238 THE ASCIDIANS. 



opposite one another and connected together by contractile 

 protoplasmic threads, which keep up a pulsating motion. 



The tail is, as might be expected, more elaborately or- 

 ganised than that of the Ascidian larva. The dorsal nerve- 

 cord is solid, and proceeds backwards from the ganglion, 

 passing to the riglit of the alimentary canal until it reaches 

 the tail, along which it is continued, lying to the left of 

 the notochord ; it possesses ganglionic enlargements at 

 intervals in the tail, from which nerves pass out. 



The caudal musculature also shows somewhat doubtful 

 traces of being segmented in correspondence with the 

 ganglionic swellings of the nerve-cord. 



In connexion with the cerebral ganglion there is a 

 sense-organ in the form of an otocyst, with an enclosed 

 otolith, and below this a ciliated pit opens into the ante- 

 rior region of the branchial sac, corresponding to the 

 hypophysis, or sub-neural organ, of the fixed Ascidians. 



According to one view, Appendicularia is the living rep- 

 resentative of the free-swimming ancestor of the Ascidians. 

 According to the other view, it is less primitive than the 

 fixed Ascidians, and was derived from the latter by the 

 gradual increase, from generation to generation, of the du- 

 ration of the pelagic existence of the larvae, until they 

 ceased to metamorphose, and so retained the larval struct- 

 ure throughout hfe, becoming at the same time sexually 

 mature.^ 



These two views are, of course, antagonistic, and the 

 former of them is held by a number of well-known author- 

 ities. As we are ignorant of the development of Appen- 

 dicularia, it is impossible to decide definitely between them. 

 With the facts which are at our disposal, however, the 

 second view — namely, that the Appendicularije represent 

 Ascidian larvae which have become secondarily adapted to 



