CHORD AT A 



447 



do not correspond with so many gill-slits, but are rather sub- 

 divisions of a single original pair. 



The Urochordata are divided into three orders : (i.) Laev- 

 ACEA, (ii.) Thaliacba, and (iii.) Ascidiacea. 



Order 1. Laevacea. 



The Larvacea include but four genera, the best-known of 

 which is the genus A;ppe7idicularia. The members of the 

 group are of extreme interest, since they have undergone no 

 degeneration, but retain those features of the active larval 

 condition which are lost in the other members of the group. 



Appendicularia, like the other Larvacea, is a minute free- 



FiQ. 261.- 



-Semi-diagrammatio view of Appendicularia frpm the right. 

 After Herdman. 



swimming organism, found near the surface of the water in all 

 parts of the ocean. Its body is divided into a short trunk and 



