasitic Rotifera, &c.,are further increased by the similar manner of life obtainingin all these animals. Ophicoma, like Lineus gesserensis andLeptoplana tremellaris, inhabits muddy bottoms. The sameis true of the limicolous Annelids and of Nebalia, on whichwe find as parasites certain degraded Rotifers (Balatro andSaccobdella). But the Orthonectida are in any case veryinferior to the most degraded Rotifers, and represent, withoutany doubt, after the Gastrseada, the first step of the sub-kingdom Metazoa.

I must add here that, according to a letter from Leuckart,the embryo of Distoma resembles the Orthonectida in a mostastonishing manner, a fact which tends to confirm theposition which I have assigned to this group among theVermes.

Beneden, ' Bulletins de l'Acad. de Belgique,' 2e serie, t. xli and xlii,No. 7, 1876 ; also Abstract, with Plate, in • Quart. Journ. Mic. Science,'vol. xvii, 1877, p. 132.

THE ORTHONECTIDA. 239

I X . — G E N E R A L REFLECTIONS.

At first sight it seems as though the discovery of theOrthonectida would bring a very solid support to the Planulatheory of Ray Lankester, or, still more, to the Parenchy-mula theory of Metschnikoff, and I do not doubt that morethan one zoologist will interpret, in that sense, the observa-tions I have recorded.

1 persist, however, for my own part, in considering theinvaginate Gastrula as the prototype of the Metazoa. Ibase this opinion on the following arguments :

1. The Orthonectida are parasitic animals, and we musttake into account the retrogression which this kind of lifemay have brought about in their structure. An organisa-tion which we consider as one of primitive simplicity is verypossibly simple only in consequence of degeneration, if wehave to deal with a parasite, and especially an internalparasite.

2 We have seen that the Planula is formed by epibol£ inRhopalura; in this case we have, then, at any rate, mo-mentarily, a real gastrula, which closes up and does notreopen, because the mode of life of the animal does notrequire the existence of a permanent digestive tube.

3. The forms which present the embryonic phase termedparenchymula by El. Metschnikoff cannot be considered, asthat naturalist would wish, as the lowest among the Spongesand Hydroids.

The forms known as Ualisarca are not low sponges, butsponges which have undergone a degeneration of theirskeletal apparatus. From the point of view of generalmorphology I have shown that we may compare them to theBotrylli, anil must assign to them a very high grade amongthe Fibrosa, analogous to that which the Botrylli occupyamong the Synascidise or the Leucones among the Calci-spongise.

The Siphonophora, we may remember, are very far frombeing Ccelentera of inferior position, and it is by no meansastonishing that they present a condensed embryogeny.The typical embryonic form is found among the Coralligenaand certain Actiniae.

The Echini and the Ophiurids with pelagic embryos havea small nutritive vitellus. They present a gastrula formedby invagination, and it is only when this gastrula is thusformed that the mesoderrn takes origin, at first by partialdelamination of the ectoderm, and later by lateral thicken-ings of the endoderm.

2 4 0 -ALFRED GIARD.

In the Ophiurids with condensed embryogeny, the eggpresents an enormous nutritive yelk, and the mesodertn isformed by abbreviation at the same time as the endodermby a general delamination of the ectoderm, which leads usto MetschnikofPs form dubbed Parenchytnula. This I haveestablished most clearly in cases of Ophiocoma neglecta andOphiothrix fragilis.

It is clearly impossible for me to discuss here the im-portant questions of general embryogeny which are raised bythe study of the Orthonectida. I will merely say that,according to my observations, a large number of calcareousand siliceous sponges present what one may well call abiconvex archigastrula. Quite recently Keller has shownthat Chalinula possesses an amphigastrula. Kowalevskyhas described, in the most exact manner, the existence of anarchigastrula in a variety of Actinia mesembryanthemum,in Cereanthus, and in various Medusae. I am able to confirmhis statement in reference to Actinia equina. Finally, Ed.van Beneden has described and figured an amphigastrulain the Dicyemida, so closely related in many respects to theOrthonectida.

These examples will suffice, I think, to justify my opinionthat the gastrula by invagination is the primitive mode;the gastrula by delamination {Planula or Parenchymula) asecondary mode of embryonic development.

I would also direct attention to the metamerisation whichis so remarkable in the Orthonectida.

We have seen that this metamerisation only affects theectoderm, and I believe that this was primitively the casealso in the Annelids. What proves this to be the case isthe highly differentiated form of the digestive -tube of theCbsetopods in which metamerisation is only well marked inthose organs derived from the ectoderm, such as the bristles,the parapodia, and the segmental organs (nephridia).

It is obvious enough that I am not alluding to that kindof metamerisation which is observed in Salmacina, Syllis,&c, which is merely the result of gemmiparous reproduc-tion. This last kind of metamerisation is only comparableto what one finds in the Cestoidea and the Rhabdocoela.