Explanation:Obelia:Obelia is a genus of animals.They include invertebrate marine animals.Obelia is a genus which belongs to the phylum Cnidaria.They are commonly found in oceans.They have a polyp, which is a stalklike attachment to the ground.They do not show alteration of generation.Phylum Cnidaria:They are marine in nature.They are diploblastic and have specialized nervous tissues.All the members of the group have radial symmetry.They show polymorphism which means their body is divided into polyp and medusa.Important PointsPolymorphic:The organisms which show polymorphism is called polymorphic.Here, Obelia is polymorphic in nature.Colonial:The organisms which form or occur in colonies are said as colonial.Example- volvox, obelia etc.Aquatic:The organisms which are found in water bodies are known as aquatic in nature.As obelia is a marine organism it is aquatic in nature.Sedentary:The organisms which move very slowly are called sedentary in nature.Example- protozoans, heliozoans etc.