THE EGHIMDEA. 



491 



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

 face is, in many cases (Amphidotm, Brissus, Spatangu8) ? 

 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 



Fia. l&.Amphidotus cordatus.A., viewed from above ; B, from behind : a, 66, 

 triTium, or anterior an 1 anterolateral ambulacra ; ec, bivium. or posterolateral am- 

 bulacra ; rf, milreporic tubercle, surrounded by the cenitnl apertures ; 0, intra- 

 petalous tanita: /, circa manal semifa ; q, an banal semita ; h. annc : t, intra- 

 semital pores of bivial ambulacra. C, semita magnified : a, semital tubercles ; 6, 

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

 *, 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 



