CLASSIFICATION OF ARTICDLATA. 81 



599. The division of this Sub-kingdom into Classes, is princi- 

 pally founded on the organs of locomotion ; which, as we have 

 seen, are so characteristic of it. We may first form two groups, 

 distinguished by the presence, or absence, of distinctly-articulated 

 members. In the former, the locomotive power is for the most 

 part consigned to these appendages ; and the trunk is usually 

 encased in a hard envelope, in which, by the union of segments 

 that were originally distinct, we sometimes almost lose the traces 

 of the characteristic division. This is especially the case with 

 the Crab and its allies ; in which the different parts of the body 

 are quite immoveable upon each other. On the other hand, in 

 the lower group, in which distinct members are wanting, the 

 locomotion of the animal is chiefly effected by the movement of 

 the body itself ; and this is permitted to the utmost 

 extent, by the softness of the integument, in which 

 the intervals of the articulations are scarcely 

 distinct from the rings themselves, as in the 

 Leech or Earth-worm, so that here, too, the 

 division into segments becomes indistinct, from 

 the opposite cause. It is in the Centipede (Fig. 

 292) that we have the best example of the divi- 

 sion of the body into segments, which is charac- 

 teristic of the entire group ; together with an 

 articulated structure in the limbs. In the classes 

 of Insects, Crustacea, Spiders, &c., the equality 

 of the segments disappears ; whilst in the Leech 

 and Worm tribes, the members disappear. 



600. The higher division of the Articulated 

 series may be arranged into the following 

 classes : 



I. INSECTS ; characterised by the division of the 

 body into three distinct portions, the head, 

 thorax, and abdomen ; by the possession of 

 FIG. 292. antenna? on the head, of three pairs of legs, and 



CENTIPEDE. * . 



(in general) of one or two pairs of wings ; and 

 by their aerial respiration. 



II. ARACHNID A, including the Spiders, Scorpions > and Mites; 



