F. foliacea has long been known to possess in the fresh state a remarkable odour, which is described, according to the fancy of the observer, as a strong odour of fish, or as the smell of violets after a shower. Others have compared it to that of the orange or verbena, or to that of a mixture of roses and geranium.
Flustrella hispida, another of our commonest Polyzoa, which may be found between tide-marks on the stalks of Fucus, consists of a softish brown encrustation, about one-sixteenth of an inch thick, covered by numerous spines. If examined undisturbed in a rock-pool, or transferred to a glass of sea-water, the brown mass will be seen to become surrounded by a delicate bluish halo, which is about as thick as the encrusting mass itself, and consists of the tentacles of the numerous individuals of the colony. The microscope shows that each individual is provided with a circlet of some thirty or more long, delicate tentacles, which together form a graceful funnel (as in Fig. 233). At the bottom of the funnel is the mouth, to which Diatoms or other minute organic particles are conveyed by the cilia which fringe the tentacles. If the tentacles be touched with a needle, the whole funnel is retracted with great rapidity, and in this retracted condition we see no more than the body-walls of the animals. After an interval the tips of the tentacles are cautiously protruded; the tentacles are gradually pushed out, at first in a close bundle, but finally separating from one another to form the funnel which we have already noticed.
There is hardly a more surprising spectacle in the whole animal kingdom than a living fragment of the genus Bugula. The colony grows in the shape of a small tree, whose height may amount to several inches; and is characterised, in many species, by a spiral arrangement of the branches, which makes the genus easy to recognise at first sight (Fig. 233, A). The stem and branches are composed of a single layer of zooecia, arranged two or more abreast. Each zooecium bears, on its outer side, a most singular body termed an avicularium, from its resemblance to a bird's head. Imagine a minute eagle's head attached by a short but flexible neck to the zooecium. Suppose further that this structure moves backwards and forwards in a deliberate but determined fashion, its lower jaw usually widely open so as to be nearly 180° distant from its position when closed. Suppose that the lower jaw is moved by powerful muscles which can be distinctly seen inside the transparent head of the avicularium, and that every now and then it closes with a snap, seizing any unfortunate worm which may happen to be within reach with a grasp of iron. The above gives a very faint idea of the appearance of a living Bugula colony, with its hundreds of swaying avicularia, and with its tentacular funnels protruding from their zooecia, and withdrawing themselves capriciously from time to time.
Fig. 233.—Bugula turbinata Alder, Plymouth. A, A small colony (natural size); B, portion of a branch (× 50): a, a', avicularia, in different positions; ap, "aperture" (see p. [524]); b, polypide-bud, attached by its stomach to b.b, brown body; m, mouth, surrounded by the circle of tentacles; two individuals to the right show the tentacles partially expanded; o, ovicell; s, marginal spine. The avicularia of some of the zooecia have been omitted in B.
General Characters.—The Polyzoa are colonies, leaf-like or tree-like in form, and often strongly resembling seaweeds, or forming encrustations on the surface of stones and water-plants, or taking on other shapes. The units of the colony are complete individuals (Fig. 234). The zooecium or body-wall encloses a body-cavity, in which lies a digestive canal, with which are closely connected the central nervous system and the retractile, ciliated tentacles. The structures other than the zooecium constitute the "polypide." The mouth (m) leads into the ciliated pharynx (ph) which is followed by the oesophagus (oe) which again passes into the stomach (s), whose walls are coloured by a characteristic yellowish pigment. The stomach gives off the intestine (in), which is lined by strong cilia, by means of which a rotatory movement is given to the faeces contained in it. This communicates by a narrow passage with the rectum (r), which opens by means of the anus (a).
Fig. 234.—Alcyonidium albidum Alder, Banyuls-sur-Mer. Diagram showing the structure of a single zooecium with its polypide retracted: a, anus; d, diaphragm; e, ectocyst; em, ectoderm; f, funiculus; g, ganglion; i, intertentacular organ; in, intestine; m, mouth; mm, mesoderm of body-wall; o, orifice; oe, oesophagus; ov, ovary; p, parietal muscles; ph, pharynx; p.v, parieto-vaginal muscles; r, rectum; r.m, retractor muscles (contracted); s, stomach; t, testis; tn, tentacles; t.s, tentacle-sheath or kamptoderm. (After Prouho.[[503]])
In the retracted condition the tentacles (tn) lie in a cavity which opens to the exterior by the orifice (o). The cavity is bounded by a thin membrane termed the "tentacle-sheath" (ts), and it is incompletely subdivided, near its upper end, by a diaphragm (d), perforated by a circular hole through which the tentacles can be protruded. The diaphragm bears the thin folded collar characteristic of the Ctenostomata, the group to which the species figured belongs (see p. [477]).
Fig. 238, B, shows the tentacles of Bowerbankia in their fully expanded and partially expanded condition. Comparing this with Fig. 234, it will be clear that when protrusion is taking place, the tentacles are forced in a bundle, tips first, through the diaphragm and next through the orifice of the zooecium, the alimentary canal offering no resistance to this movement, owing to the length of the oesophagus. A moment's consideration will show that the bases of the tentacles, in passing through the orifice, will carry with them that part of the flexible tentacle-sheath to which they are attached; and it will further be clear that so much of the tentacle-sheath as is thus protruded will be turned inside out. This process of "evagination" continues until its further progress is stopped by the retractor-muscles (r.m), and by the parieto-vaginal muscles (p.v), which pass from the interior of the body-wall to the upper part of the tentacle-sheath. The latter has now become the delicate layer which connects the expanded tentacles with the zooecium; and the anus (Fig. 238, C, a) opens directly to the exterior. Since the name "tentacle-sheath" is thus descriptive of the condition of retraction only, the term "kamptoderm"[[504]] has been suggested as an alternative name.