Eyes on stalks

The eyes of a land snail sit at the tips of the upper tentacles. They are simple eyes. They can tell light from dark and detect large shapes and movement, and they do not form a sharp picture the way a bird’s eye does. A snail does not need a sharp picture. It is not hunting by sight. Smell and taste, sampled by the lower tentacles and by the skin around the mouth, do more of the work of finding food, a mate, and the way back to a damp crack.

Watch a snail approach a finger held still in front of it. The upper tentacles reach, then pull back if they touch you, often one at a time. The withdrawal is fast because the tentacle is inverted by a muscle, turned outside-in. Give the animal a minute and the tentacle rolls out again. This is ordinary caution. If a predator, or a careless boot, takes a tentacle, a land snail can regenerate it, eye included, over days or weeks depending on the species and the season.

Light still matters. Many species are active at dusk and after rain, and they roost in shade. A sudden shadow can make a moving snail pause or withdraw. Experiments on several species show they can learn simple associations, and that they can tell a dark refuge from an open, lit surface. None of that requires a detailed image. A sensor that says “bright, open, dry” is enough to send a snail back under the pot.

Sea snails are arranged differently. Some have an eye at the base of a tentacle rather than the tip, and a few swimming sea snails have more capable eyes. The garden animal, with eyes on the ends of stalks that invert when touched, is the stylommatophoran plan. It is the one to look for on a wet evening, and it will notice you mostly as a change in the light and a new smell.