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Rh or “mandible” can be opened and closed. It is regarded as a modified zooecium, the polypide of which has become vestigial, although it is commonly represented by a sense-organ, bearing tactile hairs, situated on what may be termed the palate. The operculum of the normal zooecium has become the mandible, while the occlusor muscles have become enormous. In the vibraculum the part representing the zooecium is relatively smaller, and the mandible has become the “seta,” an elongated chitinous lash which projects far beyond the zooecial portion of the structure. In Caberea, the vibracula are known to move synchronously, but co-ordination of this kind is otherwise unknown in the Polyzoa. The avicularia and vibracula give valuable aid to the systematic study of the Cheilostomata. In its least differentiated form the avicularium occupies the place of an ordinary zooecium (“vicarious avicularium”), from which it is distinguished by the greater development of the operculum and its muscles, while the polypide is normally not functional. Avicularia of this type occur in the common Flustra foliacea, in various species of Membranipora, and in particular in the Onychocellidae, a remarkable family common in the Cretaceous period and still existing. In the majority of Cheilostomes, the avicularia are, so to speak, forced out of the ordinary series of zooecia, with which they are rigidly connected. There are comparatively few cases in which, as in Bugula, they are mounted on a movable joint. Although at first sight the arrangement of the avicularia in Cheilostomes appears to follow no general law some method is probably to be made out on closer study. They occur in particular in relation with the orifice of the zooecium, and with that of the compensation-sac. This delicate structure is frequently guarded by an avicularium at its entrance, while avicularia are also commonly found on either side of the operculum or in other positions close to that structure. It can hardly be doubted that the function of these avicularia is the protection of the tentacles and compensation-sac. The suggestion that they are concerned in feeding does not rest on any definite evidence, and is probably erroneous. But avicularia or vibracula may also occur in other places—on the backs of unilaminar erect forms, along the sutural lines of the zooecia and on their frontal surface. These are probably important in checking overgrowth by encrusting organisms, and in particular by preventing larvae from fixing on the zoarium. Vibracula are of less frequent occurrence than avicularia, with which they may coexist as in Scrupocelloria, where they occur on the backs of the unilaminar branches. In the so-called Selenariidae, probably an unnatural association of genera which have assumed a free discoidal form of zoarium, they may reach a very high degree of development, but Busk's suggestion that in this group they “may be subservient to locomotion” needs verification.

Development and Affinities.—It is generally admitted that the larva of the Entoprocta (fig. 11) has the structure of a Trochosphere. This appears to indicate that the Polyzoa are remotely allied to other phyla in which this type of larva prevails, and in particular to the Mollusca and Chaetopoda, as well as to the Rotrfera, which are regarded as persistent Trochospheres. The praeoral portion (lower in fig. 11) constitutes the greater part of the larva and contains most of the viscera. It is terminated by a well-developed structure (fg) corresponding with the apical sense-organ of ordinary Trochospheres, and an excretory organ (nph) of the type familiar in these larvae occurs on the ventral side of the stomach. The central nervous system (x) is highly developed, and in Loxosoma bears a pair of eyes. The larva swims by a ring of cilia, which corresponds with the praeoral circlet of a Trochosphere. The oral surface, on which are situated the mouth (m) an anus (a), is relatively small. The apical sense-organ is used for temporary attachment to the maternal vestibule in which development takes place, but permanent fixation is effected by the oral surface. This is followed by the atrophy of many of the larval organs, including the brain, the sense-organ and the ciliated ring. The alimentary canal persists and revolves in the median plane through an angle of 180°, accompanied by part of the larval vestibule, the space formed by the retraction of the oral surface. The vestibule breaks through to the exterior, and the tentacles, which have been developed within it, are brought into relation with the external water.

In the common and widely-distributed Cheilostome, Membranipora pilosa, the pelagic larva is known as Cyphonautes, and it has a structure not unlike that of the larval Pedicellina. The principal differences are the complication of the ciliated band, the absence of the excretory organ, the great lateral compression of the body, the possession of a pair of shells protecting the sides, the presence of an organ known as the “pyriform organ,” and the occurrence of a sucker in a position corresponding with the depression seen between (m) and (a) in fig. 11. Fixation takes place by means of this sucker, which is everted for the purpose, part of its epithelium becoming the basal ectoderm of the ancestrula. The pyriform organ has probably assisted the larva to find an appropriate place for fixation (cf. Kupelwieser, 18); but, like the alimentary canal and most of the other larval organs, it undergoes a process of histolysis, and the larva becomes the ancestrula, containing the primary brown body derived from the purely larval organs. The polypide is formed, as in an ordinary zooecium after the loss of its polypide, from a polypide-bud.

