THE ECHINIDEA. 



493 



by long radial pieces — the rotulw (c) articulated with their 

 edges. To the inner extremity of each rotula, finally, a slen- 

 der arcuated rod, presenting indications of a division in the 



Fio 144.— A, dentarv apparatus of Clyyeastrid (after Miiller): fl, alveolus; d, rotula; 

 e, tooth. B, C, D, dentary apparatus (Aristotle's lantern) of Echinus sphcera. 

 B, two of the five chief component parts of the lantern apposed and viewed lat- 

 erally. C, lateral view, and D, hack view, of a single part: a, principal piece of 

 alveolus; a', suture with its fellow; b, epiphysis ; b', suture of epiphysis with 

 principal piece ; c, rotula ; d, radius or compass ; e, tooth. 



middle of its length, is articulated, and, running outward par- 

 allel with the rotula, terminates in a free bifurcated extremity. 

 This is the radius (d). 



Altogether, then, the Lantern consists of twenty principal 

 pieces — five teeth, five alveoli, five rotula?, and five radii — of 

 which the alveoli are again divisible into four pieces each, 



