I saw the strangest thing washed up on the beach today. Does anyone out there know about these things?

It had a huge helmet for a shell. There were alot of legs underneath the "helmet" and it seemed to have some sort of antennae for a tail. My friend said that it needed to be in the sea, so he moved it back into the ocean with his foot. All of a sudden it started vigorously moving out to sea. I had originally thought it was dead. Is there anybody out there who knows about these things who could tell me what this thing was? Thanks very much.