THE ECHIXIDEA. 



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.—K, viewed from above ; B, from behind : a, bb, 

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

 bulacra ; d, malreporic tubercle, surrounded by the genital apertures ; e, iotra- 

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

 serairal 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 



