50 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



worked rapidl}- to and fro sidewaj's, so as to bring their 

 toothed edges to bear upon the morsel. The other five 

 pairs of jaws are no less active, and the}' thus crush and 

 divide the food brought to them, as it is passed between 

 their toothed edges to the opening of the mouth. 



As the alimentary canal stretches from the mouth, 

 at one end, to the vent at the other, and, at each of 

 these limits, is continuous with the wall of the body, 

 we may conceive the whole crayfish to be a hollow 

 cylinder, the cavity of which is everywhere closed, though 

 it is traversed by a tube, open at each end (fig. 6). 

 The shut cavity between the tube and the walls of the 

 cylinder may be termed the perivisceral cavity ; and it is 

 so much filled up by the various organs, which are inter- 

 posed between the alimentary canal and the body wall, 

 that all that is left of it is represented b,y a system of 

 irregular channels, which are filled with blood, and are 

 termed blood sinuses. The wall of the cj'linder is the 

 outer wall of the body itself, to which the general name 

 of integument maj' be given ; and the outermost layer of 

 this, again, is the cuticle, which gives rise to the whole 

 of the exoskeleton. This cuticle, as we have seen, is 

 extensivel}'' impregnated with lime salts ; and, moreover, 

 in consequence of its containing cldtin, it is often spoken 

 of as the chitinous cuticula. 



Having arrived at this general conception of the dis- 

 position of the parts of the factor}', we mny next proceed 

 to consider the machiner}' of alimentation which .is con- 



