Hydrozoans are solitary or colonial polyps. Most are sedentary, relying on medusae for dispersal. A few are pelagic (free-swimming). Polyps move their tentacles and stalks with simple muscle cells.
Right: The deep water solitary polyp Branchicerianthus imperator from the seas of Japan.
Below: Physalia spp. the Portuguese Man of War is a colony of polymorphic individuals specialised for catching prey, feeding, protection or reproduction. Suspended by a buoyant float, its tentacles expand and trawl for prey. Their virulent cnidocytes cause painful, sometimes lethal, stings to unwary swimmers as well as fish.