Hydrozoa

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Hydrozoa (hydroids; phylum Cnidaria) A class of multicellular, mainly marine animals in which the cells are derived from two layers, epidermis and gastrodermis (endodermis), separated by a gelatinous mesogloea. These enclose a continuous gastrovascular cavity (coelenteron), which communicates directly with the exterior by a single aperture (mouth) and is lined by a gastrodermis. The gastrodermis lacks nematocysts. Eggs and sperm are shed outside the animal and not into the gastrovascular cavity.

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