Shell shock

This is a corner of our tool shed container, which we call The White House.  Originally painted white it is gradually toning in with the landscape thanks to rust and the growth of algae on the paintwork. 

But a few days ago I noticed a strange pattern  .  .  .

White trails through the grey-green algae

I think it has been caused by a snail or snails eating off the algae and thereby cleaning the white paint.  The jagged marks must be where the snail has been moving its head from side to side as it crawls along, scraping off the algae.  The spikes suggest that some of the tracks were formed on the way up and some on the way down.

Snails eat with a sort of rasping tongue called a radula.  You can see it on this video.

Or you can see it for yourself by smearing some flour and water on a sheet of glass.  Put a snail on the glass and watch from underneath as it feeds.