278 COMPARATIVE ZOOLOGY. 
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 antenne are 
transformed into small, nipping claws, and the eyes gen- 
erally number six. Respiration is carried 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 sepa- 
rate chambers. Scorpions are confined to the warm-tem- 
perate and tropical regions, usually lurking in dark, damp 
places. 
The Harvest-men (Phalangiwm), frequently seen about 
our houses, belong to this order. They have a short, thick 
body and extremely long legs, and breathe by tracheze. 
3. Araneina, or Spiders. They are distinguished by 
