ENDOCRINOLOGY OF CRUSTACEANS 183 



material which had escaped from the sinus gland during removal, and that 

 the sinus gland might therefore be the source of this principle. 



Following Welsh's study came a series of reports which, while failing 

 to settle entirely the question of hormonal regulation with experiments in- 

 volving removal of the sinus glands, at the same time gave indication that 

 the regulation of the retinal pigments was probably more complex than 

 hitherto believed. Kleinholz (1948a,b) reported that sinus-gland removal 

 from one eye of the crayfish, followed by ablation of the other eyestalk, had 

 no effect on the ability of the proximal retinal pigment to adapt to light 

 and to darkness when such operated animals were placed under appro- 

 priate conditions of lighting; of 5 such animals placed in darkness, the 

 proximal pigment of all was in the dark-adapted position, while, of 21 

 similarly operated animals kept in an illuminated environment, the proxi- 

 mal pigment of 20 was in the typical light-adapted position and that of one 

 animal was irregularly light adapted. No reliable observations of the effects 

 on the distal pigment of the crayfish were available from Kleinholz's study 

 because of the damage caused to the retinal cones in the process of histo- 

 logical sectioning. Similar effects of sinus-gland removal were found by 

 Smith (1948) in his study of glow in the retinas of the crabs Hemigrapsus 

 and Pachygrapsus; glow, which is generally associated with the dark- 

 adapted retina, did not appear during daylight as a result of this operation, 

 while it was generally present at night, but was variable in degree. Klein- 

 holz (1949) found that in isolated eyestalks of the crayfish Astacus the 

 proximal retinal pigment could become light- or dark-adapted when placed 

 in moist chambers under appropriate conditions of illumination ; under the 

 conditions of these experiments the eyestalks were isolated from the cen- 

 tral nervous system (except for the optic ganglia contained within the 

 eyestalk) and from the circulatory system. The result of these three studies 

 do not support the view that a hormone from the sinus gland is solely re- 

 sponsible for causing the proximal pigment of the crayfish (and of the 

 crab?) to move into the light-adapted position and indicate the possibility 

 that the proximal retinal pigment cells of the crayfish retina may respond to 

 light and to darkness as independent effectors. Knowles ( 1950) found that 

 the proximal pigment cells of Leander from which the sinus glands had been 

 removed respond in normal fashion to light and to darkness ; he, too, in- 

 clined toward the interpretation that these proximal retinal pigment cells 

 were behaving as independent eff'ectors. In none of the studies cited 

 has the independent-eft'ector hypothesis been critically demonstrated to 

 the exclusion of other possibilities. One such possibility which has been 

 mentioned (Kleinholz, 1948a), and which may be indicated in the experi- 

 ment of Knowles, is a source of retinal pigment hormone outside of the 

 eyestalk. Conclusive evidence for such sources, beyond that resulting only 



