196 



ECHINODEEMATA. 



removed from the shell is represented in fig. 96, and consists of the 

 following parts : There are five long teeth (c c), each of which is 

 enclosed in a triangular osseous 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 (k Tc, i i), so as to form a pentagonal pyramid, having its apex 

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

 with several bony levers by means of numerous muscles provided for 

 the movements of the whole. These parts we must now proceed to 

 describe seriatim. The teeth (fig. 97, 1, a) resemble, at the part pro- 

 truded from the mouth, long three-sided prisms, and at this point they 

 are extremely hard and brittle : each tooth is fixed in a socket passing 

 through the jaw (fig. 97, 2, e), from which it projects by its opposite 

 extremity (fig. 97, 2, a'), that may be called the root of the tooth, 

 where, instead ' of being of glassy hardness like the point (a) which 

 issues from the mouth, it is flexible and soft, resembling fibres of 

 asbestos, and is covered by a membrane apparently connected with its 

 secretion. The jaws, which thus support and partially enclose these 

 teeth, are five in number : when examined separately, each is found to 

 resemble in figure a triangular pyramid, the external surface (fig. 97, 2,e) 

 being smooth, and presenting eminences provided for the attachment 

 of muscles ; while the other two sides (fig. 97, 1, b b) are flat, and 

 marked with transverse 

 grooves, so as to have 

 the appearance of a 

 fine file. When the five 

 jaws are fixed together 

 in their natural po- 

 sitions, they form a 

 five- sided conical mass, 

 aptly enough compared 

 by Aristotle to a lan- 

 tern, and frequently 

 described by modern 

 writers under the name 

 of the " lantern of Ari- 

 stotle." When thus 

 fitted to each other, 

 the two flat and striated 

 sides of each jaw are in 

 apposition with the 

 corresponding surfaces of two others, so that there are ten grinding 

 surfaces formed, between which the food must pass preparatory to its 

 introduction into the digestive canal. This arrangement will be easily 

 understood by referring to fig. 97, 1, in which three of these jaws, 

 each containing its incisor tooth, are represented in situ. 



Dental system of Echinus. 1. Represents three of the 

 pyramidal pieces forming the " lantern of Aristotle " in situ : 

 a a, cutting extremities of the incisor teeth, which are of 

 enamel-like hardness; a' a' a', fibrous roots of the same, 

 resembling asbestos in their texture ; 5 b, opposed flat sur- 

 faces of the jaws; d d, arched processes. 2. An isolated 

 pyramid: e, its external surface. Other letters as in fig. 1. 



