TIN-: HUU>TIHKII>I:A 



407 



and its conversion into the peritoneal cavity and the ambula- 

 cral system of vessels and nerves ; and by the metamorphosis 



Pro. 135. Diagram exhibiting the general plan of the development of the Echlno- 

 derm<. (After M tiller.) A, common form whence the vermiform Holothnrid(B, BO 

 and the pluteiform Ophiurid or Kchinid (C, C'i larvae are derived ; D, IX, younger 

 jind more adv.'iiKvd stages of the Asterid (liipiiniaria) larvse : a. month : 6. stom- 

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

 of Blpinnaria. 



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 Echinodorms 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 



