THE ECHINIDEA. 



491 ^ 



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

 face is, in many cases {Amphidotus, JBrissus, Spatangus)^ 

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

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

 are attached to these tubercles, the calcareous skeleton of 



Fig. H3.—Amphidolus cordatus.—K, viewed from above : B. from behind : a, h b, 

 triTium, or anterior anl anterolateral ambulacra ; cc, biviiim. or posterolateral am- 

 bulacra ; d, ma Ireporic tubercle, surrounded by the genital apertures ; ^, intra- 

 petalous semita : ./'. circumanal semifa ; <7, suiianal semita ; ^, anus ; i. intra- 

 semital pores of bivial ambulacra. C, semita ma2;nified : a, semital tubercles ; b, 

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

 6, 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 



