Nematodes are tiny roundworms. They live everywhere — from the sea to the soil in your garden. They are usually 2.5 millimeters or smaller, tinier than a single sesame seed. Despite being called “worms,” they aren’t related to earthworms. Instead, nematodes have their own phylum — a related group of living organisms, one of 35 in the Animal Kingdom. Scientists have discovered more than 25,000 species of these worms so far, and use some species in research. Others species help to decompose dead organisms or to kill garden pests. Nematodes also can act as nasty parasites for plants and animals, including people.