snails/worms? living on my astrea snails?

[Melina]

I don't have a picture, but living in the shells of a few of my astrea snails, are these long, thin black things. They have their own little shells (same color and texture as the snail's shell, but distinctly separate "homes", on top of, and fused to, the main shell), .. like, hitchhikers I guess... on the backs of the snails, in their own little "houses". They extend a long (1"?) "neck" and are jet black. They have two small antennae, similar to a snail, (on each side of the "neck", protruding from their shells) but their method of feeding is much different. They extend their long "necks", and "graze" (peck?) at the rocks. At the end of their necks, appear to be two tiny feelers? which open and close, or scrape? like a mouth. It extends, feeds, retracts back into it's shell, extends again. Can anyone tell me what these things are?