78 CASWELL GRAVE 



cells' which have been described in other ascidian larvae. In 

 Amaroucium, however, they are not modified cells, but are many- 

 celled bodies derived from the ectoderm. The number of test- 

 vesicles is not constant. As accurately as could be determined, 

 the numbers present in each of eight tadpole larvae are as fol- 

 lows: 62, 52, 55, 52, 53, 60, 58, and 62. 



NERVOUS SYSTEM 



In the nervous system of the tadpole the following parts may 

 be distinguised : a sensory vesicle, visceral ganglion, and nerve 

 cord which are functional during the brief larval period only; 

 an hypophysial duct, subneural gland, and definitive ganglion 

 which persist to function during the life of the sessile ascidiozooid. 

 The position of these nervous structures in the tadpole and the 

 relations they bear one to another are shown in figures C, D, and 1. 



SENSORY VESICLE 



The sensory vesicle is situated between the oral and atrial 

 siphons to the right of the median sagittal plane of the body 

 (figs. 1 and 3). It is oval in form and contains a spacious cavity 

 or ventricle filled with a clear liquid. Two sense organs are 

 developed in its wall and project into its central cavity, the 

 eye occupying a considerable portion of the left side and pos- 

 terior end, the static organ located on its right and ventral sides. 

 Except for the parts which form the sense organs and their 

 ganglia, the wall of the sensory vesicle is thin (figs. D and 5). 



THE EYE 



The following parts may be distinguished in the eye; a mass 

 of brownish-black pigment granules arranged in the form of a 

 cup, the mouth of which is directed obhquely upward and for- 

 ward; three lenses arranged in a linear series in the axis of the 

 pigment cup, and a third part which may be called the retina 

 or retinal ganglion (figs. D, 4, 5, and 6). 



A layer of pigment-forming cells in addition to true nerve 

 cells has been described by Salensky ('93) in the developing retina 

 of the embryo of Distaplia, but I have been unable to distinguish 



