182 INVERTEBRATE PHYSIOLOGY 



affected by these extracts. These observations were confirmed by Welsh 

 (1939), who investigated retinal pigment movement in Cainbarus, and 

 found, not only that injected extract was effective on the distal pigment of 

 Cambarus, but that use of more concentrated extracts also brought about 

 light adaptation of the proximal pigment as well ; the reflecting pigment 

 does not move in Cambarus. The refractoriness of the proximal pigment 

 of Palaemonetes to injected eyestalk extract (Kleinholz, 1936) may have 

 been due to inadequate concentration of the eyestalk extract, as compared 

 with Welsh's observation on the proximal pigment of the crayfish retina, 

 but the possibility should not be overlooked that the differences in response 

 of the proximal pigments of Palaemonetes and of the crayfish may be due 

 to different physiological mechanisms of regulation (see below). A sub- 

 sequent study by Welsh (1941) pointed to the sinus gland of the eye- 

 stalk as the presumptive source of the eft'ective substance in Cainbarus. 

 Additional supporting evidence for the participation of an endocrine factor 

 in retinal pigment migration was presented by Kleinholz and Knowles 

 (1938), Kleinholz (1938), and Sandeen and Brown (1952). In the 

 first study it was found that movement of the distal retinal pigment in 

 Leander was not an all-or-nothing response to illumination, but that the 

 movement of this pigment could be graded between the extremes of light 

 and dark adaptation by varying the intensity of illumination. Sandeen and 

 Brown reported a similar situation for Palaemonetes. Kleinholz (1938) 

 found further that the amount of migration of the distal retinal pigment 

 in Leander was proportional to the concentration of the injected extract, 

 and that the graded responses observed by Kleinholz and Knowles might 

 therefore be mediated by the amount of hormone released into the circula- 

 tion. Smith (1948) extended the possibility of humoral activation of the 

 retinal pigment to brachyuran crustaceans when he found that injection of 

 extracts of sinus glands, optic ganglia, and other portions of the central 

 nervous system brought about varying degrees of reduction of the glow 

 observable at night in dark-adapted crabs ; which particular retinal pig- 

 ments were affected by these injections were, however, not investi- 

 gated. 



Critical proof of endocrine regulation of crustacean retinal pigments 

 was incomplete. The evidence described above came predominantly from 

 injection experiments, and could be subject to the reservations raised 

 against evidence exclusively of this nature. In an attempt at localizing the 

 source of the retinal pigment hormone, Welsh (1941) found in Cambarus 

 that extracts of the sinus gland and extracts of medulla terminalis ( seat of 

 the X-organ) were active but those of cerebral ganglia were not. He con- 

 cluded that, although some activity was shown by extracts of medulla 

 terminalis, this might be due to residual tissue from the sinus gland or to 



