386 



ECHIXODEIOIA 



made up of a number of concentric layers, arranged in a manner that 

 strongly reminds us of the concentric lings of an exogenous tree 



FK;. C7i5. Transverse section of spine of Echinometra. 



(fig. 673). The number of these layers is extremely variable, de- 

 pending not merely upon the age of the spine, but (as will presently 



appear) upon the part of 

 its length from which the 

 section happens to be 

 taken. The centre is 

 usually occupied by a 

 very open network (fig. 

 (572) ; and this is bounded 

 by a row of transparent 

 spaces (like those at a ', 

 b b'. r c', &c., fig, 07.-.). 

 which on a cursory in- 

 fection might be sup- 

 posed to lie \ did, but are 



Ki,.. (J74. On, o\ bhe segments of the calcareous found on closer exnmina- 

 skeleton of an ambulacral disc of 



tion to be the sections of 

 solid ribs or pillars, which 



run in the direction of the length of the spine, and form the exterior 

 ol' every layer. Their solidity becomes very obvious when we 



