« AnteriorContinuar »
quadrangular meshes thus formed, is perforated by an aperture, (the stigmates branchiauw of Milne-Edwards),” surrounded by a fringe of vibratile cilia. It must, moreover, be borne in mind, that the transverse bars open into two great longitudinal canals, the “branchial” and “thoracic sinuses” of Milne-Edwards, one placed upon the neural, and the other upon the haemal side of the sac, that they constitute, in fact, a series of communicating channels, passing transversely between these two sinuses, receiving the blood from one and pouring it into the other. In advocating the homology of the respiratory sac of the Ascidian with the tentacular crown of the Polyzoon, some zoologists, among whom must be especially mentioned M. Van Benedent, maintain that the longitudinal bars of the sac correspond to the tentacula of the Polyzoon, while the transverse bars in the sac of the Ascidian must be considered as becoming extinct in the tentacular crown of the Polyzoon. Now this view is certainly untenable. I have in another place; endeavoured to show that while the respiratory sac of the Ascidian has undoubtedly its homologue in the tentacular crown of the Polyzoon, it is the transverse and not the longitudinal bars of this sac that are represented by the tentacula of the Polyzoon, Fig. 6. Fig. 7.
a. External tunic. b. Middle tunic. c. Internal tunic, d d d. General sinus system. e. Respiratory orifice. f. Cloacal orifice. g. Transverse respiratory bars. h. Longitudinal respiratory bars. i. Branchial sinus. k. Thoracic sinus. l l. Proper membrane of respiratory sac. m. Languettes n. Mouth. o. CEsophagus. p. Stomach. q. Intestine. r. Anus. s. Cloaca. t. Tentacula at entrance of respiratory chamber. u. Ganglion. v. Heart.
* See Milne-Edwards's beautiful memoir, “Sur les Ascidies Composées.” + Van Beneden sur les Ascidies simples. ‘Mém. de l'Acad. Roy. de Belgique,” tome xx, 1847. f ‘Trans. Roy. Irish Acad, Jan. 1852.
and this is a very important distinction, which from not having been recognised, rendered previous attempts at comparison between the respiratory sac of the Ascidian and the tentacular crown of the Polyzoon indefensible. Without a knowledge of the Hippocrepian Polyzoa, it would, perhaps, have been impossible to arrive at anything like a satisfactory conclusion on this point; the peculiarities of these Polyzoa, however, afford a key to the clearing up of this difficult subject, and we shall best perceive the relations in question by comparing an Ascidian Tunicate with a Hippocrepian Polyzoon, a Clavelina, for example, with a Plumatella, a comparison which the accompanying diagrams (figs. 6–9) will render easy.” Now it does not need much assistance from the imagination to see in the great branchial sinus of Clavelina, a representative of the lophophore of Plumatella, while the transverse bars which pass off at either side from this sinus, and are richly ciliated, will correspond to the ciliated tentacula of the Polyzoon; the delicate membrane which constitutes the proper walls of the respiratory sac, to the interior of which the respiratory bars of the Ascidian are adherent, and which is pierced in the intervals of these bars by the “branchial stigmata,” will have its homologue in 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 Ascidiae 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 Ascidiae 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 Ascidiae; 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 phylactolaematous 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 haemal 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.
Fig. 8. Fig. 9.
goff's ---d z a" ------------------------- o ---ont. wn ---!, 6--- --A-M+/----d -- c. to.-4------- d Fig. 8. Plan of a Hypocrepian Polyzoa— Fig. 9. Plan of a Hypocrepian Polyzoa— longitudinal section. transverse section.
a. Ectocyst. b. Endocyst. c. Tantacula sheath. ddd, Perigastric space. f + e. Orifice of cell. g. Tentacula. i. Lophophore. l. Calyx. m. Epistome. n. Mouth. o. CEsophagus. p. Stomach. q. Intestine. r. Anus. s. Cavity of tantacular sheath. u. Ganglion. w. Retractor muscle.
* In figs. 6—11, the same letters are used throughout to indicate homologous parts.
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 haemal 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 haemal 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. Fig. 11.
a + b. External and middle tunic united. c. c. Internal tunic. ddd. General sinus system. e. Respiratory orifice. J. Cloacal orifice. g. Respiratory bars. i. Gill (Salpa) and branchial sinus (Doliolum). m. Languette. n. Mouth. o. CEsophagus. p. Stomach. q. Intestine. r. Anus. s. Cloaca. u. Ganglion. v. 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 pinnae, 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 haemal 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 Ascidiae 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 Lassenella, 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 coenoecium 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 existin 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 haemal 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