TUNICATA 



68 1 



Ascidia. Solitary; without epicardium ; with the viscera at the side 

 of the body, not in an abdomen; with dorsal lamina in place of 

 languets. British. 



Perophora (Fig. 476). Colonial; the zooids are free from one 

 another but connected at their bases by a stolon with a mesodermal 

 septum; and have dorsal lamina and viscera at the side as in Ascidia. 



Botryllus (Fig. 477). Colonial; the zooids imbedded as in Poly- 

 clinum\ but with pallial budding; and with dorsal lamina, and viscera 

 at the side. 



ch 



M. 



Fig. 476. . Fig. 477. 



Fig. 476. Part of a colony of Perophora. After Lister, with modifications. 



al. alimentary canal; at. atrium; est. endostyle; M. mouth; ph. pharynx; 



sep. septum of stolon; stn. stolon; swd. seaweed on which the colony is 



growing; z. zooids; z.' young zooid. 



Fig. 477. Two groups of individuals of Botryllus violaceus. Magnified. After 



Milne Edwards, cl. opening of common cloaca of the group; M. mouth 



opening. 



Class THALIACEA 



Pelagic Tunicata in which the adult has no tail ; a degenerate nervous 

 system; an atrium which opens posteriorly; a stolon of complex 

 structure; and gill clefts which are not divided by external longi- 

 tudinal bars. 



Thus defined, the group includes the Pyrosomatida (Luciae), 

 which are transitional from the Ascidiacea, with which they are 

 usually placed, though by their essential peculiarities they belong 



