196 HOW BODY CONTROL IS BROUGHT ABOUT 



This method of escape from the unfavorable environment is 

 called the method of trial and error. It is an example of the way 

 in which some of the lower organisms react to the unfavorable con- 

 ditions of their environment. If by such methods they do not 

 escape from harmful conditions, they perish. 



Different intensities of hght, different kinds of light, the passage 

 of a current of electricity through the water, different chemical 

 substances placed in the water, as well as many other factors, cause 

 very definite responses on the part of these one-celled organisms. 

 The responses in general save the organism from harm, or help it, 

 and thus may be said to be adaptive responses. 



Sense Organs and what they do. — Most plants do not react 

 quickly to stimuli, because they have no special sense organs. 

 Nor has the amoeba any special part of the cell fitted to receive stim- 

 uli. But in animals composed of numerous cells, division of labor 



soon appears, and we have organs fitted to 

 receive light stimuli (eyes), touch stimuli 

 (tactile hairs, etc.), and sound stimuli 

 (sensory hairs, tympana of insects, and 

 the ears of higher animals). These end 

 organs or structures at the outside of the 

 Tactile hairs, claw of lob- animal, when put by nerves in communi- 

 cation with organs of movement, like 

 muscles, bring about reactions to stimuli which result in obtaining 

 food, in escaping from enemies, and in many other important 

 functions. 



Some Examples of Sense Organs. — One of the simplest sense 

 organs is a sensory hair which contains nerve cells. These cells 

 have become modified, so that when they are stimulated they send 

 a message inward to another kind of nerve cell in the central part 

 of the body. This cell in turn sends a message which stimulates a 

 muscle to work, and the animaFs body is moved either away from 

 or toward the source of the stimulus. This type of response is 

 known as a simple reflex. 



There are many kinds of sensory structures in the lower animals. 

 The antennae of insects are for feeling and for receiving odors and, 

 in some insects, sound waves. A few insects like the locust have 

 tympana, or ears. Some animals use their ears for balancing and 



