So long as the gizzard retains its spherical form the green cells and its other contents are prevented from entering the stomach by the movements of the cilia that surround its lower aperture, but every now and then, at irregular intervals, the muscles that form its outer wall contract. The chitinous lining although resilient and not inflexible is too stiff to prevent the lumen of the gizzard being obliterated, but the action of the muscles changes its contents from a spherical to an ovoid form and in so doing presses a considerable part of them down into the stomach, through the ring of the cilia.
Fig. 38.—Optical section of gizzard of Hislopia lacustris, with contained green cysts, × 240.
The contraction of the gizzard is momentary, and on its re-expansion some of the green cysts that have entered the stomach are often regurgitated into it. Some, however, remain in the stomach, in which they are turned round and round by the action of the cilia at both apertures. They are apparently able to retain their form for some hours in these circumstances but finally disintegrate and disappear, being doubtless digested by the juices poured out upon them by the glandular lining of the stomach. In polypides kept under observation in clean tap-water all the cysts finally disappear, and the fæces assume a green colour. In preserved specimens apparently unaltered cysts are sometimes found in the rectum, but this is exceptional: I have observed nothing of the kind in living polypides. Cysts often remain for several days unaltered in the gizzard.
Imperfect as these observations are, they throw considerable light on the functions of the gizzard in Hislopia. Primarily it appears to act as a food-reservoir in which the green cysts and other minute organisms can be kept until they are required for digestion. When in the gizzard certain organisms surrender a large proportion of the food-material stored up for their own uses, and this food-material doubtless aids in nourishing the polyzoon. Although the cysts in the gizzard are frequently accompanied by diatoms, the latter are not invariably present. The cysts, moreover, are to be found in the zoœcia of polypides that have formed brown bodies, often being actually enclosed in the substance of the brown body. The gizzards of the specimens of Arachnoidea I have examined contain cysts that resemble those found in the same position in Hislopia.
Hislopia is widely distributed in the southern part of the Oriental Region, and, if I am right in regarding Echinella, Korotneff as a synonym, extends its range northwards to Lake Baikal. It appears to be a highly specialized form but is perhaps related, through Arachnoidea, to Victorella.
27. Hislopia lacustris, Carter.
Hislopia lacustris, Carter, Ann. Nat. Hist. (3) i, p. 170, pl. vii, figs. 1-3 (1858). Norodonia cambodgiensis, Jullien, Bull. Soc. zool. France, v, p. 77, figs. 1-3 (1880). Norodonia sinensis, id., ibid. p. 78, figs. 1-3. Norodonia cambodgiensis, id., ibid. x, p. 181, figs. 244, 245 (1885). Norodonia sinensis, id., ibid. p. 182, figs. 246, 247. Hislopia lacustris, Annandale, J. As. Soc. Bengal (new series) iii, p. 85 (1907). Hislopia lacustris, Walton, Rec. Ind. Mus. i, p. 177 (1907). Hislopia lacustris, Kirkpatrick, ibid. ii, p. 98 (1908). Hislopia lacustris, Walton, ibid. iii, p. 295 (1909).
Zoarium. The zoarium forms a flat, more or less solid layer and is closely adherent to foreign objects. As a rule it covers a considerable area, with radiating branches at the edges; but when growing on slender twigs or the stems of water-plants it forms narrow, closely compressed masses. One zoœcium, however, never grows over another.
Zoœcia. The zoœcia are variable in shape. In zoaria which have space for free expansion they are as a rule irregularly oval, the posterior extremity being often narrower than the anterior; but small triangular zoœcia and others that are almost square may often be found. When growing on a support of limited area the zoœcia are smaller and as a rule more elongate. The orifice is situated on a slight eminence nearer the anterior than the posterior margin of the dorsal surface. It is surrounded by a strong chitinous rim, which is usually square or subquadrate but not infrequently circular or subcircular. Sometimes a prominent spine is borne at each corner of the rim, but these spines are often vestigial or absent; they are rarely as long as the transverse diameter of the orifice. The zoœcium is usually surrounded by a chitinous margin, and outside this margin there is often a greater or less extent of adherent membrane. In some zoœcia the margin is obsolete or obsolescent. The dorsal surface is of a glassy transparency but by no means soft.