286 



COLLEGE ZOOLOGY 



facets, which are the ends of long visual rods, the ommatidia. 

 The average number of ommatidia in a single eye is 2500. 

 The parts of an ommatidium are shown in Figure 203. 



Vision. — The eyes of the crayfish are supposed to produce 

 an erect mosaic or " apposition image "; this is illustrated in 

 Figure 204, where the ommatidia are represented by a-e, and the 

 fibers from the optic nerve by a'-e' . The rays of light from any 

 point a, b, or c will all encounter the dark pigment cells surround- 

 ing the ommatidia and be absorbed, except the ray which passes 



directly through the center of 

 the cornea, as d or e ; this 

 ray will penetrate to the 

 fibers from the optic nerve. 

 One ommatidium thus re- 

 ceives a single impression, 

 and since the ommatidia are 

 directed to different, though 

 adjoining, regions, the sum 

 of the resulting images may 

 be compared to a mosaic. 

 This method of image forma- 

 tion is especially well adapted for recording motion, since any 

 change in the position of a large object affects the entire 2500 

 ommatidia. 



When the pigment surrounds the ommatidia (Fig. 203, A), 

 vision is as described above; but it has been found that in dim 

 light the pigment migrates partly toward the outer and partly 

 toward the basal end of the ommatidia (Fig. 203, B). When 

 this occurs, the ommatidia no longer act separately, but a com- 

 bined image is thrown on the retinular layer. 



Statocysts. - - The statocysts of Cambarus are chitinous- 

 lined sacs situated one in the basal segment of each antennule. 

 In the statocyst are a number of sensory hairs, among which are 

 a few grains of sand, called statoliths, placed there by the cray- 

 fish. The contact of the statoliths with the hairs determines 



Fig. 204. — Diagram to explain mosaic 

 vision (see text). (From Packard, after 

 Lubbock.) 



