The muscles present in a zooecium are (i) parietal muscles (pm.) formed by bundles of two to five fibres, attached to the basal angle of the sides at one end and to the upper surface at the other. This upper surface they depress, (ii) A pair of opercular muscles (op.m.). (iii) A retractor muscle (rm.), which invaginates the tentacle-sheath and is attached at one end to the abanal side of the oesophagus, at the other to the proximal wall of the zooecium. (iv) A pair of parieto-vaginal muscles (pv.m.) attached to the base of the fold surrounding the tentacle-sheath, and situated close to the opercular muscles on the anal side of the digestive tract. (v) The muscles of the tentacle-sheath. These are arranged in two layers - a circular, which forms a sphincter near the base of the circle of tentacles; a longitudinal, the fibres of which are collected towards the base of the sheath into four bundles - two on the abanal, two on the anal side. The two first form the superior parieto-vaginal ligaments (pv.l.), and are attached to the upper zooecial wall; the two latter form the inferior parieto-vaginal ligaments (pv.l'.), and are attached to the inferior zooecial walls close to the angles of the distal end.

The parent zooecium of the colony has a rounded outline, and gives off numerous buds. In the colony, when once established, new zooecia are formed as buds in the same linear series as the old zooecia. A bud of unusual width may divide longitudinally, thus originating two parallel series in the place of one. The endocyst of the growing bud is composed of two distinct cell-layers - an outer set of columnar cells, an inner set of fusiform cells. The latter persist in the funiculi laterales, and other cords of cells, but are indistinct in other parts of the zooecium. In the fresh-water Polyzoa there is an inner layer of ciliated cells. The new zooecium is cut off in the bud by a double fold of the endocyst, which grows inwards from all sides simultaneously, and finally closes in the centre. The dividing wall of ectocyst is formed between the two halves of the fold.

If a zooecium is situated at the free edge of the object on which the colony grows, or is in contact with some obstacle, its upper surface developes a large cylindrical outgrowth three to four times as high as the zooecium is long, with a thickened ectocyst, in which, however, no calcareous matter is deposited. Such zooecia are termed by Nitsche 'Turret-zooids.' They possess neither tentacles, tentacle-sheath, nor digestive tract. Under normal conditions these organs are developed within the new-formed zooecium by a process perhaps not yet fully explained. Every zooecium remains in connection with its neighbours by means of the 'rosette' plates, the columnar cells covering them, and the system of funicular cords (supra), to which a nervous function was at one time assigned.

Flustra foliacea resembles Membranipora membranacea in all essential features. It possesses thirteen to fourteen tentacles; its tentacle-sheath and digestive tract are long, and much bent and folded in the retracted condition. It possesses Avi-cularia, but they are not much modified in shape from the ordinary zooecium. An Avicularium is a zooid in which the digestive tract is aborted with the tentacles and tentacle-sheath, whilst the rest of the zooecium and the operculum persist. In its most specialised form, e.g. in Bugula (Fig. 10. B.), one of the Cheilostomata, the avicularium is borne upon a moveable peduncle (p.); the operculum has become a moveable mandible (m.), the zooecium a hollow chamber (c.) bearing the mandible, containing posteriorly a set of divaricator (d.m.) and occlusor (o.m.) muscles, and anteriorly fashioned into a beak (b.). Between this highly specialised form and the simple form seen in Flustra all degrees of specialisation may be found. In some Cheilostomata another remarkable form of zooid, shown in Fig. 10. C, occurs. This is the Vibraculum. It possesses a moveable seta (s.), the homologue of the operculum carried on a simple chamber (c.) containing the muscles (m.) that move the seta.

A long tubular appendage originates in many instances from an aperture (a.) in the lower part of the chamber. Transitional forms from such a Vibraculum as this of Scrupocellaria to the Avicularium or zooecium are to be found. The function of the Avicularium is doubtful. In the most specialised forms the surface of the beak turned to the mandible has a small depression, from which project a number of tactile (?) setae with a cellular ganglionic (?) body at their base. The mandible is in constant motion, and the Avicularium has been observed to catch and retain small worms. In the less specialised forms, however, such an action would be impossible. The Vibracula are swept from time to time over the surface of the colony, sometimes with apparently concerted action, as e.g. in Caberea, and probably remove foreign bodies, etc. They appear to act as locomotor organs for the free colony of the Selenariidae. See Hincks, Marine Polyzoa, i. Introduction, p. lxiv. to p. lxxxiii.

