Order 2. Cryptostomata. Fossil, Palaeozoic. Zooecia usually shorter than in the Trepostomata, sometimes with transverse par titions. Orifice at the bottom of a vestibular shaft, which may be traversed by diaphragms. This order, which includes the net-like Fenestellidae, has been regarded by palaeontologists as ancestral to the Cheilostomata.
Order 3. Cyclostomata. Zooecia elongated, prismatic or cylin drical (fig. 3), with terminal, typically circular orifice, of the full width of the tubular part. The ovicells are modified, greatly enlarged zooecia, and, in the recent species investigated, contain numerous embryos, produced by the divi sion of a single, primary embryo. The polypide is protruded with the aid of a "membranous sac," which surrounds it. The Cyclostomata are known from early Palaeozoic strata (Ordovician) onwards and are represented in the Cretaceous by specially numerous species ; their highly calcareous zooecia being well preserved as fossils. They form a comparatively small proportion of the recent marine fauna, in which Crisia, Tubulipora, Idmonea, En talophora, Hornera and Liclienopora are represented by many species.
Order 4. Ctenostomata. Walls soft and uncalcified, the orifice being closed by a membranous "collar" (fig. 4), with folded walls, which surrounds it. Encrusting or erect and broadly lobed, the zooecia connected with one another (Alcyonidium, Flustrella); or erect and delicate, the zooecia arising separately from a con necting stem (Amathia, Bowerbankia). The preceding are marine, but V ictorella and Paludicella, belonging to the second group, oc cur in fresh water. Certain Palaeozoic fossils (Rho palonaria, V inella) have been referred to this order.
Order 5. Cheilostomata. Much or little calcified, the orifice closed by a chitinous operculum (fig. 5). Polymorphism usually occurs, certain individuals being modified as "avicularia" or "vi bracula." A prominent, globular "ovicell" is commonly found at the distal ("upper") end of the fertile zooecium, serving as an ex ternal brood-cavity in which an egg develops. This order is first known in the Mesozoic period (Jurassic), but its species become extremely numerous in the Cretaceous. In the Tertiary period, as at present, it is the dominant group of Polyzoa, chiefly marine but occasionally found in brackish water. Representative genera are : Membranipora, Flustra, Onychocella, Cellaria, Scrupocellaria, Bugula, Schizoporella, Retepora, Cellepora.
Tribe II. Phylactolaemata. Lophophore horseshoe-shaped (fig. 2), except in Fredericella, the mouth guarded by a lip or "epis tome." Body-wall muscular, uncalcified, the body-cavities con tinuous with one another. Reproduction sexual and by means of "statoblasts" (fig. 12), internal buds protected by a chitinous shell. Fredericella, Plumatella, Lophopus, Cristatella. The colony
can move slowly from place to place in the last two.
Structure of Ectoprocta.—The colony may assume several distinct forms: (a) Encrusting, the zooecia in close contact with one another, usually in a single layer, attached to a stone or sea weed by the basal surface, the orifices on the exposed or "frontal" surface, sometimes becoming multilaminar by the addition of new layers, each of which completely covers its predecessor, sealing the orifices; (b) erect, with broad lobes or branches, unilaminar or bilaminar, the zooecia correspondingly opening on one or both surfaces. Both these conditions are found in Flustra and its allies, in which there is little calcification and the branches are flexible. Retepora is a highly calcified unilaminar type, the branches of which form an elegant network. In other genera the branches are cylindrical, the orifices arranged all round the curved surfaces; (c) erect and more slender, resembling Hydroids, mostly unilam inar or with cylindrical branches, of ten with flexible joints; (d) not rigidly attached, unilaminar, discoidal or conical.
The zooecium of the Cheilostomata (fig. 6) resembles a shallow box, having the "orifice" near the distal end, the one further from the base or commencement of the colony. The orifice is the ex ternal opening of a thin-walled "tentacle-sheath," really a flexible, uncalcified part of the body-wall, pushed into the body-cavity. The tentacles arise from the blind end of the retracted sheath; and these parts, with the U shaped alimentary canal and the nerve-ganglion which lies be tween the mouth and anus, con stitute the "polypide." Retrac tion of the tentacles takes place rapidly by means of retractor muscles, but protrusion is a more gradual process and is effected by "parietal muscles," the contrac tion of which increases the fluid pressure in the body-cavity and forces out the tentacles. Some part of the body-wall must ac cordingly be flexible, and in the division Anasca (fig. 6) this is the whole or part of the frontal surface. In the Ascophora (fig. 7) most of the frontal wall is cal cified and rigid, but the part which lies on the proximal side of the operculum has been pushed in as a very delicate "compensation-sac," into the basal wall of which the parietal muscles are inserted. The contraction of these muscles dilates the sac, into which water enters from the outside, and the mechanism of protrusion is thus the same as in the Anasca. In the erect Ctenostomata the whole body-wall is flexible, and the parietal muscles produce their effect by passing from one part of it to another, across the body-cavity. In the Phylactolaemata the flexible body-wall is itself muscular. The calcareous Cyclo stomata have a special, rather complicated arrangement for pro trusion (see Borg, 1926).