46 



is but feebly calcined. The cuter walls, the floor and 

 roof of the branchial chamber and the roof of the prc- 

 branchial chamber are also extremely thin and 

 membranous. The chitinous linings of the fore-gut and 

 hind-gut, which are continuous with the exoskeleton, are 

 uncalcified, except in those regions of the fore-gut where 

 the ossicles are present. The gills, also, are covered by 

 an extremely fine chitinous layer. 



The integument of the crab consists of an epidermis, 

 below which lies the dermis. On the outer side of the 

 epidermis is a chitinous layer, the thickness of which 

 differs considerably in various parts of the body. This 

 outer chitinous layer is a product of the epidermis, and 

 constitutes the exoskeleton already referred to. The 

 chitinous layer may be impregnated with calcareous 

 salts. 



The epidermis [Chitogenous epithelium, Vitzou*] 

 (Text fig. 8, e.) consists of a single row of columnar cells 

 resting upon a basement membrane (/.). These cells differ 

 in their appearance in various parts of the body, and also 

 show marked changes during the interval between one act 

 of ecdysis and the next. In the dorsal integument of the 

 hard crab, for example, the cells of the epidermis are 

 only moderately columnar, but in the oesophagus of the 

 same animal the cells are extremely elongated. In the 

 soft crabs the cells are of much greater length compara- 

 tively than in the hard crabs. 



In some regions where we have two parts of the 



integument coming close together, such as at the edge of 



the carapace and also the gill lamellae, the cells of the 



epidermis sometimes become extremely elongated and 



pass across the dermis to fuse with similar cells from the 



* Vitzou, A. N. "Eecherchos sur la structure ct la formation des 

 teguments chez les Crustaces Pecapodes." Arch, de Zooloqie exp&r. et 

 g&n., T. X. [1882], p. 451. 



