ARTHROPODS 173 



2. Accra ta, the spiders 



3. Insecta, the insects. 



4. Myriapods, worm-like forms. 



The members of each of these classes have all the characteristics 

 of the arthropods, but there are additional points of resemblance 

 within each class. For instance, all the Crustacea have the head 

 and thorax united into a cephalothorax (head-thorax) which is 

 covered by a part of the external skeleton 

 called the carapace. Usually they have 

 five pairs of legs and breathe by gills 

 attached to them. 



The acerata (spiders) have no carapace, 

 have four pairs of legs and breathe by air 

 sacs or tracheae. The insects' head and 

 thorax are separate; they have three pairs 

 of legs and usually wings as well, and 

 breathe by means of tracheae. (For 

 further comparison see tabulation.) 



Classification. Each of these three 

 classes is further divided into groups called 

 orders, the orders into families, and the 

 families into still smaller groups called 

 genera (singular: genus) and genera into 

 species (singular: species also). 



As we come down in the classification, 

 the groups have more and more points of 

 resemblance, but of course include fewer 

 individuals. Take, for instance, the com- 

 mon grasshopper; it belongs to the FIG. 55. A centipede, 



Branch of the animal kingdom called Scol P end S P- (J; rom 



Specimen.) From Kellogg, 

 arthropods 



Class, insecta 

 Order, orthoptera 

 Family, acrididae 

 Genus, melanoplus 

 Species, femur-rubrum. 



