ROTIFERA. The Rotifera (or Rotatoria) form a small, in many respects well-defined, but somewhat isolated class of the animal kingdom. They are here treated of sepa-rately, partly on account of the difficulty of placing them in one of the large phyla, partly on account of their special interest to microscopists.

Now familiarly known as " wheel animalcules" from the wheeldike motion produced by the rings of cilia which generally occur in the head region, the so-called rotatory organs, they were first discovered by Leeuwenhoek (l), to whom we also owe the discovery of Bacteria and ciliate Infusoria. Leeuwenhoek described the Rotifer vulgaris in 1702, and he subsequently described Melicerta ringens and other species. A great variety of forms were described by other observers, but they were not separated as a class from the unicellular organisms (Protozoa) with which they usually occur until the appearance of Ehrenberg's great monograph (2), which contained a mass of detail regarding their structure. The classification there put forward by Ehrenberg is still widely adopted, but numer-ous observers have since added to our knowledge of the anatomy of the group (3). At the present day few groups of the animal kingdom are so well known to the micro-scopist, few groups present more interesting affinities to the morphologist, and few multicellular animals such a low physiological condition.

The surface of the body is covered by a firm homogeneous structureless cuticle. This cuticle may become hardened by a further development of chitin, but no calcareous deposits ever take place in it. The cuticle remains softest in those forms which live in tubes. Among the free-living forms the degree of hardening varies considerably. In some cases contraction of the body merely throws the cuticle into wrinkles (Notommata, Asplanchna); in others definite ring-like joints are produced which telescope into one another during contraction; while in others again it becomes quite firm and rigid and resembles the carapace of one of the Entomostraca; it is then termed a "lorica." The lorica may be prolonged at various points into spines, which may attain a considerable length. The surface may be variously modified, being in some cases smooth, mothers

General Anatomy.—The Rotifera are multicellular animals of microscopic size which present a ccelom. They are bilaterally symmetrical and present no true metameric segmentation. A head region is generally well marked, and most forms present a definite tail region. This tail region has been termed the " pseudopodium." It varies very much in the extent to which it is developed. It attains its highest development in forms like Philodina, which affect a leech-like method of progression and use it as a means of attachment. We may pass from this through a series of forms where it becomes less and less highly developed. In such forms as Brachionus it serves as a directive organ in swimming, while in a large number of other forms it is only represented by a pair of terminal styles or flaps. In the sessile forms it becomes a con-tractile pedicle with a suctorial extremity. A pseudo-podium is entirely absent in Asplanchna, Triarthra, Polyarthra, and a few other genera. The pseudopodium, when well developed, is a very muscular organ, and it may contain a pair of glands (fig. 2, A, gl) which secrete an ad-hesive material.

marked, dotted, ridged, or sculptured in various ways (fig. 1, K). The curved spines of Philodina aculeata (fig. 1, G) and the long rigid spines of Triarthra are further develop-ments in this direction. The so-called seta? of Polyarthra on the other hand are more complex in nature, and are moved by muscles, and thus approach the "limbs" of Pedalion.

FIG. 1.—A, Floscularia campanula/a, an adult male, drawn from a dead specimen (after Hudson): t, testis; oc, eye-spots. B, Floscularia appendiculata, an adult female (after Gegenbaur) : a, the ciliated flexible proboscis. C, Stephano-ceros eichhornii: a, the ureeolus. D, Microcodon clavus, ventral view (after Grenacher) : m, mouth; a, bristles ; x, architroch ; s, lateral sense-organs. E, Polyarthra platyptera: oc, eye-spot; x' isolated tufts representing a cephalo-troch; x, branchiotroch; a, b, and c, three pairs of appendages which arc moved by the muscles m. F, another figure of Polyarthra, to show the position which the appendages may take up. G, Philodina aculeata : oc, eye-spot; s, calcar. H, Actinurus neptunius. oc, eye-spot; s, calcar. I, Asplanchna sie-boldii, male, viewed from the abdominal surface: a, anterior short arms; b, posterior longer arms; m, mouth ; x', cephalotrochic tufts; a:, branchiotroch. J, Asplanchna sieboldii, female; letters as before. K, Noteus quadricornis, to show the extent to which the lorica may become sculptured. (All, except where otherwise stated, from Pritchard.)

Several genera present an external casing or sheath or tube which is termed an " ureeolus." In Floscularia and Stephanoceros the ureeolus is gelatinous and perfectly hyaline; in Conochilus numerous individuals live in such a hyaline ureeolus arranged in a radiating manner. The ureeolus, which is secreted by the animal itself, may become covered with foreign particles, and in one species, the well-known Melicerta ringens, the animal builds up its ureeolus with pellets which it manufactures from foreign particles, and deposits in a regular oblique or spiral series, and which are cemented together by a special secretion. The urceolus serves as a defence, as the animal can by con-tracting its stalk withdraw itself entirely within the tube.

