If you meet a camaenid snail, sneak a glance at the wart on its head. It lies between the two upper tentacles. Well, when I say lies, it stands out like a dog's ... er ... wart. Now check the bonce on a garden snail. No peculiarity there. It's all smooth and slimy.
Of course, there's a chance you might miss it on a camaenid. Not all species have a head wart. And of those that do, not every one parades it around. The big tropical Hadra and allies have a head wart they can tuck away into a little pocket when not needed. Here's a picture of one proudly on display. The tentacles have been retracted into those two slits in the head. (This photograph was taken with a scanning electron microscope. I don't get to play with SEMs anymore. Sob.)
So what does the head wart do? It probably has a role in courtship. Certainly, there's evidence that the same structure in bradybaenid snails has a reproductive function—it may even secrete pheromones. Neither the skin (epithelium) cells nor tissue of the camaenid head wart appear to produce much in the way of secretions, but there are big spaces that may store products from elsewhere. I dunno.
But odd as it is, the camaenid head wart fades into a mere blemish when compared with the frontal organ of the African slug Gymnarion. Like the head wart, the frontal organ sits between the upper tentacles and can be everted at will. The difference is that in some species of Gymnarion it's armed with hooks made of calcium carbonate. Apparently, the courtship of the barbed species is longer and more complex than those of the unbarbed species. I'm not surprised.
[Thanks heaps to Josh for scanning the photo]