If you look in any decent book on seashells, you will come across the genera Littoraria and Littorina in the family Littorinidae (commonly known as periwinkles). Although considered to be "seashells" or marine snails, some species in these genera spend almost all of their adult lives on land.
The snail pictured above, Littoraria angulifera, is common in mangrove forests in Florida. I photographed this one on a mangrove branch about 3 meters above the ground near Sanibel Island last July.
These snails appear to be the end of a lineage of marine gastropods that couldn't quite complete their evolution from marine to terrestrial life. For example, they breathe air even though they have gills; their activities are regulated by both the rain and the tides.
Their most vital connection to the sea not only reveals where they originated, but also explains why they can't move away from it: they must return to the sea to release their eggs.
As you can imagine, these snails offer many important clues to the transition of life from the sea to the land. I will return to them in the future.