THE HOLOTHURIDEA. 



467 



and its conversion into the peritoneal cavity and the ambula- 

 cral system of vessels and nerves ; and by the metamorphosis 



Fig. 135. — Diasrara exhibiting the general plan of the development of the Echino- 

 derm?. (After Miiller.) — A, common form whence the vermiform Holothnrid (B, W) 

 and the plnteiform Ophiurid or Echinid (C, C) larvae arc derived ; D, W^ younser 

 and more advanced stages of the Asterid (JSipinnaria) larvae ; a. month ; b, stom- 

 ach : c, intestine ; d, anus ; ?, ciliated band ; e', second or anterior ciliated circlet 

 of Bipinnaria. 



of the mesoderm into radially-disposed antimeres, the result 

 of which is the more or less complete obliteration of the 

 primitive bilateral symmetry of the animal. 



1. The Holothueidea. — The study of the structure of 

 the Echinoderms may best be commenced with the members 

 of this division, which, in many respects, deviate least from 

 such worms as the Gephyrea. 



In the SynaptcB, for example (Fig. 136), the body is 



