ROTIFERA, or ROTATORIA, the name of a class of invertebrate animals which are characterised by the absence of a medullary chord and pulsating vessels; by the possession of a simple tubular alimentary canal ; a defi nite form ; a reproduction neither fissiparous nor gemmiparous ; the reproductive organs of both sexes in the same individual. Their movements are effected by peculiar rotating organs, and they have no true ar ticulated feet, but mostly a single false foot. The creatures thus constituted are often called wheel-animalcules, from the wheel-like motion of their ciliated rotatory organs. They were formerly classed together with the polygastric animalcules (POLYGASTRIA), under the com mon name of Infusoria, on account of their frequent presence with these animals in vege table infusions. Recent researches, more especially those of Ehrenberg, have shown that the Rotifera possess a much higher and more complicated organisation than the Poly gastria ; so much so, that in any linear arrange ment of the animal kingdom, if the Polygastria were regarded as the lowest beings, several classes might properly intervene between them and the Rotifera. We shall, however, see that there are transitionary forms from the lower to the higher family, sufficiently indicative of their relations and the common circumstances under vvhich they are produced.
For the discovery, and our knowledge of the structure, of the Rotifera, as well as the Polygastria, we are almost entirely indebted to the use of the microscope. Although, gene rally, the former creatures are much larger than the latter, they were not discovered till after the Polygastria. We are, however, in debted for the first observation of both the one and the other to the sagacity of the same great observer Leeuwenhoek, who, in 1675, first saw the Vorticella convallaria, and, in 1702, described the Rotifer vulgaris. Pre vious to this period, no accurate knowledge of creatures so small existed, although the specu lations of Plato and the older Greek philoso phers, subsequently followed up by Descartes, on the doctrine of living atoms, indicated that the human mind had already felt the possibility of the existence of such conditions of organic matter. Aristotle, too (Hist. Anim. v. o. 19.), as Ehrenberg has pointed out, was not un aware of the fact, that coloured water was produced by worms of some kind, which would seem to indicate a knowledge of the existence of some of the forms of Infusoria.
As the discovery of the first Rotifer must be regarded as an era in the history of zoology, we give it in the words of Leeuwen hoek himself:—" On the 25th of August I saw in a leaden gutter, at the fore part of my house, for the length of about five feet, and the breadth of seven inches, a settlement of rain water which appeared of a red colour. . . . I took a drop of this water which I placed before the microscope, and in it I dis covered a great number of animalcules. Some of them red, and others of them green. The largest of these viewed through the micro scope did not appear bigger than a large grain of sand to the naked eye, the size of the others was gradually less and less : they- were for the most part of a round shape ; and in the green ones the middle part of their bodies was of a yenowish . colour. Their bodies seemed composed of particles of an oval shape ; they were also provided with certain short and slender organs, or limbs, which were pro truded a little way out of their bodies, by means of which they caused a kind of cir cular motion and current in the water : when they were at rest, and fixed themselves to the glass, they had the shape of a pear with a short stalk. Upon more carefully examining this stalk, or rather this tail, I found that the extremity was divided into two parts, and by the help of these tails, the animalcules fixed themselves to the glass ; the lesser of these appeared to me to be the offspring of the larger ones." This animalcule, which was the Rotifir vulgaris, is so accurately described by Leen wenhoek in the same paper, as to leave little to be added by future describers. Sub sequently to the time of Leeuwenhoek, who in addition to the Rotifer vulgaris discovered the Melicerta ringeus, a number of species were described by Joblot, Hill, Baker, Rosel, Brady, and others ; so that, in 1824, Bory St. Vincent was enabled principally, from the writings of others, to describe eighty species. -Up to this time no distinction had been made between the wheel and other animalcules as a class. This separation was effected by Ehrenberg, who has not only examined the structure of these creatures with great care, but has added many new species to the list. In his work on infusory animalcules, he de scribes 189 species in fifty-five genera and eight families.