pseudocoelomate


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pseudocoelomate

(so͞o'dōsē`ləmāt'), 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). The pseudocoelom is contrasted with the coelom of mollusks, annelid worms, and the more complex animals (including humans and other vertebrates). Pseudocoelomates lack a circulatory system, and the pseudocoelom itself lacks the endothelial lining of a coelom. The hydrostatic pressure of the pseudocoelom gives the body a supportive framework that acts as a skeleton. Nematodes or roundworms (see 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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), rotifers (see 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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), acanthocephalans (spiny-headed worms), kinorhynchs (see KinorhynchaKinorhyncha
, phylum containing about 150 species of tiny pseudocoelomate worms, it is widely distributed in marine sediments. The kinorhynch body is divided into 13 segments, each covered with a cuticle and equipped with characteristic spines.
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) and nematomorphs or horsehair worms (see NematomorphaNematomorpha
, small (about 230 species) phylum of pseudocoelomates; the horsehair worms. Most are very slender, elongated creatures found in ponds and streams, whose larvae live as parasites in arthropods. They emerge as adults for a brief time, then mate and die.
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) are pseudocoelomates.