The nematodes or roundworms (phylum Nematoda) are the most diverse phylum in the world. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. The total number of nematode species has been estimated to be about 1,000,000. Nematodes have successfully adapted to nearly every ecosystem from marine to fresh water, to soils, and from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts, oceanic trenches, and within the earth's lithosphere.
Roundworms are a member of the nemathelminths phylum or group of animals. The hookworm, pinworm and trichinella are part of this group. They are more advanced than flatworms but less advanced than earthworms. They have thin round bodies, with none of the pieces or segments that earthworms have in their bodies. Roundworms do not have skeletons or ecto-skeletons. The space between the outside of the digestive system and the muscles and skin of the worms is filled with pressurized fluid. Like a long balloon, this space tends to "rebound" back to a set shape when bent, or it changes shape ways when squeezed, giving some resistance or direction to the action of the muscles. A muscle-resistance system based upon fluid-filled spaces is called a hydro-static skeleton.
The eye worm, Loa loa, common parasite of humans and other primates in central and western Africa, a member of the phylum Nematoda. It is transmitted to humans by the deer fly, which feeds on primate blood. When the fly lands on a human victim, the worm larva drops onto the new host’s skin and burrows underneath. The larva migratesthrough the bloodstream, commonly locating in the eye or in other tissues just under the skin. The adult worm is 3–6 cm long. The movement of the worm beneath the skin may cause itching or sometimes swellings as large as a hen’s egg. As well as roundworms and other nematodes the eye worm as a smaller hydro-static skeleton.
The hook worm is a parasitic nematode that lives in the small intestine of its host, which may be a mammal such as a dog, cat, or human. Hookworms are much smaller than the roundworms, and the complications of tissue migration and mechanical obstruction so frequently observed with roundworm infestation are less frequent in hookworm infestation. Like all the other nematodes described in this page, the hookworm does not have a skeleton at all. It has a muscle structure and a hydro-static skeleton similar to the eye worm and the round worm.