The Internal Anatomy of Spiders 



According to Causard, the pulmonary veins are large hollow 

 exocardiac ligaments, which serve as veins in addition to their 

 function of holding the heart in place. In fact he regards the 

 other exocardiac ligaments as reduced pulmonary veins, which 

 have lost their venous function, and become merely ligaments. 



The venous circulation. — From the arteries the blood passes 

 into the spaces of the body-cavity between the viscera, bathing 

 the various organs. It then passes into two longitudinal sinuses, 

 one dorsal and one ventral, which lead to the base of the abdomen. 

 Here it is purified by the book-lungs; and then it passes through 

 the pulmonary veins to the pericardial cavity, from which it 

 enters the heart through the ostia. 



The blood is also purified to a greater or less extent by tubular 

 tracheae as it is in insects; but the extent of the ramifications of 

 these tracheae varies greatly in different spiders. 



THE ALIMENTARY CANAL 



The alimentary canal of arthropods is a tube extending from 

 one end of the body to the other. With some arthropods, as the 

 larvae of insects, it is a nearly straight tube; with others, as most 

 adult insects, it is greatly lengthened, being much longer than the 

 body and, consequently, more or less coiled within it. This 

 increase in the length of the alimentary canal gives an increased 

 surface for the performance of its functions. 



In spiders, and other arachnids an extended area of the 

 digestive tract is obtained in a different and peculiar manner; 

 instead of a lengthening of the intestine, there have been de- 

 veloped extensive diverticula, extending in different directions 

 from it (Fig. 143). 



Three chief regions of the alimentary canal are recognized; 

 namely, the fore-intestine, the mid-intestine, and the hind- 

 intestine. These regions differ in their histological structure, 

 a difference due to different embryological origin. The fore- 

 intestine and the hind-intestine are developed from an infolding 

 in each case of the ectoderm, the germ layer from which the body- 

 wall is formed. And these regions are lined with a chitinous 

 layer, the intima, which is a continuation of the cuticula of the 

 body-wall. On the other hand, the mid-intestine is developed 

 from the entoderm and is lined by an epithelium. 



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