The Internal Anatomy of Spiders 



name. It is situated just behind the point where the fore- 

 intestine emerges from the nervous collar and rests upon the endo- 

 sternite (Fig. 143). The chitinous intima of the fore-intestine is 

 greatly thickened in this region, affording a firm support for 

 powerful muscles. In a cross-section of the sucking stomach 

 (Fig. 161), it can be seen that the thickened intima consists of 

 four longitudinal plates; a dorsal and a ventral one, and one 

 on each side. From the dorsal plate very strong muscles extend 

 to the ental surface of the median furrow of the cephalothorax; 

 and from the ventral and the lateral plates muscles extend to the 

 endosternite. These act to enlarge the cavity of the organ. 

 Opposed to the muscles just described, the sucking stomach 

 is supplied with sphincter muscles (Fig. 161). These it will 

 be observed are outside of the wall of the intestine and do not, 

 therefore, strictly speaking, form a muscular coat of the intestine. 



b. THE MID-INTESTINE 



The mid-intestine begins a short distance back of the sucking 

 stomach; it differs greatly from the fore-intestine both in histo- 

 logical structure and in general form. 



The distinctively characteristic feature in the structure 

 of the mid-intestine is the absence of a chitinous intima; this 

 portion of the alimentary canal being 

 lined with a layer of cells, the essen- 

 tial digestive element of the organ 

 (Fig. 162, ep). This digestive epithe- 

 lium rests on a basement membrane, 

 outside of which there is a peritoneal 

 layer composed of fibres, presumably 

 of connective tissue (Fig. 162, p). No 

 muscular coat has been observed; but 



. , , . . . . . Fig. 162. CROSS-SECTION OF 



outside of the peritoneal layer there mid-intestine (after Bertkau) 

 is a cellular layer, which has been 



termed by several writers, but 1 believe incorrectly, a peritoneal 

 layer (Fig. 162, f). This so-called peritoneal layer, I believe to 

 be the fat-bodv, which is described later. 



The most striking feature of the mid-intestine of spiders is the 

 presence of an extensive system of diverticula by means of which 

 the extent of the digestive epithelium is greatly augmented. 

 In the cephalothorax, these diverticula are in the form of large 



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