MOSS ANIMAL

also called bryozoan, any member of the phylum Bryozoa (also called Polyzoa or Ectoprocta), in which there are probably more than 4,000 extant species. The bryozoans are a widely distributed, aquatic, invertebrate group of animals whose members form colonies composed of numerous units called zooids (hence the term Polyzoa, which means many animals). Until the mid-18th century, bryozoans, like corals, were regarded as plants; hence their name, which means moss animals. Seventy-five years later, the bryozoans were distinguished from the cnidarians, and the characteristic structure of the zooid was first described. Bryozoans are separated into three classes: Phylactolaemata (freshwater dwelling); Stenolaemata (marine); and Gymnolaemata (mostly marine). The order Cheilostomata (class Gymnolaemata), containing 600 genera, is the most successful bryozoan group. also called Bryozoan (phylum Bryozoa, Polyzoa, or Ectoprocta), an aquatic invertebrate whose individual members (called zooids) form colonies of varying size and shape. Bryozoa is a very diverse group. The small genus Monobryozoon lives between sand particles in the ocean and is less than one millimetre in length, while the warm-water Zoobotryon colony hangs from harbour pilings in clumps or chains that can be as much as a foot and a half across. Moss animals are inevitably found as growths or crusts on other objects, such as overhanging rocks by the shore, plants, or the hulls of ships. They are separated into three classes: Phylactolaemata (freshwater-dwelling), Stenolaemata (marine), and Gymnolaemata (mostly marine), which includes the order Cheilostomata, the most successful of all bryozoan groups. The texture of different moss animal colonies can be soft and gelatinous (as is the case in many marine varieties), tufted with leaflike fronds, or covered with slender branches formed by lime deposits in the zooid walls. Other colonies form hard, calcified skeletons, like the lace corals Retepora or Sertella, which create slender branchlike twigs. Colonies are made up of individual zooids, each of which is a complete and fully organized animal. Zooids are connected by the matrix of the colony and may communicate by tiny pores in their body walls. Most zooids have ciliated tentacles (tentacles with mobile, hairlike projections) that propel food particles toward the simple mouth of the animal. Their guts are sectioned and fold upward to end in an anus just outside the tentacle-lined mouth. These tentacles, the digestive tract, and the nervous system and muscles that control them, form the polypide of the zooid. A colony is formed from a single zooid that budsthat is, asexually duplicates itselfforming a colony of a definite shape and growth pattern. This zooid originates from a resting bud (statoblast) or from an ancestrula, which originated from the metamorphosis of a sexually produced larva. In Gymnolaemata and Phylactolaemata the zooids are generally hermaphroditic, with clusters of male and female reproductive organs either near the polypide or lining the body wall. Stenolaemates are more specialized, however, with most zooids male and only a few females per colony. The female zooids develop spacious brood chambers; a developing embryo may squeeze off secondary embryos and continue repeating the process, so that many larvae develop in a single chamber. Freshwater moss animals live primarily on leaves, stems, and tree roots in shallow water. Marine moss animals have a wide range of habitats, from coastal areas to great ocean depths, but they are most common just below the tidemarks. While the presence of currents and flowing water is important, since it brings food within reach of the colony, vigorous wave action militates against bryozoan growth, and shallow, sheltered channels are preferred. Sea slugs and sea spiders seem to be their chief predators. Moss animals have no respiratory, excretory, or circulatory system. The major role of the tentacles is to capture plankton, and they are arranged in a manner similar to a funnel with the mouth at its vertex. If disturbed, a feeding zooid will pull in its tentacles with the aid of powerful retractor muscles. To extend them, the zooid pulls in its body wall, forcing fluid and the tentacles back into the water. Since most zooidal walls are calcareous, moss animals have left a long fossil record. The earliest fossils were stenolaemates found in rocks dating from the Lower Ordovician Period (430,000,000 to 500,000,000 years ago). A majority of limestone formations, especially those alternating with strata of shale, preserve moss animal fossils. Bryozoans apparently evolved rapidly, in several cases producing orders that flourished for millions of years only to become extinct. Additional reading J.S. Ryland, Bryozoans (1970), is a general survey of fossil and living bryozoans for the nonspecialist. Advanced accounts include Libbie Henrietta Hyman, The Lophophorate CoelomatesPhylum Ectoprocta, ch. 20 in the author's Invertebrates, vol. 5, Smaller Coelomate Groups (1959); Robert M. Woollacott and Russell L. Zimmer (eds.), Biology of Bryozoans (1977); and J.S. Ryland, Physiology and Ecology of Marine Bryozoans, a review in Advances in Marine Biology, vol. 14, pp. 285443 (1976). Proceedings of the seventh international conference of the International Bryozoology Association, held in 1986, resulted in the collection of current research in June R.P. Ross (ed.), Bryozoa: Present and Past (1987). Other contributions to the subject are found in Richard S. Boardman, Alan H. Cheetham, and William A. Oliver, Jr. (eds.), Animal Colonies: Development and Function Through Time (1973); G. Larwood and B.R. Rosen (eds.), Biology and Systematics of Colonial Organisms (1979); Jeremy B.C. Jackson, Leo W. Buss, and Robert E. Cook (eds.), Population Biology and Evolution of Clonal Organisms (1985); and J.L. Harper, B.R. Rosen, and J. White (eds.), The Growth and Form of Modular Organisms (1986). Well-illustrated surveys of the bryozoans may be found on pp. 667677 of ch. 26, Lophophorates, in Vicki Pearse et al., Living Invertebrates (1987), and on pp. 238242 of ch. 14, Lesser Lights, in Ralph Buchsbaum et al., Animals Without Backbones, 3rd ed. (1987). Extensive introductions are offered in the four works belonging to the Synopses of the British Fauna series: J.S. Ryland and P.J. Hayward, British Anascan Bryozoans: Cheilostomata, Anasca: Keys and Notes for the Identification of the Species (1977); P.J. Hayward and J.S. Ryland, British Ascophoran Bryozoans (1979); P.J. Hayward, Ctenostome Bryozoans (1985); and P.J. Hayward and J.S. Ryland, Cyclostome Bryozoans (1985). A classification of extant Bryozoa is provided by J.S. Ryland, Bryozoa, pp. 743769, vol. 2, in Sybil P. Parker (ed.), Synopsis and Classification of Living Organisms, 2 vol. (1982). John S. Ryland