324 



THE ECHINOIDEA 



has a disconnected apical system, the postero-lateral interambulacry meet- 

 ing across it. In this and some other respects it resembles Collyrites ; but 

 it has a sub-anal fascicle, and is probably to be regarded as a degenerate 

 Spatangid rather than a direct descendant of the Meso/oic Collyritidae. 



J'xu XLV. 

 The Heart Urehiu (Spata/ 



Abactinal side. 



The second genus Spatagocystis agrees with Pourtalesia in the disruption 

 of the apical system, but it has no fasciole. The third genus Echinocrcpi* 

 also has no fasciole, but the apical system is compact. 



FIG. XLV I. 

 Pourtalesia Jefreysi (Wyv. Thomson). 



We are now in a position to discuss briefly, first, the relations 

 of the Echinoidea to the other classes of Echinoderma, and 

 secondly, the lines of evolution within the class itself. (Compare 

 Chapter VIII. pp. 17, 33.) 



