MONOGRAPH OF THE EXISTING CKINOIDS. 



173 



Thus it is that iii the echinoderms the course of the successive calcareous 

 rings is from the oral to the aboral eud of the animal, and the circumoral struc- 

 tures liave been drawn backward to the apical pole along with them. 



The original somatic divisions of the echinodemas have become so inert that 

 they play no part whatever m the morphology of the animals, further than indicat- 



FlO. 107.-I1OESAL VIEW OF A SPECIMEN OF CoMPSOMETRA KCOMMOD.V FROM PoRT JACKSON, NEW SOUTH WALES, SHOWINQ TUE 

 RELATIVE PROPORTIONS OF THE ARMS, PINNULES, CENTRODORSAL, AND CIRRI. 



mg the radial symmetry antl iudicatiug the paths by which the mouth structures 

 must travel backward between them. 



Withm the class the bilateral symmetry of the echinoderms is determmed 

 wholly from the digestive tube. 



In the urchins the oculars alway stand at the heatl of the ambulacral scries 

 from which they are never separated; they always remain extremely imjiortant 



