THE NERVOUS SYSTEM. 237 



apical area seen on the dorsal surface of the young forms is 

 not demonstrable in the adult. 



The " posterior " ambulacra, those between which the 

 modified basal or madreporic plate lies, are often distin- 

 guished as the " bivium," the other three form the " trivium," 

 and the middle one of the three is " anterior." 



On the shell there are obviously many spines, most 

 abundant on the inter-ambulacral areas. Their bases fit 

 over ball-like knobs, and are moved upon these by muscles. 

 But besides these, there are two modified forms of spines, — 

 (rt) the minute pedicellariae, with three snapping blades on 

 a soft stalk, and sometimes with apical glands ; and (b) small 

 globular sphseridia, which show some structural resemblances 

 to otocysts. It is said that like true otocysts they are con- 

 cerned with the perception of direction of motion. 



In front of the mouth project the tips of five teeth, which 

 move against one another, grasping and grinding small 

 particles. They are fixed in five large sockets, and along 

 with fifteen other pieces form " Aristotle's lantern," a complex 

 masticating apparatus, of whose history we know little. It 

 surrounds the pharynx, and is swayed about and otherwise 

 moved by muscles, many of which are attached to five beams 

 which project inward from the margin of the shell round 

 about the mouth. 



As in other Echinoderms, the skeleton of lime is meso- 

 dermic. The shell is covered externally by a delicate 

 ciliated ectoderm, beneath which, in a thin layer of con- 

 nective tissue, there is a network of nerve fibres, and some 

 ganglion cells. Internally, there is another thin layer of 

 connective tissue, and a cihated epithelium lining the body 

 cavity. The skeleton grows by the formation of new plates 

 around the apical disc, and also by the individual increase 

 of each. In a few forms the shell retains some plasticity. 



Nervous System. 



The nervous system consists of a ring around the mouth, of 

 radial branches running up each ambulacral area, and of the 

 superficial network. Tube feet, sphEeridia, pedicellaria;, and 

 spines are all under nervous control, and each radial nerve 

 ends in the " eye specks " of the apical " ocular plates." 

 It is probable that all the tube feet are sensory, and 



