CRAYFISH 



261 



ment in the visual field, since any motion almost inevitably results in a 

 stimulus being withdrawn from one retinula and appHed to another. 



295. Statocyst. — At the distal end of the basal segment of each 

 antennule is a sac, hned with chitin, which is continuous with the chitin- 

 ous covering over the surface of the body; this is a statocyst. On its 

 walls are sensory hairs and in its cavity are grains of sand or other hard 



Cornea/ facet 



- Cornea 

 '~Corneogen cell- 

 -Cap of cone 

 ce// 



-Disfal refinu/ar 

 pigment ce// 





■ CrystaJIins cone 



-Proximal portion 

 of cone ceJf 



-Proximo) ret I nu Ian 

 pigment- cell 



-R/iabdom 



yBasement 

 rnembrone. 



'Optic nerve fibers 



A B 



Fig. 167. — Longitudinal sections through two ommatidia, which show the different 

 arrangement of pigment. A, in dim light. B, in bright light. (After Parker.) 



objects known as statoliths. While the animal is in a normal position 

 there is no movement of these statoliths, and though they are in contact 

 with certain of the sensory hairs no stimulation is received. When, 

 however, the position of the animal is changed, their movement causes 

 them to come in contact with other hairs and this acts as a stimulus. 

 There results a sensation which causes the animal to respond in such a 

 way as to maintain its equilibrium. When the cuticula over the surface 

 of the body is shed, that which fines the statocyst is lost with the rest, 



