MOUTH AND TEETH OF ECHINUS. 



195 



most remarkable features of the history of the Echini ; it is only by a 

 minute examination of the intimate structure of oach of these parts that 

 the perfection of the mechanism conspicuous throughout can be properly 

 understood. 



(511.) The calcareous pieces surrounding the mouth of the Echinus 

 are not so immoveably consolidated as those composing the rest of the 

 shell, but, on the contrary, admit of considerable movement, whereby 

 the prehension of food is materially facilitated. The mouth itself 

 (fig. 94, l) is a simple orifice, through which the points of five sharp 

 teeth are seen to protrude. These teeth obviously perform the office of 

 incisors, and from their sharpness and extreme density are well calcu- 

 lated to break the hard substances usually employed as food. The 

 points of such incisor teeth, although of enamel-like hardness, would 



Oral apparatus of Echinus : a a a a a, pyramidal pieces forming the " lantern of Aristotle " ; 

 b b, internal projections from shell ; c c c c c, teeth enclosed in their sockets ; d d, interposed osseous 

 pieces ; e e, curved processes ; ff, g y, hh,ii,kk, muscular fasciculi for the movements of the jaws. 



nevertheless be speedily worn away by the constant attrition to which 

 they are necessarily subjected, were there not some provision made to 

 ensure their perpetual renewal ; like the incisor teeth of rodent quadru- 

 peds, they are therefore continually growing, and are thus always pre- 

 served sharp and fit for use. In order to allow of such an arrangement, 

 as well as to provide for the movements of the teeth, jaws are provided, 

 that are situated in the interior of the shell ; and these jaws, from their 

 great complexity and unique structure, form, perhaps, the most ad- 

 mirable masticating apparatus met with in the whole animal kingdom ; 

 we must therefore entreat the patience of the student while we describe 

 at some length the parts connected therewith. The entire apparatus 



o 2 



