ASCIDIA. 



293 



Ascidia is hermaphrcdite. The testes and ovaries are 

 simple paired sacs lying over the stomach 

 and leading by separafe.ducts into the atrium. 



1 he main interest attaching to Ascidia is involved in its 

 development. If the anatomical account has been carefully 



Fig. 205. — Development of an Ascidian. 



(After KowALEVSKi.) 

 Blastula. Gastrula. 



Blastoccele. — s-i.'- *> , 





Hypoblast. 

 Blastopore. ^ ' 



"J_^| Hypoblast. 

 - Notochord. 



Neuropore. ^^a,^'^^'^^3>y. 



Neural Tube, 



Archenteron. 



Notochord. 



The various stages are shown in median section. 



Development. 



followed, it will have been noticed that Ascidia differs from 

 nearly all the preceding types in having the 

 nervous syste?n confined to the dorsal region of 

 the body, in- possessing numerous paired slits in the wall of the 

 pharynx^ and in \}i\Q. presence of a ventral hearty which is (in- 

 termittently) respiratory^ involving a backward current in the 

 dorsal vessel and a forward i?i the ventral. These are all 

 characters in which the Tufiicata resemble the other members 

 of the important phylum Chordata. In addition, in the 

 structure of the pharynx, the method of feeding and the 



