Physical characteristics

Rotifers may range in size from less than 0.00394 in (100 pm) to 0.098 in (2,500 pm), but most species measure between 0.00591 and 0.0197 in (150-500 pm). The body generally is divided into a head, a trunk, and a foot region, but this basic pattern may vary greatly. The most conspicuous organ in the head is the wheel organ, also called the corona, which is composed of metachronously beating cilia that are arranged in different, distinct bands. Generally, the corona comprises a large buccal field that surrounds the mouth and a circumapical band that encircles the aciliate apical head region. Numerous modifications from this basic plan have made the corona morphological highly variable. In the class Bdelloidea, for example, the circumapical band is divided medially so that it forms two trochal discs. In live swimming or feeding animals, the ciliary beat of the disc gives the illusion that the animal carries two small, rotating wheels, and the whole phylum is named after this feature (rota, meaning "wheel," and fero, meaning "to bear"). The corona is used both for locomotion and feeding. The ciliary beat leads food particles toward the mouth opening, which always is located more or less ventrally.

The trunk may vary in shape from a very elongated form, sometimes divided into telescopic, retractable pseudo-segments, m

Rotifer embryo with parent. (Photo by Animals Animals ©P. Parks, OSF. Reproduced by permission.)

Rotifer embryo with parent. (Photo by Animals Animals ©P. Parks, OSF. Reproduced by permission.)

to a much more globular or sacciform form. The foot is composed of one to several pseudo-segments, and it often has two terminal toes with adhesive gland openings at their tips. Both the foot and the toes may be reduced in several genera and species. The rotifer integument is syncytial, which means that the epidermal cells are not separated by cell membranes. An outer cuticle, which is present in most other invertebrates, is lacking in rotifers, and instead they have an intracellular filamentous lamina. The thickness of the lamina varies, and in some taxa parts of it are so thick that it forms a heavy body armor. Such taxa are called loricate, whereas those with a thinner lamina are referred to as illoricate.

The mouth is located ventrally or apically in the corona. It leads to the pharynx, which contains a complex masticatory apparatus referred to as the mastax. The mastax is made up of hard jaw parts, called trophi, and minute muscles that connect the jaw elements. The trophi comprise four principal elements: paired rami, paired unci, paired manubria and an unpaired fulcrum. There also may be different associated elements that together are referred to as epipharyngeal elements. The rami are the central elements in the trophi and often are equipped with teeth or denticles. They are joined caudally with a flexible ligament such that they are able to open and close and in that way crush food items. The only unpaired principal element, the fulcrum, extends caudally from the articulation point of the rami. It primarily serves as a muscle attachment point for different muscle fibers, for example, the large abductor muscles that run to the rami. The unci are located rostral or ventral to the rami. They may be rod-shaped with a single sharp tooth or plate-shaped with several strong teeth, and they may be used to grab and manipulate food particles or, in some predatory rotifers, to penetrate the integument of the prey. Proximally, the unci join the manubria. The manubria often are rod-shaped with a well-

developed head and are responsible mainly for the movement of the unci. The morphological features of the four principal elements differ greatly, and the rotifer trophi have been divided into nine different types, depending on the size and shape of the principal sclerites. These types are highly significant for rotiferan taxonomy; several families and genera can be recognized solely on the basis of jaw type.

From the pharynx a short esophagus leads to the stomach, where the food is digested. Digestive enzymes are produced in syncytial gastric glands that empty into the stomach. After the stomach comes the gut, which terminates in a dorsal cloaca. Pairs of protonephridia control excretion and maintenance of osmotic balance. Each protonephridium is composed of one or more multiciliated terminal cells and multiciliated canal cells that lead to the collecting tubules, which guide the wastes on to the urinary bladder. The female reproductive system comprises one or two syncytial germovitellaria that each consists of a germinal region and a yolk-producing vitellarium, surrounded by a follicular layer. The classes Seisonidea and Bdelloidea have paired gonads, whereas the Monogononta have only a single germovitellarium. An oviduct formed by the follicular layer may connect the germovitellarium with the cloaca. The male organs comprise an unpaired testis and a penis. (See Reproductive biology for a more detailed description of the complex rotifer reproduction.)

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