TUNIC ATES. 287 



BRANCH I. TUNICATA. 



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 

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

 which communicates with a pharynx or gill-region per- 

 forated by numerous gill-slits. At the bottom of this 



*/ 



pharyngeal region is the oesophagus, which leads to stomach 

 and intestine, 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 gill-slits, is collected, and passes into the same 

 cloacal chamber. The nervous system consists of a centre 

 or ganglion between the two openings, from which nerves 

 radiate to the various parts. There is a heart at the 

 opposite side of the body, and a peculiarity of this organ 

 is that it regularly changes in its action, the blood flowing 



