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 Ctenophora
(q.v.) and the
membranelles of ciliate
Infusoria (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.
|
| From Camb. Nat. Hist. vol. ii. “Worms,” &c., by permission of Macmillan & Co. Ltd. |
|
Fig. 2.—Diagrammatic Views of Disks of Rotifers: cingulum continuous; trochus dotted; groove shaded; mouth black. 1, Simple disk of Microcodon; 2, bdelloid disk of Rotifer and of most Melicertids showing dorsal gap; 3, disk of Hydatina, with lobed ridges in the groove, bearing vibratile styles (membranelles); 4, disk of Melicerta ringens and M. conifera; the star represents the ciliated cup connected by ciliated depressions with the groove; 5, disk of Conochilus, like the Bdelloid, but with mouth antero-dorsal, the gap postero-ventral; 6, disk of Stephanoceros—cingulum broken up into setiferous lobes, groove a naked funnel, trochus a horseshoe-shaped ridge, mouth central. |
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.
|
|
From H. S. Jennings in American Naturalist, vol. xxxv., by permission of Ginn & Co. |
|
Fig. 3.—a, Stephanoceros eichornii in gelatinous tube; b, Acyclus inquietus in gelatinous tube, with eggs; c, Floscularia coronetta in gelatinous tube, with eggs; d, Apsilus bucinedax, showing lateral (distal) antennae funnel of mouth hanging into enormous crop, stomach at apical end with gastric glands, anus on postero-ventral surface, large coiled kidneys at proximal end, uniting into median duct; e, Melicerta ringens in tube; f, same, proximal end enlarged, showing a pellet in the cup proximal to the paired lateral antennae; g, Melicerta janus, tube formed of faecal pellets. |
|
|
From C. T. Hudson in Quar. Journal of Microscopical Science, vol. xxiv., by permission of J. & A. Churchill. |
|
Fig. 4.—Types of Trophi. a, malleate, with enlarged view of malleus above—the Y-shaped incus consists of a short median fulcrum bearing two large rami, each of which is in contact with a stout malleus consisting of a toothed uncus carried on a long manubrium; b, sub-malleate, with enlarged view of malleus—the manubria are twice as long as the 3- to 5-toothed unci; c, virgate—mallei rod-like, manubria and fulcrum very long, unci 1-or 2-toothed; d, forcipate—rami large and used as a forceps, mallei rod-like, unci pointed or evanescent; e, incudate—stout fulcrum, rami forming a forceps, mallei evanescent; f, uncinate—unci large, 2-toothed, manubria evanescent, incus slender; g, ramate—rami subquadrantic, fulcrum rudimentary, manubria evanescent; h, malleo-ramate—mallei fastened by their unci to the rami, manubria looped, rami large and fulcrum slender. |
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,
