CLASS ARACHNIDA, AND THE ORDER ACARINA 265 



an elongate ii -segmented abdomen, ending in a long, seg- 

 mented, filiform " tail." 



Order 7 : Solifugm, p. 300. 



Order 8 : Pseudoscorpionidea ; Book-scorpions. Small 

 Arachnida resembling small scorpions without a "tail." 

 They live under bark, stones, etc., and are sometimes found 

 in houses. They are predaceous on small insects. 



Order 9 : Phalangidea, or Opiliones. Arachnida that re- 

 semble and are commonly mistaken for very long-legged 

 spiders ; but differing from spiders in having the abdomen 

 (which is segmented) broadly united to the cephalothorax, 

 and the chelicer^e chelate. 



Order 10 : Acarina ; Mites and Ticks. 



Order 11 : Tardigrada ; Bear-animalcules. Minute 

 aberrant Arachnida that, when they are noticed at all, are 

 mistaken for mites. The only appendages are four pairs of 

 stumpy, unsegmented legs. They are found among herbage, 

 and under flower-pots, etc., in gardens. 



From the medical standpoint the only Order of outstand- 

 ing interest is the Acarina, which will therefore be taken first. 



Order ACARINA : Mites and Ticks. 

 (Gr. aKapi = a mite or tick.) 



The members of this large Order are for the most part 

 minute ; many of them are parasitic on animals and plants ; 

 many are predaceous on their own kind. Except in one small 

 family — the OpiliocaridcB — which is doubtfully included in the 

 Order, the body is not visibly segmented, but is commonly 

 a roundish or oval mass, sometimes without any distinction 

 even of regions, but sometimes showing a more or less perfect 

 division of cephalothorax and abdomen. In one group the 

 body is elongate and almost worm-like. The anterior end of 

 the body usually forms with the chelicerse a sort of beak or 

 rostrum. 



The appendages exhibit a good deal of diversity, and 

 in one family (gall-mites) the legs are reduced to two 

 pairs. The chelicerce are sometimes needle-like, more often 

 chelate or pincer-like, and sometimes grapnel-like ; they are 

 usually used for piercing ; and when they are pincer-like 



