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MESOZOA TYPE (MESOZOA)
Mesozoic are small (not more than 1 cm, usually less than 1 mm), organisms somewhat similar to flatworms, parasitizing inside some marine worms and molluscs. The Mesozoic are covered with cilia, usually dioecious, rarely hermaphrodite, have a complex developmental cycle with alternation of parthenogenetic generations with males and females (heterogony). Several dozen Mesozoic are known.
They are characterized by a very simplified structure. Outside, their body is covered with epithelium, the inner space is occupied only by generative cells, which can be considered as endoderm.