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 j^oung 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 antennae (5) 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 (Ih) 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. 



