ECHINOIDEA. 26) 



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- 

 hoemal" 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 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. 



Classification. 



The class may be divided into three sub-classes or groups of orders. 

 Sub-Class I. Regularia Endobranchiata. Mouth and anus at opposite 

 poles ; the anus surrounded by the apical system of plates if these 

 are developed ; no external gills. 



e.g. the somewhat primitive Cidaris. 



Sub-Class II. Regularia Ectobranchiata. Mouth and anus at opposite 

 poles ; a double circle of apical plates surrounds the anus ; there 

 are external gills. 



e.g. the common genera Echinus, Strongylocentratus, Arbacia. 

 The Echinothurinse have flexible tests and powerful muscles. 



e.g. Asthenosoma and Phormosoma. 



Sub-Class III. Irregularia. The anus lies outside the apical system 

 of plates in the posterior inter-radius. 



e.g. the heart-urchins, Spatangus and Echinocardium , without 

 lanterns. In the related Echinoneus there is a lantern in the 

 young forms. It is interesting to contrast the large massive 

 Clypeaster with the minute Echinocyamus pusiHus, common 

 in the stomach of cod-fishes. 



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