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Mesozoan - Britannica.com
Any of approximately 50 species of small, ciliated, multicellular animals that are parasitic in
marine invertebrates. These wormlike organisms lack digestive, respiratory, nervous, and excretory systems; their
bodies consist of two layers of as few as 20 to 30 cells each. Both sexual and asexual reproduction occur. Their
relationship to other phyla is obscure as it is not known whether their simple structure is primitive or, as a result of
their parasitic existence, degenerate (i.e., changed gradually into a lower form). Some authorities, however, have
suggested a link with the phylum Platyhelminthes, a group of flatworms. [eng]
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Invertebrate Zoology - Mesozoa-Porifera
Characters of phylum. [eng]
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Mesozoa
Mesozoa (Gr. mesos, "middle"; zoion, "animal"), phylum or superphylum of life forms, sometimes thought to be transitional from unicellular to multicellular organisms.
The body consists of a layer of outer cells surrounding internal reproductive cells and has no real organs. Mesozoans live as internal parasites of marine invertebrates
except during dispersal. Some authorities consider them degenerate flatworms; others deny that they are animals. The group contains about 50 species placed in two
classes or orders: Dicyemida and Orthonectida. [eng]
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