234 ECHINODERMA. 



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- 

 cellariae have been seen removing the feces. 



Accompanying the first coil of the gut is a canal or 

 "siphon," which opens into the gut at both ends. Accord- 

 ing to Cuenot, a current of water traverses this tube, which 

 •thus, by reason of its thin walls, carries oxygen to the 

 corpuscles of the body fluid. The spacious body cavity is 

 lined by ciliated epithelium, and contains a " perivisceral " 

 fluid, whose corpuscles have a respiratory pigment (echino- 

 chrome). When the fluid of a perfectly fresh sea-urchin is 

 emptied out, the contained corpuscles unite in plasmodia, 

 forming composite amceboid clots (cf. Proteomyxa, etc.). 



The madreporic plate communicates with a membranous 

 stone canal, which runs downwards into a circular vessel 

 near the upper end of the lantern. This gives off five inter- 

 radial transparent 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 tube-feet. When the tube-feet are made 

 tense with fluid, they extend 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 small calcareous plates regularly arranged ; 

 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. 



The blood vascular system is not leadily traced, and there is un- 

 certainty as to many points. It seems to consist of a circular vessel 

 around the gullet, connected both with five radial vessels and with two 

 vessels lying respectively on the dorsal and ventral surfaces of the 

 intestine, and forming a network over it. The fluid cannot be dis- 



