32 THE NATURAL HISTORY OF THE COMMON CRAYFISH. 



go on growing till, "in exceptional cases, they may attain 

 between seven inches and t'ght inches in length ; but at 

 what degree of longevity this linusual dimension is reached 

 is 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 craj'fish, 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 



