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, continuoiis, 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 

 materia] gives rise to the tough laminae, the softer to 

 the intermediate transparent substance. But the quan- 

 tity of the latter is at first very small, whence the more 

 external laminae 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- 



