The Internal Anatomy of Spiders 



ectoderm at the posterior end of the embryo, the proctodeum. 

 It, consequently, like the fore-intestine, resembles the body-wall 

 in its histological structure, there being a chitinous intima, an 

 epithelial layer and a basement membrane. It differs from the fore- 

 intestine in possessing a more or less well-developed muscular coat. 



The stercoral pocket. — The most striking feature of the hind- 

 intestine is the presence of a large, bladder-like diverticulum of 

 its dorsal wall, which is situated near the place of union of the 

 mid-intestine and hind-intestine (Fig. 143). It serves as a res- 

 ervoir of fecal matter and is termed, therefore, the stercoral pocket 

 (ster'co-ral). 



The Malpighian vessels. — There is a pair of tubes which 

 open into the alimentary canal near or at the place of union of 

 the mid-intestine and hind-intestine; these are the Malpighian 

 vessels (Mal-pe'ghi-an or Mal-pig'hi-an). Each of these tubes 

 is branched and the branches ramify among the alimentary 

 tubules. They are supposed to correspond to the kidneys in 

 function. 



Some writers maintain that Malpighian vessels open into 

 the mid-intestine. If this should prove to be true, they cannot 

 be homologous with the Malpighian vessels of insects, which are 

 developed from the proctodeum. 



The rectum. — The rectum is a simple tube extending from 

 the stercoral pocket to the anus. 



THE ADIPOSE TISSUE 



In spiders the greater part of the adipose tissue or fat-body 

 is intimately associated with the mid-intestine and its diverti- 

 cula. In the case of the simple caeca of the mid-intestine, situated 

 in the cephalothorax, the fat cells form a continuous investing 

 sheath, which has been termed, incorrectly, the peritoneal coat. 

 In the abdomen, the fat-body completely fills the spaces between 

 the alimentary tubules, and surrounds them as a whole. 



In many spiders the fat-body presents a chalky appearance 

 due to the presence in it of fine particles, which are believed to 

 be crystal-like remains of the food. These "fecal crystals" 

 are sometimes very abundant in the dorsal superficial portion 

 of the mass; and, showing through unpigmented portions of the 

 cuticula, produce conspicuous white markings. 



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