DEVELOPMENT OF ASCWIANS. 



205 



(Fig. 138, A, h) ; whether the primitive opening (Fig. 1.38, 

 A, «) is closed or not, Kowalevsky says is an interesting 

 question. According to analogy with many other animals 

 it probably closes. 



The larva hatches in from 

 forty-eight to sixty hours af- 

 ter the beginning of segmen- 

 tation, and is then of the 

 form indicated by Fig. 139 

 (copied with some additions 

 and omissions from Kupffer's 

 figure, being partly diagram- 

 matic). This anatomist dis- 

 covered in the larva of As- 

 cidia canina, which is more 

 transparent than Kowalev- 

 sky's Phallusia larva, not 

 only a central nervous cord 

 overlying the chorda dorsalis 

 and extending well into the 

 tail, while in the body of the 

 larva it becomes broader, 

 club-shaped, and surrounds 

 the sensitive cavity {a), but 

 he also detected three pairs 

 of spinal nerves (s) arising at 

 regular intervals from the 

 spinal cord Qi, Ji') and dis- 

 tributed to the muscles (not 

 represented in the figure) of 

 the tail ; KupfFer calls / the 

 middle and g the lower brain- 

 ganglion. The pharynx (i), 

 or respiratory sac, is now 

 very large ; it opens pos- 

 teriorly into the stomach and 

 intestine {%) ; x represents 

 one of the three appendages by which the larva fastens 

 itself to some object when about to change into the adult, 



Fig. 139.— Larval Ascidian. a, sense 

 cavity containing the eye; 6, ptiarynx or 

 respiratory sac : c, notoctiord ; e, supposed 

 auditory organ ; /, middle, ,9, lower brain- 

 ganglion; /(, A, .'jpinal cord ; s, s, f, three 

 sets of spinal nerves; i, intestine; i, 

 body-wall, consisting of epitlielial cells. — 

 Copied with some changes from Kupffer. 



