Page:EB1911 - Volume 23.djvu/800

Rh deep down in the funnel-shaped disk. The cingulum appears to be represented by the margin, usually produced into long petal-like lobes, fringed with long stiffish setae, which in Stephanoceros are vibratile at intervals, seemingly at will. In Floscularia they serve to convert the lobed funnel into an efficient casting net or clasp net: in one species (F. pelagica) there is an outer girdle of fine cilia for swimming. In Apsilus and Atrochus (fig. 3, b, d) the cingulum is a mere contractile hood. In most rotifers, on the contrary, the trochus is stronger than the cingulum, often lobed, and with some of its cilia aggregated into vibratile styles homologous with the combplates of  (q.v.) and the membranelles of ciliate  (q.v.). The trochus forms the powerful currents for locomotion, and for the supply of food material, while the cingulum produces a local current round the upper rim of the corona to bring the food articles direct to the mouth, which is displaced through a postero-ventral gap in the trochus to lie behind the disk, just as occurs in the more specialized Ciliata. The current formed by the trochus is a gigantic vortex-ring, the down stroke of the cilia being directly outwards, but the wave beats running round the organ in uniform succession in one direction. Thus the rotifer is, as it were, constantly drawn forward into the centre of this vortex ring. There is a dorsal interruption to the disk, involving both trochus and cingulum and groove in this case the two halves of the disk may be developed in lobes, flower-shaped in Melicerta ringens, but often rounded and projecting like kettledrums. These give a strong impression of two crown wheels revolving in the same sense. This appearance puzzled the older observers, who were led thereby to give the name “wheel-bearers” to the group until the true character of ciliary motion was recognized; for a wheel cannot be in organic continuity with the support on which it rotates. In Conochilus (fig. 2, 5), a Melicertan, the mouth is displaced towards the antero-dorsal side and the gap is postero-ventral.

In Melicerta ringens and M. conifera (fig. 2, 4; fig. 3, e, f) there is a glandular ciliated pit between the mouth and the chin into which the overflow water passes by a pair of gutters, and in which fine particles are aggregated into pellets, which the animal deposits, as formed, on the edge of its tube and so builds it up. M. janus builds up a tube by pellets of its own faeces (fig. 3, g). In most Ploima the dorsal gap is not well marked, and the trochus is broken up into a number of lobes, often furnished with vibratile styles; in front and at the sides, but ventrally passing into the uniformly ciliated oral funnel.

Other ciliated organs to be noticed are the proboscis cup of Bdelloidaceae, and the toes of Pedalion. Besides these Synchaetadae and Notommatidae (fig. 7) possess a pair of aurioles, great eversible ciliated pouches a little above the disk, utilized in swimming. The mouth begins as a funnel, continued into a narrow pharynx, which in Flosculariaceae is prolonged into a slender tube hanging freely down into the crop: this is followed by the crop-gizzard, also ciliated except behind, where it is hardened into a set of articulated sclerites (trophi) to form the gizzard or mastax. Thus the crop-gizzard has the same combination of structures as we find in the stomach of higher Crustacea, with which we may call it homoplastic. The trophi are (1) a median incus or anvil (fig. 4), Y-shaped, with the foot (fulcrum) distal and the arms (rami) apical, often independently jointed; (2) with the outer ends of the rami articulate two lateral pieces (mallei), and again composed of a distal longitudinal piece (manubrium) and an apical transverse piece (the uncus), the whole recalling, as the name implies, a single-clawed hammer. For the varieties and modifications of the trophi we simply refer to Hudson's figure above. The relative size of the crop to the trophi varies greatly; it is small where the trophi are well developed and complex, as well as in Bdelloidea; but in Flosculariaceae it is large, and so it is in Asplanchnaceae. Eversible trophi of the forcipate or virgate type, which can be used for nibbling, are common in Ploima, notably Rattulidae, and are used for attachment to the host in the parasitic Seisonaceae, &c. In Asplanchnaceae also,