

MONOGRAPH OF THE EXISTING CRINOIDS. 



173 



Thus it is that in the cchinodornis the course of the successive calcareous 

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

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



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

 they play no part whatever in the morphology of the animals, further than iudicat- 



FlG. 107. DORSAL VIEW OF A SPECIMEN < F CoMPSuMETRA DJCOMMODA FROM PORT JACKSON, NEW SOUTH WALES, SHOWING TUB 

 RELATIVE PROPORTIONS OF THE ARM?, PIXXVLES, CENTRODORSAL, AND CIRRI. 



ing the radial symmetry and indicating the paths by wliich the mouth structures 

 must travel backward between them. 



Within the class the bilateral symmetry of the echinoderms is determined 

 wholly from the digestive tube. 



In the urchins the oculars alw:iy stand at tho head of the ambulacra! sn-ic-, 

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



