Hydrozoans are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The colonies of the species can be large, and in some cases the specialized individual animals cannot survive individually. A few within this class live in fresh water. Hydrozoans are related to jellyfish and corals, and 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.
Portuguese man-of-war |
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The Portuguese man-of-war fish is found in the Atlantic, Indian, and Pacific Oceans. It's tentacles can deliver a venomous and painful, sometimes fatal, sting. It is not a jellyfish but a siphonophore, which is distinguished from jellyfish in that it is not a single multicellular organism, but a colonial organism made up of specialized minute individual organisms called zooids. They have no independent means of propulsion and either drift on the currents or catch the wind with their pneumatophores.