CLASSIFICATION AND EVOLUTION 515 



2. Phylum Annelida. 



Bilaterally symmetrical, segmented Triplo- 

 blastica, with a closed blood-vascular 

 system, a well-developed coelom, a double 

 ventral nerve cord parting in front to 

 enclose the gut, and a thin cuticle. 



a. Class Oligoch.<eta. 



Annelida without parapodia, with setae, 

 e.g. Lumbricus. 



b. Class Polych^eta. 



Annelida with parapodia and numerous setae, 

 e.g. Nereis, Arenicola. 



c. Class Hirudinea. 



Annelida without parapodia or setse, with two 

 suckers, and with canalicular ccelom. 

 e.g. Hirudo. 

 \i\J^2>- Phylum Arthropoda. 



Bilaterally symmetrical, segmented Triplo- 

 blastica, with an open blood-vascular 

 system, a very restricted ccelom, a double 

 ventral nerve cord parting in front to 

 enclose the gut, a thick cuticle, and paired, 

 jointed limbs, some of which serve as jaws. 

 ^ a. Class Crustacea. 



Aquatic Arthropoda with two pairs of 

 antennae, and usually with gills. 

 ■^ e.g. Astacus, Cyclops, 



b. Class Hexapoda or Insecta. 



Land Arthropoda without gills, but with in- 

 ternal air tubes for breathing, with one 

 pair of antennae, three pairs of legs, and 

 usually two pairs of wings. 



The characteristic features of the following groups of 

 insects and examples of their members are given in 

 Chapter XVII. 



Orders : Orthoptera, Neuroptera, Coleop- 

 tera, Hymenoptera, Hemiptera, Diptera, 

 Aphaniptera, Lepidoptera. 



yjc. Class Mvriapoda. 



Land Arthropoda without gills but with in- 



