II.] THE CRAYFISH AND LOBSTER. 189 



castings of the outer, chitinous, layer of the integument, or 

 whole exoskeleton if calcined. The shedding of this is 

 preceded by a process of disintegration along certain 

 definite lines, such as the edge of the chela and the inter- 

 articular membrane between the cephalo-thorax and abdo- 

 men; these become, during the period at which the animal 

 is freeing itself, so many points of least resistance and a 

 consequent rupture of them ensues. After each ecdysts, the 

 body is soft, being invested in a continuous chitinous cuticle, 

 and the animal retires into shelter until, by calcification of 

 this, the ' shell ' is reproduced. 



As the hard parts of the exoskeleton are the result of re- 

 placement in earthy matter of portions of an originally con- 

 tinuous chitinous cuticle, it follows that the uncalcified areas, 

 which remain soft and flexible, are but persistent portions of 

 the cuticular predecessor of the whole; they stand related 

 to the calcified portions of the exoskeleton, as do the arti- 

 cular surfaces of the long bones of the Frog's endoskeleton 

 to their ossified shafts. Special thickenings or ingrowths 

 of the exoskeleton take place at all points, for purposes 

 of furnishing the surfaces requisite for the insertion of 

 muscles: these ingrowths or tendons are, for the most part, 

 mere tongue-shaped involutions, densest where resistance is 

 greatest. The maximum of specialization is reached in the 

 thoracic region where, from metameric ingrowths of the 

 sternal and epimeral walls, a complicated endophragmal 

 system is formed. This is fully described in the Laboratory 

 work. 



