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).
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).
An advantage of having a, let’s say, ‘volatile’ memory is that you can sit back and do microscopy… on your own hard drive.