I found this on a rock in the creek. The rock was covered with many hair like filaments. Upon reviewing the photos I could see these tiny caterpillar-like things on its shell. When I flipped it over it seemed to be ...delivering one? No, this is the immature form, therefore i guess the worm-like things must be parasitic.

Especially at higher resolution, the ventral side shows something that look like wings, but are probably the tracheal gills.

It might be worth mentioning that the penny did move. It moved at about the same pace as a snail.