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the calyx-like membrane adherent to the outer side of the tentacula in Fredericella and the Hippocrepian Polyzoa. This correspondence will be rendered more obvious by imagining the branchial sinus to be depressed towards the neural side of the Ascidian, by a rotation round its oral extremity through an angle of 90° in a vertical plane passing through the mouth and anus; its position from longitudinal will thus be changed to transverse, while the transverse bars will become longitudinal, and the branchial sinus and its bars will then have the same direction as the exserted lophophore and tentacula of Plumatella ; while it is interesting to observe, that during the retracted state of the Polyzoon, the lophophore assumes the constant direction of the branchial sinus in the Tunicate.
That the tentacula of the Polyzoa are not homologous with the unciliated rudimental tentacula at the entrance of the respiratory sac in the Ascidiæ is apparent also, not only from the difference of structure, but from the fact, that while the tentacula of the Polyzoa are in immediate relation with the digestive tube, those of the Ascidiæ are mere appendages of the internal tunic. The real homology of the Ascidian tentacula is, in fact, to be found in the tentacular mantel-fringe of a lamellibranchiate mollusc. It is true that, in accordance with this view, we can find no homologue in the Polyzoa for the tentacula of the Ascidiæ; we must therefore conclude, that these organs have absolutely disappeared in the Polyzoa, a circumstance for which we have been already prepared by their absence in Salpa and other Tunicates.
In connection with the tentacular crown, there is another part of the organization of the Polyzoa for which we have still to find an equivalent, and which without comparison with the Tunicata would remain inexplicable, namely the epistome of the phylactolæmatous Polyzoa. Now for the determination of the homological import of this somewhat enigmatical organ, the key is at once afforded by the Tunicata. The epistome is plainly homologous with the tonguelike organs, the “languets” of Milne-Edwards, which are attached along the branchial sinus in Clavelina, and certain other Tunicates, and thence project into the cavity of the branchial sac. In Salpa, the languets are reduced to a single one; that, however, which remains in this genus is not, as we might be led to expect from the comparison we have made between these organs and the epistome of Plumatella, the languet nearest to the mouth, but on the contrary (if we may judge from its position), the one most remote from this part of the animal. It is, however, particularly worthy of attention, that both the existing languet of Salpa, and the epistome of the hippocrepian Polyzoa, are quite similarly related to the great nervous ganglion. This ganglion is certainly homologous in the Tunicata and Polyzoa, and it is manifestly it, and not the mouth, that determines the place of the persistent languet.
We now need only a few unimportant modifications in order to complete the resemblance between the branchial sac of Clavelina and the tentacular crown of Plumatella ; we have only to imagine the oral extremity of the branchial sinus to be prolonged, with its bars, for a short distance towards the hæmal side, so as to surround the mouth, the transverse bars to become free at their extremities, where opposite to the branchial sinus they communicate with the “thoracic sinus,” the longitudinal bars to be suppressed, and the languets to be reduced to one, situated in the immediate vicinity of the mouth-a series of changes involving no essential modification of structure—and we shall then have an organ only wanting in a deep crescentric depression of the distal extremity of the branchial sinus to resemble, even in minute details, the tentacular crown of Plumatella.
Now nearly all the changes which we have thus hypothetically supposed to take place in Clavelina, in order to convert its branchial sac into the tentacular crown of Plumatella, do actually occur in other genera of Tunicata, some in one, and some in another. The predominant importance of the transverse over the longitudinal bars of the branchial apparatus in the Tunicata is sufficiently manifest; in most cases, they are larger and more evident than the longitudinal ; in Pyrosoma, they are not only the better developed, but they alone carry cilia. In this genus, moreover, the hæmal extremities of the transverse bars of one side are separated from the corresponding extremities of those of the other by a considerable space, and thus present a marked approach to the open condition which characterises the tentacular crown of the Polyzoa. According to Lesieur, in a species described by him, they even hang free into the branchial sac for some distance from their extremities, an important fact not easily reconcilable with the view that the branchial sac of an Ascidian is nothing more than a perforated pharynx permeated by a vascular network.
In Doliolum, the longitudinal bars actually disappear.
In Salpa, both longitudinal and transverse bars have disappeared, and the gill consists merely of a tubular rod passing obliquely across the thoracic chamber, and furnished with transverse bands of cilia. It is the homologue of the branchial sinus” in Clavelina ; and the mouth, situated at its posterior and hæmal end, is related to it exactly as the mouth to the “ branchial sinus” in Clavelina. The following two diagrams will render apparent the relations of the parts in Salpa and Doliolum.
The relation of the parts in Doliolum is particularly interesting, and of great importance
Fig. 10. Plan of Salpa.
Fig. 11. Plan of Doliolum. a + 6. External and middle tunic united. cc. Internal tunic. d d d. General sinus system. e. Respiratory
orifice. f. Cloacal orifice. g. Respiratory bars. i. Gill (Salpa) and branchial sinus (Doliolum). m. Languette. n. Mouth.
