The 1st detailed account of the little semi-terrestrial snail Pomatiopsis lapidaria was published by William Stimpson (1832-1872) in 1865. Stimpson's descriptions of the anatomy and behavior of Pomatiopsis lapidaria are surprisingly accurate. For example, he described the snail's eye as follows.
The eye is situated on the outer side of a rather prominent swelling out or protuberance of the head at the base of the tentacle. On the upper and inner side of these protuberances there is a conspicuous longitudinal fusiform spot of flake-white or yellow, which is a prominent character, probably, however, of specific importance only.When I first read this passage, I was on the train home. I put a note on the margin of the paper to remind myself to check this in my photographs of Pomatiopsis lapidaria. Sure enough, an elongated, yellowish band is clearly visible above each eye in the pictures showing the heads of the snails I photographed.
I don't know what exactly those spots are and what function, if any, they may have. The "pigment layer" lining the eye chamber of Pomatiopsis lapidaria that Dundee (1957) mentioned may have been the same structure. Davis (1967) referred to them as "glandular units".
Here is the entire picture showing the rest of the snail. The whitish oval plate on the snail's foot below the shell is its operculum.
Davis, G. 1967. The systematic relationship of Pomatiopsis lapidaria and Oncomelania hupensis formosana (Prosobranchia: Hydrobiidae). Malacologia, 6:1-143.
Dundee, D.S. 1957. Aspects of the biology of Pomatiopsis lapidaria. Misc. Pub. Mus. Zool. U. Michigan #100.
Stimpson, W. 1865. Researches Upon The Hydrobiinae And Allied Forms. Smithsonian Miscellaneous Collections #201 (Smithsonian Miscellaneous Collections vol. 7, 1867).