UROCHORDA. 15 



straightened out. It has a striking resemblance to a tadpole 

 (vide fig. 10). 



In the free larval condition the Ascidians have in many 

 respects a higher organization than in the adult state._ It is 

 accordingly convenient to divide the subsequent development 

 into two periods, the first embracing the stages from the con- 

 dition represented in fig. 8 V. up to the full development of the 

 free larva, and the second the period from the full development 

 of the larva to the attainment of the fixed adult condition. 



Growth and Structure of the free larva. 



The nervous system. The nervous system was left as a 

 closed tube consisting of a dilated anterior division, and a 

 narrow posterior one. The former may be spoken of as the 

 brain, and the latter as the spinal cord ; although the homologies 

 of these two parts are quite uncertain. The anterior part of the 

 spinal cord lying within the trunk dilates somewhat (fig. 8 V. and 



FIG. 9. LARVA OF ASCIDIA MENTULA. (From Gegenbaur; after Kupffer.) 

 Only the anterior part of the tail is represented. 



N'. anterior swelling of neural tube; N. anterior swelling of spinal portion of 

 neural tube; . hinder part of neural tube; ch. notochord; K. branchial region of 

 alimentary tract; d. oesophageal and gastric region of alimentary tract; 0. eye; 

 a. otolith ; o. mouth ; s. papilla for attachment. 



VI. Rg) and there may thus be distinguished a trunk and a 

 caudal section of the spinal cord. 



The original single vesicle of the brain becomes divided by 

 the time the larva is hatched into two sections (fig. 9) (i) an 

 anterior vesicle with, for the most part, thin walls, in which 



