THE ECHINODERMATA 



245 



tion. The radii half-way between these are called 

 intermdii, and organs which lie upon them are said to be 

 interradial (e.g., the basal plates). These alternating 

 positions are of fundamental importance in all echino- 

 derms, except the lowest forms in which five-rayed 



a 



FIG. 67. COPRESSOCRINUS. 



a, C. gracilis Goldfuss ; b, c, C. abbreviates Goldfuss ; both Middle 

 Devonian, Eifel. a, Side view of complete crown, with arms closed, 

 and part of stem ; b, oral view ; c, basal view of a cup without arms or 

 stem. (All natural size.) (Original.) A, Anus; A.C., axial canal; 

 Ax, axial canal of stem ; B, basal plate ; Br. i, 2, first and second 

 brachial plates ; IB, infra-basal plate ; M, mouth ; O, oral plate ; 

 R. , radial plate. 



symmetry is not fully developed. It is a universal rule 

 that the mouth and anus, unless they are central, are 

 respectively perradial and interradial in position. With 

 the mouth, however, the central position is the rule, w.ith 

 the anus it is the exception. 



