346 



COMPARATIVE ANATOMY 



CHAP. 



the imbrication of the ambulacra is adoral, i.e. the oral edge of each plate overlaps 

 the apical edge of the next in order below it, whereas, in the interambulacra, the 

 imbrication is apical. Lateral imbrication also occasionally occurs. 

 Slight imbrication is also found in certain Spain ii<jo'ln. 



(/) Special Modifications of the Ambulacra. 



In all Echinoidea, in which the mouth remains at the centre of the oral surface, 

 the five ambulacra are alike in length, breadth, and in the arrangement of their 



Fio. 306. System of plates of a Clypeastroid (Encope Valenciennes!, Agass.), spread out 



(after Loven). 



pores, prominences, etc. They only vary in length when the apical system, towards 

 which, radiating from the peristome across the ambitus, they converge, is shifted 

 from the centre of the apical hemisphere to a somewhat anterior (less frequently 

 posterior) position. If the test of such an Echinoid, in which the ambulacra 



