398 



ZOOLOGY 



SECT. 



Firmly united to the base of the alveolus is a stout bar, the 

 epiphysis (b). Adjacent epiphyses are in close contact with one 

 another, and running inwards from their points of union are five 

 radially-directed, stout bars, the rotulce (c), the inner ends of which 

 unite to bound a circular aperture through which the oesophagus 

 passes. With the inner end of each rotula is movably articulated 

 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 pyramid, at the apex of which project the five teeth ; the 

 pyramid is hollow, containing a passage which is the beginning of 



the oesophagus. The 

 base has the appear- 

 ance of a wheel, the 

 tyre of which is re- 

 presented 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 be- 

 tween the various os- 

 sicles of the lantern, 

 and from them to the 

 auricles, are systems 

 of muscles by- means 

 of the contractions of 

 some of which the 

 lantern as a whole 

 can be protruded 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 ambulacral area, is a radial nerve (Fig. 325, rad. 

 ne). Within the ring of auricles the five radial nerves are con- 

 nected with a nerve-ring [nerv. r) surrounding the mouth. At its 

 distal end each radial nerve is connected with the so-called eye (oc), 

 borne by the corresponding ocular plate. These parts correspond 

 to the epidermal nervous system of the Starfish, which, owing to 

 the ambulacral grooves having become closed in to form narrow 

 canals — the eyineural canals (Fig. 326, ep.), covered over by the 

 plates of the corona — is here more deeply situated ; the deep and 

 ccelomic systems are only feebly developed. 



radne 

 rada, 



Fig. 325.— Lateral view of the internal organs of a Sea- 

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

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

 canal ; amp.&mpullx ; an. anus ; aur. auricle ; roil, ccelome ; 

 int. intestine ; int. ve«.iiitestinal haemal strands ; mad. mad- 

 reporite ; 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. ovoid gland ; 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. (Prom Leuckart, after Hamann.) 



