Have you ever found something like this on the beach?
Many people are surprised to learn that this is a cluster of calcareous tubes made by snails. That's right, it's not a group of worms, but snails that live in tubes permanently attached to the substrate.
These are vermetid gastropods called Petaloconchus montereyensis. Here's what they look like when they're alive. These specimens are from the low rocky intertidal zone on Bodega Head. (The following images were taken under a microscope. Each tube opening is ~2 mm across.)
With an even closer view, you can see the snails inside the tubes.
On the right, the brown, saucer-shaped operculum (or trap door) is visible. On the left, look for the tentacles and the mouth above the operculum. Below is an even closer view. Note the bright blue highlights and the dark eye at the base of the uppermost tentacle.
There are so many interesting things about Petaloconchus, it's hard to know where to start. Here are a few amazing attributes:
Possibly unique among gastropods, they periodically shed and replace their operculum. Here's a close-up of the chitinous operculum (below). It almost looks like a contact lens (with a spiral in the center) perched on top of the snail's foot.
This diagram shows the stages of operculum replacement.
Adapted from Hadfield, M.G. 1970. Observations on the anatomy and biology of two California vermetid gastropods. Veliger 12: 301-309. (Op = operculum)
They produce mucous nets to capture food particles suspended in the water. Although it may be tough to see, look for the small mucous mass between the tentacles in the photo below (it's at ~ 3 o'clock).
Females brood egg capsules within their tubes and the young hatch as crawl-away juveniles. Although I'm not sure that the tiny snail below is a juvenile Petaloconchus, they would look similar so it seemed useful to include it as a way to visualize a newly hatched Petaloconchus starting to explore its world and searching for a place to attach to the rock.
P.S. To compare with the somewhat similar looking spirorbid tube worms, see the original "In the tube" post from 6 February 2012.