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PHYLUM ECHINODERMA 



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 communi- 

 cate with the internal ampullae and thence with the external 

 tube-feet. 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. 



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. There is also a " pseud- 

 hsemal " system consisting of a circum-oesophageal 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 starfishes. 

 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 indistinguishable externally. 

 Five large 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 

 adjacent sea-urchins, and the free-swimming larva is called 

 a Pluteus. 



