VERTEBRATA 11 



divided ; mostly pelagic ; Thaliacea. 



Cloaca dorsal ; caudal appendage caduc- 

 ous; protostigmata divided into rows 

 of secondary stigmata ; mostly fixed ; Ascidise. 



The Tunicata have sac-like bodies, and are marine in 

 habitat. Some of the Tunicata are free-swimming, and 

 these may consist of single or conjoined individuals. Others 

 are attached, some by a peduncle and others sessile, separ- 

 ately or in colonies. No extinct species of Urochorda are 

 known. The Thaliacea and Aseidiae are degenerates from 

 the vertebrate line. The families are : 



Perbnnichoedata : Appendiculariida'. 



Thaliacea : Salpidfe, Pyrosomidse. 



AsciDi/E : Botryllidfe ; Ascidiidas ; Clavelliuidae. 



Superclass HI.— CEPHALOCHORDA. 



The only class of the Cephalochorda is the following : 



Walls of the body muscular myotomes ; no 

 jaws nor extremities ; pharyngeal walls 

 fissured ; heart a longitudinal vessel, 

 which gives off" branchial vessels which 

 unite into an aorta ; a liver and vena 

 cava present ; Acrania. 



Class I.— ACRANIA. 



Of this class but one order is yet known : 



Pharyngeal fissures enclosed externally by a 

 fold of the integument, which encloses a 



