RETINAL PIGMENT OF PLANORBIS 375 



the exact periods of time necessary to complete light- and dark- 

 adaption becomes the next object of consideration. 



Snails which had xjreviously been thoroughly adapted to 

 light (fig. 2) or to darkness (fig. 5), were subjected to opposite 

 conditions of illumination. Indi^'iduals were removed and 

 killed at half-hour intervals until the experiments had con- 

 tinued for 5 hours. Microscopical examination of the retinas 

 gave the following results: 



Dark-adapted Planorbis subjected to light: 



Incoinplctc light-adaption occurred in 3 hours 



Complete light-adaption occurred in 4 hours 

 Light-adapted Planorbis subjected to darkness: 



Incomplete dark-adaption occurred in 4 hours 



("omi)lete dark-a<Iaption occurred in 5 hours 



Our suspicion as to the cause of Smith's failure to procure 

 satisfactory and consistent results is corroborated, since the 

 time alloted foi- his ox]ierimentation was quite uiadeciuate. 



From the comjiarative standpoint, it is interesting to see how 

 the rai)idi1y of pigment migration in Planorbis agi'ees with 

 corres]ionding movements in the eyes of other animals. 



In the compound eyes of the prawn, Palaemonetes, Parker 

 ('97) gives the following data for the time consujued in tlu^ 

 adaption of the proxnual, or retuial, pigment: 



Darkness to ligiit :iO to 4.^ minutes 



Light to darkness 45 to (U) minutes 



The writer (Arey, 16"), working upon several fishes, has shown 

 that the comi)lete adaption of the retinal pigment re(iuires 

 longer periods of time than have generally been siq^posed. 

 These determinations may be combined in the following sum- 

 marization: 



Darkness to light 45 to 60 minutes 



Light to darkness 30 to 60 minutes 



An intoi-esting comparison with the cephalopod type of eye 

 is made possible through the values determuied by Hess ('05), 

 from whose work it appears that many hours (48 or more) are 



THK .lornVM Ol- C<>\ir \HATIVE NEUROLOGY, VOL. 26, NO. 4 



