The zooids, or polypides, form a colony, or zoarium, which is sometimes erect and either lamellate, or branching, and plant-like, or else adherent to some foreign object, either living, e.g. crab, sea-weed, or inanimate. The colony is fixed with rare exceptions. In Cristatella mncedo it creeps about on a flat sole, and in the Selenaridae it is free and moves about by the action of the vibracula. The polypide itself is covered by a cuticle or ectocyst secreted by the ectoderm. This cuticle always remains thin and flexible on the anterior part of the zooid which forms the so-called tentacle-sheath, and can be invaginated and evaginated by the action of special muscles (p.236). The cuticle of the posterior part generally thickens, and becomes resistent: it is gelatinous (Alcyonidinm; Lopkopns), or chitinoid, and usually more or less opaque and sometimes partially, or wholly, calcified. The calcareous matter in Lepralia, etc, is, however, said to be laid down between two layers of ectoderm cells, which are covered externally by a thin cuticle. The posterior region of the cuticle is known as zooecium, and the collection of zooecia forming a colony constitute the coenoecium. The zooids are often polymorphic.
In both Ctenostomata and Cheilostomata certain of them may be modified into stem-cells, and in the latter as Avi-cularia, and Vibracula (p. 237). The Ooecia and Root-fibres are probably to be regarded as organs (Vigelius). The ectocyst is lined by a soft layer or endocyst formed apparently in marine Polyzoa by the fusion of two layers of cells. It has recently been stated, however, that an ectoderm layer may always be distinguished by the use of silver nitrate. In the Phylactolaemata a layer of ciliated epithelium lines the coelome, and distinct muscular layers, external circular, and internal longitudinal, intervene between it and the ectoderm, at least in Alcyonella fungosa. There is a common coelome for all the zooids in the colony of the Phylactolaemata, but in the Gymnolaemata the coelome of every zooid is isolated by septa. In these septa and in the walls of the zooecia, where they touch one another there are in some forms, groups of perforations, or communication-plates, which are covered by a columnar epithelium and are connected by the funiculi (p. 235).
The mouth and anus are approximated, and the line between them, perhaps, marks a mid-ventral surface. The ganglion is situated between them. It is minute and double in Phylactolaemata where it has a narrow commissure encircling the oesophagus, and supplies nerves to the lophophore, tentacles (?) and digestive tract. Sensory cells resembling those of the Entoprocta have been detected on the tentacles of Alcyonella fungosa, etc. The mouth in the Phylactolaemata is overhung by a small ciliated mobile lobe, the epistome, perhaps homologous with the same structure in Entoprocta. It lies in the centre of a disc, or lophophore, either circular (Gymnolaemata), or horse-shoe shaped, i.e. crescentic (Phylactolaemata), along the edges of which are ranged in single series a row of tentacles which are postoral in position. They are hollow, and their cavities are extensions of the coelome: they are covered on the aboral and adoral surfaces by ciliated epithelium, on the action of which depends the food supply. Their number varies much; they are flexible and move swiftly bending to one or the other side, or coiling and uncoiling together in a spiral. Muscular bands have been detected inside them on the aboral and adoral surfaces.
In the Phylactolaemata the bases of the tentacles are protected externally by a thin membrane or calyx. The alimentary canal retains a U-shaped curvature. It consists of a pharynx, followed in many instances by a ciliated oesophageal region, separated by a slight constriction from a stomach, the cells lining which contain a brown pigment. There is interposed between the stomach and the oesophagus in Ctenosto-mata a gizzard with thickened muscular walls, and containing gastric teeth or pointed processes. The stomach is tied to the endocyst by one or more funiculi, and is often V-shaped, as in Membranipora (Fig. 10 A, p. 235). The bent up pyloric portion is always ciliated in Gymnolaemata. The length of the intestine varies. The coelome is roomy, and contains a liquid in which float corpuscles derived, it is said, from the funicular tissue.
The Phylactolaemata are hermaphrodite, as are some Gymnolaemata; in others this appears to be the case with certain zooids in a given colony, whilst the remaining zooids possess a ripe ovary or a ripe testis only. It is possible, however, that a testis or ovary may be developed at some other time in these cases. Both ovary and testis are produced by the growth of cells derived from the funicular tissue or the endocyst. The ovary is often placed at the upper end of the zooecium, the testis at the lower. The ripe products are set free into the coelome, where the ovum may be impregnated by the spermatozoa of the same individual. In two species of the genera, Alcyonidium and Membranipora, the spermatozoa have been seen to escape through a flask-shaped and ciliated inter-tentacular organ placed on the anal aspect of the body, and opening both externally and into the coelome. It has been found in female as well as male zooids, and is perhaps to be considered as a nephridium. The number of ova in the ovary is very variable (2 to 30). Some of the ovarian cells form a follicle for the growing ovum.
The ripe ova pass into an Ooecium or marsupial chamber in Cheilostomatay where they are probably impregnated by the spermatozoa of another zooid, and develope into larvae which escape through the Ooecial aperture. In Farrella and Hypophorella the ovum escapes by a special aperture at the base of the tentacles. In some Phylactolaemata, e. g. Alcyonella, and some Gymnolae-mata> e. g. Valkeria, a rudimentary tentacular and digestive system is produced by budding in a zooecium in which the perfect organs have been lost (infra). The ripe ovum passes into the tentacular sheath thus formed, is fertilised and developed into a larva which escapes through the opening of the sheath. In many instances, however, the ovum or the larva must be set free solely by the death and decay of the parent. A special individual - a gonoecium, or a gonocyst - is sometimes developed in which ova are found. The former occurs in Crista (Cyclostomata), some Cheilostomata, and in Alcyonidium (Ctenostomata), the latter in some Cyclostomata, but the anatomy of these structures is not understood. The ovum undergoes total segmentation. In Phylactolaemata it forms a hollow cyst ciliated externally, with walls formed of two layers of cells. The colony is derived by gemmation from this cyst.