252 PH YL UM E CHI NODE RMA . 



globular sphreridia, 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 or pyramids, 

 and along with five stout "braces" and five curved "com- 

 passes " 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 and form a 

 " girdle " round about the mouth (Fig. 120). 



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 ciliated 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. 



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, sphreridia, 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 lie near 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 debris. It 

 ends near the centre of the apical disc, whence the pedi- 

 cellarire have been seen removing the faeces. 



The spacious body cavity is lined by ciliated epithelium, 

 and contains a " perivisceral " fluid, whose corpuscles have 

 a respiratory pigment (echinochrome). When the fluid of 

 a perfectly fresh sea-urchin is emptied out, the contained 

 corpuscles unite in plasmodia, forming composite amoeboid 

 clots (cf. Protomyxa, etc.). 



