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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; it 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 haemal 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 Ascidiae, 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 oesophagus 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 oesophagus, 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 haemal side,t 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.

f : 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.

view a number of important facts, and has defended it with more ingenuity than it has received from any other anatomist.* Imagining an ideal molluscan archetype, Mr. Huxley derives from this general plan all the special modifications presented by the molluscan sub-kingdom. From this archetype he thus derives the polyzoal form. “Conceive the abdomen of the archetype to be greatly prolonged, the neural region with its appendages, the organs of sense, and the heart, remaining undeveloped, so that the anus comes into close apposition with the oral extremity, while the edges of the latter are produced into long ciliated tentacles; and the result will be a Polyzoon, which needs only the power of gemmation to give rise to those composite aggregations which are so characteristic of the group.”f In these words we have an excellent and easily comprehensible enunciation of the essential morphological characters of a Polyzoon. In deriving the ascidian form from the archetype, “it is to be remarked,” he says, “in the first place, that the pharynx, large in the Polyzoa, becomes comparatively enormous in the Ascidians; while the tentacles, which were very large in the Polyzoa, are in the Ascidians comparatively small. Next with the development of a post-abdomen, the intestine acquires a haemal flexure; but instead of the anal aperture remaining on the haemal side, it is bent round by the same process as in Spirialis and Limacina, but in the inverse direction. Suppose, with all this, that a mantle has been developed, and that its free margin, remaining small and narrow, has followed the anus to the neural side, while its cavity has extended up on each side of the pharynx to the middle line of the haemal surface of the latter, carrying to a great extent a process of which the outline may be seen in Cymbulia, and giving rise to the atrium; —imagine also that the sac thus constituted externally by the inner surface of the mantle (third tunic) and internally by the pharynx, becomes perforated by minute apertures—and the result would be an Ascidian.”f Now, if the above be an exact statement of the true morphology of the Ascidian, it is inconsistent with the views attempted to be demonstrated in the present paper. After a very careful consideration, however, I have not been able to adopt it. Its essential feature consists in the peculiar explanation it offers of the formation of the “atrium,” the term given by Mr. Huxley to the space, including the cloaca, which intervenes between the branchial sac and the third tunic of the Ascidiae; and it necessarily involves the supposition that the walls of this cavity wrap themselves round the pharynx in the manner of a serous sac, just as the serous lining of the pericardium in the higher animals wraps itself round the heart; and that the pharynx is therefore, properly speaking, eaternal to the cavity of the atrium. I have never succeeded in satisfying myself of the reality of this condition, and must still believe that the walls of the atrium simply surround the branchial sac, without being reflected on its sides; that the branchial sac is, therefore, properly within the cavity of the atrium, just as the mouth and labial palps are within the mantle-cavity of the lamellibranchiate Mollusca,

and the tentacular crown of the Polyzoa within the tentacular sheath during the retracted state of the polypide.

* * British Association Reports’ for 1852, and ‘English Cyclopaedia,” 1855, article “Mollusca.”
+ Article “Mollusca” in “English Cyclopaedia, 1855, column 858.
Id, column 864.

From what has now been said, it will perhaps be admitted that the only homological view deducible from the comparison which we have endeavoured to institute between the two great sections of the Molluscoida, is that insisted on in the present memoir. But no view of the homological relations between two sets of organs can be accepted, unless it can also be shown that the conclusions derived from anatomy are confirmed by the history of development, or at least unless it appears that this history presents nothing irreconcileable with the conclusions which rest on anatomical evidence alone.

Unfortunately, the difficulties which beset this part of the subject render us still deficient in the class of facts here needed. We owe to Krohn” some of the most recent investigations into the development of the Ascidiae, and, so far as these go, I believe them to be in accordance with the views advanced in the present memoir. In the account given by Krohn, some observations are still wanting before we can determine the exact nature and sequence of all the steps in the development of the Ascidian embryo. From this account, however, it appears that a cavity, which represents that of the future respiratory sac, shows itself at a very early period in the interior of the embryo, having immediately behind it the first indication of the alimentary canal as a tube of uniform diameter bent into a loop. The walls of this cavity are at first in close apposition with those of the thoracic chamber, which completely encloses it, so that the two cavities at this stage appear as one, being exactly co-extensive; we soon perceive, however, that the proper branchial sac becomes perforated by four apertures, the first appearance of the branchial stigmata, and at the same time becomes detached, in the region of these apertures, from the walls of the thoracic chamber, which had previously itself been perforated by the respiratory and cloacal orifices (the latter at first double, but afterwards single by coalescence). The branchial stigmata subsequently increase in number, the branchial sac at the same time detaching itself more and more from the walls of the thoracic chamber, while the space thus produced, and bounded internally by the walls of the branchial sac and externally by those of the thoracic chamber, becomes the cloacal chamber of the adult. In the intervals of the branchial stigmata, the transverse and longitudinal respiratory bars make their appearance, but the mode in which they originate is not evident; nor have we any account of the mode of formation of the branchial or thoracic sinus, the first indication of their existence being the current of blood which is seen to flow through them after the heart has begun to act. The alimentary canal, as has just been said, had already shown itself. It has now become more developed, and presents a division into the three regions of oesophagus, stomach, and intestine; while the mouth opens into the bottom of the branchial sac, and the opposite extremity is found in the cloacal chamber, or space formed by the separation from one another of the walls of the branchial sac and thoracic chamber.

