Fig. 238.—Bowerbankia pustulosa Ell. and Sol., Plymouth. A, Fragment of a colony, natural size, showing the branching stem, bearing tufts of zooecia: B, one of these tufts, with the growing apex of the stem (st), × 27; b, young zooecia (buds); c, the "collar" characteristic of Ctenostomata; t, tentacles; C, a single zooecium, with expanded tentacles, more highly magnified; a, anus; c, collar; g, gizzard; i, intestine; o, oesophagus; s, stomach.
The Ctenostomata rarely have even the slightest trace of calcareous matter. Alcyonidium and its allies form soft encrustations, or may even grow into erect masses six inches or more in height (A. gelatinosum). In this type the zooecia are often so closely united that it may be difficult or impossible to make out their limits in the living colony. Many of the dendritic or branching Ctenostomes (Fig. 238) are characterised by an extreme delicacy of habit. The zooecia in these cases are sharply marked off from the stem. They are either cylindrical or ovoid, being commonly attached by a very narrow base, so that in some species they readily fall off, and may thus be completely absent in certain parts of the colony. In such forms as Vesicularia spinosa, it requires considerable experience to recognise a stem which has lost its zooecia as being part of a Polyzoon. In Mimosella the zooecia possess a remarkable power of movement on the stem, similar to that possessed by the leaflets of the Sensitive Plant.[[532]] In certain forms (Bowerbankia, Amathia) the zooecia occur in groups separated by intervals which are devoid of zooecia, but in other cases they may have a more irregular arrangement. The collar to which this group owes its name is by no means a conspicuous feature. Its position when retracted has been shown in Fig. 234, while Fig. 238 further illustrates its relations.
The Cheilostomata grow in a great variety of forms, and also show a wide range of character in their zooecia. The orifice is commonly surrounded by stiff spines (Fig. 257, p. [524]), which perhaps have the function of protecting the delicate polypides from the sudden impact of foreign bodies. These spines may attain an enormous development, as in Bicellaria ciliata, and some forms of Electra (Membranipora) pilosa (Fig. 256, A).
The operculum is usually, though by no means always, a conspicuous feature of the Cheilostome zooecium. It is invariably of chitinous consistency, and is more or less semicircular in outline, the straight edge forming a hinge on which the operculum opens. In some cases the orifice is surrounded by a raised margin or "peristome" (Fig. 255, B, C); the operculum is then situated at the bottom of a depression of the surface, and may be concealed from view. In others, in which the front wall of the zooecium is membranous (Bugula, Fig. 233), the operculum is merely a part of this membrane, and so is quite inconspicuous; and in cases of this kind the membranous wall may be protected by an arched spine, the "fornix," developed from one side of the zooecium (Fig. 254, f). The ovicells are commonly a conspicuous feature of this group, although they are believed to differ fundamentally from those of Cyclostomata. They have the form of a helmet-like covering overhanging the orifice (Figs. 240, 241), and may be either prominent or more or less concealed by the growth of adjacent parts of the zooecia. The presence of ovicells of this description is perfectly distinctive of the Cheilostomata.
Avicularia and Vibracula.—Most singular of the external appendages of the Cheilostomata are the extraordinary "avicularia" and "vibracula" of some genera.[[533]] By the comparison of a carefully selected series of genera, it has been established that the avicularium is a special modification of a zooecium. One of its least modified forms is found in Flustra foliacea (Fig. 232), where the avicularia (a) are small zooecia with a conspicuously large operculum ("mandible"). Avicularia of a similar type occur in Cellaria (Fig. 239, A), Schizotheca, etc., the avicularium occupying the place of an ordinary zooecium. These are the "vicarious" avicularia of Mr. Busk.[[534]]
Fig. 239.—Forms of avicularia. A, Cellaria fistulosa L., Plymouth, × 43; a.z, avicularian zooecium, with closed mandible; o, operculum of zooecium: B, Schizoporella unicornis Johnst., Scilly Is., × 43; zooecium bearing two avicularia; m, opened mandible of avicularium; s, sinus of orifice: C, zooecium of Smittia landsborovii Johnst., Plymouth, × 43; the operculum is seen at the bottom of a depression surrounded by a thin collar or "peristome," in an emargination of which is seen an avicularian zooecium (a.z); m, mandible (opened); p, pores; t, tooth.
In the next stage (Figs. 239, B, 256, B) the avicularian zooecium is further reduced; it has in most cases lost its place in the series of individuals, and is found instead seated on some part of an ordinary zooecium ("adventitious" avicularia). The avicularium now consists of a much reduced zooecium, bearing the well-developed operculum or mandible.
Having arrived at this point, the avicularia seem to lose all sense of the propriety of remaining in the positions once occupied by zooecia. They have become degraded to the rank of appendages of the zooecia, and as such they may occur in an astonishing variety of positions. Sometimes one occurs on each zooecium in the middle line, or asymmetrically, or even on the top of the ovicell; in other cases the orifice is flanked by an avicularium on each side (Fig. 239, B). Sometimes (Cellepora) the avicularia are of more than one kind, some being large and some small, some having a pointed mandible and others a mandible with a rounded spoon-like end.