ENDOCRINOLOGY OF CRUSTACEANS 181 



older literature but have in the more recent studies been designated as the 

 distal retinal pigment, the proximal retinal pigment, and the reflecting 

 retinal pigment ( Fig. 1 ) . 



These retinal pigments may undergo movements in response to light 

 and to darkness, but the numbers of retinal pigments showing such move- 

 ments and the extent of their movement in response to light or darkness 

 may be a species characteristic. Thus, for example, in Palaemonetes all 

 three retinal pigments undergo such movements ; in Astacus the reflecting 

 pigment of the retina is fixed in position above the basement membrane, 

 while the distal and the proximal retinal pigments do undergo photo- 

 mechanical movements ; in Homarus the distal and the reflecting pigments 

 are fixed in position and only the proximal retinal pigment moves in re- 

 sponse to light and to darkness. 



The movements of the distal and proximal pigments have been ex- 

 plained as functioning to screen the sensory component of the ommatidium, 

 the rhabdome, in bright light, and to uncover this rhabdome in low light 

 intensity and in darkness. Thus, in bright illumination the proximal pig- 

 ment moves above the basement membrane of the eye and the distal pig- 

 ment migrates centrally, so that the two sets of pigments form a collar 

 around the rhabdome ; in this condition light entering the ommatidium di- 

 rectly will stimulate the rhabdome, and light rays which enter obliquely 

 from adjacent ommatidia will be screened out by the collar of light-absorb- 

 ing black pigment, presumably melanins, which are contained in the distal 

 and proximal cells. In darkness or in dim light the distal and the proximal 

 pigments move away from the rhabdome so as to leave it relatively un- 

 screened, and dim light which enters the eye may pass readily through 

 several adjacent ommatidia to stimulate the sensory receptors of several 

 units, and are thus more effective in stimulation than would be the case 

 were the rhabdomes screened by the pigments. In those species where it 

 undergoes photomechanical movements, the reflecting pigment lies above 

 the basement membrane in darkness, and below the basement membrane 

 of the retina in light. These white pigment granules, which appear to be a 

 mixture of purines and pterins (Kleinholz and Henwood, 1953; Klein- 

 holz, 1955), are believed to reflect dim light that enters the eye over sev- 

 eral adjacent receptors, thus increasing the effectiveness of the dim light 

 as ,a stimulus. 



The first experimental evidence that the movement of these retinal pig- 

 ments might be under hormonal regulation was shown by Kleinholz ( 1934, 

 1936), who found that injection of extracts prepared from the eyestalks of 

 a variety of crustaceans into dark-adapted Palaemonetes used as the test 

 animals brought about light adaptation of the distal and the reflecting 

 retinal pigments (see Fig. 1) ; the proximal pigment was apparently un- 



