ARTICULATED ANIMALS. 



79 



stitutes a sort of external skeleton. In tlie lowest 

 forms the body is extremely elongated, the segments 

 proportionately soft and numerous, and, as a neces- 

 sary consequence, limbs either do not exist, or are 

 feeble and imperfect. Such is the srtucture met 

 with in the Annelida, or Worms, as lor example, in 

 the leech (Fig. 53). 



As we advance, we find the tegumen- 

 tary rings become less numerous, and 

 the skin of a denser and moi-e firm tex- 

 ture, adapted to sustain the action of 

 stronger and more powerful muscles; 

 the limbs likewise become more elabo- 

 rately formed, their movements more 

 free and energetic. Moreover, the in- 

 struments of sight and touch begin to 

 assume considerable perfection of struc- 

 ture. This state of develoi^ment we 

 find in the Myriapoda or Centipedes 

 (Fig. 55). 



In the Insects the perfection of the 



Fro. 5.3. 



Fig. 54. — DR'isioxs of a beetle. 



external skeleton is still more remarkable, and the 

 integument acquires a hardness and solidity pro- 



