886 



ECHINODEKMA 



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

 strongly reminds us of the concentric rings of an exogenous tree 



FIG. 678. Transverse section of spine of Eclitnoinctra. 



(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. 

 672) ; and this is bounded 

 by a row of transparent 

 spaces (like those at a a! , 

 b b f , c c', &c., fig. 675), 

 which on a cursory in- 

 spection might be sup- 

 posed to be void, but are 

 found on closer examina- 

 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 

 of every layer. Their solidity becomes very obvious when we 



FIG. 674. One of the segments of the calcareous 

 skeleton of an ambulacral disc of Echinus. 



