CHAPTER III: THE INTERNAL ANATOMY OF 



SPIDERS 



The more general features of the internal anatomy of spiders 

 are illustrated by the accompanying plate (Fig. 143) upon which 

 the different systems of organs are indicated by different colours, 

 except that the muscles and the endosternites are omitted, in 

 order that the diagram should not be too complicated. This 

 plate represents the organs projected upon a vertical, median 

 section of the body. 



The black outline of the figure represents the body-wall; 

 the respiratory organs which are, morphologically, infoldings 

 of the body-wall, are indicated by the same colour; the alimentary 

 canal is coloured yellow; the blood vascular system, red; the 

 nervous system, blue; the poison gland, the silk glands, and the 

 Malpighian vessels, green; and the reproductive organs, purple. 



THE BODY-WALL 



a. THE THREE LAYERS OF THE BODY-WALL 



In that division of the Animal Kingdom to which spiders 

 belong, the phylum Arthropoda, the outer covering of the body 

 serves as a skeleton as well as a protecting shield. It is more 

 or less firm in texture, and to its inner surface and to inward 

 projections of it are attached the muscles that serve to move the 

 body and its appendages; and within the cavity bounded by it 

 are located the viscera. In other words, the body-wall is a firm 

 tube containing the softer structures. The appendages of the 

 body, that is, the legs, the mouth-parts, and the spinnerets, are 

 also tubular, and the cavity of each communicates with the 

 general body cavity. 



Three, more or less distinct, layers can be recognized in the 

 body-wall of a spider; first, the outer protecting layer, the cuticula; 

 second, an intermediate cellular layer, the bypodcrmis; and third, 

 an inner, delicate, membranous layer, the basement membrane. 



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