Mesozoa

Mesozoa [‚mez·ə′zō·ə]

(invertebrate zoology)

A division of the animal kingdom sometimes ranked intermediate between the Protozoa and the Metazoa; composed of two orders of small parasitic, wormlike organisms.

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Mesozoa

A division of the animal kingdom sometimes ranked as intermediate between the Protozoa and the Metazoa. These animals are unassignable to any of the better-known phyla, as usually defined. In the absence of proof concerning their relationships, and in view of the disagreement among zoologists relative to their affinities and even with respect to the facts and interpretation of their structure and life cycle, they are treated as a small phylum somewhere between Protozoa and Platyhelminthes. No particular phylogenetic interpretation should be attached to this placement.

The Mesozoa comprise two orders of small, wormlike organisms, the Dicyemida and the Orthonectida. Both are parasitic in marine invertebrates. The body consists of a single layer of ciliated cells enclosing one or more reproductive cells. These body cells are rather constant in number and arrangement for any given species. The internal cells do not correspond to the entoderm of other animals, as they have no digestive function. The life cycles are complex, involving both sexual and asexual generations (metagenesis).