Nematodes are a varied and ubiquitous group, but fall into the general types of predators, saprophages, plant parasites and animal parasites. Even these categories are problematic, however, since some species occupy one niche during their juvenile stages and another at adult stages. Nematodes are found in almost every environment on earth, but their small size and their habits make them rarely seen by humans.
Nematodes are also known as roundworms and are distinct from other groups such as flatworms and annelids. They have long, thin, cylindrical bodies. Nematodes are often microscopic, but some parasitic species reach a meter in length. They are very simple organisms, with a straight, tube-like gut and no circulatory system. They exchange gases with their environment via diffusion through their skin, but they engage in sexual reproduction — not asexual reproduction — with some species producing up to 100,000 eggs per day.
Nematodes are extremely widespread in almost every environment, but all live concealed, whether under dirt or burrowed into another organism. They are very important in breaking down dead organic matter in many environments. Some species harmlessly infest living animals and remain dormant until they die, and then they eat the dead tissue.
The various species that are parasitic can be devastating, both to heath and to the economy. Parasitic nematodes that eat plants can lead to major crop losses, and parasitic diseases caused by nematodes can be deadly.