A pair of worms under the microscope shot in dark-field.
The long, thin and somewhat yellow-ish worm is an Aelosoma, which are related to leeches and earthworms (part of the Annelid phylum). They are quite common in freshwater and are voracious feeders of algae and soil.
The smaller white worm is a turbellarian, which is a type of free-living flatworm (as opposed to parasitic flatworms, which make up around 50% of all flatworms). This species in particular was a predator, as I saw siphoning up protists and rotifers.
Although I refer to both of these two as “worms”, this does not mean they are closely related! Many life forms on earth have evolved a worm-like body. In science vernacular, we call this body shape “vermiform”. This just goes to show you that morphology alone is not enough to parse evolutionary relationships.