ARTHROPODA 



167 



Senses of the Crayfish. — Touch is the most 

 valuable sense. Crayfish are sensitive, to touch 

 over the whole body, especially on the chelae 

 and chelipedsy mouth parts, the ventral surface 

 of the abdomen and the edge of the telson. 



Vision. — The compound eyes are almost 

 worthless for detecting the forms of stationary 

 objects, but good for moving objects. The re- 

 sponse is not due to any change in the intensity 

 of light such as that caused by a shadow falling 

 on the animals, for they react to a movement 

 made on the opposite side of them from a 

 window. Crayfish are sensitive to a strong light 

 and hide during the day under stones, among 

 the roots of plants near the bank, or burrow 

 into the bank. They retreat from a strong 

 light, but approach a dim one. 



Smell and Taste. — Bell applied meat juice to 

 various parts of the body of the crayfish and 

 found that the antennae^ antennules, mouth 

 parts and chelipeds were especially sensitive. 



Holmes and Homuth found that the outer 

 rami of the antennules bearing the olfactory 

 setae were especially sensitive to olfactory 

 stimuli, that the inner rami of the antennules 

 and the antennae^ the mouth parts, and the tips 

 of the chelipeds were all sensitive to some extent 

 to olfactory stimuli. Crayfish have a highly 

 developed topochemical or contact-odor sense. 

 The crayfish is sensitive to food when not in 

 contact with it. Experiments with freshly cut 

 meat and with meat on which the cut surfaces 

 had dried showed that the crayfish prefers fresh 

 meat, probably because it locates it sooner. 



Hearing. — Bell learned that the crayfish has 

 no sound reactions, but is sensitive to vibrations 



Fig. 74. The central nervous system of the lobster. 

 (After Calkins, Biology. Courtesy of Henry Holt & Co.) 



