The Hydrozoa is a subgroup of cnidarians containging approximately 3700 species. It is a diverse group with a variety of life cycles, growth forms, and specialized structures. Like many cnidarians, hydrozoans have both polyp and medusa stages in their life cycle. They are distinguished from other groups by their complex life cycle, by the growth of medusae from buds rather than strobilae or from metamorphosis, by the presence of a velum inside the bell of the medusa, and by the production of gametes from ectodermal, rather than endodermal, tissue. Most hydrozoans are marine, and hydrozoan species are found in nearly every marine habitat type; a very few species live in freshwater. Most hydrozoans form colonies of asexual polyps and free-swimming sexual medusae. Colonies are usually benthic, but some, notably the siphonophores, are pelagic floaters. Colonial polyps often have some division of function, with certain polyps specialized for defense, feeding, or reproduction. Most hydrozoans are predators or filter-feeders, though a few have symbiotic algae (zooxanthellae), in the same way that other other groups of cnidarians do.
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