ON CRUSTACEA. 
301 
difference between it and that of the mollusca 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 moulting, 
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. 
Tt is extremely difficult to stock a stream with aslaci, and 
