STRUCTURE OF SPIDERS 



419 



The nervous system is of the usual Arthropod type, but 

 shows much centraUsation. Thus the ventral ganglia are 

 fused into one large centre in the cephalothorax (see Fig. 

 232), a condition comparable to that in crabs. There 

 are two or three rows of simple 

 eyes on the cephalothorax, 

 whose focal distance is very 

 short, spiders trusting most 

 to their exquisite sense of 

 touch, by which they dis- 

 criminate the various vibra- 

 tions on a web line. The 

 senses of sm.ell, 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 p^^, 232.— Dissection of My-a/^ 

 five pairs of long caeca, a pair from the ventral surface. — 

 running forwards and a pair After Cuvier. 



passing into the bases of each i, Chelicerae ; 2, pedlpalps cut short ; 



TM\\r nf lp(J<; and then hark 3-6, walking legs ; g.i, large thoracic 

 pair 01 .egS, ana tnen OaCK ganglion; g.2, ganglion at base of 



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 proteins. 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 



abdomen ; c.t., chambered tracheae 

 or lung-books — at the left side the 

 anterior is cut open to show the 

 lame41ae (/.) ; m., muscle of abdomen ; 

 si.i and st:^, stigmata of lung-books ; 

 ov., ovary ; sp., spinnerets. 



