SECTION X. PHYLUM ARTHROPODA 



IF we examine and compare, even quite superficially, a 

 crayfish, a scorpion, a centipede, and a blue-bottle fly, we 

 see at once that, while they manifestly do not belong to any 

 of the groups of animals studied hitherto, they are all con- 

 nected together by certain broad common features. They 

 all have a hard, or at least tough, integument ; they all have 

 the body more or less clearly divided into segments, and 

 they all have a system of appendages, feelers, jaws, legs, etc., 

 adapted to different uses in the different animals mentioned, 

 and in different parts of the body of the same animal, but 

 agreeing in being covered with a hard or tough integument 

 like that of the body itself, and in being divided into seg- 

 ments by a number of joints. These features, together with 

 certain points in the arrangement and structure of the 

 internal parts, are characteristic of the members of the phy- 

 lum Arthropoda, a group of very great extent, comprising, 

 among others, four large classes, each exemplified by one of 

 the four familiar animals above referred to. 



Of these the crayfish differs from the rest in being an 

 aquatic animal and in having organs of respiration, gills, or 

 branchiae adapted to this mode of life. The remaining 

 three are, with a few exceptions, air-breathers. The cray- 

 fish is a representative of the class Crustacea of the phylum 

 Arthropoda ; the scorpion of the class Arachnida, the cen- 



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