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 zooecia 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 zooecium 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 zooecia 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 zooecia are provided with a collar-like membrane which is pleated vertically and closes together above the polypide inside the zooecium when the former is retracted; it is thrust out of the zooecium and expands into a ring-shaped form just before the tentacles are extruded. The zooecia 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._ The zooecia are provided neither with a lip nor with a collar-like membrane. They are tubular and usually have circular orifices. The Cyclostomata are exclusively marine. Order II. PHYLACTOLÆMATA. Ectoproctous polyzoa the polypides of which have a leaf-shaped organ called an epistome projecting upwards and forwards within the circle of tentacles and between the mouth and the anus. The zooecia are not distinct from one another, but in dendritic forms the zoarium is divided irregularly by chitinous partitions. The Phylactolæmata are, without exception, freshwater species. II. SYNOPSIS OF THE LEADING CHARACTERS OF THE DIVISIONS OF THE SUBORDER CTENOSTOMATA. Suborder B. _CTENOSTOMATA._ The suborder has been subdivided in various ways by different authors. The system here adopted is essentially the same as that proposed in a recent paper by Waters (Journ. Linn. Soc. London, Zool. xxi, p. 231, 1910), but I have thought it necessary to add a fourth division to the three adopted by that author, namely, the Alcyonellea, Stolonifera, and Vesicularina. This new division includes all the freshwater genera and may be known as the Paludicellina. In none of these divisions are the tentacles webbed at the base. The four divisions may be recognized from the following synopsis of their characteristic features:-- Division I. ALCYONELLEA.