424 CEPHALOCHORDA. 



but this is in part explained by the very marked degenera- 

 tion displayed by the adult Ascidians. 



The following important resemblances should be noticed : 

 — In both cases the walls of the pharynx are perforated by 

 numerous slits, which open, not directly to the exterior, but 

 into an atrial or peribranchial chamber, formed from the 

 ectoderm, and with a single external aperture. In both, the 

 pharynx has a distinct ventral glandular endostyle, and a 

 dorsal fold (Tunicates) or groove {Amphioxus), connected 

 anteriorly to the endostyle by means of a ciliated band ; 

 the process of food-taking seems also to be similar. Again, 

 the olfactory pit of Amphioxus is apparently homologous 

 with the sub-neural gland of the Ascidians, and although 

 there is little in common between the nervous system of the 

 adult Ascidia and of Amphioxus, yet the nerve-cord of the 

 larval Ascidian, in its origin, structure, and relations, shows 

 a close resemblance to that of Amphioxus. Similarly, the 

 larval notochord, although never attaining the development 

 which it does in Amphioxus, is an essentially similar structure. 



On the other hand, the Ascidians differ from the lancelets 

 in many ways, e.g. the sessile habit, the presence of the 

 test, of a heart, and of genital ducts ; the absence of seg- 

 mentation, of nephridia, and any trace of ccelom in the 

 adult ; the U-shaped alimentary canal, the power of budding, 

 so common in sedentary animals, and the hermaphroditism. 



The detailed study of development yields similar series 

 of facts ■ — marked resemblances coupled with marked 

 differences ; among the latter, the absence in Ascidians of 

 the segmented coelomic pouches of lancelets is especially 

 noteworthy. In spite of these differences, most morpho- 

 logists are agreed that the resemblances are due to true 

 homology, and that lancelet and Tunicates are descended 

 from a common ancestor, which was at least nearly related 

 to the forms from which the true Vertebrates sprang. It is 

 noteworthy, however, that the inclusion of Amphioxus and 

 Tunicates among the Chordata does not bridge over the 

 gap between Vertebrates and Invertebrates, for the rela- 

 tion of Tunicates to the latter remains obscure. It is 

 possible that Balanoglossus and the related forms may 

 help to bridge the gulf, but as yet there is much un- 

 certainty. 



