Real Dictionary

hydrozoan in American English
any animal of this class, as a hydroid or Portuguese man-of-war
of a class (Hydrozoa) of cnidarians having a saclike body consisting of two layers of cells, and a mouth that opens directly into the body cavity
adjective: belonging or pertaining to the Hydrozoa
noun: any freshwater or marine coelenterate of the class Hydrozoa, including free-swimming or attached types, as the hydra, in which one developmental stage, either the polyp or medusa, is absent, and colonial types, as the Portuguese man-of-war, in which both

hydrozoan in British English
adjective: of, relating to, or belonging to the Hydrozoa
noun: any colonial or solitary coelenterate of the class Hydrozoa, which includes the hydra, Portuguese man-of-war, and the sertularians