54 



ment is continuous with the cliitinous lining of the fore- 

 gut and hind-gut. The body, therefore, may he said to 

 be enclosed in an inflexible coat, which prevents the 

 tissues from expanding. The growth of the animal cannot 

 be gradual, but can only take place when the animal 

 breaks through the stiff outer covering. Immediately 

 after exuviation, the animal, which is then only covered 

 by an extremely thin flexible membrane, will increase in 

 size. This process of casting, or ecdysis, is characteristic 

 of all Arthropods. Ecdysis takes place periodically, and 

 growth can only take place while the animal is in a 

 " soft " condition. 



In Cancer, when ecdysis is about to take place, the 

 carapace opens along the pleural groove at each side. 

 These two longitudinal splits become connected posteriorly 

 with a transverse opening, which makes its appearance 

 between the posterior border of the carapace and the 

 tergum of the first abdominal somite. Thus the tergal 

 region of the carapace is free from the remainder of the 

 exoskeleton, except along a line marking the posterior 

 boundary of the first cephalic sternum. The carapace, 

 therefore, acts like a lid of a box, and is hinged anteriorly. 

 The first part of the body to be withdrawn from the old 

 shell is the abdomen, which is followed by the various 

 legs. When all the parts are completely free the crab 

 emerges from beneath the hinged carapace. 



On the dorsal sides of the basi-ischium and meros of 

 the chela there are faint grooves (PL II, fig. 12, abs.). 

 These are the " lines of absorption," and at the time of 

 ecdysis the exoskeleton of the chela loses its calcification 

 at these points. In this way the withdrawal of the large 

 claw is effected, as it would be extremely difficult for the 

 chela to be withdrawn if the integument at the base of the 

 limb remained hard. 



