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 laj^er. 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 laminae, 

 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 l-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 



