THE ECHINIDEA. 



493 



by long radial pieces the rotulce (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 



PIG. 144. A, dentary apparatus of Clypeastrid (after Mflller) : a, alveolus; d, rotula; 

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

 B, two of tlie flve chief component parts of the lantern appoBed and viewed lat- 

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

 alveolus; o>, suture with its fellow; ft, epiphysif? ; &', suture of epipbysis with 

 principal piece ; c, rotula ; of, 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 rotuLi?, and five radii of 

 which the alveoli are again divisible into four pieces each, 



