IX 



PHYLUM ECH1NODERMATA 



a it abrves 



rad.ne 

 rada. 



ot other* is to cause the movements of the alveoli by which the 

 teeth arc 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 

 fi/i/i/t/ nerve(Fig. 297, 

 r(t(/. nc). Within the 

 ring of auricles the 

 five radial nerves are 

 connected with a 

 nerve-ring (ncrv.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 coelo- 

 mic systems are only 

 feebly developed. 



Ambulacra! Sys- 

 tem. - - Internal to 



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

 nnllal umbulnrral -vessel (red. amb). 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 ampulla; 

 (it'iap), 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 canal (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, 

 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 



nerv.r 



FIG. i'JT. Latei'al view of the internal organs of a Sea- 

 urchin as suen on the removal of a half of the shell. 

 afo.r.ves. abm-al ring blood-vessel; antji. ampulla;; an. 

 anus; ni'i: auricle; int. intestine; /'///. ves. intestinal 

 bli id- vessels ; mini, madreporite; mad. c-an. madreporic 

 canal; ,/>. mouth: max. muscles passing from the 

 auricles to Aristotle's lantern; nere. /. nerve ring; oc. 

 ocular plate ; or.- r. r. oral ring blood-vessel; ,/</<..<. ovoid 

 gland ; pol. n:n. Polian vesicle ; rail. muh. radial ambulacral 

 vessel; rad. ne. radial nerve; *i/>li. siphon; *ji. radial 

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

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



