This animal having selected a piece of floating wood for its permanent abode, attaches itself to it by the head, which is immovably fixed by a tenacious glue exuded from glands at the base of the antennæ. The bivalve shell is subsequently thrown off, a portion of the head becomes excessively elongated to form the peduncle of the Barnacle or Lepas, and in that state it is exactly like the Lucifer Stomapod. In the Balanus, on the contrary, the head expands into a broad disk of adhesion, and the animal resembles the Mysis or Opossum Shrimp.
From the first segment of the throat a prolongation is sent backwards which covers the whole body, and the outer layer is converted into the multivalve shell; and the three pairs of cirrhated feet, which were formed in the larval state, now bend backwards from the other three rings of the throat.
Though the Cirripeds lose their eyes in their mature state, they are sensitive to light. They draw in their cirrhated feet, and the Balanus even closes the lid of its shell under the shadow of a passing cloud.
SECTION IX.
BRYOZOA, OR POLYZOA.
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. Cells of Lepraliæ.—A, L. Hyndmanni; B, L. figularis; C, L. verrucosa.
Fig. 159. A, Cellularia ciliata; B, ‘bird’s head’ process of Bugula avicularia, highly magnified, seizing another.