Fig. 1.
Flustra foliacea. A, natural size; B′, portion magnified in B; B, magnified 30 diameters.
a, avicularium; o, ovicell.
[‘The Cambridge Natural History.’]
The brown horny fronds, which vary in width, branch upwards from a narrow flat stem attached at its base to stones and shells. Both surfaces of the fronds show a fine network pattern formed by the edges of little oblong boxes or cells termed zoœcia,[[10]] arranged in longitudinal parallel rows and forming a double layer back to back. The cells are broad and rounded above, narrow and truncate below, and each is roofed in by a transparent membrane with a semicircular lid or operculum situated near the upper end; four short stout spines spring from the margin in this neighbourhood. When the surface of a living frond is examined in sea-water, here and there a bundle of tentacles may be observed pushing up a lid, slowly emerging and expanding into a bell-shaped coronet; on the least alarm the tentacles are rapidly withdrawn into the cell and the lid shut. The flexible protrusible region of the cell is termed the tentacle sheath. The relation of the cell to the tentacle sheath (Figs. 2, 3) may be roughly compared to a glove finger, stiff below, but flexible at the end, and surmounted by a crown of bristles; on pulling down the glove-finger tip, the tentacles will also be drawn in, and will lie in a sheath formed by the invaginated portion of glove finger. The lid which closes over the tentacle sheath is only found in the Sub-order Chilostomata to which Flustra belongs. The area of the tentacle sheath whence the tentacles arise is termed the lophophore.[[11]]
Figs. 2, 3, diagrams representing polypide in cell. Fig. 2, tentacle-sheath protruded. Fig. 3, ditto, retracted; a, tentacles; b, tentacle-sheath; c, mouth; d, gullet; e, stomach; f, vent; g, retractor muscle; h, funiculus; l, ovary; k, testis; l, lid or operculum; nerve ganglion is between mouth and vent. Fig. 4, polypide extracted from cell; d, pharynx; e, stomach; f, anus (after Van Beneden). Fig. 5, section (partly diagrammatic) of frond of Flustra, showing cells back to back.
The mouth is situated in the centre of the lophophore, surrounded by the circle of tentacles; and the latter, by the action of their cilia, set up currents which convey food to the mouth.
The mouth leads into a pharynx and gullet, the latter opening into a stomach, whence the intestine ascends to terminate in the vent opening below and outside the circle of tentacles; the intestines, in fact, form a U-shaped tube (Figs. 4, 5) suspended in the body-cavity in the interior of the cell. A cord, the funiculus, passes from the stomach to the base of the body-cavity. A small nerve ganglion is situated within the upper part of the loop of intestine.
The tentacles, intestines, and other organs constitute the “polypide,” the cell being simply the protective house formed by the latter.
The body-cavity, which contains fluid, is in direct communication with the interior of the tentacles, which are hollow, and which act as respiratory organs by bringing the fluids of the body-cavity in proximity to the water. In Flustra the body-cavities of the cells are shut off from each other, but pores and sieves in the partition walls allow of the junction of the inner linings of these cavities. The male and female reproductive elements are formed in the body-cavity. The egg develops in a helmet-shaped brood-pouch, the ovicell, situated at the summit of the cell and almost immersed in the cell above. The ciliated embryo swims about for a few hours and settles down to form the first polypide and cell; from the latter there arise buds which remain attached, and produce other buds, till a colony like that of Flustra results.
Among the ordinary cells are certain smaller cells (Fig. 1, a) slightly raised above the general level, different in shape from the ordinary kind and with thicker lids. These peculiar cells are termed avicularia, and chiefly contain muscles for opening and shutting the lid. They arise by modification of the ordinary cells, whereby all the organs of the polypide have become atrophied except the muscles. The Polyzoa[[12]] were so named by Vaughan Thompson, who, in 1820, discovered that certain plant-like animals, which had previously been classed with the zoophytes, possessed a much higher organisation, in that the intestine was separate from the body-cavity and not continuous with it as in Sea-Firs, Sea-Anemones, and Corals. In 1834, Ehrenberg named the group Bryozoa[[13]] or Moss Animals.
With the exception of one genus (Loxosoma), all Polyzoa form colonies, which arise by the continual budding of the cells, the buds remaining attached to the parent cells. The colonies vary endlessly in form and habit, occurring as crusts on rocks, etc., masses, broad fronds, branching tree-like growths, bushy tufts, etc.