62 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Fig. 24. Pentacrmoid Larva of 

 Antedon. 



A, Quite young larva, before the open- 

 ing of the cup, and the appearance 

 of the radial plates; B, Nearly 

 mature : 6, basals; o, orals; r, first 

 radial s. (After Carpenter.) 



organs, or should, in other 

 words, have before our 

 eyes representatives of all 

 the more important sys- 

 tems of organs in the body. 

 It is this phenomenon 

 which has led to the 

 theory once held by Cuvier, 

 re-presented by Duvernoy, 

 and, in our times, sup- 

 ported with much vigour 

 by Haeckel, that the Echi- 

 noderm is a colony of 

 bilaterally symmetrical 

 metazoic animals which 

 have become connected to- 

 gether by their anterior 

 ends. It is more in 

 accordance with the facts, 

 as at present known to 

 us, to suppose rather that 

 the radiate form has been 

 brought about by a return 

 to a fixed habit, and that 

 this mode of symmetry 

 has been retained by in- 

 heritance. In those forms 

 which stand farthest from 

 the Crinoids the radial is 

 again obscured by a 

 secondarily acquired bila- 

 teral symmetry (Spatan- 

 gus, Synapta) ; a close in- 

 vestigation into the char- 

 acters of most members of 

 the phylum enables us 

 to distinguish a plane 



