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.
[Illustration: Fig. 158. Cells of Lepraliæ.—A, L. Hyndmanni; B, L.
figularis; C, L. verrucosa.]
[Illustration: Fig. 159. A, Cellularia ciliata; B, ‘bird’s head’ process
of Bugula avicularia, highly magnified, seizing another.]
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.