646 THE INVERTEBRATA 



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- 

 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 



FiR- 453- A diagram of a section across a radius of Echinus. From Shipley 

 and MacBride. amh. ambulacral plate; amp. ampulla; ho. boss for articula- 

 tion of spine ; ect. ectoderm ; epin. epineural canal ; m. muscles which move 

 the spine; ra.n. radial nerve cord; ra.peh. radial perihaemal canal; ra.wv. 

 radial water canal ; sp. spine ; t.f. cavity of tube foot. 



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

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

 course under the shell by the radial nerve cord, epineural canal, and 

 perihaemal canal (Fig. 453). It is said that a small radial "blood 

 vessel" (not shown) runs between the perihaemal canal and water 

 vessel. From the water vascular ring the stone canal, which is not 

 calcified, ascends vertically to the madreporite, accompanying the 

 axial organ, which surrounds the small axial sinus. Under the 