0. Esophagus. p. Stomach. 9. Intestine. r. Anus. 8. Cloaca. u. Ganglion. 0. Heart.
in the determination of the present question. In this remarkable little Tunicate, the gill consists essentially of a tubular band, extending transversely across the thoracic chamber,
and communicating with a series of secondary tubes, which pass off from it at each side like the leaflets of a pinnate leaf from the common petiole. The main tube, with its lateral pinnæ, thus constitutes an imperfect diaphragm, which divides the great thoracic chamber into an anterior or branchial portion, and a posterior or cloacal portion. Now the main tube may be obviously compared to the great “ branchial sinus” of Clavelina, and the lateral branches to the transverse respiratory bars of that tunicate. The longitudinal bars have totally disappeared, and the posterior extremity of the main tube or “branchial sinus” is continued across the alimentary canal on each side, until it reaches the hæmal side of the thoracic chamber, thus causing the mouth, which perforates this tube near its centre, to be related to it and to its lateral branches, exactly as the mouth in the Polyzoa is related to the lophophore and tentacula in these. Another point of correspondence between the gill of Doliolum and the tentacular crown of the Polyzoa is to be found in the fact, that in Doliolum the "thoracic sinus” is absent, and the remote extremities of the respiratory bars of one side are quite separate from those of the other, and thus present the open condition which characterises the tentacular crown in the Polyzoa. The gill of Doliolum thus constitutes the exact link by which the branchial sac of the Ascidiæ passes immediately into the tentacular crown of the Polyzoa.
However interesting the hippocrepian Polyzoa may be in directly indicating the relations here dwelt on, the infundibulate genera present no difficulty, for the orbicular lophophore, after all, is but an unimportant modification of the crescentic, and is connected to it by a series of intermediate forms. The arms of the lophophore in Plumatella have already become obsolete in Fredericella, in which, however, the lophophore still retains a bilateral figure, which is rendered still more decided by the presence of the epistome. In the marine genus Lagenella, the epistome has disappeared, but the lophophore still retains a slight bilaterality. Finally, in the fresh-water genus Paludicella, and most of the marine genera, not only has the epistome disappeared, but all trace of bilaterality has now vanished from the lophophore.
A comparison of the tunics of the Tunicata with the cænæcium of the Polyzoa will render still more obvious the relations here insisted on, and show how easily the structure of the one can be explained by the study of the other. M. Milne-Edwards has proved by the anatomy of Clavelina that there exist in the Tunicata three distinct envelopes, which, however, may be variously united with one another in the different genera.* Now all these have their homologues in the Polyzoa; the external sac or test of the Tunicata corresponds to the external investment or ectocyst of the Polyzoa; the middle sac, or “mantle,” of the Tunicata to the internal investment or endocyst of the Polyzoa; and the internal or third tunic of the Tunicata, which surrounds the branchial sac, and forms the
thoracic chamber” of Milne-Edwards (and which is divided into two portions, one hæmal containing the proper branchial sac, and the other neural, constituting the cloacal chamber), will be equivalent to the tentacular sheath of the Polyzoa. The homology of the two 'outer tunics of the Tunicata with the ectocyst and endocyst of the Polyzoa is obvious, and need not here be further dwelt on. The homology of the third or innermost tunic of the Tunicata may perhaps, at first sight, not appear quite so manifest; it is, however, equally
* See Huxley, “ Observations upon the Anatomy and Physiology of Salpa and Pyrosoma, together with remarks on Doliolum and Appendicularia.” “Phil. Trans.,' 1851.
decided, and is a relation of great importance in the present question. If we examine this tunic in Clavelina (see figs. 6, 7, page 44) we shall find that it is continuous with the mantle at the respiratory and cloacal orifices, and becomes attached to the alimentary canal just behind the mouth and anus; il thus holds to the surrounding parts in the Tunicata exactly the same relation that the tentacular sheath or inverted tunic in the Polyzoa (see figs. 8, 9, page 45) does to the corresponding parts of these during the retracted state of the animal ; it scarcely differs, in fact, from the tentacular sheath of the Polyzoa in anything except its being united to the branchial sac along the hæmal side, in the region of the thoracic sinus, and in its therefore not admitting of eversion. In the Polyzoa there is, properly speaking, but one external orifice, namely, that through which the tentacular crown is projected and retracted; but this is equivalent to the respiratory and cloacal orifices of the Tunicata united; and the point where the rectum opens externally in the Polyzoa is not therefore, as supposed by Van Beneden and others, the homologue of the cloacal orifice in the Tunicata with the cloacal chamber itself become extinct-a view which evidently originated in the too exclusive contemplation of the Polyzoon in its exserted state--but rather corresponds to the point where the rectum penetrates the internal tunic in the Tunicata. The cloaca of the Tunicata is nothing more than the dorsal portion of the thoracic chamber in these animals, and is plainly represented in the retracted Polyzoon by the dorsal portion of the cavity of the tentacular sheath, the whole of the cavity of this sheath becoming obliterated in the exserted state of the polypide. The thoracic chamber of the Tunicata is obviously homologous with the mantle cavity of ordinary molluscs, as has been already maintained by Huxley,* and the cavity of the tentacular sheath in the Polyzoa has precisely the same signification.
