THE CRAYFISH AND ARTHROPODS IN GENERAL 



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retinular cells (8) which form a rhabdom (7) consisting of four 

 rhabdomeres, and (/) a number of pigment cells (4, 6). Fibers 

 from the optic nerve enter at the base of the ommatidium and 

 communicate with the inner ends of the retinular cells (10). 



VISION. The eyes of the crayfish are supposed to produce 

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

 ure 104 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 surrounding 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 retinulae, and thence to the fibers from the optic nerve. 

 " Thus the retinula of one ommatidium receives a single resultant 

 impression from the light which reaches it. But the adjacent 

 ommatidia being directed to a different, though adjoining, region 

 of the outer world, may transmit a different impression, and the 

 stimuli from all the omma- 

 tidia which make up a com- 

 pound eye will correspond in 

 greater or less degree to the 

 whole of the visible outer 

 world which subtends their 

 several optic axes. The sum 

 of the resulting images which 

 we may thus suppose to be FIG. 104. Diagram to explain mosaic 

 transmitted to the brain may vision ( see text ) ( From Packard 

 be compared to a mosaic in 



which the effect is given by a large number of separate pieces, 

 of one size and each of uniform colour " (183, p. 332). This 

 method of image formation is especially well adapted for record- 

 ing motion, since any change in the position of a large object 

 affects the entire 2500 ommatidia. 



When the pigment surrounds the ommatidia (Fig. 103, A), 

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

 light the pigment migrates partly toward the outer and partly 



