Nematodes are bilaterally symmetrical, unsegmented animals without a coelom. Their body cavity is the primary body cavity, which is not lined by epithelia but an extracellular matrix (ECM). Nematodes have colonized all conceivable habitats, including the deep sea, the soils of aquatic and terrestrial biotopes, and, as parasites, the body cavities or tissues of animals and plants. Some researchers estimate that there may be as many as one million nematode species on Earth. However, only about 80,000 species have been taxonomically recorded so far. Although not the most species-rich group, nematodes are considered the most abundant metazoan animals on the planet.