Nematodes are microscopic animals commonly known as eelworms or roundworms. They comprise the phylum Nematoda members of which are present in every habitat aquatic or terrestrial.

Nematodes have varied life style and they occupy all trophic levels in the soil ecosystem. They may feed on bacteria (bacteriovorous), fungi (fungivores), other soil microorganism (predators), plants (herbivores) while some may have diverse feeding habitats (omnivores).

Scientist conveniently classify them into two groups, parasitic (plants and animals including man) and free living (encompassing all other groups). A great majority of nearly 50% are marine inhabitants and a smaller proportion of about 25% are free-living.

Approximately 10% are plant parasites and 15% animal parasites (Viglierchio, 1991). They are widespread in the polar seas, oceans, deserts, hot water springs, mountain tops and the frozen Antarctica.

Nematodes are considered to be one of the most abundant groups among the invertebrates on the earth along with the arthropods. Their numbers in different habitats have been estimated by several workers and range from 1.

5 billion in the upper 20 mm of an acre of marine beach sand to 3 billion in a single acre of soil (Chitwood & Chitwood, 1950). Nearly 16 million are estimated in a square metre of intertidal area (Teal & Wieser, 1966) and 380 million in a square metre of leaf litter (Wasilewska, 1979).

Approximately 28,000 nematode species have been described so far and estimates of the probable number of species ranges from 100,000 to 10 million (Poinar, 2011). Their physiology, structure, adaptability and reproductive patterns have helped them to colonize every habitat on land and in water.

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