188 



ECHINODEEMATA. 



between the primitive segment and the margin of the disk, the original 

 segment retaining its size and figure unaltered, while the succeeding 

 ones differ in their shape, assuming a polygonal form, which varies in 

 different species. The places where all new segments are formed are 



Fig. 93. 



1. Ophiura in a still more advanced stage of development, showing the larva portion (Pluteus) 

 in great part obliterated : first appearance of the mouth and tentacles. 2. The larva has entirely 

 disappeared, and the feet and spines of the Ophiurus begin to develope themselves. 3. Shows the 

 mode of growth of one of the rays : the terminal or primitive segment is easily recognizable, to 

 which the following segments succeed in the order of their formation. (After Miiller.) 



in the shield itself, at points situated upon the ventral aspect, between 

 the inter-radial spaces ; and each successive segment produced, being 

 at the base of the ray, is of course larger than all that preceded it 

 (fig. 93, 3, 4). 



(496.) In order to complete the history of the Asteridce, we have yet 

 to mention the nervous apparatus wherewith they are furnished. This 

 consists of a simple circular cord that runs around the mouth of the 



