THE SHEDDING OF THE SKIN. 33 



hardens. This sort of moulting is what is teohnicallj' 

 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 Eeaumur, 

 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 Ecvaumur's two Memoirs, " Sur les diverses reproductions qui 

 fie font dans les ecrevisses, les omars, les crabes, etc.," " Histoire de 

 rAoademie royale des Sciences," annee 1712 ; and "Additions aux ob- 

 servations sur la mue des ecrevisses donnees dans les Mcmoires de 1712." 

 Ibid. 1718. 



