STRUCTURE OF A SEA-URCHIN. 211 



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, — 

 the minute pedicellariae, with three snapping blades on a soft 

 stalk, and small globular sphaeridia, which contain many 

 nervous cells, and show by their motions that they detect 

 chemical changes in the water. 



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 connect- 

 ive tissue, there is a network of nerve-fibres, and some 

 ganglion-cells. Internally, there is another thin layer of 

 connective tissue, and a ciliated epithelium lining the body- 

 cavity. The skeleton grows by the formation of new plates, 

 and also by the individual increase of each. In a few forms 

 the shell retains some plasticity. 



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, sphaeridia, pedicellariae, 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 this is 

 certainly the main function of ten which adjoin the mouth. 



The Alimentary Canal passes through Aristotle's lantern, 

 and the intestinal portion lies in two and a half coils around the 

 inside of the shell to which it is moored by mesenteries. It 

 contains fine gravel, sand, and some organic ddbris. It ends 

 near the centre of the apical disc, whence the pedicellariae 

 have been seen removing the faeces. On the inner wall of the 



