ARTHROPODA. 
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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 tp 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 
their soft, unjointed abdomen, separated from the thorax 
by a narrow constriction, and provided at the posterior 
end with two or three pairs of appendages, called “ spin- 
Fig. 260.—A, female Spider; B, male of same species; C, arrangement of the eyes. 
nerets,” which are homologous with legs. The office of 
the spinnerets is to reel out the silk from the silk-glands, 
the tip being perforated by a myriad of little tubes, 
through which the silk escapes in excessively fine threads. 
An ordinary thread, just visible to the naked eye, is the 
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