LIFE PROCESSES IN ANIMALS 281 



as legs or wings, are present, the movement of which 

 makes locomotion possible. Sponges and corals, on the 

 other hand, are always attached and can not move from 

 place to place. 



Sensitiveness. Even the simplest one celled animals 

 respond to certain stimuli. If a point on one side of 

 the body is touched in some way, a message is con- 

 ducted to all parts of the cell and the animal moves. 

 As animals increase in complexity they become more 

 sensitive, and special organs are developed for special 

 senses. An earthworm, for example, is sensitive to 

 light and darkness. It has no eyes, but all of the skin 

 near the head end is sensitive to light. In higher 

 animals well developed eyes are present, which can not 

 only distinguish between light and darkness, but also 

 between colors. Animals also have special organs which 

 are sensitive to touch, others to sound, and still others to 

 odors. From these special structures the message is 

 not transferred from cell to cell and thus to all parts 

 of the body, but it travels along definite tracts (nerves) 

 which carry it to a central organ, in higher animals 

 the brain. From this central organ messages are sent 

 out to muscles which cause them to act and produce 

 motion. 



The brain and nerves not only make it possible for 

 animals to see. hear, and feel, but they also keep all 

 parts of the body working together harmoniously. 



Reproduction. The length of time which animals live 

 varies greatly. Many smaller forms, such as insects, live 



