Recent Nematoda includes 3 classes, 31 orders, 267 families, 2829 genera and 42783 species, with fossil taxa represented in 2 genera by 10 species.There are 7 genera and 7 species known only as fossils (Hodda 2011).
The phylum Nematoda, commonly known as the roundworms, included 24,793 described species as of 2011.Described species, however, make up only a small fraction of total nematode number, which is thought to be around 1 million species (although some estimate it at up to 75 million).Tremendously diverse, nematodes occupy every area on earth. The smallest nematodes are microscopic, other species reach over a meter in length (Hodda 2011).
Living either as parasites or free-living organisms, nematodes generally eat living material.Free-living species eat bacteria or other small-bodied organisms.They play an important role in the decomposition process and the recycling of nutrients in marine environments.Hodda (2011) estimates that parasitic nematodes exist in every other animal phylum larger than themselves, and all plants, with the exception of some marine plants. Several common human nematode parasites cause intestinal and subcutaneous disease including the ascarids, filarias, ancylostomids (hookworms), Enterobius (pinworms/threadworms) and Trichuris (whipworms). Nematode parasites in humans often occur in warm/tropical environments.Plant-parasitic nematodes can affect crops, causing damage to roots, formation of galls, and transmission of viruses (Hodda 2011; Wikipedia 2014).
A particularly important nematode, the soil dwelling, free-living Caenorhabditis elegans, was first investigated by Sydney Brenner in the 1960s as a model organism for scientific exploration. This nematode has since been extensively studied and has contributed to scientific discoveries in a huge range of disciplines including cell, molecular, developmental, evolutionary and neuro- biology.Researchers have sequenced the C. elegans genome, determined the developmental fate of every cell, and mapped every neuron (wormclassroom.org).
Many free-living nematodes are carnivorous, they feed on animals that are even smaller than they are including other nematodes.
Other free living nematodes feed on phytoplankton such as diatoms, algae and fungi. Many terrestrial species feed on plant roots, penetrating the cells and sucking out the contents. They are considered parasitic in the way that fleas are parasitic on other animals, they can cause great damage to the plants in this manner.
Species that live in sediments and other aquatic environments ingesting particles of the substrate when they digest associated bacteria and / or organic material.
Others feed more directly on dead organic material such as decomposing plants and animals or dung where they may actually be eating the bacteria or fungi that are feeding on the decomposing material rather than being a decomposer directly.
