Experts prefer to call those sticky-out bits (on the head of Cornu aspersum) tentacles rather than feelers or antennae. To me tentacles belong on an octopus and our special snail deserves a term that connects him individually to the modern age since he has survived millennia because of his adaptability.
At first you observe the upper, longer pair which give this snail its characteristic appearance often portrayed in cartoons or drawings. (Also doodled in the borders of medieval manuscripts fighting knights - but that's for another post). These are the ocular tentacles and have rudimentary eyes at their tips. They are sensitive to light and shade and obvious movement but cannot perceive detail. They are sufficiently acute for the snail to detect predators and withdraw into its shell. In this way they are an essential part of its survival mechanism. Studies have shown that they also find food by smell. Astonishingly, if one is damaged in a predator attack, it regrows completely including a new eye, optic nerve and lens. This is particularly the case with juveniles. Hard to believe but true.
Lower down on the head is a second, shorter pair which are the sensory tentacles and often emerge after the upper ones. These are packed with tactile cells and chemoreceptors. The snail uses them to find food sources, taste and smell its surroundings and decide if surfaces are safe to slide onto. They also track mucus trails left by the owner and other snails which can mark regular food sources. When one snail finds another's shiny track of signals its heart rate rises measurably from 47 to 51 beats per minute.
Because our friend moves so slowly, it needs this equipment to stay alive: when it senses danger it can retreat inside its carapace - since it is unable to run away. I find that mine extend their upper ones (when I talk to them) which it quickly withdraws if they encounter an obstacle but, if the creature feels safe generally, out they come again. Very endearing.
