SPIDERS. 



369 



the various vibrations on a web line. The senses of smell, 

 hearing, and taste are also present, but little is known in 

 regard to the organs. 



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. Farther 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 FIG. 193. Dissection of Mygak 

 said to be endodermal in 

 origin. 



A three-chambered heart, 

 containing colourless blood, 

 lies within a pericardium near 

 the dorsal surface of the 

 abdomen. It gives off an 

 anterior and a posterior aorta 

 and lateral vessels; and the 

 circulation corresponds in general to that of the scorpion. 



In a few forms (Tetrapneumones) respiration is effected 

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

 (Fig. 193). In the vast majority (Dipneumones) there are 

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



2, pedipalps cut short ; 

 !egs ; j".*j large thoracic 



from the ventral surface. After 

 Cuvier. 



Chelicerae ; 



3-6, walking legs"; , 

 ganglion ; . 2 , ganglion at base of 

 abdomen; c.t., chambered tracheae 

 or lung-books at the left side the 

 anterior is cut open to show the 

 lamellae (/.); #*> muscle of abdomen; 

 st. 1 and J/. 2 , stigmata of lung-books ; 

 ov., ovary ; sfl., spinnerets. 



