Bryozoa

Bryozoa, a class usually placed near the Brachiopoda, but of which the exact position in the animal kingdom is as yet undecided. Except one genus (Loxosoma) the members of this class are compound, and live in colonies which may encrust shells or stones, but which more often grow into irregular plant-like tufts; when, as is the case with most of the English species, the skeleton is composed of the horny material known as "chitin," the colonies are usually mistaken for seaweeds. In their mode of life they also closely resemble the Zoophytes of the class Hydrozoa, and it was not till 1830 that their great differences were first discovered by Thompson of Cork; the term "Polyzoa" which he used in describing them is, by some English authors, adopted as the name of the class.

Though the colonies (or "polyzoaria," as the whole skeletons are called) are often of considerable size, the separate individuals (zooids or polypites) are minute. Each zooid is composed of two coats forming a small sac, open at one end; here are placed the mouth and anus; around the former (in the Ectoprocta) or around both (in the Entoprocta) is a circle or crescent of arms, forming the "lophophore;" by the lashing of the cilia (q.v.) with which the arms are clothed, currents of water are set up, by which the food is obtained and respiration effected. The outer coat (ectocyst) may be calcareous, chitinous, or gelatinous. It was at one time suggested that the zooid, as here described, consisted of two individuals, the cell or Cystid, and the digestive animal or Polypid. Though this is improbable, a certain amount of dimorphism (i.e. specialisation of certain individuals for special functions) does occur; thus some zooids are modified into "avicularia" (q.v.) or "bird's-head processes," others into "vibracula," (q.v.) and others into "oecia," or chambers which serve as marsupial pouches for the protection of the eggs. In some fresh water species reproduction sometimes occurs by "statoblasts," i.e. winter eggs which are not fertilised and may be regarded as internal buds. The larvae undergo a metamorphosis. The Bryozoa are mainly marine. The position of the class in the animal kingdom is rendered doubtful owing to some peculiar forms which some authors include among the lower Chordata (q.v.); such are the two remarkable genera that form the group of the Pterobranchia, and Phoronis, the only genus of the Vermiformia, but it is probable these are not as closely related to the true Bryozoa as was once thought. The true Bryozoa are divided into two groups, the Entoprocta and the Ectoprocta, to which reference should be made for the further subdivisions.