Vol. 3] Torrey. — Biological Studies on Corymorpha. 



273 



the bottom or suspended freely in the aquarium; but they do 

 not grow to their usual length, and their ends retain a cylin- 

 drical shape, unless they come in contact with the substratum, 

 which provides the mechanical aid necessary for their later 

 development. 



In the adult the frustules occupy a definite zone about one 

 sixth the length of the stem from the proximal end, arranged 

 in from two to four vertical rows on each peripheral canal. 

 They become progressively older proximally. 



The origin of the frustules doesj. not differ essentially in larva 



b 



Fig. 19. Two optical sections of same develojjing frustule from larva 

 with six proximal tentacles. X 1210. 



and adult. In both the body wall evaginates and epithelial 

 endoderm cells are incorporated as a portion of the axial tissue, 

 in which a cavity is wanting from the beginning. In so far the 

 development closely parallels the development of the tentacles. 

 There is also a preliminary change in the ectoderm to be evag- 

 inated, which in the larva is almost identical with the thickening 

 of the ectoderm where the distal tentacle is to appear. 



