The Rotifera, or "Wheel-animalcules," constitute a very natural group, the exact position of which has been a good deal disputed, and is still doubtful. They are looked upon here as a distinct division of the Scolecida ; but they are very frequently placed with the Annelida amongst the next division 6f the Annulosa (Anar-thropoda).

The Rotifera are minute animals, never parasitic, inhabiting water, and usually provided with an anterior ciliated disc, capable of inversion and eversion. In the females there is a distinct mouth, intestinal canal, and anus. A nervous system is also present, consisting of ganglia, situated near the anterior extremity of the body, and sending fila7nents backwards. A water-vascular system is also present. The sexes are distinct.

Most of the Rotifera are entirely invisible to the naked eye, and they are all extremely minute, none of them attaining a greater length than 1-36th of an inch. Nevertheless, as remarked by Mr Gosse, "so elegant are their outlines, so brilliantly translucent their texture, so complex and yet so patent their organisation, so curious their locomotive wheels, so unique their apparatus for mastication, so graceful, so vigorous, so fleet, and so marked with apparent intelligence, their movements, so various their forms and types of structure," that they form one of the most interesting departments of zoological and microscopical study. They are all aquatic in their habits, and in the great majority of cases are free-swimming animals, some, however, being permanently fixed, as is the case with Stephanoceros, Melicerta (fig. 122, B), and Floscularia. They are usually simple, but are occasionally composite, forming colonies, as in Megalotrocha. As a rule, the male and female Rotifera differ greatly from one another, the males being smaller than the females, destitute of any masticatory or digestive apparatus, and more or less closely resembling the young form of the species. The most characteristic organ in the great majority of the Rotifera, is the so-called "wheel-organ," or "trochal disc," which is always situated at the cephalic or distal end of the body, and consists of a retractile disc, surrounded by a circlet of cilia, which, when in action, vibrate so rapidly as to produce the illusory impression that the entire disc is rotating.

The disc, which carries the cilia, is capable of eversion and inversion, and may be circular, reniform, bilobed, four-lobed, or divided into several lobes. It serves the purpose of locomotion in the free-swimming forms, acting somewhat like the propeller of a screw-steamer; and in all it serves to produce currents in the water, which convey the food to the mouth.

In Chaetonotus, and some other forms (Gastrotricha), there is no true wheel-organ, capable of protrusion and retraction, but the cilia are variously disposed over the surface of the body. The Chaetonoti or Hairy-backed Animalcules have no jaws, and have the ventral surface of the body clothed with cilia. They have often been placed in the Turbellaria, but there seem to be good reasons for regarding them as an aberrant group of Rotatoria. Balatro and Apsilus are non-ciliated in the adult condition.

The proximal extremity of the body in the free forms terminates in a caudal process, or "foot," sometimes telescopic, which ends in a suctorial disc, or in a pair of diverging "toes," which act as a pair of forceps (fig. 122, A).

The mouth usually opens into a pharynx, or "buccal funnel," which is generally provided with a muscular coat, constituting the "mastax" or "pharyngeal bulb," and which generally contains a very complicated masticatory apparatus.* The parts of this apparatus are horny, and are believed by Mr Gosse to be homologous with the parts of the mouth in insects. In the females of almost all known species of Rotifera the intestinal canal is a more or less simple tube, extending through a well-developed perivisceral cavity, and terminating posteriorly in a dilatation, or "cloaca," which forms the common outlet for the digestive, generative, and water-vascular systems.

In both sexes there is a well-developed water-vascular system, usually consisting of the following parts:

* The lower jaws, or "incus," consist of a fixed portion, the "fulcrum," to which are attached two movable blades - the "rami." The upper jaws, or "mallei," consist each of a handle, or "manubrium," to which is hinged a toothed blade, or "uncus."

