MOLLUSCOIDA I TUNICATA. ' 243 



With the exception of Doliolutn and Appendicularia, all the 

 Tunicata are hermaphrodite. The reproductive organs are 

 situated in the fold of the intestine, and their efferent duct 

 opens into the atrium. The embryo Tunicate is at first free, 

 and is shaped like the tadpole of a frog, swimming by means 

 of a long caudal appendage. 



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 Tunicata 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 some- 

 thing 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. 



DIVISIONS OF THE TUNICATA. By Professor Huxley the fol- 

 lowing arrangement of the Tunicaries is adopted : 



CLASS TUNICATA. 



Order I. Ascidia branchialia. 



Branchial sac occupying the whole, or nearly the whole, length of 

 the body ; intestine lying on one side of it. (Ascidiadce, Botryllus, $c.} 

 E 2 



