184 INVERTEBRATE PHYSIOLOGY 



from injection experiments, would present serious limitations to the 

 classical methodology of demonstrating endocrine regulation of these 

 effectors. 



The evidence for the sinus gland as the source of a hormone causing 

 light adaptation of the distal retinal pigment in the Natantia seems more 

 favorable. In addition to the evidence from the injection experiments of 

 Kleinholz (1936, 1938), Knowles (1950) found that the distal retinal 

 pigment of Leander, from w^hich sinus glands had been removed, attained 

 maximal dark adaptation and w^as not affected by changes of illumination ; 

 these results, however, were not unequivocal, for in a few such operated 

 individuals the distal retinal pigment underwent a slight proximal migra- 

 tion (toward light adaptation). The latter responses might have been due 

 to slight injury to the optic ganglia, caused during removal of the sinus 

 glands, a condition which Smith ( 1948) found to result in varying degrees 

 of light adaptation in the retina of crabs, or they might have been due to 

 some of the other physiological possibilities mentioned above. 



In recent years it has been proposed (Brown, Fingerman, and Hines, 

 1952; Brown, Hines, and Fingerman, 1952; Brown, Webb, and Sandeen, 

 1953) that, in addition to a hormone which brings about light adaptation 

 of the distal retinal pigment of Palaemonetcs, an antagonistic hormone, 

 which causes dark adaptation of the distal retinal pigment, may be pres- 

 ent. The basis for this hypothesis lies in two kinds of observations ; 

 Palaemonetes from which one eyestalk has been removed show less light 

 adaptation than normal animals, while similar one-eyed animals show the 

 same dark adaptation as normal animals ; the other kind of observation 

 resulted from studies of the kinetics of light and dark adaptation of the 

 distal retinal pigment of animals which had been successively dark-adapted 

 for varying periods, given light stimuli of different durations, and then re- 

 turned to darkness. It was found that the ensuing variations in the rate 

 of readaptation to darkness could be explained in terms of a hormone that 

 causes dark adaptation of the distal retinal pigment cells. By way of experi- 

 mental test of this hypothesis, these authors studied the effects on the 

 kinetics of dark adaptation of injecting — into previously dark-adapted 

 animals given a light stimulus and then returned to darkness — extracts of 

 eyestalks, of central nervous system, of tritocerebral commissures, and of 

 sea water. It was found that the rate of subsequent readaptation to dark- 

 ness was greater after injection of the eyestalk extract and of the central- 

 nervous-system extract than with the sea-water control, and that Palae- 

 monetes injected with extracts of tritocerebral commissure showed less 

 light adaptation than the controls. These authors had no success in in- 

 ducing dark adaptation of the distal retinal pigment in light-adapted ani- 

 mals, and point out that the only condition under which it was possible in 



