ECHINODERMATA. 169 



so as to resemble so many bony arches ; and from them, as well as 

 from the spaces which separate them, numerous muscles derive 

 their origin. Of these muscles, ten (/>/->) arise from the spaces 

 between the arches, two being inserted into the outer edge of the base 

 of each jaw ; so that the effect produced by their contraction, when 

 they all act in concert, will be to approximate the whole mass of 

 the mouth to the oral aperture of the shell, and of course cause 

 the points of the incisor teeth to protrude externally ; or, if they 

 act separately, they can draw the base of the lantern in any di- 

 rection, or cause the grinding surfaces of the jaws to work against 

 each other. 



The antagonists to the muscles last mentioned are ten others, 

 (> >) arising from the extremities of the arches themselves, and 

 running in a radiating manner towards the apex of the lantern, so 

 that the point of each piece or jaw receives a muscle from two of 

 those processes. These fasciculi, from the manner in which the 

 arches project into the cavity of the shell, will draw inwards the 

 entire mass ; or, if they act separately upon the jaws to which they 

 are individually fixed, they will produce movements precisely op- 

 posite to those caused by the contractions of the muscles derived 

 from the spaces between the bony processes ; or, if both sets should 

 act in concert, they become the antagonists of the muscles (i,i,k, &,) 

 which connect the jaws to each other, and by causing the separa- 

 tion of the different pieces they necessarily enlarge, not only the 

 opening of the mouth, but all the passage leading to the oesopha- 

 gus through the axis of the lantern. 



Yet even these are not all the muscles which act upon the 

 masticating apparatus ; ten others, (A, A,) arising in pairs from the 

 middle of the interspaces between the arches, are connected with 

 the bifurcated extremities of the slender curved processes, (e, e,) 

 each of which receives a muscle from two contiguous spaces ; and, 

 from the length of the levers upon which these muscles act, we 

 may well conceive the force with which they will influence the 

 motions of the whole mass of the jaws. 



Such is the complex structure of the mouth of Echinus escu- 

 lentus ; a piece of mechanism not less remarkable on account of the 

 singularity of its construction, than as exhibiting an example of 

 the sudden developement of a dental system, of which not a 

 vestige is visible in any other of the Echinoderm families. In 

 others of the Echinida having the shell much depressed, the 

 dental lantern is modified in form, and proportionately flattened, 



