PHYLUM ARTHROPODA 



375 



are also usually present, but do not ramify to all parts of the body 

 as in the insects (p. 320, Fig. 242). 



The excretory organs are the Mai pig/nan tubules (Fig. 314, 4), 

 which open into the intestine, and two coxal glands in the cepha- 

 lothorax. The coxal glands are sometimes degenerate, and their 



FIG. 316. Orb web of a spider, Epeira. a, first spiral line; b, second 

 spiral line; c, line to nest. (From Davenport, after Emerton.) 



openings are difficult to find ; they are homologous with the green 

 glands of the crayfish (p. 284, Fig. 202, 40-42). 



The nervous system consists of a bilobed ganglion above the 

 oesophagus (Fig. 314, p), a subcesophageal ganglionic mass (10), 

 and the nerves which arise from them. There are sensory hairs 

 on the pedipalps and probably on the walking legs, but the prin- 

 cipal sense-organs are the eyes. There are usually eight eyes 

 (Fig. 313, B; Fig. 314, 21), and these differ in size and arrange- 



