STRUCTURE OF SEA-URCHIN 303 



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

 move against one another, grasping and grinding small 

 particles. They also help locomotion on a flat surface. 

 They are fixed in five large sockets or pyramids, and 

 aiong with five stout " braces " (rotulae) and five curved 

 " compasses " (radii) form " Aristotle's lantern," a 

 masticating apparatus. 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 " of 

 auriculae, also called standards and perignaths. 



The shell is covered externally by a dehcate ciliated 

 ectoderm, beneath which, in a thin layer of connective 

 tissue, there is a network of nerve cells. Internally, there 

 is another thin layer of connective tissue, and a ciliated 

 epithelium lining the body cavity. The whole complex 

 test starts from a few triradiate spicules in the Pluteus 

 larva. 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, sphaeridia, pedi- 

 cellariae, and spines are all under nervous control, and each 

 radial nerve ends in the sensitive tube-foot that is pro- 

 truded through each ocular plate. It is probable that 

 all the tube-feet are sensory, and tasting is 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 l:he apical disc, whence 

 the pedicellariae have been seen removing the fasces. 



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.). 



The madreporic plate communicates with a membranous 



