POLYZ OA. A group of animals so called from the fact that numerous individuals, formed by budding, remain united in a colony. They were mostly included in the Zoophyta of the older naturalists, in consequence of their plant-like appearance, a feature which is emphasized in their alternative name, Bry ozoa, or moss-like animals. They are here regarded as a primary group of inverte brates, although they have been placed with the Brachiopoda in a larger group, Molluscoidea—an association which rests on insufficient evidence. Certain characters of their larvae suggest that they may be distantly related to the Mollusca and Roti fera. They are pre-eminently marine, but a small proportion are confined to fresh water. A colony may have a diameter of a foot, but this size is exceptional in recent forms ; and in many cases the longest meas urement does not exceed an inch, the individuals ("zooecia") being commonly less than a millimetre long. The walls are gen erally stiffened with calcareous matter, and the Polyzoa are ac cordingly represented by numerous fossil species. It is probable that as their study advances they will take an increasingly import ant part in the determination of the geological age of strata.
They may be defined as aquatic animals, forming colonies by bud ding, with ciliated tentacles which can be infolded or retracted into a depression of the body-wall, with a U-shaped alimentary canal and a ganglion lying be tween the mouth and anus.
Specific excretory organs are found in the Entoprocta alone.
, Classification.—Sub-class I. Entoprocta.—Lophophore (the part bearing the tentacles) circular, including both mouth and anus. Tentacles infolded, during retraction, into a "vestibule," which can be closed by a circular muscle. Body-wall not calci fied, body-cavity absent. Definite excretory organs present. Re productive organs continuous with ducts, which open into the vestibule.
Loxosoma (fig. I), marine, is unique among the Polyzoa in the fact that the colony consists merely of a single individual, with its system of buds, which are produced in two series, each bud breaking off on reaching maturity. It is of minute size, and nearly
always occurs on other animals, Sponges, Polychaet and Sipun culid worms, Ascidians or even Polyzoa. As in other Entoprocta, the individual consists of a muscular stalk bearing a "calyx" or body, which contains the viscera and bears the tentacles. Pedicel lina (fig. 1), marine, has the individuals arranged in a linear series on an attached, narrow "stolon." The calyces are deciduous and can be regener ated, a process analogous to the formation of "brown bodies" in the Ectoprocta.
Urnatella, a fresh water species.
Sub-class II. Ectoprocta.—Lophophore circular or shaped like a horseshoe (fig. 2), including the mouth but not the anus. Tentacles retractile into a delicate, flexible "introvert" ("tentacle-sheath") of the body-wall (fig. 6). Remainders of the body-wall membranous or calcified, the body-cavity spacious, containing the re productive organs, which are not con tinuous with ducts. Specific excretory organs absent. Zooecia usually closely apposed to their neigh bours, with which they are in organic connection by means of threads of living tissue, traversing "communication-pores" in the separating walls. There is naturally no evidence as to the anatomy of the two exclusively fossil Orders.
Tribe I. Gymnolaemata. Lophophore circular (fig. 2), without an "epistome" or lip. Body-cavities not continuous with one an other, body-wall not muscular.
Order 1. Trepostomata. Fossil, Palaeozoic. Zooecia long, co herent, their cavity traversed by many transverse partitions, which become more numerous near the terminal orifice. Surface of col ony with regularly distributed elevations or "monticules." The reference of these organisms, which include the Monticuliporidae, to the Polyzoa has been disputed, but the characters of the pri mary individual of the colony are in favour of this association.