50 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
worked rapidly to and fro sideways, 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 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 by a system of 
irregular channels, which are filled with blood, and are 
termed blood sinuses. The wall of the cylinder is the 
outer wall of the body itself, to which the general name 
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- 
