324 



THE ECHINOIDEA 



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

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

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

 Spatangid rather than a direct descendant of the Mesozoic Collyritidae. 



FIG XLV. 

 The Heart Urchin (Spcdangus purftireus). Abactinal side. 



The second genus Spatagocystis agrees with Pourtalesia in the disruption 

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

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



FIG. XLVI. 

 Pourtalesia Je/reysi (\Vy v. 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.) 



