INTEGUMENT 145 



which is highly resistant to most chemical reagents. The cu- 

 ticulaof the crayfish also contains considerable quantities of lime 

 salts, which add much to its hardness. The integument of 

 insects is similar to that of the crayfish, but the cuticula lacks 

 the lime. 



342. This type of integument is extremely effective as a 

 protective structure, and it will be noted that within the group 

 of Arthropods, we first find animals which can withstand 

 exposure to dry air. A disadvantage of the arthropod type of 

 integument is that it is too rigid to permit great freedom of 

 motion. For this reason the integument of both body and 

 appendages is divided into segments which are connected by 

 zones of more flexible tissue. Flexure of the body and appen- 

 dages can only occur at these points. (See'ecdysis, page 346.) 



343. The skin of vertebrates, especially that of Mammals, 

 is much more complex. The epidermis consists of many layers 

 of cells, which are continually increasing in number by the active 

 growth and division of the deeper lying ones. The superficial 

 layers are composed of lifeless cells which have been transformed 

 into flattened, horny scales. This horny layer takes the place 

 of a cuticula. In addition to the epidermis there is a deeper 

 layer, the dermis, which is much thicker and consists chiefly 

 of felted connective tissue fibres. 



outlines; C, a sectional view of the thick epidermis of the ventral side of nereis, 

 with the underlying circular and longitudinal muscles. In A the cells are 

 cubical and the cuticula very thin; in C the cells are columnar, the cuticula is 

 very thick and pierced by pores through which glands open and sensory hairs 

 project. The lumina of the gland cells are represented as light oval spaces. 

 D, a sectional view of the epidermis of the crayfish; E, the epidermis of a mammal 

 with a hair follicle in which a hair (h) is beginning to develop; F, the epidermis 

 of a mammal highly magnified and showing its layers horny layer, granular 

 layer and Malpighian layer; G, a section through the skin of a mammal including 

 epidermis, dermis, and a hair follicle; H, diagram of the epidermis of a reptile 

 with its outer layers (black) solidified into horny scles; /, a similar diagram of 

 the bony scales beneath the epidermis of a fish; /, a section of the skin of a 

 bird to show a feather in an early stage of development. Bl.V., blood-vessels; 

 C.M., circular muscles; Cu., cuticula; D, dermis; Ep, epidermis; h, hair; H.L., 

 horny layer; l.m., longitudinal muscles; M, muscles; M.L., Malpighian layer; 

 s (in G), sebaceous glands; s (in H and 7), scales; S.C., sensory cell. 



