completely fused, so that only two body sections are apparent. The spiders, 

 which are so often found as unwelcome invaders of our homes, belong to this 

 group, and so do the mites, harvestmen, scorpions and their relatives. 



The Myriapoda, commonly called centipedes and millipedes or hundred' 

 legs and thousand-legs, make up another division of the arthropods and are 

 common in damp, dark places on land. 



The remaining division of the Arthropoda, the Insecta, has as almost its 

 only common character the possession of six walking legs in the adult. It 

 comprises such a gathering of almost innumerable kinds and forms that the 

 study of them. Entomology, has attained an almost independent status, cor' 

 responding somewhat to the position that Bacteriology holds in Botany. Studies 

 of the methods of controlling insects are now occupying much attention, for 

 it is estimated that insects take a toll of at least one-tenth of all food crops, 

 and occasionally heavy infestations of insects destroy entire crops over large 

 areas. In addition to the loss of food there are many insect-carried or insect- 

 spread diseases, such as malaria and typhoid. 



ANNOTATED LIST OF THE CLASSES OF ARTHROPODA 



Class CRUSTACEA 



Gill-breathing; usually with two pairs of antennae 

 Class MYRIAPODA 



With tracheae, one pair of antennae, many similar legs 

 Class ARACHNOIDEA 



No antennae ; with two main divisions, cephalothorax and abdomen 

 Subclass ARACHNIDA 



With tracheae, book lungs or book gills 

 Class INSECTA (or Hexapoda) 



With tracheae, one pair of antennae, three pairs of legs 



ANNOTATED LIST OF THE SUBCLASSES AND ORDERS OF 



CRUSTACEA 



Subclass ENTOMOSTRACA 



Without abdominal appendages; small, often minute 

 Order BRANCHIOPODA 



Thoracic appendages flat and leaf-like 

 Suborder PHYLLOPODA 



Body distinctly segmented; with nine to forty pairs of ap- 

 pendages 

 Suborder CLADOCERA 



No distinct segmentation; usually with a bivalve carapace, which 

 may or may not completely cover the body; seldom more than 

 six pairs of appendages 



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