We have here, then, a series of facts which admit of an easy comparison, so far as they go, with the corresponding steps in the development of the Polyzoa. It will be recollected that, in the development of the bud of a Polyzoon, a minute anterior cavity, closed on all sides, is the first evidence of differentiation. From the walls of this cavity the lophophore subsequently detaches itself, and then gives rise to the tentacles which gradually develop themselves from its margins. The cavity is here to become that of the tentacular sheath,

* Krohn in Müller's ‘Archiv,” 1852-3; translated in the new series of Taylor's ‘Scientific Memoirs,” August, 1853.

which we have assumed as homologous with the thoracic chamber of the Ascidian ; and the lophophore, which represents the thoracic sinus of the branchial sac of the Ascidian, becomes separated from it just as the branchial sac separates itself from the walls of the thoracic chamber of the Ascidian. The development of Salpa has been especially studied by Leuckart.” From the greater simplicity of the respiratory system in the Salpas, it is easier to compare the Salpae and Polyzoa with regard to the development of this system, than it is to institute a similar comparison between the Ascidiae and the Polyzoa. Now, the researches of Leuckart fully bear out the views here taken. As in the Ascidians so also here does the first trace of special organs show itself in the formation of an internal closed cavity, the primary sketch of the branchial chamber. From the walls of this cavity the rod-like gill is then separated, leaving behind it a space which is to become the cloaca. The alimentary canal, about the same time, makes its appearance; and it is not until a comparatively late stage of the development that the branchial chamber communicates with the exterior by the branchial and cloacal orifices. It is plain that, in its essential points, the resemblance here with what we have seen to occur in the Polyzoa is complete. The closed cavity, constituting the early stage of the branchial chamber in the Salpa, resembles even in minute particulars the early stage of the cavity of the tentacular sheath in the Polyzoa, as will be at once evident from an inspection of Leuckart's figures, while the rod-like gill is developed in the interior of the branchial chamber, in all essential points just as the lophophore of the Polyzoa is developed within the cavity which is ultimately to become that of the tentacular sheath in these animals. In the Polyzoa the development is continued by the shooting out from the lophophore of hollow processes which are to become the tentacula. In the Salpa these processes are never developed, and the gill continues in the condition of a polyzoal lophophore, with the tentacula suppressed. It appears to me, then, that the points of connection attempted to be established in the present memoir between the Polyzoa and the Tunicata are those which really exist in nature; but there still remain to be considered some other relations of great interest presented by the Polyzoa. We know that the affinities of organized beings are not confined to those which directly unite them to some single allied form. On the contrary, they stream forth from them in various directions, and bring into a community of plan many different types. Besides the close affinity between the Polyzoa and the Tunicata, another series of homologies can be pointed out, bringing the Polyzoa into intimate relation with the Brachiopoda. Mr. Hancockt had maintained the resemblance between the arms of the lophophore in the hippocrepian Polyzoa and the oral arms of the Brachiopoda, a resemblance singularly striking, and pointing to an important homological relation. In confirmation of this view, he has further called attention to the resemblance between the parieto-vaginal muscles of the Polyzoa and the pedicle muscles of Terebratula, and has compared their action, in one case in closing the orifice of the cell, in the other in closing the valves of the shell. The resemblance between the two sets of muscles is certainly obvious, and may possibly

* ‘Zoologische Untersuchungen, Zur Anatomie und Entwickelung der Tunicata,’ 1854. + On the Anatomy of the fresh-water Bryozoa, by Albany Hancock, Esq. ‘Annals and Mag. of Nat. Hist.,’ 1850.

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