192 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
A notable exception to this generalisation, however, 
obtains in the case of the cuticular structures, in which 
no cellular components are discoverable. In its simplest 
form, such as that presented by the lining of the in- 
testine, the cuticle is a delicate, transparent membrane, 
thrown off from the surface of the subjacent cells, either 
by a process of exudation, or by the chemical transfor- 
mation of their superficial layer. No pores are discern- 
ible in this membrane, but scattered over its surface 
there are oval patches of extremely minute, sharp conical 
processes, which are rarely more than 1-5,000th of an 
inch long. Where the cuticle is thicker, as in the 
stomach and in the exoskeleton, it presents a stratified 
appearance, as if it were composed of a number of lamin, 
of varying thickness, which had been successively thrown 
off from the subjacent cells. 
Where the cuticular layer of the integument is un- 
calcified, for example, between the sterna of the abdo- 
minal somites, it presents an external, thin, dense, 
wrinkled lamina, the epiostracum, followed by a soft 
substance, which, on vertical section, presents numerous 
alternately more transparent and more opaque bands, 
which run parallel with one another and with the free 
surfaces of the slice (fig. 56, D). These bands are very 
close-set, often not more than 1-5000th of an inch apart 
near the outer and the inner surfaces, but in the middle 
of the section they are more distant. 
If a thin vertical slice of the soft cuticle is gently 
