UROCHORDA. 13 



convenient to divide the subsequent development into two periods, 

 the jfirst embracing the stages from the condition represented in 

 fig. 8 V. up to the full development of tlie 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 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) — (1) an anterior 

 vesicle with, for the most part, thin walls, in which unpaired auditory 

 and optic organs make their appearance, and (2) a posterior nearly solid 

 cephalic ganglion, through which there passes a narrow continuation 

 of the central canal of the nervous system. This ganglion consists 

 of a dorsal section formed of distinct cells, and a ventral section 

 formed of a punctated material with nuclei. The auditory organ ^ 

 consists of a ' crista acustica ' (fig. 9), in the form of a slight promi- 

 nence of columnar cells on the ventral side of the anterior cerebral 

 vesicle ; to the summit of which a spherical otolith is attached by 

 fine hairs. In the crista is a cavity containing clear fluid. The 

 dorsal half of the otolith is pigmented : the ventral half is without 

 pigment. The crista is developed in situ, but the otolith is formed 



Fig. 9. Labva 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; n. hinder jmrt 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. 



from a single cell on the dorsal side of the cerebral vesicle, which 

 forms a projection into the cavity of the vesicle, and then travels (in 



^ For a fuller account of the organs of sense inde the chapters on the eye and ear. 



