Polyzoa may be collected with ease on almost any part of our coasts. The fronds of the "sea-mat" (Flustra foliacea) are thrown up by the waves in thousands in places where the bottom is shallow and sandy. The bases of the larger seaweeds growing on rocks between tide-marks are nearly always thickly covered with encrustations of Flustrella hispida or of species of Alcyonidium, in places where they are kept moist by being covered with a sufficiently thick layer of other algae. Rocks which are protected from the sun may be coated with calcareous Cheilostomes; and these are also found, in company with branching Polyzoa of various kinds, on the bases of the Laminaria thrown up by gales or exposed at spring tides. The graceful spirals of Bugula turbinata (Fig. 233, A) may be found hanging from the rocks at extreme low water; while colonies of Scrupocellaria, remarkable for their vibracula (see p. [484]), are common in many places between tide-marks. Certain species affect the mouths of estuaries.
Membranipora membranacea commonly covers many square inches of the frond of Laminaria with its delicate lace-like encrustation. Nitsche[[527]] has shown that this species has its calcareous matter deposited in plates, separated by intervals of uncalcified ectocyst. The effect of this arrangement is to make the colony flexible, and to enable it to adapt its shape to the movements of the Laminaria, which is swayed to and fro by the action of the waves. Many of the calcareous forms growing on Laminaria have no special arrangement of this kind, and they accordingly grow in colonies whose area is so small that the greatest movements to which the seaweed is liable are not sufficient to crack or break the colony.
Many species show a decided, or even exclusive, preference for particular situations; as, for instance, species of Triticella, which are only found on certain Crustacea. Many encrusting forms prefer the inside of dead shells of Pecten, Cyprina, etc., to any other habitat. Terebripora[[528]] excavates tubular cavities in the substance of the shells of Molluscs. Hypophorella[[529]] inhabits passages which it forms in the walls of the tubes of the Polychaets, Lanice and Chaetopterus. Lepralia foliacea, one of the Cheilostomata, forms masses which may reach a circumference of several feet, simulating a small coral-reef. Its contorted plates are a regular museum of Polyzoa, so numerous are the species which delight to find shelter in the quiet interstices of the colony. The exquisite little colonies of Crisia eburnea are commonly found on red seaweeds, or on the branches of the Hydroid Sertularia.
The Polyzoa are found at all depths, certain Cheilostomes having been recorded from 3000 fathoms. The Cyclostomes dredged by the "Challenger" were all found in depths of 1600 fathoms or less, while the Ctenostomes are a distinctly shallow water group, most having been found at less than 40 fathoms, and only three at so great a depth as 150 fathoms.[[530]]
A few forms (Membranipora pilosa, Scrupocellaria reptans, etc.) are known to be phosphorescent;[[531]] but it is not known what is the purpose of this phenomenon.
External Form.—The Polyzoa may be roughly divided into (1) encrusting forms, usually calcareous, but sometimes soft; and (2) erect forms, which are either rigid or flexible. This flexibility can coexist with a highly calcified ectocyst, as in Crisia (Fig. 237), Cellaria, and others in which the branches are interrupted at intervals by chitinous joints. The coral-like forms may assume the most exquisite shapes, pre-eminent among which are the lovely net-like colonies of Retepora. Polyzoa of this type are seldom found between tide-marks, where their brittle branches would be liable to be snapped off by the waves. The erect species which occur in such positions are flexible, although flexible species are by no means restricted to the zone between tide-marks.
Fig. 237.—Crisia ramosa Harmer, Plymouth. A, End of a branch, × 1; B, another branch, × 20, showing the chitinous joints, the tubular zooecia characteristic of Cyclostomata, and the pear-shaped ovicell with a funnel-shaped orifice at its upper end.
Although the form of the colony is very different in different Polyzoa, a pocket-lens will usually show whether a given specimen belongs to the group or not. The surface is nearly always more or less distinctly composed of zooecia, or at least shows their orifices. The entire colony may be built up of these zooecia; and this is by far the commonest arrangement, both in encrusting and in erect forms. In certain genera, however, and particularly in some Ctenostomes (Fig. 238), and in most of the Entoprocta, the individuals grow out at intervals from a cylindrical stem or "stolon" (st), which is not composed of zooecia.
The Cyclostomata may assume an erect or encrusting habit. Their zooecia are always more or less cylindrical; the upper ends being often completely free, although in many cases the whole zooecium is closely adnate to its neighbours. In the breeding season the forms which belong to this group are provided with curious "ovicells," which contain the embryos. These may either be pear-shaped swellings on the branches (Crisia, Fig. 237), or they may form inflations of the surface, between the zooecia. The mature ovicell is provided with one or more openings, through which the larvae escape.