2 INTRODUCTION 



only important thing to consider in studying its life. 

 The life of each plant or animal may be studied under 

 eight headings, known as life processes. These are sensa- 

 tion or irritability, locomotion, food getting, digestion, assimi- 

 lation, respiration, excretion, and reproduction. 



1. Sensation (irritability') is that life process by means 

 of which an organism comes to know of things outside of 

 itself. Through sensation (irritability) it becomes aware 

 of its food. By the help of the senses the higher animals 

 are able to see and hear one another, are conscious of heat, 

 cold, light, sound, and many other things, all of which 

 are called stimuli. 



2. Locomotion is the life process by which animals move, 

 and is closely related to sensation. It is the means by 

 which animals secure food. In the higher animals stimuli 

 are sent through the nervous system to the various muscles, 

 which contract and so cause the animal to move. 



3. Food getting needs no definition. Man gets his food 

 from many sources. He eats animals, minerals, and vege- 

 tables. Lower animals live by hunting or grazing, and 

 plants get their food through their leaves and roots. 



4. Digestion is the life process which prepares the food 

 to pass to all parts of the body. It takes place in all 

 animals and plants, but we are most familar with it in 

 man. Man chews his food in the mouth, thus softening 

 it and mixing it with saliva ready for the stomach. Di- 

 gestion is continued in the stomach and completed in the 

 intestine. 



As soon as the food is digested, some of it passes through 

 a thin membrane in the wall of the intestine into the 

 blood vessels and thus is ready to furnish energy in the 

 body. This passage of the dissolved food through a mem- 

 brane is called osmosis (os-mo'sis). 



5. Assimilation is the building of the digested food 



