ARTHROPODA. 



233 



CLASS III. INSECTA. 



In the Insects there is a distinct head con- 

 sisting of four segments; respiration is effected 

 by tracheae opening along the sides of the body, 

 while the reproductive organs open near the tip 

 of the abdomen. 



SUBCLASS I. HEXAPODA (p. 213). 

 SUBCLASS II. CHILOPODA (Centipedes). 



In the Chilopods, which include the centi- 

 pedes and similar forms, the head is succeeded 

 by a long series of body-segments, each with a 

 pair of locomotor appendages (legs), and with 

 no distinction between thorax and abdomen. 

 Most of the group are carnivorous, and the 

 larger forms, at least, are provided with poison- 

 glands which open in the last pair of cephalic 

 appendages. The chilopods of northern lati- 

 tudes are small, insect-feeding forms, but in the 

 tropics occur the centipedes, the larger species FIG! no. A 

 of which are said to be extremely venomous. 



Usually the Chilopods are associated with another group, 

 the DIPLOPODA (thousand-footed worms), as a class or sub- 

 class, Myriapoda, but the differences between them are too 

 great for this. The Diplopods have but three segments in 

 the head, and, after the first three, each segment of the 

 body bears two pairs of legs, while the reproductive organs 

 open far forward. The thousand-legged worms live in 



