§ 3.222 EFFECTORS WITH MOVABLE PIGMENT GRANULES 79 



The distal retinal cells contain dark, melanin-like pigment 

 granules. The pigment movement can be observed in the intact 

 eye (Fig. 3-12, d-f), and is accurately adapted to the light 

 intensity, especially over the lov^er ranges, such as inshore, under- 

 water animals are most likely to encounter. The adaptive move- 

 ment is, however, achieved in two different ways : (i) by the usual 

 migration of pigment granules in stationary cells (Fig. 3-12), and 

 (ii) by contractile fibres which change the shape of the cells, 

 thereby causing redistribution of the pigment in relation to the 

 rhabdome (Fig. 3-13). This latter process overrides any sign of 

 pigment migration (Parker, 1932). 



Migration of pigment granules in stationary distal retinal cells 



In the crayfish Astacus and Camharus and the crabs Dromia 

 and Maia^ the pigment granules move inwards to meet the out- 

 ward movement of the proximal pigm.ent, as they become light- 

 adapted; they move outwards, towards the surface of the eye, to 

 reach the dark-adapted position, which occurs in dim light, rather 

 than in complete darkness. The latter movement appears to be 

 comparable to concentration of pigment in the cell body of a 

 chromatophore ; the former inward movement is like dispersal 

 of the pigment into the stationary but outlying cell-processes 

 (Fig. 3-12C). 



In Cambarus, inward migration, or dispersal, of retinal pigments 

 to their light-adapted position is induced by injection of an eye- 

 stalk extract, the extent of the migration being dependent on the 

 quantity of the retinal-pigment-dispersing hormone, RPDH, 

 used. The threshold for response of the distal pigment is lower 

 than that for the proximal pigment (Fig. 3-12, b and c). No 



shows a bright orange centre to the cornea (cm), when seen by 

 reflected light, because the rhabdome (rh) is unscreened by the 

 proximal pigment, (b) and (e) In response to the injection into 

 a dark-adapted animal of retinal-pigment-dispersing hormone 

 extracted from one eyestalk, the distal pigment has partially dis- 

 persed inwards but the proximal has not moved towards the light- 

 adapted position, (c) Extract from two eyestalks has been injected 

 and pigment in both sets of retinal cells has dispersed to the fully 

 light-adapted position. In surface view (/) this eye appears black 

 all over, as in a naturally light-adapted animal (from Welsh, 1939). 



