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 omnipresent 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. They represent, for example, 90% of all life forms on the ocean floor. Their numerical dominance, often exceeding more than 1 million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity in lifestyles and their presence at various levels point at an important role in many ecosystems. Their many parasitic forms include pathogens in most plants and animals (including humans).
Ascaris
Ascaria lack a distinct circulatory system. Circulation occurs when fluids in the body cavity slosh/move randomly back and forth in the pseudocoel. They use diffusion for circulation of substances around their body.Muscular movements by the animal during locomotion can facilitate hemolymph movement, but equal flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores.
Hookworm
The hookworm is another example of an open circulatory system. Hemolymph in a cavity called the hemocoel bathes the organs directly with oxygen and nutrients and there is no distinction between blood and interstitial fluid; this combined fluid is called hemolymph. Hemolymph fills all of the interior hemocoel of the body and surrounds all cells. Hemolymph is composed of water, inorganic satls, and organic compunds( mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.
Filaria
As in Ascaria, filaria lack a distinct circulatory system. Circulation occurs when fluids in the body cavity slosh back and forth in the pseudocoel. They use diffusion for circulation of substances around their body.Muscular movements by the animal during locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores.