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( Nematoda)
Adenophorea
&_160;&_160;&_160;Subclass Enoplia
&_160;&_160;&_160;Subclass Chromadoria
Secernentea
&_160;&_160;&_160;Subclass Rhabditia
&_160;&_160;&_160;Subclass Spiruria
&_160;&_160;&_160;Subclass Diplogasteria
&_160;&_160;&_160;Subclass Tylenchia The nematodes or roundworms (Phylum Nematoda from Greek ??µa (nema) "thread" + -?d? -ode "like") are one of the most common phyla of animals, with over 80,000 different described species (over 15,000 are parasitic). 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 Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants, animals, and also in humans. The nematodes were originally named nematoidea by Rudolphi (1808). They were renamed nematodes by Burmeister 1837 (as a family; Leuckart 1848 and von Siebold 1848 both promoted them to the rank of order), then nematoda (Diesing 1861), though Nathan Cobb (1919) argued that they should be called nemata or nemates (and in English 'nemas' rather than 'nematodes'). After some confusion which saw the nematodes placed (often together with the horsehair worms, nematomorpha) as a class or order in various groups such as Aschelminthes, Lankester (1877) definitively promoted them to the level of phylum. Nematodes are unsegmented, bilaterally symmetric and triploblastic protostomes with a complete digestive system. Roundworms have no circulatory or respiratory systems; most of them are essentially anaerobic. Although they lack a circulatory system, nutrients are transported throughout the body via fluid in the pseudocoelom. They are thin and are round in cross section. Nematodes are one of the simplest animal groups to have a complete digestive system, with a separate orifice for food intake and waste excretion, a pattern followed by all subsequent, more complex animals. The body cavity is a pseudocoelom (persistent blastula), which lacks the muscles of coelomate animals used to force food down the digestive tract. Nematodes thus depend on internal/external pressures and body movement to move food through their digestive tracts. The mouth is often surrounded by various flaps or projections used in feeding and sensation. The portion of the body past the anus or cloaca is called the "tail." As they grow, their cells get larger, but the total number is constant, called eutely. The epidermis secretes a layered cuticle made of three layers of collagen[1] that protects the body from drying out, from digestive juices, or from other harsh environments. Although this cuticle allows movement and shape changes via a hydrostatic skeletal system, it is very inelastic so does not allow the volume of the worm to increase. Therefore, as the worm grows, it has to molt and form new cuticles. The cuticles don't allow volume to increase so as to keep hydrostatic pressure inside the organism very high. For this reason, the roundworms do not possess circular muscles (just longitudinal ones) as they're not required. This hydrostatic pressure is the reason the roundworms are round.
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