268 ARTICULATA. 



from their hardness, their muscular power, and the 

 stout teeth with which they are armed, a very for- 

 midable weapon, as well as a substitute for a hand. 



There is no part of the economy of these ani- 

 mals more interesting than the periodical rejection 

 of their crust. Solid and unyielding as this is, it 

 is obvious that it would soon present an obstacle 

 to the increase of the growing animal, which could 

 be surmounted only by its removal ; the necessity 

 for which is the more pressing, as some species 

 attain a very large size, living to the age of twenty 

 years, and growing till death. To the indefati- 

 gable Reaumur, we owe most of the information 

 we possess on the mode in which this singular 

 operation is performed. It seems usually to take 

 place in the summer season, when food is abundant 

 and the weather warm. The naturalist above-men- 

 tioned placed several specimens of the Cray-fish 

 (Astacus Fluviatilis) in perforated boxes, some of 

 which were immersed in the river, and others kept 

 in his house. In a state of freedom, the animal 

 retires on the approach of its change into clefts of 

 the rocks, holes, and similar places, to be free from 

 interruption and danger during its helpless state. 

 Remaining for some time without eating, it be- 

 comes thin ; and meanwhile a new skin the future 

 shell, but yet soft and expansible is formed be- 

 frvveen the old shell and the body. When this is 

 fully formed, the animal becomes violently agitated, 

 and throwing its body into various postures, rubbing 

 its feet against each other, endeavours to loosen the 



