INSECTS. 271 



4. By fusion of head and thorax , a cephalo thorax may 

 be produced. 



5. The eyes are either ocelli or compound eyes. 



6. The abdomen may bear simple swimming-feet or it 

 may lack distinct appendages. 



7. The Arthropoda are divided into Crustacea, Acerata, 

 and Insecta. 



8. The CRUSTACEA respire by gills. They have two pairs 

 of antennae, usually two-branched feet; the reproductive 

 ducts open near the middle of the body. 



9. The Crustacea are divided into Entomostraca and 

 Malacostraca. The extinct Trilobites were near relatives 

 of the Crustacea. 



10. The ACERATA lack antenna?, they have a cephalo- 

 thorax and abdomen, they respire by gills, lungs, or 

 tracheae. The reproductive ducts open near the middle 

 of the bodv. 



/ 



11. The IXSECTA have four pairs of appendages on the 

 head; they breathe by tracheae, and the reproductive 

 ducts open at the end of the body. 



12. The Insecta are divided into Chilopoda and Hex- 

 apoda. 



13. The Chilopoda have numerous body-segments, each 

 with a pair of legs, and with no distinction of thorax and 

 abdomen. 



14. Chilopoda and Diplopoda are frequently united as 

 a group of Myriapoda. 



15. The DIPLOPODA differ from Insecta in having a head 

 with three pairs of appendages, most of the body-segments 

 with two pairs of appendages, and the opening of the re- 

 productive ducts in front of the middle of the body. 



16. The Hcxapoda have the body divided into head, 

 thorax^ and abdomen. 



