RETINAL PIGMENT OF PLANORBIS 385 



SUMMARY 



1. Light causes the retinal pigment of nornuil Planorbis to 

 migrate distally (toward the source of ilhmiination). Dark- 

 ness causes a proximal migration. 



2. About 4 hours is necessary for the pigment to assume the 

 distribution characteristic of light-adaption, whereas about 5 

 hours is demanded in accomplishing the reverse process of 

 dark-adaption. 



3. The pigment of dark-adapted, excised eyes exhibits mi- 

 gratory movements when exposed to light, and is, therefore, 

 capable of direct stinmlation. In the converse treatment (light 

 to dark) of excised eyes, on the contrary, no change in the posi- 

 tion of the pigjuont occm-s. It is possible that the absence of 

 positional change iji darkness is indicative of an anaesthetic 

 effect, produced by the imremoved products of catabolism, the 

 more vigorous influence of light, however, being able to o\er- 

 come this inhibitory tendency. 



4. High tem])rrature (30°C.) induces proximal migration of 

 the normal retinal ]Mgment both in darkness and in light. Low 

 temperature (3°C\) hiduces distal migration both in dai-kness 

 and hi light. 



5. Upon light-adapted excised eyes, low temperature likewise 

 favors proximal migration and high temperatm-e favors distal 

 pigment migration. 



G, Planorbis afifords the first case among gastero])()ds in which 

 the occurrence of positional changes in the retmal pigment 

 has been correlated with the presence and absence of light. 

 For the first time among molluscs, a thenno-mechanical in- 

 fluence upon the retinal pigment has been demonstrated. 



7. It is probable that low concentrations of anaesthetics, such 

 as carbon dioxide or ether, are capable of completely arresting 

 migratory mo\'ements of the Planorbis retinal jjigment. This 

 suggests that carbon dioxide, as a product of catabolism, may 

 be the inhibiting factor that prevents migratory movements 

 of the pigment when excised eyes are brought from light into 

 darkness. 



