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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 skiu, 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 
observed 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 
