The protozoans on the snails are not Vorticella. However there are other similar stalked protozoans that will look like Vortecella. Examples are Opercularia, Epistylis, Carchesium, and Campanella. The stalked protozoans on the snails I don't think are parasites. I think they are termed - commensalism or commensal organisms - a relationship between two species in which one species obtains food or other benefit from the other without causing harm or benefit to the latter. Snails are just forming a subsrtate or surface for attachment for the protozoans.

I question if the green spheres in above pics are Volvox. I cannot see the colonies with good clarity and resolution. There are several other genera of green algae that form spheres with individual cells composing the colony matrix. Sharper images would help with identification.

Have you ever found fingernail clams in stomachs of perch? I have seen snails in YP stomachs.