A Blind Snail

A closer view of the large tentacles shows: This snail really
does not have any eyes. [RN]

A Roman snail (Helix pomatia) without eyes. [RN]

Snails can be blind as well. Mica from Berlin found a Roman snail (Helix pomatia), whose longer tentacle pair was complete and intact, except it lacked an eye.

The snail very obviously does not see anything. Also the shadow reflex seems afflicted, as the snail reacts to incoming shadows only very slowly. Concerning the rest of its behaviour, the snail is not very handicapped: It likes climbing things and touches everything with its tentacles.

For a snail, the loss of its eyes is not a large handicap. The eyes may be important for the optical information on the immediate surroundings of the snail, but as a snail only possesses one type of optical sense cells, it is only able to see black and white. And because the lens in a snail's eye does not have any lenticular muscle, it cannot focus. So the picture a snail sees, is not only black and white, but also blurred.

A Roman snail has got more important sense organs, such as its tentacles full of cells that sense smell, as well as its lips, on which there are cells that sense taste.