The name Nematode means, "thread form".  The nematodes are also known as round worms.   These animals have a cylindrical tapered bodies and range is length from few millimeter to a meter and live in diverse habitats.

Free-living nematodes eat microorganisms; other nematodes may feed on decaying organic material.  Vinegar eel is a common free-living nematode. All roundworms have a bilateral symmetry and a mouth and an anus.

The parasitic species are: pinworms, hookworms, heartworms (in dogs), and ascaris (intestinal roundworm).  Humans can contract parasitic worms by eating under cooked beef or pork.