ARTHBOPODA. 289 



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 

 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- 



Fia. 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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