A Bryozoon is a microscopic polype, inclosed in an open horny or calcareous sheath, out of which it can protrude and draw in the anterior part of its body. It is seldom or never seen alone, on account of its tendency to propagate by budding. When the buds spring from the sides of the sheath or cell, it is known as the Sea Mat, or Flustra. The Flustra, which is common on our coasts, spreads its hexagonal cells like a delicate network over sea-weeds, shells and other marine substances. Sometimes the polypes are so closely arranged on both sides of a leaf that a square inch may contain 1,800. In the calcareous genera, Eschara and Cellipora, the cells have a lid movable by two muscles, so that the polypes can close the orifice, and shut themselves in.

In the greater number of the Polyzoa the polype has a cylindrical form, a mouth at its anterior extremity surrounded by an annular disk, which forms the roof of the internal cavity containing the stomach and the other digestive organs. The disk is furnished with eight, ten or a greater number of tubular tentacles, which surround the mouth, their tubes being continuations of the internal cavity below. The mouth leads into a funnel-shaped space, separated by a valve from the gullet; and the gullet ends in a capacious stomach. Short vibratile cilia are arranged like a fringe on the opposite sides of each tentacle, which form two currents in the surrounding water—an ascending stream on the outside, and a descending one on the inside. When any particles of food that may be carried down the inner surface of the tentacles arrive at the mouth, a selection is made, the rejected particles being carried off by the stream, while those that are chosen are received by the funnel-shaped mouth, and pass through a valve in the gullet into the stomach, where they are kept in continual motion by cilia, and the refuse is ejected by an orifice near the mouth.

Fig. 158 represents the cells of different species of the genus Lepralia, which form crusts upon marine objects. Other genera grow as independent plant-like structures, and some take an arborescent form, and creep over rocks and stones. The Cellularia ciliata, of which fig. 159 is a magnified portion, rises in upright branching groups like little shrubs; and as many are commonly assembled together, they form miniature groves, fringing the sides of dark rocky sea pools on our coasts. Most of the Polyzoa have pedicellariæ attached to their stems, either sessile or stalked. Their forms are various: a jointed spine, a pair of pincers, &c. But on the Cellularia they are like a bird’s head with a crooked beak, opening very wide, and attached by a stem. B, fig. 159, represents a highly magnified pedicellaria in the act of seizing another. These bird’s head appendages are numerous on the Cellularia ciliata, and in constant motion; the jaws are perpetually snapping little worms, or anything that comes in their way, while the whole head nods rhythmically on its stalk. Two sets of muscles move the jaws; when open, a pencil of bristles projects beyond them, which is drawn in again when they are closed; they are supposed to be organs of feeling. The Polyzoa have organs also called vibracula, which are bristle-shaped, as on A, fig. 158; these sweep over the surface of the Polyzoon to remove anything that might injure the polypes.

It is believed that the polypes of the Polyzoa are male and female, and that the ciliated locomotive larvæ which appear in spring are developed from eggs. The fresh-water Polyzoa are as worthy of microscopic examination as the marine.