HOMOLOGIES. 49 



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 rela- 

 tion 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 con- 

 templation 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 tiie 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 

 Ascidise, 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 sheatli of the Polyzoa. t 



The difference of position between tiie 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. 1&. 



^ The peculiar tunicate organ called " Eiitlostyle" by Huxley, and its accompanying ciliated 

 furrow liave no representative among the Polyzoa. 



