50 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



worked rapidly to and fro sidewa3's, so as to bring their 

 toothed edges to bear upon the morsel. The other five 

 pairs of jaws are no less active, and they 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 bod}^ 

 we may conceive the whole cra^'fish 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 by a system of 

 irregular channels, which are filled with blood, and are 

 termed blood sinuses. The wall of the cjdinder is the 

 outer W'all of the body itself, to which the general nanje 

 of integument may 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 

 extensively impregnated with lime salts ; and, moreover, 

 in consequence of its containing chitin, it is often spoken 

 of as the chitinous cuticula. 



Having arrived at this general conception of the dis- 

 position of the parts of the factory, we may next proceed 

 to consider the machinery of alimentation which is con- 



