ANATOMY OF ECHINUS 



505 



small papilla, projects from it on one side of the centre. The 

 lower area of flexible skin sm-rounds the mouth, and is called the 

 "peristome" (Fig. 229), though it corresponds to considerably 

 more than the peristome of Asteroidea. In the mouth the tips 

 of tlie five white chisel -like teeth can be seen. 



The plates forming the corona are, like all the elements 

 of the skeleton of Echinodermata, products of the connective 

 tissue which underlies the ectoderm, which in Echinoidea remains 



Frci. 223. — Aboral view of Echinus esn'/nitits. x h. (After Mortensen.) 



in a fully developed condition covering the plates, and does not, 

 as in Ophiuroidea, dry up so as to form a mere cuticle. The 

 ectoderm consists of the same elements as that of Asteroidea, 

 viz. delicate tapering sense-cells with short sense-hairs, somewhat 

 stouter supporting cells and glandular cells. It is everywhere 

 underlaid by a plexus of nerve fibrils, which, in part, are to be 

 regarded as the basal outgrowths of the sense-cells and partly 

 as the outgrowths of a number of small bipolar ganglion-cells, 

 found intermixed with the fibres. 



Just as the muscular arm has been the determining factor in 

 the structure of the Ophiuroidea, so the movable spine has been 