Locomotor Organs.—While, as mentioned above, several genera or individual species present long spines, these become movable, and may be spoken of as appendages, in two genera only. In Polyarthra (fig. 1, E, IT) there are four groups of processes or plumes placed at the sides of

FIG. 2.—Floscularia appendiculata. A and B represent the same animal, some of the organs being shown in one figure and some in the other, oc, eye-spots; g, nerve ganglion ; p, pharynx (the mouth should he shown opening opposite the letter); ma, the mastax; e, (Esophagus; si, stomach; a, anus, opening the cloaca; gl, mucous glands in the pseudopodium; n, nephridia; /, flame-cells; bl, contractile vesicle ; m, m, muscles.
the body, each of which groups can be separately moved up and down by means of muscular fibres attached to their bases, which project into the body. The processes them-selves are unjointed and rigid. In Pedalion (fig. 3), a remarkable form discovered by Dr C. J. Hudson in 1871 (12, 13, 14, and 15), and found in numbers several times since, these appendages have acquired a new and quite special development. They are six in number. The largest is placed ventrally at some distance below the mouth. Its free extremity is a plumose fan-like expansion (fig. 3, A, a, and H). It is (in common with the others) a hollow process into which run two pairs of broad, coarsely trans-versely striated muscles. Each pair has a single insertion on the inner wall—the one pair near the free extremity of the limb, the other near its attachment; the bands run up, one of each pair on each side and run right round the body forming an incomplete muscular girdle, the ends approximating in the median dorsal line. Below this point springs the large median dorsal limb, which termin-ates in groups of long setiE. It presents a single pair of muscles attached along its inner wall which run up and form a muscular girdle round the body in its posterior third. On each side is attached a superior dorso-lateral and an inferior ventro-lateral appendage, each with a fanlike plumose termination consisting of compound hairs, found elsewhere only among the Crustacea; each of these is moved by muscles running upwards towards the neck and arising immediately under the trochal disk, the inferior ventro-lateral pair also presenting muscles which form a girdle in the hind region of the body. Various other muscles are present: there are two complete girdles in the neck region immediately behind the mouth; there are also muscles which move the hinder region of the body. In addition to these the body presents various processes which are perhaps some of them unrepresented in other Botifers. In the median dorsal line immediately below the trochal disk there is a short conical process presenting a pair of muscles which render it capable of slight move-ment. From a recess at the extremity of this process spring a group of long setose hairs the bases of which are connected with a filament probably nervous in nature. This doubtless represents a structure found in many Rotifers, and variously known as the "calcar," "siphon," "tentaculum," or "antenna." This calcar is double in Tubicolaria and Melicerta. It is very well developed in the genera Rotifer, Philodina, and others, and is, when so developed, slightly retractile. It appears to be repre-sented in many forms by a pit or depression set with hairs. The calcar has been considered both as an intromittent organ and a respiratory tube for the admission of water. It is now, however, universally considered to be sensory in nature. Various forms present processes in other parts

FIG. 3.—Pedalion mira. A, Lateral surface view of an adult female: a, median ventral appendage; b, median dorsal appendage; c, inferior ventro-lateral appendage : d, superior dorso-lateral appendage ; /, dorsal sense-organ (calcar) ; g, " chin; " x, cephalotroeh. B, lateral view, showing the viscera : oc, eye-spots ; n, nephridia; e, ciliated processes, probably serving for attachment; other letters as above. C, ventral view: x', cephalotroeh; x, branchiotroch; other letters as above. D, ventral view, showing the musculature (c/. text). E, dorsal view of a male: a, lateral '_ appendages; b, dorsal appendage. F, lateral view of a male. G, enlarged view of the sense-organ marked/. H, enlarged view of the median ventral appendage. (All after Hudson.)

of the body which have doubtless a similar function, e.g., Microcodon (fig. 1, D, s) with its pair of lateral organs. Pedalion presents a pair of ciliated processes in the posterior region of the body (fig. 3, B, C, and D, e), which it can apparently use as a means of attachment: Dr Hudson states that he has seen it anchored by these and swimming round and round in a circle. They possibly represent the flaps found on the tail of other forms. Pedalion also has a small ciliated muscular process (fig. 3, A, g) placed immediately below the mouth, and termed a " chin," which appears to be merely a greater development of a sort of lower lip which occurs in many Rotifers.

Muscular System.—All the llotifera present a muscular system which is generally very well developed. Transverse striation occurs among the fibres to a varying extent, being well marked in cases where the muscle is much used. The muscles which move the body as a whole are arranged as circular and longitudinal series, but they are arranged in special groups and do not form a com-plete layer of the body-wall as in the various worms. Some of the longitudinal muscles are specially developed in connexion with the tail or pedicle. Other muscles are developed in connexion with special systems of organs,—the trochal disks, the jaw apparatus, and the reproductive system. The muscles in connexion with the trochal disk serve to protrude or withdraw it, and to move it about, when extruded, in various directions. The protrusion is probably, however, generally effected by the elasticity of the integument coming into play during the relaxation of the retractor muscles, and by a general contraction of the body wall. The tentaculiferous apparatus of Polyzoa and Gephyrea is protruded in the same manner.

