Roundworms are shapeless worms with two pointed ends. They are very common. Most are tiny, and live in water or damp soil. You can hold up to 1000 in just one handful. One kind of roundworm, the potato-root eelworm, is a serious pest. It weakens the plant by living in its roots. Other roundworms, such as the hookworm of the southern USA, live inside humans as parasites.