196 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
The whole exoskeleton of the crayfish is, in fact, 
produced by the cells which underlie it, either by the 
exudation of a chitinous substance, which subsequently 
hardens, from them; or, as is more probable, by the 
chemical metamorphosis of a superficial zone of the 
bodies of the cells into chitin. However this may be, 
the cuticular products of adjacent cells at first form a 
simple, continuous, thin pellicle. A continuation of the 
process by which it was originated increases the thick- 
ness of the cuticle; but the material thus added to the 
inner surface of the latter is not always of the same 
nature, but is alternately denser and softer. The denser 
material gives rise to the tough lamine, the softer to 
the intermediate transparent substance. But the quan- 
tity of the latter is at first very small, whence the more 
external lamine are in close apposition. Subsequently 
the quantity of the intermediate substance increases, and 
gives rise to the thick stratification of the middle region, 
while it remains insignificant in the inner region of the 
exoskeleton. 
The cuticular structures of the crayfish differ from 
the nails, hairs, hoofs, and similar hard parts of the 
higher animals, insomuch as the latter consist of aggrega- 
tions of cells, the bodies of which have been metamor- 
phosed into horny matter. The cuticle, with all its 
dependencies, on the contrary, though no less dependent 
on cells for its existence, is a derivative product, the 
formation of which does not involve the complete meta- 
