THE ECHINIDEA. 



491 



In the Spatangoida, when the skeleton is cleaned, its sur- 

 face is, in many cases (Amphidotus, Brissus, Spatangus), 

 marked by one or more symmetrical bands of close-set, mi- 

 nute tubercles (Fig. 143, e,f, g). During life, slender spines 

 are attached to these tubercles, the calcareous skeleton of 



Fig. 143.— -Amphidotus cordatus.—A, viewed from above ; B, from behind : a, bb, 

 triviuru, or anterior and anterolateral ambulacra ; cc, bivium. or posterolateral am- 

 bulacra ; d, madreporic tubercle, surrounded by the genital apertures ; e, int.ra- 

 petajous semita : /, circnmanal semita ; </, subanal semita; h, anus ; i, intra- 

 eeraital pores of bivial ambulacra. C, semita magnified : a, semital tubercles ; b, 

 ordinary tubercles. D, semital spine : a, terminal enlarged, non-ciliated portion ; 

 o, ciliated stem. 



which is clothed with a thick coat of integument, which sud- 

 denly enlarges at the apex (Fig. 143, D) ; long and close-set 



