ON CRUSTACEA. 301 



difference between it and that of the molhisca is, that in the 

 latter there is much calcareous earth, and but little gelatine ; 

 and in the former there is much gelatine, and but little calca- 

 reous earth. * 



The astacus being thus left covered with a soft membrane, 

 does not, however, long remain in this state : the skin within 

 four and twenty hours often assumes all the consistence of the 

 old one. The general period, however, for this to take place 

 is from two to three days. 



The astaci, ready to moult, have always two stones, which 

 are placed at the sides of the stomach, but which are no longer 

 to be seen in those which have moulted. It appears that 

 these stones are destined to furnish the matter, or a portion of 

 the matter of the testa ; for, if on the day after the moulting, 

 when the testa is as yet but half hardened, an astacus be 

 opened, it will be remarked that those stones are diminished 

 one half. If opened on the third day, but a mere atom will 

 be visible, and afterwards no trace of them whatsoever. This 

 method, employed by nature to consolidate promptly the 

 envelope of an animal exposed, while naked, to so many 

 dangers, is extremely worthy of observation. 



Reaumur has measured astaci before and after the moultinsr, 

 and has attained the proof that they augment about one-fifth 

 in bulk. He does not say that this augmentation is the same 

 at all ages ; but it seems probable that it gradually diminishes, 

 hence it may be concluded that these animals grow slowly, 

 and the reports of fishermen confirm the supposition. 



The fresh-water astaci principally delight in the running 

 and gravelly waters of the mountains. They are also found in 

 lakes and ponds ; but there their flesh, unless these collec- 

 tions of waters be augmented by neighbouring sources, is not so 

 good. They conceal themselves, during the day, in holes 

 which they excavate, or under stones, the roots of trees, &c. 



It is extremely difiicult to stock a stream with astaci, and 



