major group of calcified bryozoans (small, colonial, aquatic invertebrate animals) that first appeared during the Jurassic period (200 to 146 million years ago). Individual members of the cheilostome colony are small (usually less than 1 mm 【0.04 inch】) and protected by a calcareous or chitinous covering that may be closed by a lidlike structure, an operculum. The cheilostomes are exclusively marine animals, and the colony frequently has an attractive, plantlike appearance. The cheilostomes are the most abundant and varied of modern bryozoans; there are about 2,800 described species.