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.