CHAPTER XVII 



THE LOWEST VERTEBRATES AND THEIR 

 NEAREST ALLIES 



The various groups of animals thus far studied are col- 

 lectively known as the Invertebrates on account of the ab- 

 sence of a vertebral column or back bone. We now pass 

 to the Vertebrates in which a vertebral column is one of the 

 most characteristic features of structure. In any verte- 

 brate such as a fish, frog, bird, or horse we find that there 

 are several fundamental characters which are very differ- 

 ent from those prevailing among the in- 

 vertebrates; the central nervous system is 

 dorsal in position, the heart lies below 

 (ventral to) the alimentary canal instead 

 of above it; and the skeleton is an inter- 

 nal one, although in some vertebrates as 

 in turtles, an outer skeleton may be FIG. 124. A 



i tunicate. 



present also. 



While most animals may be classed without hesitation 

 as vertebrate or invertebrate there are a few of more or 

 less intermediate position. Some of these are so different 

 in appearance from the true vertebrates that their re- 

 lationship to the latter would never be suspected upon 

 ordinary observation. Such is the case, for instance, 

 with the tunicates or "sea-squirts." Most of these 

 animals are sac-like creatures living attached to rocks and 

 sea-weed, and they derive the name " sea-squirt" from 

 their habit of squirting out water when they are irritated; 



153 



