ECHINOIDEA. 253 



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 ambulacra! 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 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. 



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

 certainty as to many points. A " dorsal or axial organ " lies beside the 

 stone canal, and seems to be connected with a "genital ring " and with 

 a circular vessel around the gullet. The latter is connected with five 

 radial vessels and with vessels on the intestine. There is also a 

 " pseud-haemal" system consisting of a circum-cesophageal sinus with 

 radial branches. The fluid cannot be distinguished from that of the 

 body cavity ; it contains corpuscles, some of which are pigmented. 



On the area round about the mouth there are ten hollow 

 outgrowths, which resemble the skin-gills of star-fishes. 

 There are also five large vesicles at the top of the lantern 

 (" Stewart's organs ") which may function as internal gills. 

 As already mentioned, the pigmented cells of the body 

 cavity fluid seem able to absorb oxygen. There is no 

 doubt that the water vascular system plays a very important 

 part in respiration. It probably also aids in excretion. 



The sexes are separate, and like one another. Five 

 branched yellow-brown ovaries or rose-white testes lie inter- 

 radially under the apex of the shell, and open by separate 

 ducts on the five genital plates. In spring the apical disc 

 may be seen covered with orange ova or milky-white 

 spermatozoa. 



The eggs are fertilised externally by sperms wafted from 



