168 



ECHINODERMATA. 



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 



a 



m l 



Jig. 71, 1, in which three of these jaws, each containing its in- 

 cisor tooth, are represented in situ, the two others having been 

 removed. 



The five curious jaws described above are fixed together by a 

 set of muscles, (Jig. 70, &, A:,) consisting of short fibres passing 

 between the external edges of the -contiguous segments of the 

 lantern, and evidently capable of powerfully approximating the 

 grinding surfaces and rubbing them upon each other. The jaws, 

 moreover, are provided with five other osseous pieces (rf, d,) 

 arranged in a radiating manner between the bases of the different 

 segments, with which they are connected by ligaments, and like- 

 wise by the pentagonal muscle (z, i,) which runs from one to 

 the other. 



The above described parts complete the apparatus required for 

 connecting the different portions of this remarkable mouth, but 

 the movements of the whole are effected by a very complicated set 

 of levers and muscles which must next be noticed. 



The levers attached to the jaws are five long and slender pro- 

 cesses, (Jig. 71, 1 d, d,) each arising from the central extremity of 

 one of the radiating osseous pieces, (c, c,) and arching outwards con- 

 siderably beyond the base of the lantern, to terminate by a forked 

 extremity. But there are likewise other processes projecting from 

 the inner surface of the shell ; these, two of which are seen in 

 (Jig. 70, 6, ,) are also five in number, and are placed around the 

 orifice of the mouth : they are generally perforated in the centre, 



