272 ELEMENTARY SCIENCE 



family. The marten family includes our small fur-bearers: 

 otter, mink, weasel, wolverene, skunk, badger. You should 

 learn to know all these animals by studying them in a 

 museum or zoological garden if you have a chance. 



The third order of mammals you should remember are 

 the Rodents, or gnawers. Here belong rats and mice, 

 and also squirrels, rabbits, and gophers. The fourth im- 

 portant order is composed of the hoofed animals Un- 

 gulates. Cattle, deer, sheep, swine, and horses are the most 

 important members of this great order. 



Among mammals belonging to the smaller orders are 

 seals, moles, bats, whales, and opossums. Now as you 

 stop to think of all these forms you realize what a great 

 and successful class the mammals are, and how they have 

 spread all over the world. 



Think then of the fishes in the water, the birds in the 

 air, amphibians on both land and water, and you realize 

 that these animals with skeletons have come to occupy 

 the earth as far as large animals can go. All other great 

 groups of animals are small in stature as compared with the 

 vertebrates. 



Now, having in mind all this wonderful population of 

 vertebrates, let us think how they are built to do their work 

 in the world. They must all move actively about in search 

 of their food. Their skeletons form a solid framework to 

 which muscles are attached, and the movements are all ac- 

 complished by muscular contractions acting on the bones. 



But what is it that moves the bones? And what acts 

 on the food and oxygen that enter the body? How are 

 they changed so that they can be used ? Perhaps you can 

 answer these questions as to your own body, but do all 

 these other vertebrates have lungs, hearts, nerves, blood- 



