Members of the phylum Nematoda are roundworms, and many are microscopic.

Roundworms have many of the same characteristics as flatworms; for example, bilateral symmetry and cephalization. They also have a digestive tract open at both the mouth and the anus. This tract is suspended in a body cavity that is said to be false, so it is called a pseudocoel.

Many species of roundworms are free-living organisms that help consume dead plant and animal matter. Several parasitic roundworms cause human diseases, including trichinosis, hookworm disease, and elephantiasis.