The Internal Anatomy of Spiders 



As the outer and inner layers are derived from the hypodermis, 

 it will be described first. 



The hypodermis. — The hypodermis is the active, living part 

 of the body-wall. It consists of a single layer of cells (Fig. 

 144, by). 



Certain of the hypodermal cells become highly specialized 

 and produce the hollow hairs with which the body is clothed; 

 such a hair-forming cell is termed a trichogen (Fig. 144, tr), each 

 trichogen remains connected through a pore in the cuticula, 

 the trichopore, with the cavity of the hair it produces. 



Other hypodermal cells are modified so as to form glands, 

 which open through pores in the cuticula, either directly or 

 through the tubular hairs. These glands may be either unicellular 

 or multicellular. 



The cuticula. — Outside 

 is a firm layer, which pro 

 as a support for the inter 

 cuticula. The cuticula con 

 an outer, usually thinner, 

 pigments that produce the 



of the hypodermis, there 

 tects the body and serves 

 nal organs; this is the 

 sists of two layers: first, 

 layer, which contains the 

 colours of the outer sur- 



Fig. 144- DIAGRAM OF A SECTION OF THE BODY-WALL 



cu.i, primary cuticula at. 2, secondary cuticula hy, hypodermis 



bm, basement membrane tr, trichogen s, seta tp, trichopore 



face, the primary cuticula (Fig. 144, cu. /); and second, an inner, 

 usually thicker, layer, devoid of pigment, the secondary cuticula 

 (Fig. 144, cu. 2). The secondary cuticula is laminate; the laminae 

 remind one, when the cuticula is seen in section, of the lines of 

 growth of an exogenous tree. 



The well-known firmness of certain parts of the body-wall 

 is due to the presence in the cuticula of a substance which has 

 been termed chitin. This substance bears some resemblance 

 in its physical properties to horn; but it is very different from 

 horn in chemical composition. 



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