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 



Fig 144.— a, dftntary apparatus of Hypeastrkl (after Miiller) : a, alveolus ; d, rotula ; 

 e, tooth. B, C, D, deiitary apparatus (Ari!?to(le's lantern) of Echinus sphcera. 

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

 endlv. C. lareral view, and D, back view, of a single part: a, prit cipal piece of 



alveolus; «', suture with its fellow; b, epiphysis 

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



b^, suture of epiphysis with 

 tooth. 



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

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

 This is the radws (d). 



Altogether, then, the Lantern consists of twenty principal 

 pieces — five teeth, five alveoli, five rotulce, and five radii — of 

 which the alveoli are again divisible into four pieces each, 



