252 SUPPLEMENT 



membranaceous parts; 3d. a third stratum, still thicker, of a 

 less compact texture, in which the calcareous molecules are 

 deposited, which give solidity to the testa ; 4th. a last stratum, 

 altogether external, composed of colouring matter, or pi gmen- 

 tum, and an epidermic layer. 



The name of moulting has been given to the renovation of 

 the testa in Crustacea. These moultings are more or less fre- 

 quent according to the age of the animals, and the more or 

 less rapid degrees in which their growth is developed. 



In the decapod Crustacea, the moulting takes place every 

 year, towards the middle of the spring. Reaumur has studied 

 that of the river crawfish, or astaci, and to him we are in- 

 debted for every thing we know concerning the mode in 

 which this operation takes place. When these Crustacea are 

 desirous of changing skin, they rub their feet one against the 

 other, and put themselves into very considerable motion. 

 They afterwards swell out their body in a very sensible degree, 

 and the first segment of the tail appears more separated than 

 usual from the posterior edge of the carapace. The membrane 

 which unites them breaks, and the body, with its new skin, 

 appears. After a short term of repose, they begin to agitate 

 themselves afresh, inflate and raise themselves more than they 

 did at first ; the carapace rises, is detached, and remains ad- 

 herent in no place except towards the mouth. Soon after, the 

 eyes are disengaged from their old skin, which remains fixed 

 to the former testa, then the antenna), as well as the parts of 

 the mouth, and finally the carapace, are almost totally sepa- 

 rated. At last, after divers reiterated movements, the astaci 

 strip their claws and feet in an indeterminate order. Then 

 they quit their carapace altogether, and suddenly extending 

 their tail, they disengage themselves entirely from their old 

 envelope. 



After the moulting, the astaci are very soft, and remain in a 



