374 COLLEGE ZOOLOGY 



Internal Anatomy and Physiology (Fig. 314). - -The food of 

 the spider consists of juices sucked from the bodies of other ani- 

 mals, principally insects. Suction is produced by the enlarge- 

 ment of the sucking stomach (Fig. 314, 2], due to the contraction 

 of muscles attached to its dorsal surface and to the chitinous 

 covering of the cephalothorax (7). The true stomach, which 

 follows the sucking stomach, gives off five pairs of caca or blind 

 tubes (8) in the cephalothorax. The intestine passes almost 

 straight through the abdomen; it is enlarged at a point (j) where 

 ducts bring into it a digestive fluid from the " liver," and again 

 near the posterior end, where it forms a sac, the " stercoral pocket " 

 (5). Tubes, called Malpighian tubes (4), enter the intestine near 

 the posterior end. The alimentary canal is surrounded in the 

 abdomen by a large digestive gland or " liver." This gland se- 

 cretes a fluid resembling pancreatic juice and pours it into the 

 intestine through ducts (3). 



The circulatory system consists of a heart, arteries, veins, and 

 a number of spaces or sinuses. The heart (Fig. 314, u) is situ- 

 ated in the abdomen and is surrounded by the digestive glands. 

 It is a muscular, contractile tube lying in a sheath, the peri- 

 cardium (22), into which it opens by three pairs of ostia. It 

 gives off posteriorly a caudal artery, anteriorly an aorta which 

 branches and supplies the tissues in the cephalothorax, and three 

 pairs of abdominal arteries (24). The blood, which is colorless 

 and contains mostly ameboid corpuscles, passes from the ar- 

 teries into sinuses and is carried to the book lungs (12) where it 

 is aerated; it then passes to the pericardium by way of the 

 pulmonary veins (25), and finally enters the heart through the 

 ostia. 



Respiration is carried on by tracheae and book lungs ; the latter 

 are peculiar to arachnids. The book lungs (Fig. 314, 12], of 

 which there are usually two, are sacs, each containing generally 

 from fifteen to twenty leaf-like horizontal shelves through which 

 the blood circulates. Air entering through the external openings 

 is thus brought into close relationship with the blood. Tracheae 



