49 



in the fully-formed exoskeleton. It is colourless and 

 richly impregnated with calcareous salts. Like the 

 previous layer, it exhibits striations parallel to the 

 surface, but the laminae are generally broader than in the 

 pigmented layer. It is to this layer that the great thick- 

 ness and hardness of the shell in a hard crab are due, as 

 new laminae are constantly being added to this region 

 until the exoskeleton attains its maximum thickness. 



(4) The non-calcified layer (d.) is a very thin layer 

 composed of delicate laminae parallel to the surface. This 

 layer remains in a very soft condition, and is not formed 

 until the calcified layer has attained its maximum width. 



Vertical sections through the integument reveal the 

 fact that there are striations in the chitin at right angles 

 to the surface, as well as the horizontal lamellae already 

 referred to. Also, as Vitzou has pointed out, in horizontal 

 sections the chitinous integument is divided up into small 

 hexagonal areas, and in each of these areas small pores are 

 present. Vitzou determined that these areas were of the 

 same size and shape as the horizontal sections through 

 the cells of the epidermis. He concluded, therefore, 

 that the exoskeleton is composed of innumerable hexagonal 

 prisms packed side by side, having their long axes at 

 right angles to the surface of the body. Furthermore, 

 each of these chitinous prisms is in contact with the 

 outer end of an epidermal cell. So that for every cell of 

 the epidermis there is a corresponding prism forming a 

 unit of the chitinous exoskeleton. Such an explanation 

 accounts for the presence of the vertical striations in 

 vertical sections, and for the polygonal areas in the 

 horizontal sections. The small pores in the middle of 

 these areas are due to the presence of numerous fine 

 canals traversing each prism from the epidermis to the 

 exterior. 



