There exists a wonderful book with an illustrated key to the more common protists you can find in freshwater: Free-Living Freshwater Protozoa by Paddy Patterson. However, as many good things in life, it is out of print and thus very expensive (though relatively cheap-ish in the above link at the time of writing this).
An advantage of having a, let’s say, ‘volatile’ memory is that you can sit back and do microscopy… on your own hard drive.
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).