218 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
the ridge, on the other hand, remains relatively short, and 
constitutes the free posterior margin of the carapace. 
As these changes take place, the abdomen and the 
sternal region of the thorax are constantly enlarging in 
proportion to the rest of the ovum; and the food-yelk 
which lies in the cephalothorax is, pari passu, being 
diminished. Hence the cephalothorax constantly becomes 
relatively smaller and the tergal aspect of the carapace 
less spherical; although, even when the young crayfish 
is ready to be hatched, the difference between it and the 
adult in the form of the cephalothoracic region, and in the 
size of the latter relatively to the abdomen, is very marked. 
The simple bud-like outgrowths of the somites, in 
which all the appendages take their origin, are rapidly 
metamorphosed. The eyestalks (fig. 59, 1) soon attain 
a considerable relative size. The extremities of the 
antennules (2) and of the antenne (8) become bifurcated ; 
and the two divisions of the antennule remain broad, 
thick, and of nearly the same size up to birth. On the 
other hand, the inner or endopoditic division of the 
antenna becomes immensely lengthened, and at the same 
time annulated, while the outer or exopoditic division 
remains relatively short, and acquires its characteristic 
scale-like form. 
The labrum (ib) arises as a prolongation of the middle 
sternal region in front of the mouth, while the bilobed 
metastoma is an outgrowth of the sternal region be- 
hind it. 
