CH. Il] THE CRAYFISH 67 



larger in proportion to the abdomen ; the chelae are more 

 slender, their tips are more sharply curved inwards, and 

 there are hooks on the ends of the last two walking legs. 

 These last modifications are of value in enabling the young 

 animal to cling to the empty eggshell or to the swimmerets 

 of its parent for a time. In about 10 days it moults, and 

 again four times at intervals of about 3 weeks until 

 September; the skin then obtained lasts through the 

 winter, but in the following spring and early summer three 

 more moults occur, making 8 moults during the first year 

 of life. During the 2nd year there are 5 moults, 2 in the 

 3rd. The female is mature in her fourth year, and thence- 

 forth moults only once yearly. The male is mature in his 

 3rd year, and is said to continue moulting twice every 

 year. It is not known to what age crayfishes attain, nor 

 at what age growth ceases. It must be borne in mind 

 that moulting may well continue after growth has ceased ; 

 the casting-off of the skin is an act of excretion, and 

 probably accounts for the singularly feeble development 

 of nitrogenous excretory organs in the crayfish and other 

 Arthropods. The need for nitrogenous excretion does not 

 cease when growth ceases, though probably the moults 

 become less frequent. We need exact observation on this 

 point. 



The phenomena preceding a moult are complex, and 

 affect organs other than the mere cuticle that is to be cast 

 off. About 5 weeks before a moult, in an adult, or for 

 a shorter time in younger individuals, the wall of the 

 stomach on each side of the entrance of the oesophagus 

 begins to lay down sheets of calcified chitin which 



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