CRAYFISH. 283 
inward prolongations of the cuticle to which some of the muscles are 
attached, are all got rid ofand 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 setze 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 pvofopodite, and two jointed branches rising 
from this—an internal exdopodite 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excised eye has been replaced by an antenniform structure suggest that 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-sta/k may be of the nature of an appendage. 
With many of the thoracic appendages, gills, plate-like epipodites, 
and sete 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. 
