What is the X in X-mas?
free-swimming or crawling larval type common in many species of the phylum Cnidaria (e.g., jellyfish, corals, and sea anemones). The planula body is more or less cylindrical or egg-shaped and bears numerous cilia (tiny hairlike projections), which are used for locomotion. Planulae are produced by both of the basic body forms found in cnidarians-polyps, the stalklike, attached adult forms, and medusae, the bell- or umbrella-shaped, free-swimming adult forms. See also polyp (zoology); medusa.