With regard to their geographical distribution, they have been thought to be potentially cosmopolitan, and it is true that a rela tively small number of species seem to occur in every country whose fauna has been investigated with respect to them. But for the overwhelming majority of known species there exist only isolated or few records, and while this is the case, such views seem lacking in foundation. Even in those countries which have been most carefully searched, a mere fraction of the possible investiga tion has yet been carried through. After discarding a host of im perfectly described and unrecognizable forms, there are over 1,0m reliable species known to present-day microscopists.
BIBLIOGRAPHY.—The literature relating to the Rotifera is somewhat voluminous and deals with them from every conceivable view-point, from that of the possessor of a microscope of moderate capacities to that of the highly trained zoologist equipped with the most modern instrument and the newest and most powerful lenses. A remarkably complete and useful bibliography of the subject, as at the end of 1912, is included in H. K. Harring's Synopsis of the Rotatoria (United States Nat. Museum, Washington, 1913) and comprises over 1400 titles of works of minor or major importance, mainly by European authors. In view of the vast improvement in lenses, instruments and:technique, few works more than fifty years old have other than historical value or as the home of the original descriptions of particular genera or species. Of those of more recent date the following may be indicated as having greatly influenced current thought on structure and conse quently on classification: C. Zelinka, Studien weber Raederthiere, three papers (Leipzig, 1886, i888, 1891) ; C. Wesenberg-Lund, Danmarks Rotifera (Copenhagen, 1899) ; P. D. Beauchamp, Recherches sur les Rotiferes, les formations tegumentaires et l'appareil digestif (Paris, On the faunistic side may be noted as outstanding: C. Wesenberg Lund, Contributions to the Biology of the Rotifera I: The Males of the Rotifera (Copenhagen, 1923) ; and, in course of publication, in instalments, H. K. Harring and F. J. Myers, The Rotifer Fauna of Wisconsin (Madison, 1922 onwards). The latter, with a scope much wider than is implied in the title and embodying the very latest ideas as to classification, promises to be the standard work of its kind for a long period to come. (D. L. B.)
The systematic affinities of the Rotif era have been much dis cussed without any general measure of agreement being arrived at. Since Ehrenberg in 1838 distinguished them from the ciliate Protozoa they have been approximated in turn to nearly every one of the major divisions of the animal kingdom except the Chordata. In 1851 Huxley compared them with the free-swim ming ciliated larvae of Annelids and, more particularly, of Echinoderms. In 1858, Semper's discovery of Trochosphaera gave fresh support to the comparison with the larvae of Annelids and for a long time the view that the Rotif era were persistent trochophores may be said to have held the field. In 1871 Pedalion was described by Hudson, and this remarkable form with its three pairs of hollow limbs moved by muscles, giving it a superficial re semblance to a crustacean Nauplius larva, revived an older view that the Rotifera were in some way related to the Arthropoda. Lankester included them with the Annelida and Arthropoda in his phylum Appendiculata. But the resemblances between Trochosphaera and the trochophore larva break down when ex amined in detail until little more is left than the common posses sion of a preoral ciliary wreath which they share with the Perit richous Infusoria; and on the other hand the fact that two of the appendages of Pedalion are median and unpaired seems to preclude any close comparison with the other "Appendiculata." More recently Wesenberg-Lund and de Beauchamp have argued that the ciliary wreath is a secondary development and that the most primitive Rotifers are those like Notommata in which there is a ventral uniformly ciliated field surrounding the mouth. From these it is easy to pass to the ventrally ciliated Gastrotnicha and to imagine the derivation of both from a uniformly ciliated Turbellarian-like stock.
With organisms like the Rotifera, however, where palaeon tology can give no help, phylogeny must remain a matter of speculation. All that we can be sure of is that they are un segmented Metazoa without definite mesoderm or coelom, with branching excretory canals furnished with flame-cells, and having a single pre-oral nerve-ganglion. They are, therefore, on the same grade of organization as the Platyhelminths and the early larvae of several groups of higher Metazoa. It is likely that the exact arrangement of the locomotive appendages, whether ciliary or appendicular, is without any important phylogenetic significance.
(W. T. C.)