Definition of hydrozoan
: any of a class (Hydrozoa) of cnidarians (such as the hydra, fire coral, and Portuguese man-of-war) that includes solitary and colonial polyps and medusae but often having the medusa stage reduced or absent and lacking nematocysts in the digestive cavity — see siphonophore
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Merriam-Webster unabridged
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Time Traveler for hydrozoan
The first known use of hydrozoan was in 1877
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