ARTHROPODA 



121 



ried on by four pairs of pulmonary sacs which open on 

 the under surface of the abdomen. The heart is a 

 strong artery, extending along the middle of the back, 

 and divided into eight separate chambers. Scorpions 

 are confined to the warm-temperate and tropical regions, 

 usually lurking in dark, damp places. 



FIG. 80. Scorpion (under surface) and Centipede. 



2. Phalangida, the harvest men, or " granddaddy 

 longlegs " (P/ialangium), frequently seen about our 

 houses, belong to this order. They have a short, thick, 

 unsegmented body, extremely long legs, and no spinning 

 glands. 



3. Araneida, or spiders. They are distinguished by 

 their soft, unjointed abdomen, connected to the thorax 

 by a narrow neck, and provided at the posterior end 

 with two or three pairs of appendages, called "spinner- 

 ets," which are homologous with legs. The office of 

 the spinnerets is to reel out the silk from the silk glands, 



