ARTHROPODA. 
289 
long, tail-like abdomen and the abnormal jaws place them 
in a lower rank. The abdomen consists of twelve seg¬ 
ments : the anterior half is as large as the thorax, with no 
well-marked division between; the other part is compara¬ 
tively slender, and ends in a hooked sting, which is perfo¬ 
rated by a tube leading to a poison-sac. The mandibles 
are transformed into small, nipping claws, and the eyes 
generally number six. Respiration is carried on by four 
pairs of pulmonary sacs which open on the under surface 
Fig. 259—Scorpion (under surface) and Centipede. 
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. 
The Harvest-men ( Phalangium ), frequently seen about 
our houses, belong to this order. They have a short, thick 
body and extremely long legs, and breathe by tracheae. 
3. Araneina, or Spiders. They are distinguished by 
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