California Horn snail
These snails were quite numerous in the mudflats at the Palo Alto Baylands. I was surprised to see how quickly they could move. Their mouth is located in the long black tube near the opening of the shell. These snails are the first of three steps in the life cycle of several species of parasitic trematode worms. The snails are eaten by fish, which are then eaten by birds, where the worms lay their eggs, birds excrete the eggs into the water to be ingested by the snails, and cycle is then completed.