128 CRUSTACEANS. 



combined into classes. The centiped, spider, and all 

 the orders of six-footed insects already studied, belong to 

 the class Insecta. Make a list of the characters which all 

 these have in common. This will approximately charac- 

 terize the class. 



The crawfish, asellus, and cyclops represent separate 

 orders, which are included in another class, the class Crus- 

 tacea. Make a list of the characters these have in common. 



Classes are in like manner combined to form branches. 

 Thus the two classes, Insects and Crustaceans, constitute 

 the branch Arthropoda (or jointed-footed animals). 



Branches are the primary divisions of the animal 

 kingdom. 



All animals having the body made up of a definite 

 number of rings or segments with the hard parts outer- 

 most, and with jointed appendages, comprise the branch 

 Arthropoda. 



All arthropods that breathe by trachese (or tracheal 

 gills) comprise the class Insecta. 



All insects with scaly wings and complete metamor- 

 phosis comprise the order Lepidoptera, etc. 



It often happens that the constituent members of one 

 of these groups show different degrees of relationship 

 toward each other. For example, in the class Insecta the 

 orders of six-footed insects show much closer relationship 

 to each other than they show toward the orders to which 

 the spider and the centiped belong. In a case like this 

 it is convenient to recognize intermediate groups. Accord- 

 ingly all the orders of six-footed insects are combined into 

 a subclass, Hexapoda; the spiders are placed in another 

 subclass, Arachnida; and the centiped and thousand-legs, 

 etc., are placed in still another, the subclass Myriapoda 

 (or many-footed insects). So intermediate groups, when 

 found in nature, may be interpolated between any of the 

 other groups. 



