SENSE ORGANS 



197 



some for both balancing and hearing. In the shrimp the " ear " 

 consists of a tiny pit, the wall of which is lined with sensory hairs. 

 In this pit are small grains of sand or other substances, which move 

 about as the animal changes its position, and thus assist in making 

 the animal aware of its position in space. A German named 

 Kreidl (kri'd'l) showed in an experiment that shrimps, after molt- 

 ing, place small grains of sand in their stat'ocysts or balancing pits. 

 He kept the shrimps in an aquarium containing small particles of 

 iron which the shrimps took in place of sand. Using a magnet, 

 Kreidl then found that its 



pull affected the shrimps '^''^J'^'"' 



as did the force of gravity 

 when sand grains were in 

 the statocysts. This 

 showed that the stato- 

 cysts are balancing or- 

 gans. 



Light-receiving devices 

 are of various kinds, from 

 the eyespot in euglena 

 or small groups of sensory 

 cells to the complicated compound eye of insects and the camera- 

 like structure of man. 



The Sense Organs of Man. — We have seen that simpler forms 

 of life perform certain acts because outside forces acting upon 

 them cause them to react to che stimulus from without. All 

 many-celled animals, including man, are put in touch with their 

 surroundings by what we call the special sense organs. The senses 

 of man, besides those we commonly know as those of sight, hearing, 

 taste, smell, and touch, are tnose of temperature, pressure, and 

 pain. It is obvious that such organs, to be of use, must be at the 

 outside of the body. Thus we find eyes and ears in the head, and 

 taste cells in the mouth, cells in the nose for smelling, and others in 

 the skin which are sensitive to heat or cold, pressure or pain. 



The Nervous System of Man. — But this is not all. Strangely 

 enough, we do not see with our eyes or taste with our taste cells. 

 These organs receive the stimulations, which are sent inward by 

 means of a complicated system of greatly elongated cell structures, 



H. NEW CIV. BIOL. — 14 



Diagram of a simple reflex : the stimulus of a 

 burn, through the action of two nerves, causes a 

 muscle to contract promptly. 



