THE ECHIXIDEA. 



491 



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

 face is, in many cases (Amphidotiis, Urissus, 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. liS.—Amphidofus cordatus.—K, viewed from above ; B, from behind : a. 5 5, 

 tririuni, or anterior and anterolateral ambulacra ; cc. bivinm. or posterolateral am- 

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

 petajous semita : f. circumanal semita; q, subanal semita ; h, anus ; i. iutra- 

 semital pores of bivial ambulacra. C, semita magnified : a, semital tubercles ; 5, 

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

 5, 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 



