428 MAN BECOMES THE CONQUEROR OF THE WORLD 



The fine hairs of the appendages of the lobster 

 are organs of touch and they make the animal 

 sensitive to its surroundings. 



stimuli (sensory hairs, tympana of insects, and the ears of higher 

 animals). These end organs or structures at the outside of the 

 animal, when connected by nerves to organs of movement, like 

 muscles, bring about reactions to stimuli which result in obtaining 



food, in escaping from ene- 

 mies, and in many other im- 

 portant 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 cen- 

 tral part of the body. This 

 cell in turn sends a message 

 which stimulates a muscle to 

 work, and the animal's body is involuntarily 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 

 tympanums, or ears. In some animals the " ear " assists in bal- 

 ancing while in other animals the ear is an organ of hearing as 

 well as for balancing. In the. shrimp, the " ear " is 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 molting, place small grains 

 of sand in their statocysts (stat'6-sists) 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 against gravity affected the shrimps as did the 



