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Mesozoan - Britannica.com - English
URL: http://www.britannica.com/bcom/eb/article/3/0,5716,53533 1 52227,00.html

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.

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Mesozoa - English
URL: http://www.fwkc.com/encyclopedia/low/articles/m/m016001505f.html

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.

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Invertebrate Zoology  - Mesozoa-Porifera - English
URL: http://www.marietta.edu/~mcshaffd/invert/mesozoa.html

Characters of phylum.

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