238 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



themselves to a rock or some other firm substance, and all of 

 their organs become very much reduced and simplified. The 

 adult ascidian is a degenerate, sac-like organism looking as 

 much like a plant as an animal, and showing in no way the 

 relation to the vertebrates that is suggested by the larva. 



As these two classes are so unlike each other and so different 

 from the vertebrates they are often considered as two distinct 

 subbranches of the Chordata and all the other classes are 

 included in a third subbranch, the Vertebrata. In almost all 



FIG. in. An ascidian or sea-squirt from the coast of California. 



(After Jordan and Kellogg.) 



of the Vertebrata, the notochord, which is present in the early 

 stages of development, is replaced, in the later stages, by 'a 

 cartilaginous or bony backbone or spinal column. 



The class Leptocardii (Gr. Icptos, small ; kardia, heart) includes 

 the primitive lancelet, in which the notochord is persistent and 

 unsegmented. Lancelets occur in the sand in shallow water 



