300 SUPPLEMENT 



manner as the feet. But this is not the case with the tail, any 

 more than in the crabs. 



The Crustacea which live for many years, and appear to 

 increase in bulk during their whole life, are invested, as has 

 been already said, with a solid crust, incapable of distension 

 without being broken, and consequently calculated to put an 

 insurmountable obstacle to their growth, if nature had not 

 provided a means, by the moulting or changing of this crust, 

 which, if less surprising than the reproduction of individual 

 parts, is not less worthy of the meditations of the observers of 

 nature. 



When at the end of spring the birth of a multitude of ani- 

 mals has furnished prey for the Crustacea, easy to procure, 

 when they find themselves too much confined in their ancient 

 envelope, there is found between their testa and their flesh an 

 empty interval, which increases, so much so, that if at this 

 period their back be pressed with the finger, it will be found 

 to bend perceptibly, and a little after they are to be found 

 with a soft skin, and the remains of the old one are to be seen 

 in the neighbourhood. 



These facts have been known from all time ; but it is again 

 to Reaumur that we are indebted for having them confirmed 

 by direct experiment. 



When we inspect the spoil of an astacus, nothing is want- 

 ing to the completion of its exterior. Even the cartilage 

 which serves to the movement of the mobile finger is to be 

 found there. Each hair was a sheath which covered another 

 hair. The lower articulations of the limbs, which are smaller 

 than the upper, are divided into two in their length, by a 

 suture which separates in the operation, but which is not 

 obseiTed while the animal is living. 



The chemical analysis of the testa of the astaci proves that 

 its composition is gelatine united to calcareous earth : the only 



