THE HO LOT HUR IDEA. 



467 



and its conversion into the peritoneal cavity and the ambula- 

 cral system of vessels and nerves ; and by the metamorphosis 



Fig. 135.— Diasram exliibiting the general plan of the development of the Echino- 

 derms. (After Miiller.)— A, common form whence the vermiform Holothurid (B, B') 

 and the pluteiform Ophiurid or Echinid (C. C) larviB arc derived ; D, D', younser 

 and more adv;uiced stages of the Asterid (BiiAnnaria) larvae ; a, month ; 6, 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 Holothuridea. — 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 Synaptce, for example (Fig. 136), the body is 



