INTRODUCTION. 



The tunicates include a large number of marine animals 

 which show their chordate features most plainly in the young, 



FlG. i. Diagram of a larval tunicate, after Seeliger. A, atrial opening ; T, 

 notochord ; E, endostyle; G, gill slits ; //, heart ; J/, mouth ; A r , nervous system ; 

 S, adhesive disks ; SV, sensory vesicle. 



the adults being remarkably degenerate. These young have 

 tadpole-shaped bodies, with a central nervous system dorsal in 



position, a notochord which 

 occurs only in the caudal 

 region, while the gill slits 

 occur on the side of the 

 pharyngeal region. In the 

 course of development in all 

 except the Copelatae {Ap- 

 pendicularia, etc.), the tail 

 becomes absorbed, the noto- 

 chord being lost, while the 

 body becomes so twisted 

 that both gill slits and vent 

 empty into a common atrial 

 chamber. The body is usu- 

 ally fixed, and is covered by 

 an outer coat or tunic. 



The Cephalochordia are 

 represented by AnipJuoxns, 

 and one or two other allied 

 genera which are decidedly 

 fish-like in their general ap- 

 pearance. The body is distinctly segmented ; the gill slits are 

 numerous, extending back along the alimentary canal to 



Fin. 2. Diagrammatic section of adult 

 tunicate. <i, atrial opening ; /', branchial 

 chamber; //.heart; /', intestine; ///.mouth; 

 , nerve centre ; r, reproductive organ 

 and duct ; /, tunic ; v, vent. 



