The phylum Nematoda consists of only roundworms. There are two main types of Nematoda roundworms; free-living and parasitic. Parasitic roundworms live within the bodily fluids of a host organism until eventually the host organism dies and the roundworm moves on to the next host. Free-living roundworms eat out the roots of plants and cause them to wither and die. Both parasitic and free living roundworms have a cylindrical body, straight tail, and a dull head.