The base from which the whole set of tentacles springs is known as the "lophophore."[[520]] In the Entoprocta (Fig. 236, 1) the lophophore is circular; the mouth is situated near the margin of the area surrounded by the tentacles; and the anus is found within the circlet, near the end opposite to the mouth.
In (2) and (3), representing the Ectoprocta, the anus is outside the series of tentacles. In the majority of cases, including all the marine Ectoprocta and one or two of the fresh-water forms, the lophophore is circular (2), the mouth occurring at the centre of the circle, and not being provided with a lip. These forms of Ectoprocta constitute the Order Gymnolaemata,[[521]] the dominant group of the Polyzoa in respect of number of genera and species. The remaining Ectoprocta belong to the exclusively fresh-water Order Phylactolaemata,[[522]] in which the mouth is protected by an overhanging lip or "epistome"; the ground-plan of the tentacles is, except in Fredericella, horse-shoe shaped (Fig. 236, 3), and the tentacles themselves are usually much more numerous than in the other cases.
Fig. 236.—Ground-plan of the lophophore in (1) Entoprocta, (2) Gymnolaemata, (3) Phylactolaemata: a, anus; ep, epistome; m, mouth. The tentacles are represented by shaded circles.
The general characters of these divisions will be more easily understood by referring to the figures given of living representatives of the groups. The Entoprocta are illustrated by Figs. 243-245; the Gymnolaemata by Figs. 238, 240; and the Phylactolaemata by Figs. 247, 248.
The Gymnolaemata include three Sub-Orders:—
1. Cyclostomata.[[523]]—Body-wall densely calcareous, the zooecia being more or less tubular, usually with a circular orifice (Fig. 237).
2. Cheilostomata.[[524]]—Body-wall of varying consistency. The orifice is closed, in the retracted state of the polypide, by a chitinous lip or "operculum," which is more or less semicircular (Figs. 239, 241).
3. Ctenostomata.[[525]]—Body-wall always soft. The cavity into which the tentacles are retracted is closed by a frill-like membrane, the edges of whose folds have some resemblance to the teeth of a comb. This membrane, the "collar," is seen in different conditions of protrusion or retraction in Figs. 234, 238. The stomach may, in this group, be preceded by a muscular gizzard (Fig. 238, C, g).
Occurrence.—By far the larger number of the Polyzoa are inhabitants of the sea. A recently published catalogue[[526]] of marine Polyzoa includes nearly 1700 living species; and of these, the great majority belong to the Gymnolaemata. This group is further known to include an enormous number of fossil forms. Not only do we find that in living Polyzoa the members of a single Order largely outnumber the remainder of the Polyzoa, but we may further notice that the Cheilostomata, one of the sub-Orders of the dominant group, are at present largely in excess of the whole of the rest of the Polyzoa taken together.