PHYLUM ECH1NODERMATA 



367 



coei 



others is to cause the movements of the alveoli by which the 

 teeth are brought to bear on the food. 



Nervous System. Passing outwards through each auricle, 

 and running along the inner surface of the corona opposite the 

 middle of each am- 

 bulacral area, is a 

 radial nerve (Fig. 297, 

 rad. ne). Within the 

 ring of auricles the 

 five radial nerves are 

 connected with a 

 nerve-ring (nerv.r) sur- 

 rounding the mouth. 

 At its^distal end each 

 radial nerve is con- 

 nected with the eye 

 (oc), borne by the 

 corresponding ocular 

 plate. These"" parts 

 correspond to the epi- 

 dermal nervous sys- 

 tem of the Starfish ; 

 the deep and ccelo-l 

 mic systems ;uv only 

 feebly developed. 



Ambulacral Sys- 

 tem. Internal to T 

 each radial nerve, and pursuing a corresponding course, runs a 

 radial ambulacral vessel (rad. ami)). From this are given off on 

 each side a series of short branches to the tube-feet, with each of 

 which is connected one of a series of compressed sacs, the ampullae 

 (amp), by two canals, one passing through each of the two pores. 

 At their oral extremities the five radial ambulacral vessels unite 

 with a ring-vessel surrounding the oesophagus. [Appended to the 

 ring-vessel are five Polian vesicles (pol. ves). in the form of small 

 mammillated bodies. A madreporic caila.1 (mad. can), corresponding 

 to that of the Starfish, but with soft membranous walls devoid of 

 ossicles, runs from the madreporite at the side of the periproct 

 to the ring-canal surrounding the mouth. 



Accompanying the madreporic canal is an ovoid gland (plex) 

 similar in essential character to that of the Starfish and having \ 

 similar relations, except that the connection with the reproductive \ 

 organs has disappeared in the adult. 



The enteric canal (Fig. 298, ali) is devoid of the radial caeca 

 which it presents in the Starfish : it is a wide, soft-walled tube, \y 

 which winds round the interior of the corona in its passage from 

 the mouth to the anus held in place by a band of threads the 



v rad.n 



rada 



FIG. 297. Lateral view of the internal organs of a Sea- 7*5 nl~ Ut\ 

 urchin as seen on the removal of a half of the shell. ^ 

 ab. r. -res. aboral ring blood-vessel ; amp. ampulla} ; an. 

 anus , tiv.r. auricle; int. intestine; int. ves. intestinal 

 blood-vessels ; mad. madreporite ; mad. can. madreporic 

 canal ; mo. mouth ; mus. muscles passing from the 

 auricles to Aristotle's lantern ; nen: r. nerve ripg ; oc. 

 ocular plate ; o/v /. /. oral ring blood-vessel; plex. ovoid 

 gland ; pol. ves. Polian vesicle ; rad. amb. radial ambulacrail 

 vessel ; rat', ne. radial nerve ; siph. siphon ; sp. radial 

 extension of the coelome surrounding the nerve ; t. f. 

 tube-feet. (From Leuckart, after Hamann.) ( 



C? 



