56 



Before ecdysis the cells of the epidermis become 

 greatly elongated, and in the underlying dermis the cells 

 of Ley dig, which are rich in glycogen, become extremely 

 numerous. The supply of reserve food material in these 

 cells is evidently of the utmost importance at a time when 

 growth and regeneration of the tissues is taking place. 

 At the time when the crab is preparing to cast, a new 

 chitinous layer is formed by the epidermis. This new 

 layer is separated from the old shell by a gelatinous 

 fluid. The chitinous layer, which is the first appearance 

 of the new exoskeleton, consists of two parts — an outer 

 structureless cuticular layer, and an inner chitinous layer 

 containing pigment. This inner layer represents the 

 pigmented layer. The calcified and non-calcified layers 

 are not produced until after ecdysis. The calcified layer 

 grows throughout the greater part of the period until the 

 next ecdysis, and it is to this layer that the hardness and 

 increasing thickness of the shell is due. 



Vitzou's theory explaining the method of formation 

 of the exoskeleton has been described above (see section 

 on Integument). 



The frequency of casting and other problems 

 connected with ecdysis are discussed below in the section 

 on Economics. 



AUTOTOMY AND REGENERATION OF LlMBS. 



One of the most interesting and characteristic 

 features in the natural history of the crab is the power 

 the animal possesses of throwing off injured limbs 

 (autotomy) and of forming new limbs to replace the old 

 (regeneration). 



The processes associated with these phenomena may 

 be briefly stated as follows : — 



If the distal portion of one of the pereiopods be 



