The Early Vertebrates and the Fishes 



itt.ap. 



ex.ap. 



which leads into a large pharynx, the walls of which are perforated 



by numerous gill slits. This is 



surrounded by the mantle cavity, 



which connects with the outer 



water by means of the second pore. 



The gut is continued into a simple 



stomach and intestine, the latter 



bending back upon itself and opening 



into the bottom of the mantle cavity, 



as shown in the diagram in Fig. 63. 



In the adult animal there is no trace of 



the notochord, and only a remnant of 



the nerve cord; and there are either no 



special sense organs or only traces of 



these. On the other hand, the tunicates 



possess a centralised heart. They 



are hermaphrodite, and, very curiously, 



a number of forms multiply like corals, 



by a simple process of budding. 



Now the remarkable fact has been 

 made out that the young tunicate (see 

 Fig. 65) bears a most striking re- 

 semblance to the immature lancelet. 

 It is a free-swimming, tadpole-like creature, and possesses a 



FIG. 64. A Tunicate. 

 in.ap., Opening leading to mouth ; 

 c.r.ap., opening of peribranchial 



chamber. 



From Dendy's Outlines of Evolutionary 

 Biology (Constable). 



\n 



FIG. 65. Larva of Tunicate. 



7*, Nerve cord ; s, sense organs ; m, mouth ; kd, pharynx ; 

 I'd, alimentary canal ; ch, notochord. 



