RETINAL PIGMENT OF PLANORBIS 373 



that anaesthetics are probably capable of exerting an inhibition 

 on the migration of pigment, even in normal snails. 



Spaeth ('13^) found the melanophores on the isolated scale 

 of Fundulus to be responsive to ultra-violet rays but not to 

 those of the \'isible spectrum. Parker ('97), working upon 

 Palaemonetes, obtained responses from all three types of pig- 

 ment cells when excised eyes were brought from light into dark- 

 ness, or the reverse. This result was observed equally well 

 when stalks containing ganglia were used or when merely retinas, 

 exclusive of ganglia were employed. Hamburger ('89) reported 

 that the pigment of enucleated frog's eyes exhibits migratory 

 movements both in darkness and in light. In another paper 

 (Arey, '16") I have described the occurrence of a pigment migra- 

 tion when the previously dark-adapted excised eye of the com- 

 mon horned pout, Ameiurus, is brought into the light; in the 

 reverse exposure (hght to dark), however, no response ensues; 

 furthermore, in several other fishes which were studied, even 

 the direct influence of light could not be shown. 



Experiments on Planorbis were niiide upon animals that had 

 been adapted to light or to darkness for 6 and 24 hours, respective- 

 ly. Such animals were beheaded and small pieces of tissue, 

 bearing a tentacle, were then placed in a watch glass containing 

 an abundance of Ringer's solution and subjected to light or 

 to darkness according to desire. The exposure to light lasted 

 4 hours, whereas in the dark, eyes were left for 5 hours. Table 

 2 sununarizes the results of these determinations. 



The results of both these sets agree very closely with the 

 values given previously for nornml light-adapted eyes (11.0 /x 

 and 16.5 n for zonal and process measurement respectively). 

 The obvious conclusion to be drawn from these experiments is 

 that light has a direct influence on a previously dark-adapted 

 pigment cell, but that light-adapted retinas do not change in 

 darkness. The direct action of light on the retinal pigment 

 of this animal, therefore, is identical \\'ith that of Ameiurus. 



I am inclined to interpret these results in the following way. 

 The conditions under which an isolated eye is placed undoubtedly 

 favor the accumulation of catabolic products. This unremoved 



