434 



ZOOLOGY 



cells. The relations of the notochord, a solid diverticulum 

 of the pharynx, correspond with those of Balanoglossus and 

 Gephalodiscus. 



Class II. Urochordata (Tunicates or Ascidians). 



ChakActeristics. — HermaphrodUe Chordata, with free-swimming 

 larvae, which usiMilly lecome sessile and degenerate adults. 

 Tliese are either solitary or form colonies. In the adult the 

 nervous system is, with few exceptions, reduced to a single 

 ganglion, the elongated nervous system and the twtochord 

 being confined to the larval stages. A metamorphosis usually 

 occurs. 



Or 



Fig. 251. — Ascidiamentula, from the right side. 

 After Herdman. 



hr. Branchial aperture. 

 at. Atrial aperture. 

 t. Test. 



~t 



The simple Ascidians — Ascidiae simplices — have a more 

 direct life-history, and to some extent a less modified structure 



