350 ECHINODERMATA [CH. 



inner side of the skeleton (8, Fig. 162). Through the auriculae the 

 radial water-tube and nerve pass, and thus they correspond in 

 position to the ambulacral plates of star-fish. 



The food of the Urchin consists ordinarily of seaweed which it 

 gnaws with its teeth. No doubt the little worms and molluscs 

 always found in abundance on the surface of the weed add a flavour 

 to the repast and as we have seen it can devour small animals 

 which happen to be caught by its ophicephalous pedicellariae. 

 The alimentary canal is exceedingly unlike those of the Echinoderms 

 so far studied. The gullet ascends vertically between the teeth and 

 passes into the intestinal tube which runs in a spiral right round the 

 body and then turns sharply back and describes one turn of a spiral 

 in the opposite direction, after which it bends inwards and runs 

 straight up to the anus (Fig. 165). For the first part of its course 

 a small tube, the so-called siphon, runs parallel to it, opening into 

 it at both ends. 



fhe water-vascular ring is situated above the masticatory ap- 

 paratus and is thus widely separated from the nerve-ring, which is 

 situated below it : the radial tubes, in consequence, run downwards 

 along the " lantern " before bending outwards under the auriculae 

 (see Fig. 163). The water-vascular ring bears small pouches which 

 have been termed Polian vesicles. They seem, however, to corre- 

 spond to Tiedemann's bodies in an Asteroid. The first pair of 

 tube-feet in each radius are different from the rest, in that they are 

 short and not capable of extension, and that their discs are oval. 

 These tube-feet protrude through the peristome and are called the 

 buccal tube-feet; they function as tasting organs, and are 

 thrown into violent excitement if a piece of edible matter is put 

 near them (Fig. 166). 



In describing the Asteroidea it was mentioned that the genital 

 stolon or "dorsal organ" had been mistaken by former authors for a 

 heart, and that true blood-vessels were unknown amongst Echino- 

 dermata. If by blood-vessel is meant a tube with well-defined 

 walls in which there is a definite circulation of fluid this is strictly 

 true. 



It must be remembered, as was pointed out in the chapter on 

 Arthropoda, that blood-vessels and connective tissue have been 

 derived from the same primitive tissue, which may be compared to 

 the jelly of Coelenterata. Now Echinodermata probably represent a 

 stage before the evolution of either blood-vessels or proper connective 

 tissue. Apart from the plates of the skeleton the substance of the 



