222 AN INTRODUCTION TO ZOOLOGY 



to turn the body over. From a large number of experiments it 

 has been proven that the statocysts are the organs of equilibra- 

 tion. The structure of these organs is described on page 208. 

 The contact of the statoliths with the statocyst hairs furnishes 

 the stimulus which causes the animal to maintain an upright posi- 

 tion. 



When placed on its back, the crayfish has some difficulty in 

 righting itself. Two methods of regaining its normal position 

 are employed. The usual method is that of raising itself on one 

 side and allowing the body to tip over by the force of gravity. 

 The second method is that of contracting the flexor abdominal 

 muscles which causes a quick backward flop, bringing the body 

 right side up (168). In general, the animals " right themselves, 

 when placed on their backs, by the easiest method; and this is 

 found to depend usually upon the relative weight of the two sides 

 of the body. When placed upon a surf ace which is not level, they 

 take advantage, after a few experiences, of the inclination by turn- 

 ing toward the lower side " (188, p. 577). 



SENSES AND THEIR LOCATION. The sense of touch in crayfishes 

 is perhaps the most valuable, since it aids them in finding food, 

 avoiding obstacles, and in many other ways. It is located in 

 specialized hairs on various parts of the body (164). Vision in 

 crayfishes is probably of minor importance, since the compound 

 eyes are almost useless in recognizing form, and are of real value 

 only in detecting moving objects. No reactions to sound have 

 ever been observed in crayfishes, and apparently they do not 

 hear. " The reactions formerly attributed to audition are prob- 

 ably due to tactile reflexes " (181, p. 244). In aquatic animals 

 it is so difficult to distinguish between reactions of taste and smell 

 that these senses are both included in the term chemical sense. 

 The end organs of this sense are distributed all over the body. 



REACTIONS TO STIMULI. THTGMOTROPISM. The crayfish " is 

 sensitive to touch over the whole surface of the body, but espe- 

 cially on the chelae and chelipedes, the mouth parts, the ventral 

 surface of the abdomen, and the edge of the telson " (164, p. 644). 



