as the basis of "reference". Cats, dogs, horses, cows, and other famiHar 

 mammals (animals that suckle their young) do resemble the human body 

 in many ways. They all have a two-sided symmetry, the right and left sides 

 being almost mirror-images of one another (see illustration, p. 13). They 

 all have the same body "plan" (see illustration, p. 13). 



On the head are three pairs of special structures — the eyes, the ears, and 

 the nostrils — which seem to relate the animal to the outside world. The 

 mouth or food opening is in the middle line, below the nostrils. At the 

 posterior or hind end of the trunk are special openings that are related to 

 removing wastes from the body, and to reproduction. 



The skin of mammals usually has a more or less complete hairy cover. 

 Although the limbs of common mammals are jointed or hinged, the body 

 covering shows no distinct breaks over die joints. The forward part of the 

 trunk, the thorax or chest, has a firm wall made up of curved bones, the 

 ribs. The hind part of the trunk, the abdomen, has no such enclosing 

 framework (see illustration, p. 48). 



An Insect In the grasshopper, a representative insect, the general plan 

 of structure is that of a main body with distinct regions and several kinds 

 of outgrowths or attachments (see illustration below). 



The head bears two feelers, or antennae (singular, a^itenna), projecting 

 forward. The eyes occupy a large part of the surface of the head. Since 

 each of these consists of numerous complete eyes, it is called a compound 

 eye (see illustration, p. 15). In addition, there are three tiny simple eyes 



THE BODY PLAN OF AN INSECT 



In the grasshopper, as in other insects, the bilateral body is made up of a rather dis- 

 tinct head at the front end; the main "trunk", or abdomen; and, between these, 

 the thorax, which bears both the legs and the wings. The grasshopper has a rather 

 large eardrum near the front end of the abdomen 



14 



