266 PHYLUM ECHINODERMA. 



new plates around the apical disc, and also by the indi- 

 vidual 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 " 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 pedicellarige 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 oi 

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

 corpuscles unite in plasm odia, forming composite amoeboid 

 clots (cf. Protomyxa, etc.). 



The madreporic plate communicates with a membranous 

 stone canal (calcareous in Cidaris) which runs downwards 

 into a circular vessel near the upper end of the lantern. 

 This gives off five inter-radial transparent " Polian " vesicles 

 and five radial vessels, which run down the sides of the 

 lantern and up each ambulacral area. Each radial vessel 

 gives off numerous lateral branches, which communicate 

 with the internal ampullae and thence with the external 

 tuberfeet. When the tube-feet are made tense with fluid, 

 they extend far beyond the limit of the spines, and are 

 attached to the surface of the rock over which the sea- 

 urchin slowly drags itself. The sucker at the tip of each 

 tube-foot bears a rosette of small calcareous plates ; indeed, 

 there is hardly any part of an Echinoderm in which lime 

 may not be deposited. Before bending upwards from 

 the base of the lantern, each radial vessel gives off a branch 

 to two large tentacle-like tube-feet without attaching discs. 

 The five pairs lie near the mouth, and are sensitive. 



