Understanding A Snail's Eye at a Snail's Pace

Abstract

This paper is cast in the form of answers to pertinent (sometimes impertinent) questions which a reader might ask of persons devoting the better part of a scientific career to studying the eye of a snail. Two structures in the eye of Helix aspersa are emphasized: the microvilli and microvesicles of type I photosensory cells. Evidence is presented to support the conclusions that the microvilli are the light-sensitive organelles and that the microvesicles, termed photic vesicles, bring to the microvilli substances needed in photoreception, such as photopigment and perhaps calcium. The evidence consists of: uptake of radioactive vitamin A by the villi and vesicles, formation of a black precipitate by both structures upon prolonged staining with osmium tetroxide, the effects of light and dark upon the microvilli and photic vesicles, a comparison between the photic vesicles of a light-tolerant slug and those of a nocturnal slug, and a preliminary finding of a high concentration of calcium in the somatic layer of the retina. The movement of the photic vesicles from the Golgi centers, where they are formed, to the microvilli is attributed to rhythmic, light-dependent pulsations of the eye caused by contractions of smooth muscle cells embedded in the optic capsule.