THE ECHINIDEA. 



491 



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

 face is, in many cases {Amphidotus, J^rlssus, ^pataugus), 

 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. US.—Amphidofus cordatus.—A^, viewed from above : B, from behind : a.bb, 

 trlTium, or anterior and anterolateral ambulacra ; cc, biviiim. or posterolateral am- 

 bulacra ; f^, ma^lreporic tubercle, surrounded by th(! genital apertures; ^. intra- 

 petalous semita : /'. circunifnial semita ; <7, subanal semifa ; ^, anuf ; i, intra- 

 pemital pores of bivial ambulacra. C, semita masfnified : 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 



