SPIDERS. 



353 



Body cavity, endosternite, and coxal glands generally 

 resemble those of scorpions. 



The spider usually sucks the blood and juices of its prey, 

 and behind the gullet lies a powerfully suctorial region, 

 strengthened by chitinous 

 plates, and worked by muscles. 

 From the small mid-gut arise 

 five pairs of long caeca, a pair 

 running forwards and a pair 

 passing into the bases of each 

 pair of legs, and then back 

 again. These caeca sometimes 

 anastomose. Further back the 

 mid -gut gives off numerous 

 digestive outgrowths, which fill 

 a large part of the abdomen. 

 Their secretion digests pro- 

 teids. Terminally there is a 

 large cloaca, and where the 

 intestine joins this, four much- 

 branched excretory Malpighian 

 tubes are given off, which are 

 said to be endodermal in 

 origin. 



A three - chambered heart, 

 containing colourless blood, 

 lies within a pericardium near 

 the dorsal surface of the abdo- 

 men. It gives off an anterior 

 and a posterior aorta and 

 lateral vessels ; and the cir- 

 culation corresponds in general 

 to that of the scorpion. 



In a few forms (Tetrapneu- 

 mones) respiration is effected 



by four " lung-books," e.g. in the large bird-catching Mygafe 

 (Fig. 172). In the vast majority (Dipneumones) there are two 

 lung-books, and tubular tracheae in addition. The stigmata 

 of the lung-books lie on the anterior ventral surface of the 

 abdomen ; the tracheae open posteriorly near the spinnerets, or 

 just behind the opening of the lung-books, or at both places. 



23 



FIG. 172. Dissection of Mygale 

 from the ventral surface. After 

 Cuvier. 



i, Chelicerae ; 2, pedipalps cut short '> 

 3-6, walking legs ; g*-, large thoracic 

 ganglion ; g%, ganglion at base of 

 abdomen ; c.t., chambered trachea; 

 or lung-books at the left side the 

 anterior is cut open to show the 

 lamellae (/.) ; ///., muscle of abdomen ; 

 sfl. and si-., stigmata of lung-books ; 

 <?<'., ovary ; sfi., spinnerets. 



