“Dr. Diana Six shows leech to students” by GlacierNPS is marked with CC PDM 1.0

The group of worms is a very large group of many kinds of animals, and it includes leeches. Most leeches live in slow water or lakes (still water). If you find a lot of leeches (or flatworms, another group of animals that look a little like leeches), and not much else, then you know that the water quality is not very good.