OBELIA

genus of invertebrate marine animals of the class Hydrozoa (phylum Cnidaria). The genus, widely distributed in all the oceans, is represented by many species. The animal begins life as a polypa stalklike form attached to the ocean bottom or some other solid surface. The polyp produces medusae, or jellyfish, which in turn, produce polyps. Rootlike filaments sent out by the polyp serve as organs of attachment. The stalk lengthens and develops a mouth encircled by tentacles. Buds arise from the stalk, developing into additional polyps, until a treelike colony forms. Specialized polyps (blastostyles), lacking mouth or tentacles, develop buds that separate as medusae and swim away. Eggs and sperm produced by the medusae unite and give rise to ciliated larvae (planulae) that swim freely for a time, then settle permanently onto a surface and develop into new polyps.