aschelminths


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aschelminths

(ăsk-hĕlmĭn`thz), large assemblage of loosely related, wormlike organisms of extremely varied structure and habits. Formerly considered an animal phylum, these organisms are now more commonly referred to as pseudocoelomatespseudocoelomate
, any of a group of invertebrates with a three-layered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers).
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 and are divided in to a number of different phyla, including NematodaNematoda
, phylum consisting of about 12,000 known species, and many more predicted species, of worms (commonly known as roundworms or threadworms). Nematodes live in the soil and other terrestrial habitats as well as in freshwater and marine environments; some live on the deep
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, Gastrotricha, and RotiferaRotifera,
phylum of predominantly free-living, microscopic, aquatic or semiterrestrial pseudocoelomates. Each rotifer has a head bearing a crown of cilia, the corona, at the anterior end; most rotifers feed with the aid of currents generated by the coronal cilia.
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. All are covered by a noncellular coat, or cuticle, and have a pseudocoelom, i.e., a fluid-filled cavity separating the body wall from the gut but lacking a peritoneal lining. In many species the digestive, excretory, and reproductive systems join in a cloaca, or discharge chamber, near the posterior end. Many aschelminths also show cell constancy, a condition in which each organ of the adult contains the precise number of cells characteristic of the species.
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