HIGH SCHOOL ZOOLOGY. 



203 



are the most important. Both have the two pairs of mouth- 

 appendages and the four pairs of walking legs, but the form of 

 the body is very different in the two groups, on account of 

 the separation of the abdomen in the spiders pi-oper, by a 

 slender stalk, and the presence at its extremity of the spin- 

 nerets (Fig. 137). 



Some of the chief structural peculiarities of the spiders 

 may be gathered from Fig. 138. The two-jointed cheliceriB 

 terminate in a powerful claw, perforated by the duct of a 



I 



Fig. 13S. — ^Diayramiiiatic section of a'spider — Epcifu. (After Etuerton). 



a, h, upper and lower lips ; c, a3soi)hagus ; d, /, upper and lower muscles of the suck- 

 in;^ stomach ; e, stomach ; (j, lif^auieiits attached to diajjhrag'ni under the stomach ; 

 k, upper, j, lower, lu'rve-jranijlion ; I, nerve to leys and palpi ; m, in, branches cf stom- 

 acli ; n, poison-sland ; o, intestine ; i), licurt; r, luny ; s, ovary; t, trachea ; u, spinning 

 ^dLinds. 



poison-gland. Between the bases of the pedipalpi is the mouth, 

 wliich leads by an oesophagus into a sucking stomach, dilatable by 

 muscles, and provided with lateral cceca. The abdominal part of 

 the intestine is provided with a liver, and with Malphigian tubes 

 (slender cceca arising ^from the hinder end of the intestine in 

 air-breathing Arthropods, and discharging the function of 

 kidneys). The heart is elongated like that of the scorpion and 

 of the lower Crustacea, but the nervous cord is concentrated 

 into the thorax. Above the oesophagus is the brain, which 

 sends nerves to the simple (not facetted) eyes, the arrangement 

 of which on the head is of great use to systematists. The 

 numerous lungs of the scorpion are only represented here by 

 two air-sacs (four iu tlie J^rap-door spider.s), while, in addition, a 



