Members of this phylum are grouped into three classes. Stenolaemata
is an ancient class including one living order, Cyclostomata,
comprising calcified marine species, with tubular zooids
and robust colonies, such as Tubulipora (Figure A). Zooids are
typically monomorphic, and the terminal aperture, through
which the lophophore is protruded, is closed by a sphincter
muscle. Class Gymnolaemata consists of two orders: the Ctenostomata
(Figure B), in which the zooids are uncalcified, and the
aperture is closed by a sphincter muscle, and the Cheilostomata
(Figures C and D) in which zooids are more or less calcified and
the aperture is closed by a chitinized, hinged flap of body wall, the
operculum. Most extant marine bryozoans, and a few freshwater
species, are grouped in this class. Ctenostomate zooids show limited
polymorphism; variously shaped kenozooids (empty chambers
lacking polypides) are modified as attachment structures,
or in series from cylindrical stolons. Many cheilostomate species
show striking zooid polymorphism. The autozooid, with feeding
lophophore, is the basic unit of the colony, and kenozooids of
varying size and shape are modified as spines and stolons, and as
structural units that contribute to the architecture of the colony.