218 



THE RISE OF ANIMAL LIFE 



Fig. 11-16. Nervous system of the lobster, dorsal view. The eye and statocyst have been greatly enlarged and 

 simplified in order to show their manner of function. The eye is further divided into two parts: the upper por- 

 tion shows how it functions in the dark; the lower portion, how it functions in the light. 



conditions. The sensory structures involved 

 here are the tactile hairs which are stimu- 

 lated whenever any part of the body comes 

 in contact with an object. Tactile hairs are 

 particularly abundant on the chelipeds, at 

 the end of the telson, on mouth parts, and 

 underneath the body. They may also be 

 sensitive to vibrations in the water, thus 

 providing the animal with a "hearing" 

 mechanism which functions somewhat like 

 the ears of vertebrates. 



The animal is continually responsive to 

 gravity, even when swimming freely, which 

 means that it must have organs of equi- 



librium. These organs are the statocysts, 

 located at the base of the antennules. They 

 consist of small cuticular sacs, containing 

 tiny grains of sand, which are glued to 

 small sensory hairs (Fig. 11-16). The nerve 

 fibers from these sensory hairs join to form 

 a large nerve leading to the brain. The 

 proof of the function of these statocysts can 

 be determined experimentally. During the 

 molt the cuticular lining of the statocysts, 

 together with their grains of sand, are lost, 

 so that the crayfish must replace its sand 

 grains (statoliths) after each molt. Shortly 

 after molting, the crayfish buries its head 



