THE ECHINIDEA. 



491 



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

 face is, in many cases {Amphidotus, ^rissus, 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. liZ.—Amphidotus cordatus.—K, viewed from above ; B, from bebind : a, 5 5, 

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

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

 petajous semita : /.circumanal semita ; (/, suhanfil semita ; ^. anup ; i, intra- 

 seniital 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 



