Argonaut

The day before yesterday, Kirill found a nice looking shell with what he thought was something dead and slimy inside it on the sand of our favorite beach. He picked it up and carried it to show us; while he was walking, he felt something touching his fingers, and when he looked at it, a sad blue eye popped out to look at him. He started dipping the whole thing in the water as he walked; at which point the shy dweller climbed out of the shell and made bid to escape. The animal abandoned the shell; it looked like a tiny octopus. Kirill released it into water and it swam away, changing the color as it went. It  left the shell with us. According to my limited knowledge of octopuses at that point, they don't have shells, so we decided that the animal was probably carrying someone else's shell around for protection, rather like a hermit crab.

To my surprise, the very short google search on "octopus shell" proved me wrong. At least one group of them do, and they are called argonauts. Apparently, the name was given to the group because at that point it was thought that they are using thin, paper-like shell as a sail.

Judging by the form of the shell, it is probably a very young female of Argonauta argo.
It also has the blue tint to the eye and around one pair of tentacles, mentioned in the wikipedia article.

I hope that once back in the water, it can generate another shell for itself. I feel a bit guilty for taking away the first one, but I seriously doubt that it would have taken it back even if we managed to catch up with it.

images on black mirror at shutterstock
 Flag beach images on shutterstock