CHEILOSTOMATA

major division of the bryozoans (small, colonial, aquatic invertebrate animals) that first appeared during the Jurassic period (208 to 144 million years ago). Individual members of the cheilostome colony are 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 plantlike appearance of the colony is frequently very attractive. The cheilostomes are the most abundant and varied of modern bryozoans.