THE MYRIAPODA OR CENTIPEDES AND MILLIPEDES 129 



tropical countries may reach a foot in length, and their bite is 

 painful and even dangerous to man. 



FIG. 73. A group of animals that live under bark. At the right a centipede. 

 At the left a pill bug, a sow bug, and a snail. (After Davenport.) 



Characteristics and Classification. The chief distinguishing 

 characteristics of the Myriapoda are : (i) a distinct head with 

 one pair of tentacles and one pair of mandibles, (2) numerous 

 body segments bearing similar leglike appendages, (3) tracheae, 

 (4) excretory organs (malpighian tubules) opening into the in- 

 testine. 



The two principal orders are as follows: 



Order i. DIPLOPODA. Millipedes. 



Order a. CHILOPODA. Centipedes. 



