Free-living roundworms are abundant in many environments, including soils, fresh and salt water. These roundworms absorb oxygen contained in the air or water by diffusion, when their internal levels of oxygen are low. The diffusion of oxygen always occurs from an area of higher concentration to an area of lower concentration, in a passive way, without the use of the worm's energy. The diffusion of oxygen occurs thorough the whole skin surface. Once inside the body, the oxygen is distributed within the inter-cellular liquid to all cells.
Parasitic roundworms can attack plants, animals and humans, causing serious diseases such as ascariasis and ancylostomiasis, or hookworm disease. Most parasitic roundworms are adapted to live in oxygen-rich environments -- but they are also able to survive with the lack of oxygen -- when they infect the tissues of plants and animal organs, such as the intestines. Without oxygen, roundworms change their metabolism to anaerobic, which literally means " without oxygen." Parasitic roundworms have adapted mouths with sharp structures similar to teeth, which they use to suck liquids from plant tissues or animal organs.
Most roundworms species have separate sexes, with male and female animals. In free-living species, the eggs hatch releasing larvae, which undergo several moltings, before reaching sexual maturity. Parasitic species have complicated life cycles. The eggs of Ascaris lumbricoides can live in the soil for up to 10 years. They only develop inside an animal or human host. When ingested with water or food, these eggs penetrate the intestine wall, reaching the blood stream. In the lungs, the eggs hatch. After three weeks, the larvae migrate to the pharynx, where they are swallowed, returning to the intestines. There, they grow and reproduce. Females lay about 200,000 eggs per day, which are eliminated with the feces.
In addition to Ascaris, which causes ascariasis, other species of parasitic roundworms are harmful to humans, including Wuchereria bancrofti, which causes elephantiasis or the swelling and thickening of the skin of legs and male genitals, as well as Ancylostoma duodenale and Necator americanus, which cause intestinal infection and anemia. Several roundworm species also attack plants, damaging roots and transmitting viruses. However, some roundworms are beneficial, because they are parasites of agricultural pests, such as cutworms.