MOLLUSCOIDA: TUNICATA. 269 



rally free, and is mostly shaped like the tadpole of a frog, 

 swimming by means of a long caudal appendage. In one case 

 the larval form appears to be destitute of a tail ; and in several 

 instances the larval caudal appendage has been shown to ex- 

 hibit a cylindrical rod-like body, which has been paralleled 

 with the chorda dorsalis of Vertebrates. 



Amongst the Salpians a species of alternation of generations 

 has been observed. A solitary Salpian produces long chains 

 of embryos, which remain organically connected throughout 

 their entire life. Each individual of these associated specimens 

 produces solitary young, which are often very unlike their 

 parents, and these again give rise to the aggregated forms. 



The Timicata are often spoken of as exhibiting three main 

 types of structure, which give origin to as many sections, 

 known respectively as the solitary, the social, and the compound 

 forms. In the " solitary " Tunicaries, the individuals, however 

 produced, remain entirely distinct, or, if not so primitively, 

 they become so. In the " social " Ascidians the organism con- 

 sists of a number of zooids, produced by gemmation and per- 

 manently connected together by a vascular canal, or " stolon," 

 composed of a prolongation of the common tunic, through 

 which the blood circulates. Finally, in the " compound" forms, 

 the zooids become aggregated into a common mass, their tests 

 being fused together, but there being no internal union. 



HOMOLOGIES OF THE TUNICATA. The general resemblance 

 between a solitary Ascidian and a single polypide of a Polyzoon 

 is extremely obvious ; each consisting of a double-walled sac, 

 containing a freely suspended alimentary canal, with a distinct 

 mouth and anus, and a nervous ganglion placed between the 

 two. The chief feature in the Tunicata, as to the exact nature 

 of which there is much difference of opinion, is the branchial 

 or respiratory sac. By Professor Allman this is believed to 

 be truly homologous with the tentacular crown of the Polyzoa, 

 and the oral tentacles of the Tunicaries are believed to be 

 something superadded, and not represented at all in the 

 Polyzoa. By Professor Huxley, on the other hand, the 

 branchial sac is looked upon as an enormously developed 

 pharynx, and the oral tentacles are regarded as a rudimentary 

 representative of the tentacular crown of the Polyzoa. Pro- 

 bably the most correct view of the homologies of the Tunicata 

 is taken by Rolleston, who regards the " branchial sac " as the 

 homologue of the gills of the ordinary Bivalve Molluscs (La- 

 mellibranchiatd], whilst the oral and atrial apertures are looked 

 upon as corresponding to the respiratory apertures of these 

 same animals. 



