ROTATORIA, or RoTIFEata, popularly known as WHEEL-ANIMALCULES, derive their name from the Latin word rota, a wheel. They have received these names on account of the apparent rotation of certain disk-like ciliated organs which surround the mouth. Although some of the larger forms may he detected with the naked eye, they arc as a class microscopical. They are widely diffused over the surface of the earth, inhabiting both salt and fresh water, and occurring in all climates. There has been much discussion as to their true place in nature. Ehrenberg regarded the-in as infusoria, and Dujardin adopted a similar view. There is, however, no doubt that their organization is far more complex than that of the iufusoria, and the main question of dispute at the present day is whether they are most closely allied to the worms or to the crustaceans. Huxley main tains that they form a link connecting the echinoderms with the nematoid (or thread) worms, and that they constitute the lowest step of the echinoderm division of the anne]ida; while Leydig endeavors to show that on various anatomical, physiological, and embryological grounds they more nearly resemble crustaceans than worms, and proposes to call them ciliated crustaceans. Science is indebted to Leeuwenhoek for the discovery of this remarkable class of animals. In the Philosophical Transactions for 1702 he described one of the commonest of these animals, now known as rotifer valgaris, his attention having been especially directed toward its power of retaining its vitality after more or less complete desiccation—a fact which has been since confirmed by many other observers. and which is noticed in the article on DORMANT VITALITY. The rota toria have usually an elongated form, and are in most cases covered with a smooth hard skin, which is thrown into folds by the contractions of the subcutaneous tissue. The animal consists of a head mid body. The body usually terminates in a prolongation which, till recently, was termed the tail, but which is now known as the foot, and into which the intestines are never prolonged.• The foot is composed of muscular and gland ular structures, and often terminates in a pair of forceps, by winch the animal can attach itself to leaves, etc. The body generally presents six segments, which are more or less distinctly marked in different genera. The head presents the characteristic rota tory organs and the mouth, which always lies in the midst of them, so as to receive particles drawn in by their whirlpool action. It is by means of these organs that they swim freely about, revolving on their axis, or when at rest producing vortex-like dis turbanir of the water. The form, number, and arrangement of these organs varies extremely in different genera, and has been made a basis of classification by Ehrenberg and others. The rotatory organ may be single, double, or multiple. It often consists of a disk supported by a pedicel, on whose borders are successive rows of regularly arranged cilia, the motion of which gives the appearance of rotation to the disk itself. In the genera floscularia and stephanoceros these organs undergo peculiar modifications. In the former there are five or six button-like processes about the mouth, covered with very long bristles, which move feebly and scarcely give rise to vortices; while in the latter the rotatory apparatus con sists of five tentacle-like ciliated and the animal thus closely resembles the polyzoa (q.v.). The ciliated rotatory organs, unlike ordinary vol atile cilia, are entirely under the animal's control. The digestive apparatus differs extremely in the two sexes, which are always distinct in these animals. In the female the digestive apparatus is well developed, consisting of a mouth opening into a muscular pharynx, which has two horny masticating organs which move laterally upon each other.
The pharyngeal masticating apparatus is of a roundish form, and is composed of two jaws having one or several teeth, which are brought together laterally hy the action of special muscles. For further information on the subject the reader is referred to a very exhaustive memoir by Mr. Gosse, "On the Structure, Functions, and Homologies of the Manducatory Organs of the Class Rotifera," in the Philosophical Transactions for 1856. Succeeding the pharynx is a narrow esophagus, which leads into a dilated stomach, from which proceeds an intestine, which opens externally by an anus. In all the males that have been hitherto discovered there is an entire absence of digestive organs, a rudimentary pharynx being the most that is ever observed. The nervous. system in the rotatoria consists of a cerebral ganglion, with filaments radiating from it. No heart or vessels have been discovered hut the respiratory organs are well developed. The sexual organs of the female are better known than those of the male. The ovary is round or oval, usually lies by the side of the stomach, and the oviduct proceeding from it usually opens into the cloaca. The ovaries only develop a few eggs at a time, and the nearly mature eggs maybe readily observed in the body of the animal when examined under the microscope. These animals produce two distinct kinds of eggs, which are similar in their primary formation, but which differ in their ultimate destiny—namely, thin-shelled summer eggs and thick-shelled winter eggs. The young are liberated from the former immediately after their discharge, while they remain unhatched in the latter during the winter weather. As far as has hitherto been observed, the males, which are much fewer in number than the females, are developed only from summer eggs. Ex cept in regard to their being totally devoid of a stomach or intestine, and iu relation to the sexual organs (which in the male have been carefully examined by Mr. Gosse in his memoir On the Dimcious Character of the Rotifera," in the Philosophical Transac tions for 1857), the organization of the males is similar to that of the females. The sexes are, however, so unlike that they would be taken for widely remote genera if their actual hatching bad not been observed; the males and the eggs from which they spring being much smaller than the females and the eggs from which they are pro duced. (In bruchionus umphicerds, the female eggs were in. in length, while the male eggs were only The length of a female braehionus dorcas an hour after birth was in., while the diameters of the empty shell were only X iu.—a mar velous increase iu so short a period. " Whether," says Mr. Gosse, " certain individ uals produce only male and others only female young, or whether separate impreg nations are required for the production of the separate sexes, I do not know; but front all my observations I gather that the development of the one sex never takes places coetaueously with that of the other; for male and female eggs are never seen attached to the same parent, and the immature eggs in the ovary invariably develop themselves into the same sex as those which are already extruded. The duration of life in the male is always very brief; I have never been able to preserve one alive for twenty-four hours. Their one business is to impregnate the females, and for this momentary occupation no supply of loss by assimilation of food is wanted, and hence we can understand the lack of the nutritive organism."