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 ; H, heart ; M, mouth ; N, 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. 



MM0 



The Cephalochordia are 

 represented by Amphioxus> 

 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 

 very numerous, extending back along the alimentary canal to 



FIG. 2. Diagrammatic section of adult 

 tunicate, a, atrial opening ; />, branchial 

 chamber ; h, heart ; i, intestine; m, mouth; 

 n, nerve centre ; r, reproductive organ 

 and duct ; t, tunic ; v t vent. 



