STRUCTURE OF THE CRAYFISH. 233 



product of epidermic cells, it is dead and cannot expand. 

 Hence the necessity for periodic moulting as long as the 

 animal continues to grow. The old husk becomes thinner, 

 a new one is formed beneath it, a split occurs across the back 

 just behind the shield, the animal withdraws its cephalo- 

 thorax and then its abdomen, and an empty but complete 

 shell is left behind. The moulting is preceded by an 

 accumulation of animal starch or glycogen in the tissues, 

 and this is probably utilised in the growth which intervenes 

 between the casting of the old and the hardening of the new 

 shell. How thorough the cuticle-casting is, may be appre- 

 ciated from the fact, that the covering of the eyes, the hairs 

 of the ears, the lining of the fore-gut and hind-gut (both of 

 them inturnings of skin), the gastric mill, and the tendinous 

 inward prolongations of the cuticle to which some of the 

 muscles are attached, are all got rid of and renewed. The 

 moults occur in the warm months, eight times in the first 

 year, five in the second, two in the third, after which the 

 male moults twice, the female once a year, till the uncertain 

 limit of growth is reached. It is not clearly known in what 

 form the animals procure the carbonate of lime which is 

 deposited in the chitinous cuticle, but experiments made by 

 Mr Irvine at Granton Marine Station, proved that a carbonate 

 of lime shell could be formed by ci;abs even when the slight 

 quantity of carbonate of lime in sea-water was replaced by 

 calcium chloride. Not the least remarkable fact in regard to 

 moulting is the mortality which is often associated with the 

 process itself or with the defenceless state which follows. 

 Inequalities in the legs are usually due to losses sustained in 

 combat, but these are gradually repaired by new growth. 



The surface of the body bears hairs or bristles of various 

 kinds. These have their roots in the epidermis, and are 

 made anew at each moult. There are simple glands beneath 

 the gill-flaps, and on the abdomen of the female there are 

 cement-glands, the viscid secretion of which serves to attach 

 the eggs. 



Appendages. — The limbs of a Crustacean usually exhibit 

 considerable diversity ; in different regions of the body they 

 are adapted for different work ; yet they all have the same 

 typical structure, and begin to develop in the same way. In 

 other words, they are all homologous. Typically each con- 



