362 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



attached form, which the second hypothesis regards as salpa's pro- 

 genitor, would probably have lost the greatly developed sense-vesicle 

 which we see in the ascidian larva, so that the free swimming primitive 

 salpa would have to develop an eye in a new region of the brain. At the 

 same time it would be most natural that the new eye should develop 

 about the same structure as is found in the eye possessed by its remote 

 free swimming ancestor (the ascidian tadpole). This hypothesis, then, 

 explains perfectly the fact that the eye of salpa, while having essentially 

 the same structure as the larval ascidian eye, is found in a different 

 part of the brain and that the two are in no way homologous. 



The secondary manner of formation of the ganglion itself in pyro- 

 soma, doliolum and salpa would argue that they are descended from 

 some common form which derived its ganglion from a secondary modi- 

 fication of the central nervous system, i. e. from an ascidian-like, rather 

 than a tadpole-like form. 



