The Class Hydrozoa is a group of mostly small, mostly marine cnidarians that includes hydras, Portuguese man o' war, and many colonial species. Hydrozoans can exist as polyp colonies, medusae, or alternate between both in their life cycles. They are characterized by stinging cells (cnidocytes) for capturing prey, simple body structures without true organs, and a mostly predatory lifestyle feeding on plankton and small animals. Hydrozoans play important roles in aquatic ecosystems as both predators and prey.