THE SHEDDING OF THE SKIN. 33 
hardens. This sort of moulting is what is technically 
termed ecdysis, or exuviation. It is commonly spoken of 
as the “shedding of the skin,” and there is no harm in 
using this phrase, if we recollect that the shed coat is not 
the skin, in the proper sense of the word, but only what 
is termed a cuticular layer, which is secreted upon the 
outer surface of the true integument. The cuticular 
skeleton of the crayfish, in fact, is not even so much a 
part of the skin as the cast of a snake, or as our own nails. 
For these are composed of coherent, formed parts of the 
epidermis; while the hard investment of the crayfish con- 
tains no such formed parts, and is developed on the out- 
side of those structures which answer to the constituents 
of the epidermis in the higher animals. Thus the cray- 
fish grows, as it were, by starts; its dimensions remaining 
stationary in the intervals of its moults, and then rapidly 
increasing for a few days, while the new exoskeleton is 
in the course of formation. 
The ecdysis of the crayfish was first thoroughly 
studied a century and a half ago, by one of the most 
accurate observers who ever lived, the famous Réaumur, 
and the following account of this very curious process is 
given nearly in his words.* 
A few hours before the process of exuviation com- 
* See Réaumur’s two Memoirs, “Sur les diverses reproductions qui 
se font dans les écrevisses, les omars, les crabes, etc.,” “ Histoire de 
l’Académie royale des Sciences,” année 1712 ; and “ Additions aux ob- 
rervations sur la mue des écrevisses données dans les Mémoires de 1712.” 
Tbid. 1718. 
D 
