Nematodes also called as the roundworms belong to the phylum Nematoda and are one of the most diverse groups of all the animals. The species that belong to the nematodes are very difficult to distinguish from the others since more than 80,000 species have been described. Of the 80,000 species around 15,000 species are parasitic. Contrasting to the flatworms, roundworms have a tube like digestive system at both the ends.
Nematodes have successfully adapted to all the surroundings from marine to the fresh water, from polar to the tropic and as well to the highest to the lowest elevation. They are omnipresent in all the habitats and they outnumber all the other animals as an individual as well as in groups.
Nematodes are slender multicellular organisms that are less than 2.5 mm. The smallest nematodes species are microscopic and the largest can grow up to 5 cm. The body of the nematodes has a distinctive structure and is decorated with ridges, rings, bristles, and warts.
The body of the nematodes is bilaterally symmetrical whereas the head is radially symmetrical. The body is tapered at both the ends. And in many species solid head shields radiating which is projected outwards is found around the mouth. The mouth has a either three or six lips with a series of teeth at the inner edge.
In most of the species sexes are separate and some are Hermaphroditic i.e. both the male and female reproductive organs are found in an individual. The parasitic nematodes are found in all most all the organs of the body. But the most common organs where these species can be found are the alimentary, circulator, and respiratory systems.
Some of the common Nematodes are hookworms, lungworms, pinworm, eelworm, whipworm, and threadworm. These Nematodes have the ability to cause many diseases like filariasis, ascariasis, and trichinosis. These parasitic species may also cause damage to the plant and animals.
The roundworms reproduce by laying eggs. The egg or larvae of the Nematodes are found in the soil and get into the person body comes in contact with it. The parasitic infections are most common in warm tropical climate.