In the hinder part of the body, close to the cloaca, and opening into it, is a sac or vesicle, which is termed the "contractile bladder," and exhibits rhythmical contractions and dilatations. From the contractile bladder proceed two tubes - "the respiratory tubes" - which pass forwards along the sides of the body, and terminate anteriorly in a manner not quite ascertained. Attached to the sides of the respiratory tubes, in all the larger Rotifera, is a series of ovate or pyriform vesicles, each of which is furnished internally with a single central cilium, which is fixed to the free end of the vesicle. It is asserted, however, that these ciliated vesicles communicate internally with the perivisceral cavity with its contained corpusculated fluid. The exact function of this water-vascular system is not known, but it is most probably respiratory and excretory. Dr Leydig believes that water enters the perivisceral cavity by endosmose, where it mingles with the absorbed products of digestion, to form the so-called "chylaqueous fluid;" and that the effete fluid is excreted by the respiratory tubes, and ultimately discharged into the cloaca by the contractile bladder. Taking this view of the subject, Mr Gosse believes that the "respiratory tubes represent the kidneys, and that the bladder is a true urinary bladder ; " and consequently that the "respiratory and urinary functions are in the closest relation with one another." This observer, further, finds a decided analogy between the above system in the Rotifera and the long and tortuous renal tubes of the Insecta, to which class he believes the Rotifera to be most nearly allied.

Rotifera Sub Class Rotifera Rotatoria 158Fig. 122.   Rotifera. A, Diagrammatic representation of Hydatina senta (generalised from Pritchard): a Depression in the ciliated disc leading to the digestive canal; b Mouth; c Pharyngeal bulb or mastax, with the masticatory apparatus ; d Stomach ; e Cloaca; f Contractile bladder; g g Respiratory or water vascular tubes; h Nerve ganglion giving filament to ciliated pit (k); o Ovary. B, Melicerta ringens. (After Gosse.)

Fig. 122. - Rotifera. A, Diagrammatic representation of Hydatina senta (generalised from Pritchard): a Depression in the ciliated disc leading to the digestive canal; b Mouth; c Pharyngeal bulb or mastax, with the masticatory apparatus ; d Stomach ; e Cloaca; f Contractile bladder; g g Respiratory or water-vascular tubes; h Nerve-ganglion giving filament to ciliated pit (k); o Ovary. B, Melicerta ringens. (After Gosse.)

No central organ of the circulation, or heart, and no organs of respiration are present, but the perivisceral cavity is filled with a corpusculated fluid.

The nervous system of the Rotifera constitutes a bilobate cerebral mass, "which for its proportionate volume may compare with the brain of the highest vertebrates." It is placed anteriorly, and usually on the dorsal aspect of the body, and the eye - in the shape of a red pigment spot or spots - is invariably situated like a wart upon it. Other sense-organs, probably tactile, are often present, in the form of two knobs surmounted by tufts of bristles, placed at the back of the head. The ovaries constitute conspicuous organs in the female Rotifera, and in summer the eggs are produced by the females without having access to the males. Development is direct.

The muscular system of the Rotifera is well developed, consisting of bands which produce the various movements of the body and foot, whilst others act upon the various viscera, and others effect the movements of the jaws.

The typical group of the Rotifera is that of the Notommatina (Hydatinea of Ehren-berg.) In this group (fig. 123) the animals are all permanently free, and are never combined into colonies, while the integument is flexible, and the body is never encased in a tube.

Stephanoceros and Floscularia, on the other hand, are fixed, and are enclosed in a gelatinous tube which is secreted by the animal. Melicerta (fig. 122, B) inhabits a tubular case, which the animal forms for itself by means of a special organ for the purpose ; whilst Polyarthra and Triarthra are protected by a stiff shell, or "lorica."

In Triarthra there are ensiform fins, jointed to the body by distinct shelly tubercles, and moved by powerful muscles. These natatory organs are considered by Mr Gosse to be homologous with the articulated limbs of the Arthropoda. Locomotive appendages are also present in Hexarthra, Polv-arthra, and Pedalion.

In Asplanchna, whilst the masticatory organs, gullet, and stomach are well developed, there is no intestine, the stomach "hanging like a globe in the centre of the body-cavity," but not communicating with the body-cavity.

As regards their distribution in space, the Rotifera have an almost world-wide range. The majority of the known forms are inhabitants of fresh water, but a few live in the sea.

Fig. 123.   Rotifera. Eosphora au rita, one of the Wheel animalcules. Enlarged about 250 diameters. (After Gosse.)

Fig. 123. - Rotifera. Eosphora au-rita, one of the Wheel-animalcules. Enlarged about 250 diameters. (After Gosse.)