PHYLUM ARTHROPODA 



615 



by a constriction into two parts, head and thorax, the latter made 



up of three segments. The chelicerse are chelate ; the pedipalpi 



resemble the legs, and are used in 



locomotion. The first pair of legs 



are attached to the head. The 



abdomen is distinctly segmented, 



and there is no caudal appendage. 



A pair of poison-glands open at 



the bases of the chelicerae. There 



are two simple eyes on the head. 

 In the true Spiders (Fig. 505) 



the abdomen is rounded, unseg- 



mented, and separated off from the 



cephalothorax by a constriction. 



~~he chelicerse (Fig. 506) are sub- 

 elate, and the duct of a large 



poison gland opens at the ex- 

 tremity. The pedipalpi (Fig. 506, B) are elongated, and end in 



simple extremities; in the male (Fig. 507) the terminal joint 



ce 



2 



FIG. 505. Spider Epeira diadexna). 



hb 



FIG. 506. A, Chelicerae, and B, pedipalpi of female of Epeira diadexna. (After Leuckart.) 



Is modified to serve for the reception and transference of the 



sperms. At the extremity of the abdomen is the spinning 



apparatus or arachnidium (Fig. 513, 

 arach.). This consists of four or six 

 elevations, the spinnerets, sometimes 

 jointed, probably derived from em- 

 bryonic rudiments of abdominal ap- 

 pendages. On the surfaces of these 

 open the numerous fine ducts of the 

 spinning glands (sp. glds.), secreting 

 the material of which the spider's 

 web is composed. The fine threads 

 of viscid secretion issuing from the 

 ducts harden on exposure to the air, 



and are worked up into the web by means of the posterior legs. 



There are six or eight eyes on the carapace. 



In the spider-like Phalangida, or " Harvest-men," the cephalo- 



PIG. 507. Pedipalpi of male of 

 Epeira diadema. II. 111. 

 IV. V. podomeres ; bb, sac ; sph. 

 spiral tube. (After Leuckart.) 