The view of the homologies here taken is still further borne out by a comparison of the muscles in the two groups. Those muscles on which devolves the office of the retraction of the polypide in the Polyzoa are of course absent in the Tunicata. In the middle tunic of the Ascidiæ, however, there is, as is well known, a large development of muscular tissue in the form of circular and longitudinal fibres, which give to this tunic its characteristic contractility. Now, these muscles are exactly represented by equivalent fibres, which are developed in the homologous tunic or endocyst of the Polyzoa, and constitute the “parietal muscles” of these animals. The circular bands of Salpa and Doliolum, on the other hand, appear to be developed in the internal tunic, and have their representatives in the sphincters occurring in the inverted tunic, or tentacular sheath of the Polyzoa.t
The difference of position between the nervous ganglion in the Polyzoa and the Tunicata may seem at first to invalidate the homological views here taken, and we can easily imagine its being said, that if the branchial sac of an ascidian be homologous with the tentacular crown of a Polyzoon, in the sense here maintained, the ganglion in the Polyzoa ought to be situated, not between the esophagus and rectum, but upon the neural edge of the lophophore. If however, we carefully consider the difference of position between the two ganglia, we shall find that it is, after all, unimportant; in the Tunicata, while the ganglion is always placed between the two external orifices, it is at the
* British Association Reports,' 1852. Transactions of the Sections, p. 76.
+ The peculiar tunicate organ called “Endostyle” by Huxley, and its accompanying ciliated furrow have no representative among the Polyzoa.
same time situated in the interval between the internal and middle tunic; it is, consequently, within the great “ sinus system "* of the Tunicata, which corresponds exactly with the perigastric space of the Polyzoa. In the Polyzoa the two orifices coalescing, the ganglion can no longer occupy the position it held in the Tunicata ; it is therefore carried backwards, and, still bathed in the fluid of the sinus, now becomes situated on the æsophagus, a difference of position which, it will easily be seen, involves no important change of relations, and which is necessarily connected with the difference in the arrangement of the other organs in the respective groups. In the Polyzoa, from their constant motions of retraction and exsertion, the ganglion could not occupy the fixed position which it does in the Tunicata, and therefore comes to be situated on the polypide itself, all whose motions it then necessarily follows.
To render more complete our comparison between the Tunicata and the Polyzoa, one interesting point of difference must be here noticed, namely, that while in the Tunicata the first bend of the intestine is always, as originally insisted on by Huxley, turned towards the hæmal side,+ or that opposite to the ganglion, its whole course in the Polyzoa is as invariably towards the neural or ganglionic side.
To the uniformity of plan now attempted to be demonstrated among the various members of the molluscoid series, the curious tunicate genus Appendicularia affords the most important exception. The singular little animals constituting this genus, which I agree with Huxley* and Gegenbauer in viewing as an independent form rather than as the larval state of an ascidian, have the thoracic chamber formed on the ascidian type, but in consequence of the non-development of a branchial sac, or of any form of gill,|| this chamber is not divided into a branchial and cloacal portion, and has only a single external orifice. The mouth opens into it below just as in the Ascidians, but the intestine, instead of opening into any part of the thoracic chamber, runs directly to the outer walls of the sac, and terminates by perforating these walls on the ventral side. Appendicularia, then, cannot be viewed as forming a connecting link between the Tunicata and the Polyzoa; it is altogether anomalous, and the most important points in which it differs from the normal Tunicata are those also which separate it at the greatest distance from the Polyzoa.
A different view of the nature of the parts now under consideration has been taken by Mr. Huxley. Advocating the homology of the branchial sac of the Ascidian, not with the tentacular crown, but with the pharynx of the Polyzoon, he has brought to bear upon this
* This name has been given by Huxley to the whole of the space included between the internal and middle tunics in the Tunicata. It is that through which the blood unenclosed in proper vessels vaguely circulates. Loc. cit.
† With this generalisation, however, some recent descriptions (see Gegenbauer on Doliolum, in Siebold and Kölliker's · Zeitschrift,' vol. vii, 1856) are scarcely consistent. I Phil. Trans.,' 1851.
& Siebold and Kolliker's ' Zeitschrift,' Band vi, 1855. || Gegenbauer has shown that the thoracic chamber in Appendicularia is pierced just above the mouth by two ciliated apertures, from each of which he traced a tube leading into the interior of the body, but was unable to follow it to its destination. It is quite certain that these apertures are in no way homologous with the branchial stigmata of the Ascidians ; it is not improbable that they form the entrance to an aquiferous system in this anomalous Tunicate.