ECHINOIDEA 569 



upwards into the perivisceral cavity. These are known as Stewards 

 organs or internal gills and when they are present external gills are 

 often lacking. 



The oesophagus enlarges into a flattened tube, the stomach (Fig. 427), 

 which runs horizontally round the body in a clockwise direction as 

 viewed from below, suspended from the shell in festoons, by strings 

 of tissue. At its beginning, there is a short caecum^ and it is accom- 



cump: 



St. 



Fig. 427. A view of a sea urchin, with part of the shell removed to show the 

 course of the alimentary canal. After Cuvier. amp. ampullae at base of tube 

 feet; b.r. " blood" ring; d.b.v. " dorsal blood vessel" ; Ian. lantern of Aristotle 

 (displaced) ; M. mouth surrounded by five teeth (tth.) ; oe. oesophagus, coiled 

 intestine and rectum; ov. ovaries with oviducts; per. fold of peritoneum 

 supporting genital rachis; sip. siphon; v.b.v. "ventral blood vessel"; arc. 

 arch; /, jaw; rm. rectum; st. stomach. 



panied by a small, cylindrical tube, the siphon, which opens into it at 

 either end. From its distal end a tract similar to it, the intestine, re- 

 turns in the opposite direction and then ascends as the narrower 

 rectum to the anus. The food consists chiefly of seaweed. 



The water vascular ring has five Tiedemann's bodies. It is situated 

 above the lantern, and the radial vessels run downwards and out- 

 wards from it between the jaws and under the auriculae and then 

 meridionally under the radial plates of the corona, to end each in the 

 pigmented tentacle of an ocular plate. Each is accompanied in its 



