82 THE NATURAL HISTORY OF THE COMMON CRAYFISH. 
go on growing till, in exceptional cases, they may attain 
between seven inches and eight inches in length; but at 
what degree of longevity this unusual dimension is reached 
ig uncertain. It seems probable, however, that the life of 
these animals may be prolonged to as much as fifteen or. 
twenty years. They appear to reach maturity, so far as 
_ the power of reproduction is concerned, in their fifth or, 
more usually, their sixth year. However, I have seen 
a female, with eggs attached under the abdomen, only 
two inches long, and therefore, probably, in her second 
year. The males are commonly larger than females of 
the same age. 
The hard skeleton of a crayfish, once formed, is 
incapable of being stretched, nor can it increase by in- 
terstitial addition to its substance, as the bone of one 
of the higher animals grows. Hence it follows, that the 
enlargement of the body, which actually takes place, 
involves the shedding and reproduction of its invest- 
ment. This might be effected by insensible degrees, and 
in different parts of the body at different times, as we 
shed our hair; but, as a matter of fact, it occurs periodi- 
cally and universally, somewhat as the feathers of birds 
are moulted. The whole of the old coat of the body is 
thrown off at once, and suddenly; and the new coat, 
which has, in the meanwhile, been formed beneath 
the old one, remains soft for a time, and allows of a 
rapid increase in the dimensions of the body before it 
