Rotifer (rotifer)

[NL.(Leeuwenhoek, 1702), having a wheel, wheel or wreath and the forked foot are unmarked. distinguished by their circles of cilia, sometimes single, sometimes double, which through the microscope appear like revolving wheels, whence they have been called wheel - animalcules and Rotator i a. They are a small but well-marked group, whose true position in the evolutionary series is much questioned. Some of the forms have been known for nearly two centuries, and many others have only recently been brought to light. Being all of microscopic size, and often appearing in infusions, the rotifers that were known up to about 1838, the period of Ehrenberg's researches, were considered to be protozoan, and they were placed with some vegetable organisms in the old fashioned infusories. (See Infusoria, 1.) Their readily discernible complex organization gave one of the reasons for supposing that infusorians reach a comparatively high grade of development. Rotifers present great attractions to the microscopist, and have been much studied; and the organization of few of the low invertebrates is better known. They are true metazoans, of microscopic size, bilaterally symmetrical, usually without metameric segmentation, always with an intestinal canal and a body-cavity or cœlom, and with an anus as well as a mouth (except in one group). Head and tail are generally well marked; the former bears, under many modifications, the characteristic wheel-organ which gives name to the group, and is technically called the trochal disk (see cut under trochal ); the tail or foot body, called pseudopodium, is variously modified as a locomotory organ for swimming, skipping, creeping, or root rotour ing (see pseudopodium, 2, and cut under Rotifer ); in a few genera it is wanting. The body is covered with a firm cuticle, and sometimes also sheathed in a protective case (see urceolus ); it often presents peculiar spinose or setose appendages. The muscular system may be quite higlily developed, as in Pedal i on, where it consists of several symmetrically disposed bands. In the alimentary canal may usually be distinguished a mouth, pharynx, esopha gus, stomach, intestine, and anus. The pharynx contains the mastax with its teeth or trophi, among which are parts called malleus, incus, uncus, fulcrum, ramus, and manubrium (see these names, and cut under uncus). All true rotifers have a mastax; its homologies are disputed. Both the pharynx and the esophagus are chitin i zed. The intestine is lined with ciliated epithelium. Nephridia are present; a nervous system is demonstrable; and various sense-organs, as eye-spots, are recognized. Rotifers were supposed to be hermaphroditic; but separation of sex has been determined for most members of the cla , the males being in all such cases small and degenerate in comparison with the females. Details of the reproductive proce vary in different cases. The cla ification of Rotifer a, as well as the taxonomic rank and systematic position of the group, is not yet settled, as some equivocal or aberrant forms remain to be accounted for. Exclusive of these, a recla ification given by C. T. Hudson in 1884, and generally accepted, is into four orders: (1) Rhizota, rooted rotifers, with families Flosculariidæ and Melicertidæ; (2) Bdelloida (or Bdelligrada ), creeping rotifers, with one family, called Philodinidæ, though containing the original ge nus Rotifer; (3) Scirtopoda, skipping rotifers, the Peda lionidæ, with one genus (see cut under Rotifer ); and (4) Ploi ma, or swimming rotifers, the rest of the cla . These are either ill or i cate (the Hydatinidæ, Synchætidæ, Notomтаtidæ, Triarthridæ, and As planch nidæ ) or loricate (the Brachionida, Pterodinidæ, and Euchlanida ). Ranked as a supercla or phylum, the rotifers have also been divided into two cla es: Para pod i at a, represented alone by the genus Pedal i on; and Lipopoda, all the rest. One of the commonest rotifers is Hydatina sent a, belonging to the ill or i cate ploimate group. rotiferal (ro-tif'e-ral), a. [ ΧΧΙ. 8. rotiferan (rō-tif'e-ran), n. [