RESPIRATORY SYSTEM. 



405 



the lamprey arises as an evagination of the floor of the pharynx, and for 

 a long time, i.e. , until the metamorphosis into the lamprey, has a glandular 

 structure opening on the floor of the pharynx, remarkably like the endo- 

 style of the Tunicate Oikopleura. 



Respiratory System. 

 The water which enters by the mouth, after being deprived 

 of some of its food particles, passes through the gill slits to 

 the peribranchial chamber. On the walls of the pharynx 



Ph. 



G.D. 



Fig. 129.— Diagram of Ascidian. (After Herdman.) 



The arrows indicate the two openings, the dark border the tunic. 

 Ph., ph.aryn.\, with gill slits; G., reproductive organs; //., heart, 

 with blood vessels ; G.D., genital ducts ; /?., rectum ending in cloacal 

 chamber. Surrounding the pharyn.x the peribranchial cavity is shown. 