Flustra also possesses Ooecia, or marsupial chambers, into which the ovum passes and undergoes its development. These Ooecia are produced at the distal end of the zooid. For their structure and development, see Nitsche, Z. W. Z. xx. 1870, p. 3, pl. i. figs. 10-13, and Vigelius, Biol. Centralbl. iii. 1883-84, p. 710. The zooecial character of these organs is doubtful. In some Polyzoa the zooecium is modified into a Stem-cell or a generative zooid (=Gonoecium or Gonocyst). The 'Root-cells,' like the Ooecia, are regarded by Vigelius as organs, not as modified zooecia (Bijdrag tot de Dierkunde, xi. 1884). They are, however, often jointed and branched, and may serve as connecting links between adjoining branches in a colony. See Busk's article on Kinetoskias, Q. J. M. xxi. 1881.

The Avicularium, Vibraculum, and Stem-cell, together with the Ooecium and Radicle-fibre according to some authorities, are zooids modified from the ordinary structure to discharge certain special functions. Modifications of this character constitute Polymorphism. They must be carefully distinguished from modifications of outward form not accompanied by a specialised function, such as are found in many Calcispongiae, and have been termed by Haeckel Polymorphosis.

When only two organisms or three, differ by way of polymorphism, the terms Dimorphism or Trimorphism are sometimes used. Instances are not uncommon in the animal kingdom. There can be no doubt that the Medusa and Hydroid polype are dimorphic forms, and there are other modifications of the hydroid, such as the Blastostyle, the Dactylozooid, the Snake-like zooid of Ophiodes, etc. The worker bee is a dimorphic female; the soldiers and workers among the Termites are trimorphic with the fully-formed male and female.

The phrase sexual dimorphism is used to denote the differences other than the usual anatomical characters which separate the two sexes. Such differences are exceedingly common, and are sometimes carried to an extreme; e. g. among Lepidoptera species or genera have been based on characters which are really only distinctive of sex. And in the Insectan order named the individuals of broods appearing at different times of the year often differ from one another in a marked manner. In this case the phrase seasonal dimorphism is employed.

Polyzoa, Ray Lankester, Encyclopaedia Britannica (ed. ix), xix. British Marine Polyzoa, Hincks, 2 vols. London, 1880. British Fresh-water Polyzoa, Allman (Ray Society), 1856; cf. Hyatt, Proc. Essex Institute (U.S.), iv. 1865; cf. Allman, Recent progress of knowledge of Fresh-water Polyzoa, J. L. S. xiv. 1879; of Marine Polyzoa, ibid. xv. 1881. Vigelius, Bijdrag tot de Dierkunde, xi. 1884 (not seen).

Membranipora (=Flustra) membranacea, Nitsche, Z. W. Z. xxi. 1870-1871. Flustra membranaceo-truncata, Vigelius, Biol. Centralbl. iii. 1883-84, and Id. Bijdrag tot de Dierkunde, xi. 1884. Kinetoskias, Busk, Q. J. M. xxi. 1881. Hypo-phorella, Ehlers, Abhandl. k. Gesellsch. der Wissenschaften. Gottingen, xxi. 1876 (not seen). Halodactylus diaphanus, Kohlwey (Inaug. diss.), Halle, 1882. Alcyo-nellafungosa (freshwater), Nitsche, Archiv fur Anat. Physiol. 1868.

Avicularium and Vibraculum. Hincks, Introduction to 'Marine Polyzoa' (supra). Relation of two. Id. A. N. H. (5) ix. 1882. Movements of Vibracula in Caberea. Id. Q. J. M. xviii. 1878.

Endosarc, etc. Joliet, Bryozoaires d. Cotes de France, A. Z. Expt. vi. 1877.

Formation of generative products in Cheilostomata. Vigelius, Biol. Centralbl. ii. 1882-83.

Ooecium. Vigelius on Flustra membranaceo-truncata (supra); Nitsche, Z. W. Z. xx. 1870; Hincks, xiii. 1873, P. 3°.

Budding in Polyzoa. Haddon, Q. J. M. xxiii. 1883.

Discussion of Affinities. Harmer on Loxosoma, Q. J. M. xxv. 1885, p. 304 et seqq.