AKTHROPODA. 



83 



same animal (Fig. 54, 1). The segments often fuse 

 together, which brings about the formation of a 

 smaller number of subdivisions to the body ; or even 

 all the segments may become united (mites). In the 

 last case the Arthropod characters are only to be seen 

 in the jointing of the limbs. Segmented animals 

 (e.g. the common earthworm) are also found among the 

 worms (Sub-kingdom III. of the Animal Kingdom) ; 



Fig. 54. — Wood-borer (^Sirex) ; 1, larva ; 2, adult insect. 



but these worms have either no limbs, or only small 

 unjointed foot-stumps, never jointed limbs as in the 

 Arthropods. These last are limbless or with unjointed 

 foot-stumps only when young; when adult they 

 always have: jointed limbs. The head of Arthropods 

 bears several successive pairs of jaws, which move to 

 and fro from one side towards the other. The cover- 

 ing of the body consists of hard protective pieces ; it is 

 oidy in the young condition that the skin of several 

 species is soft. Arthropods have no internal skeleton ; 

 the muscles are attached to the skin. The central 

 part of the nervous system (p. 10), which consists in 



