APPENDAGES. 255 



The moulting is preceded by an accumulation of glycogen 

 in the tissues, and this is probably utilised in the rapid 

 growth which intervenes between the casting of the old and 

 the hardening of the new shell. 



How thorough the ecdysis or cuticle casting is, will be appreciated 

 when we notice that the covering of the eyes, the hairs of the ears, the 

 lining of the fore gut and hind gut, 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 times in the second, 

 thrice 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 crabs even when the slight quantity of carbonate of lime 

 in sea water was replaced by the chloride. Moulting is an expensive 

 and exhausting process, and great mortality is associated with the 

 process itself or with the defenceless state which follows. The process 

 is a disadvantage attendant on the advantage of armature. 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 all have the same typical structure, 

 and begin to develop in the same way. In other words, 

 they are serially homologous organs, illustrating division of 

 labour. Typically each consists of a two-jointed basal piece 

 (protopodite), and two jointed branches rising from this an 

 internal endopodite and an external eocopodite ; but in many 

 the outer branch disappears. The protopodite has usually 

 two joints, a basal or proximal coxopodite, and a distal 

 basipodite; the five joints which the endopodite frequently 

 exhibits are named from below upwards ischio-, mero-, 

 carpo-, pro-, dactylo-podites details of some use in the 

 comparison and identification of species. 