The Cyphonautes type has been shown by Prouho (24) to occur in two or three widely different species of Cheilostomata and Ctenostomata in which the eggs are laid and develop in the external water. In most Ectoprocta, however, the development takes place internally or in an ovicell, and a considerable quantity of yolk is present. The alimentary canal, which may be represented by a vestigial structure, is accordingly not functional, and the larva does not become pelagic. A pyriform organ is present in most Gymnolaemata as well as the sucker by which fixation is effected. As in the case of Cyphonautes, the larval organs degenerate and the larva becomes the ancestrula from which a polypide is developed as a bud. In the Cyclostomata the primary embryo undergoes repeated fission without developing definite organs, and each of the numerous pieces so formed becomes a free larva, which possesses no alimentary canal. Finally, in the Phylactolaemata, the larva becomes an ancestrula before it is hatched, and one or several polypides may be present when fixation is effected.

The development of the Ectoprocta is intelligible on the hypothesis that the Entoprocta form the starting-point of the series. On the view that the Phylactolaemata are nearly related to Phoronis (see ), it is extremely difficult to draw any conclusions with regard to the significance of the facts of development. If the Phylactolaemata were evolved from the type of structure represented by Phoronis or the (q.v.), the Gymnolaemata should be a further modification of this type, and the comparative study of the embryology of the two orders would appear to be meaningless. It seems more natural to draw the conclusion that the resemblances of the Phylactolaemata to Phoronis are devoid of phylogenetic significance.

.—For general accounts of the structure and development of the Polyzoa the reader's attention is specially directed to 12, 14, 6, 25, 1, 2, 17, 26, 18, 23, 3, in the list given below; for an historical account to 1; for a full bibliography of the group, to 22; for fresh-water forms, to 1-3, 7-10, 17; for an indispensable synonymic list of recent marine forms, to 15; for Entoprocta, to 10, 11, 24; for the classification of Gymnolaemata, to 21, 14, 4, 13, 20; for Palaeontology, to 27, 22.

References to important works on the species of marine Polyzoa by Busk, Hincks, Jullien, Levinsen, MacGillivray, Nordgaard, Norman, Waters and others are given in the Memoir (22) by Nickles and Bassler. (1) Allman, “Monogr. Fresh-water Polyzoa,” Ray Soc. (1856). (2) Braem, “Bry. d. süssen Wassers,” ''Bibl. Zool.'' Bd. ii. Heft 6 (1890). (3) Braem, “Entwickel. v. Plumatella,” ibid., Bd. x. Heft 23 (1897). (4) Busk, “ Report on the Polyzoa,” “Challenger” Rep. pt. xxx. (1884), 50 (1886). (5) Caldwell, “Phoronis,” ''Proc. Roy. Soc.'' (1883), xxxiv. 371. (6) Calvet, “Bry. Ectoproctes Marins,” ''Trav. Inst. Montpellier (new series), Mém.'' 8 (1900). (7) Cori, “Nephridien d. Cristatella,” ''Zeitschr. wiss. Zool.'' (1893), lv. 626. (8) Davenport, “Cristatella,” ''Bull. Mus. Harvard'' (1890-1891), xx. 101. (9) Davenport, “Paludicella,” ibid. (1891-1892), xxii. 1. (10) Davenport, “Urnatella,” ibid. (1893), xxiv. 1. (11) Ehlers, “Pedicellineen,” ''Abh. Ges. Göttingen'' (1890), xxxvi. (12) Harmer, “Polyzoa,” ''Cambr. Nat. Hist.'' (1896), ii. 463; art. “Polyzoa,” ''Ency. Brit.'' (10th ed., 1902), xxxi. 826. (13) Harmer, “Morph. Cheilostomata,” ''Quart. Journ. Mic. Sci.'' (1903), xlvi. 263. (14) Hincks, “Hist. Brit. Mar. Pol.” (1880). (15) Jelly, “Syn. Cat. Recent Mar. Bry.” (1889). (16) Jullien and Calvet, “Bryozoaires,” ''Rés camp. sci. prince de Monaco'' (1903), xxiii. (17) Kraepelin, “Deutsch. Süsswasser-Bry.,” Abh., Ver. Hamburg (1887), x.; (1892), xii. (18) Kupelwieser, “Cyphonautes,” Zoologica (1906), Bd. xix. Heft 47. (19) Lankester, art. “Polyzoa,” ''Ency. Brit.'' (9th ed., 1885), xix. 429. (20) Levinsen, “Bryozoa,” ''Vid. Medd. Naturh. Foren.'' (Copenhagen, 1902). (21) MacGillivray, “Cat. Mar. Pol. Victoria,” ''P. Roy. Soc. Victoria'' (1887), xxiii, 187. (22) Nickles and Bassler, “Synopsis Amer. Foss. Bry.,” Bull. ''U.S. Geol. Survey'' (1900), No. 173. (23) Pace, “Dev. Flustrella,” ''Quart. Journ. Mic. Soc.'' (1906), 50, pt. 3, 435. (24) Prouho, “Loxosomes,” ''Arch. Zool. Exp.'' (2) (1891), ix. 91. (25) Prouho, “Bryozoaires,” ibid. (2) (1892), x. 557. (26) Seeliger, “Larven u. Verwandtschaft,” ''Zeitschr. wiss. Zool.'' (1906), lxxxiv. 1. (27) Ulrich, “Fossil Polyzoa,” in Zittel's Text-book of Palaeontology, Eng. ed. (1900), i. 257.
 * (S. F. H.)