THE STRUCTURE OF ANIMALS. 155 



CHAPTER XXXIII. 



THE STRUCTURE OF ANIMALS. 



PLANTS AND ANIMALS. What is the difference between a 

 plant and an animal ? It is sometimes very difficult to deter- 

 mine whether some of the lower forms of living matter are 

 plants or animals. In the higher forms the distinction is easily 

 made by us. What is the difference ? A horse requires air, 

 water, and food. So does a tree. The horse takes in oxygen 

 from the air and breathes out carbonic acid gas ; the tree takes 

 in carbonic acid gas and gives out oxygen. The horse can 

 move about at will to seek food ; the tree remains fixed, and 

 the food comes or is brought to it. The horse feeds upon 

 plants organized food; the plant feeds upon crude material, 

 such as mineral compounds unorganized food. The plant, 

 therefore, is built up from the simple substances in the air, soil, 

 and water. The animal builds itself up largely by feeding 

 upon the material formed by the plants. This distinction, 

 however, does not hold in the case of all plants and all animals. 

 Can you state any exceptions to the above ? 



BONES. These are the framework or foundation. They 

 largely make the general form of the animal. The bones of a 

 young animal are pliable, but they become more rigid as the 

 animal grows older. They are the support of the animal, but 

 to enable the animal to move, they are in parts joined together. 

 Notice how every bone is suited to its place. The skull covers 

 the top and back of the head, thereby protecting the brain. 

 The ribs, front and back, protect the heart and lungs. Why 

 are they not closely joined together, as the bones of the skull ? 

 The bones arj suited in size and length to the uses required 



