CHAPTER XXIII 



The Class Arachuida, and the Order Acariua 



The Arachnida (apdxvri9 = a spider) are a much more diversi- 

 fied group than the Insects, and from the strictly medical 

 standpoint are considerably less important. They differ 

 from all other Arthropoda in not having antennae. 



Some Arachnida live in the sea or in fresh water, and 

 some of these aquatic forms breathe by gills ; but the majority 

 of the Class are terrestrial and breathe air direct, sometimes 

 by tracheae of a sort, sometimes by the general surface of the 

 body, and in the case of many of the larger forms by 

 structures known as " lung-books " — structures resembling 

 " dry " gills, which are enclosed in pockets that open on the 

 ventral surface of the abdomen. 



In some Arachnida, for instance the Mites and Ticks, the 

 segmentation of the body is obliterated ; but in the typical 

 members of the Class the segments, or many of them, are 

 distinct, and are grouped in two regions, namely, a cephalo- 

 thorax that carries the true appendages, and an abdomen 

 that either has no appendages at all, or has appendages that 

 have been modified out of ordinary recognition. 



The true appendages are never more than six pairs. The 

 first pair, or chelicera, lie in front of the mouth, and the second 

 pair, or pedipalps, are placed one on either side of the mouth. 

 Both the chelicera; and the pedipalps show much diversity of 

 form. The remaining appendages — usually four pairs, but in 

 the gall-mites only two pairs — are legs for locomotion. 



Eyes when present are always simple, and vary in number 

 from two to twelve. 



The mouth is a chink, since most Arachnida live on the 

 juices of animals or plants. In some forms the bases of some 



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