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,f, g). During life, slender spines 

 are attached to these tubercles, the calcareous skeleton of 



Fig. U3.—Amphidotu$ cordatus.—A, viewed from above ; B, from behind : a, b b, 

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

 bulacra-, d, madreporic tubercle, surrounded by the genital aperture? ; e, intra- 

 petalous semita: /.circumanal semita ; g, subanal semita; h, anus ; i, intra - 

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

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

 b, 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 



