Polymorphism — the existence of two or more structurally and functionally different individual types (zooids) within the same species colony — is a hallmark of Cnidaria. Colonial cnidarians (e.g. Physalia — Portuguese man-o'-war, Obelia) have polymorphic zooids: polyps (sessile, for feeding/reproduction: hydranths, gonozooids, dactylozooids) and medusae (free-swimming, for sexual reproduction). Each zooid type is structurally adapted for its specific function.