166 ECHINODERMATA. 



each particle of every layer composing them appears to have been 

 deposited in its proper place : indeed, if the zootomist would fully 

 appreciate the minuter details connected with their organization, 

 it is only by the employment of the microscope that he will 

 arrive at adequate ideas concerning them ; for it is not in the num- 

 ber and variety of the pieces entering into the composition of the 

 skeleton of one of these animals, the extraordinary apparatus of 

 prehensile suckers with which they are furnished, or the singular 

 locomotive spines upon the exterior of the shell, that he will find the 

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

 a minute examination of the intimate structure of each of these 

 parts that the mechanism conspicuous throughout can be properly 

 understood. 



(207.) The calcareous pieces which surround 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 move- 

 ment, by which the prehension of food is more easily effected. The 

 mouth itself (Jig- 68, 1) 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 calculated to break the hard sub- 

 stances usually employed as food. The points of such incisor 

 teeth, although of enamel-like hardness, would nevertheless be 

 speedily worn away by the constant attrition to which they are 

 necessarily subjected, was there not some provision made to ensure 

 their perpetual renewal ; like the incisor teeth of rodent quadrupeds, 

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

 served sharp and fit for use. In order to allow of such an arrange- 

 ment, as well as to provide for the movements of the teeth, jaws 

 are provided, which are situated in the interior of the shell ; and 

 these jaws, from their great complexity and unique structure, form 

 perhaps the most admirable masticating apparatus met with in the 

 whole animal kingdom ; we must therefore entreat the patience of 

 our readers while we describe at some length the parts connected 

 therewith. The entire apparatus removed from the shell is repre- 

 sented in (Jig. 70), and consists of the following parts : There are 

 five long teeth, (c, c,) each of which is enclosed in a triangular os- 

 seous piece, (a, a,) that for the sake of brevity we will call the jaws. 

 The five jaws are united to each other by various muscles, 

 (&, &, i, i 9 ) so as to form a pentagonal pyramid, having its apex 

 in contact with the oral orifice of the shell, while its base is con- 



