The Internal Anatomy of Spiders 



which facilitates the shedding of the old cuticula; but I find no 

 reference to the moulting fluid glands. Doubtless there are in 

 spiders, as there are in insects, modified hypodermal cells that 

 produce the moulting fluid. 



Several glands have been described connected with the diges- 

 tive organs that may function as salivary glands. These occur 

 in the endites and in the rostrum. But the most prominent of 

 the glands connected with the mouth-parts are those that open 

 through the chelicerae and produce the venom. 



Glands which open in the coxal segment of the third pair of 

 legs in tarantulas and in some true spiders have been observed; 

 these are termed coxal glands. 



a. THE POISON GLANDS 



The poison-glands are two in number and are situated in the 

 true spiders in the anterior part of the cephalothorax (Fig. 143). 

 Each gland discharges its product through a long slender duct 

 which opens near the tip of the claw of the chelicera of the cor- 

 responding side of the body. In the tarantulas each poison gland 

 is situated in the basal segment of a chelicera. 



The glands are sac-like in form; the lumen of the sac serves 

 as a reservoir of venom; the wall is composed of excreting cells, 

 supported by a layer of connective tissue, and there is a layer 

 of muscle fibres surrounding the sac. The fibres of the muscle 

 layer are arranged in a spiral manner. 



b. THE SILK-GLANDS 



The term silk is ordinarily used to designate the thread spun 

 by the silkworm, Bombyx mori, from which nearly all of our 

 silken fabrics are made; but by entomologists the term is also 

 applied to other similar products. In this sense, silk is produced 

 by various animals and is used by them for many different pur- 

 poses, 



Among insects there is considerable variation in the structure 

 and position of the silk organs; thus while with caterpillars 

 the silk is produced by modified salivary glands, and is spun from 

 a spinneret situated near the mouth; with ant-lions it is produced 

 by modified Malpighian tubes and is spun from the caudal end 

 of the alimentary canal. It is obvious, therefore, that silk-pro- 



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