390 



^OOLOGY 



SECT. 



a more slender bar, the radius (d), which runs outwards, parallel 

 with, and closely applied to, the rotula, to end in a free, bifurcated 

 extremity. Aristotle's lantern as a whole is in the shape of a five- 

 sided pyra- 

 mid, at the 

 apex of which 



Project the 

 ve teeth; 

 the pyramid is 

 hollow, con- 

 taining a pas- 

 sage which is 

 the beginning 

 of the oeso- 

 phagus. The 

 base has the 



FlO."329. Lantern of Aristotle of Echinus. A , fwo of the five chief appearance of 

 component parts apposexl and Viewed laterally. B, lateral, and , TT 1,1 4-1* n 



C internal view of a single part. a. alveolus, and a', suture with its <* Wilt,ei, LUC 



fellow; b. epiphysis ; b'. suture with alveolus ; c. rotula ; d. radius ; f vrp n f wTiirli 



e. tooth. (From Huxley's Invertebrates, after Miiller.) V 1 



is represented 



by the five epiphyses, the spokes by the five rotulse with the five 

 radii in close contact with them, and the hub by the rounded 

 central aperture. Passing between the various ossicles of the 

 Fan tern, and from oc af , ^ r ^ s 



them to the auricles, 

 are systems of muscles 

 by means of the con- 

 tractions of some of 

 which the lantern as a 

 whole can be pro- 

 truded or retracted, 

 while the action of 

 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 ambu- 

 lacral area, is a radial nerve (Fig. 330, rad. ne). Within the 

 ring of auricles the five radial nerves are connected with a 

 nerve-ring (nerv. r) surrounding the mouth. At its distal end 



rad.ne 

 rada, 



FIG. 330. Lateral view of the internal organs of a Sea- 

 urchin as seen on the removal of a half of the shell, ab. r 

 ves. haemal strand, aboral ring ; amb. r. ambulacral ring- 

 canal ; amp. ampullae ; an. anus ; aur. auricle ; cod. 

 ccelome ; int. intestine ; int. res. intestinal haemal strands; 

 mad. madreporite ; mad. can. madreporic canal ; mo. 

 mouth ; mus. muscles passing from the auricles to 

 Aristotle's lantern ; nerv. r. nerve-ring ; oc. ocular plate ; 

 or. r. ves. haemal strand, oral ring ; plex. axial organ ; pol. 

 ves. Polian vesicle ; rad. amb. radial ambulacral vessel ; 

 rad. ne. radial nerve ; siph. siphon ; sp. radial extension 

 of the ccelome surrounding the nerve ; t. f. tube-feet. 



