Ectoproctous polyzoa the polypides of which have no epistome; the zoœcia are in nearly all cases distinctly separated from one another by transverse perforated plates.
Most of the Gymnolæmata are marine, but species belonging to two of the three suborders into which they are divided often stray into brackish water, while a few genera that belong to one of these two suborders are practically confined to fresh water. The three suborders are distinguished as follows:—
Suborder A. CHEILOSTOMATA.
The zoœcia are provided with a "lip" or lid hinged to the posterior margin of the orifice (see fig. 33, p. 175). This lid closes automatically outside the zoœcium or in a special chamber on the external surface (the "peristome") when the polypide retracts and is pushed open by the tentacles as they expand. The majority of the zoœcia in each zoarium are more or less distinctly flattened, but some of them are often modified to form "vibracula" and "avicularia."
The Cheilostomata are essentially a marine group, but some species are found in estuaries and even in pools and ditches of brackish water (fig. 33).
Suborder B. CTENOSTOMATA.
The zoœcia are provided with a collar-like membrane which is pleated vertically and closes together above the polypide inside the zoœcium when the former is retracted; it is thrust out of the zoœcium and expands into a ring-shaped form just before the tentacles are extruded. The zoœcia are usually more or less tubular, but in some genera and species are flattened.
The majority of the Ctenostomata are marine, but some genera are found in estuaries, while those of one section of the suborder live almost exclusively in fresh water.
Suborder C. CYCLOSTOMATA.