154 ELEMENTS OF COMPARATIVE ZOOLOGY. 



at the end of the body. The animals are about two or three 

 inches long, are almost perfectly transparent, and bury 

 themselves in the sand, only the mouth end, encircled by a 

 fringe of delicate filaments, appearing above the surface. 

 They are without any economic importance, but their 

 extremely simple structure makes them intensely interesting 

 to the naturalist. 



BRANCH II. TTOICATA. 



The fact that these forms had any relationship to the 

 Vertebrates would never have been suspected had one 

 studied only the adults. When, however, the development 

 was studied, it was perceived that these forms had larvae in 

 which there was a notochord, gill-slits, and a nervous system 

 much like that of the Vertebrates ; in short, that in shape 

 and in structure these young Tunicates were decidedly 

 tadpole-like. Then these tadpoles settled down upon some 

 object and passed through a metamorphosis in which the 

 tail was lost, the nervous system was contracted into a mass, 

 and the body became more or less saccular and covered 

 with an external envelope or " tunic," which gives the 

 name to the group. 



Of these Tunicates there are many varieties, but the 

 essential features of the adult can be made out from the 

 generalized figure given. The body is globular, and shows 

 on the outside two openings. One of these is the mouth, 

 which communicates with a gill-region perforated by 

 numerous gill-slits. At the bottom of this pharyngeal 

 region is the oesophagus, which leads to stomach and intes- 

 tine, the latter twisting so as to terminate at the bottom of 

 a cloacal chamber, which opens to the exterior by the other 

 aperture mentioned. The water, which passes through the 



