As penance for irregular posting, have a pair of seemingly-symbiotic rotifers in a cnidarian (jellyfish) polyp. There were several of them on several polyps, and they seemed not to mind the tentacles (loaded with stinging cells containing a harpoon-like weapon with paralytic abilities). Unlike the dividing ciliate in the corner -- standing still as it appears to have been paralysed by the stinging cells. Curiously, instead of blowing up, it just stops moving. The morphology remains intact. Perhaps a potential method for immobilising protists for better imaging?
Here's a toe of the rotifer rooted deep in the polyp. The rotifers were alive and feeding. Might be some sort of a commensal relationship where they take advantage of the polyp's easier access to drifting detritus, as well as protection from some predators. The rotifers might also help generate extra current. This is all baseless hypothesising out of my ass though, so please forget everything I just wrote!
Since I'm apparently on an invert spree, just for fun: a ctenophore, aka comb jelly. Despite being called jellies and appearing vaguely jellyfish-like, they seem to not only be a completely separate branch from cnidarians (containing "true" jellies), but perhaps a lineage as deep as sponges in the animal tree. Fascinating organisms. Also, two anuses.
Finally, a protist. This is Noctiluca, a bioluminescent dinoflagellate that is also fairly big for a single cell -- 1-2mm. You can easily see them in a planktonic water sample. This picture was taken through a dissecting scope. They're basically bloated bubbles with a little bit of actual cytoplasm. Marine biology stations are busy and highly-distracting places. I have some rare/new organisms to show too, but later...