CRA YFISH. 283 



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 Irvine's experiments have shown 

 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. It is the necessary tax 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 setae 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, illus- 

 trating division of labour. Typically each consists of a 

 basal piece or protopodiU^ and two jointed branches rising 

 from this an internal endopodite and an external exopodite ; 

 but in many the outer branch disappears. 



The protopodite has usually two joints a basal or proximal coxo- 

 podite, 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. 



The stalked eyes are not included in the above list, since their develop- 

 ment is not like that of the other appendages ; but cases where an 

 excised eye has been replaced by an antenniform structure suggest that 

 the eye-stalk may be of the nature of an appendage. 



With many of the thoracic appendages, gills, plate-like epipodites, 

 and setae are associated. 



It is interesting to connect the structure of the appendages with their 

 functions. Thus it may be seen that the great paddles are fully spread 

 when the crayfish drives itself backwards with a stroke of its tail, while 

 in straightening again the paddles are drawn inwards, and the outer 

 joint of the exopodite bends in such a way that the friction is reduced. 



19 



