ASCIDIA. 



295 



Fig. 207. — Transverse Section of Larva of Ascidian. 



(After Van Beneden and Julin.) 



Nerve Cord. 



Mesoblastic Sac, 



Notochord. 



Hypoblast. 



hypoblast, and a pair 

 less broken up. 



This larva is, characteristic 

 of the Chordata though only 

 found as a larva in the 

 Atriozoa^ being represented 

 by an embryonic stage in 

 Vertebrata, The further 

 development of the Ascidian 

 diverges from that of the next 

 class. The larva becomes 

 divided into a body and a 

 tail, nearly all the notochord 

 and mesoblast being carried 

 back into the tail (hence 

 Urochorda), whilst the tail 



of lateral mesoblastic masses more or 



Fig. 208. — Chordula Larva 

 OF AN Ascidian. 



(After KowALEVSKi.) 

 Neuropore. 



^Neural Tube. 



Dorsal view. 



Fig. 209. — An Ascidian Tadpole. 



Neural Tube. 



— . Mesenteron. 



Notochord. 

 A median section. 



