_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up a lid, slowly emerging and expanding into a bell-shaped coronet; on the least alarm the tentacles are rapidly withdrawn into the cell and the lid shut.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] [Illustration: 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.