434 ZOOLOGY 
cells. The relations of the notochord, a solid diverticulum 
of the pharynx, correspond with those of Balanoglossus and 
Cephalodiscus. 
Cuiass Il. Urochordata (Tunicates or Ascidians). 
CHARACTERISTICS.— Hermaphrodite Chordata, with free-swimming 
larvae, which usually become sessile and degenerate adults. 
These 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 notochord 
being confined to the larval stages. A metamorphosis usually 
OCCUPS, 
Fig. 251.—<Ascidia mentula, from the right side. 
After Herdman. 
br. Branchial aperture. 
at. Atrial aperture. 
Pomnests 
The simple Ascidians—Ascidiae simplices—have a more 
direct life-history, and to some extent a less modified structure 