Trochal Disk.—This structure is the peculiar characteristic of the class. It is homologous with the ciliated bands of the larva? of Echinoderms, Chsetopods, Molluscs, &c, and with the tenta-culiferous apparatus of Polyzoa and Gephyrea, and has been termed in common with these a "velum." This velum presents itself in various stages of complexity. It is found as a single circum-oral ring {pilidium), as a single prse-oral ring (Chaetopod larva?), or as a single prse-oral ring coexisting with one or more post-oral rings (Chaetopod larva;, Holothurian larvae). We may here assume that the ancestral condition was a single circum-oral ring associated with a terminal mouth and the absence of an anus, and that the exist-ence of other rings posterior to this is an expression of metameric segmentation, i.e., a repetition of similar parts. With the develop-ment of a prostomiate condition a certain change necessarily takes place in the position of this band: a portion of it comes to lie longitudinally; but it may still remain a single band, as in the larva of many Echinoderms. How have the other above-mentioned conditions of the velum come about ? How has the prse-oral baud been developed ? Two views have been held with regard to this question. According to the one view, the fact whether the single band is a prae-oral or a post-oral one depends upon the position in which the anus is about to develop. If the anus develops in such a position that mouth and anus lie on one and the same side of the band, the latter becomes prse-oral ; if, however, the anus develops so that the mouth and anus lie upon opposite sides of the band, the band becomes post-oral. If we hold this view we must consider any second band, whether prse- or post-oral, to arise as a new development. The other view premises that the anus always forms so as to leave the primitive ring or "architroch" post-oral, i.e., between mouth and anus. Concurrently with the development of a prostomium this architroch somewhat changes its position and the two lateral portions come to lie longitudinally ; these may be supposed to have met in the median dorsal line and to have coalesced so as to leave two rings—the one prae-oral (a " cephalo-troeh"), the other post-oral (a " branchiotroch "); this latter may atrophy, leaving the single prse-oral ring, or it may become further developed and thrown into more or less elaborate folds. The exist-ing condition of the trochal disk or velum in the Rotifera seems to the writer of this article to bear out the latter view as to the way in which modifications of the velum may have come about.

In its simplest condition it forms a single circum-oral ring, as in Microcodon (fig. 1, D). The structures at the sides of the mouth in this form are stated to be bristles, and have therefore nothing to do with the velum (fig. 4, A, p). This simple ring may become thrown into folds, so forming a series of processes standing up around the mouth; this is the condition in Stephanoceros (fig. 4, s,p). There are, however, but few forms presenting this simple condi-tion ; and it must be remembered that the evidence for the assump-tion here made, that this is a persistent architroch and not a bran-chiotroch persisting where a cephalotroch has vanished, is not at present conclusive. This band, may, while remaining single and perfectly continuous, become prolonged around a lobe overhanging the mouth—a prostomium. This condition occurs in Philodina (fig. 4, E, F, p); the two sides of the post-oral ring do not meet dorsally, but are carried up and are continuous with the row of cilia lining the "wheels." There is thus one continuous ciliated band, a portion of which runs up in front of the mouth. This condition corresponds to that of the Auricularian larva. The fold-ing of the band has become already somewhat complicated; a hypothetical intermediate condition is shown in fig. 4, c, D. The next stage in the advancing complexity is that the prostomial por-tion of the band (fig. 4, G, H, p') becomes separated as a distinct ring, a cephalotroch ; we find such a stage in J-acinularia (fig. 4, then discontinuous (fig. 4, K, L, c), and further it may become so reduced as to be represented only by a few isolated tufts, as in Asplanchna (fig. 1, I, x and x'); in such a form as Lindia (fig. 6, o) the branchiotroch has vanished and the cephalotroch has become reduced to the two small patches at the sides of the head.

The trochal apparatus serves the Rotifera as a locomotive organ and to bring the food particles to the mouth ; the cilia work so as to produce currents towards the mouth.
Digestive System.—This consists of the following regions:—(1) the oral cavity ; (2) the pharynx ; (3) the oesophagus ; (4) the stomach ; (5) the intestine, which terminates in an anus. The anus is absent in one group.

The pharynx contains the mastax with its teeth ; these are calcareous structures, and are known as the trophi. In a typical mastax (8, 9) (Bra-chionus, fig. 5, A) there are a median anvil or incus and two hammer-like portions, mallei. The incus consists of two rami (e) resting upon a cen-tral fulcrum (f); each malleus con-sists of a handle or manubrium (c) and a head or uncus (d), which often presents a comb-like structure. Fig. 5 shows some of
the most important FlG 5]_Tropni o{ various forms: A, Brachionus; B, modificationswllicil Biglena forcipata; C, Asplanchna ; D, Philodina. f, the apparatus may fulcrum, and e,e, rami, forming the incus; c, manubrium, exhibit The parts an<^ ^> uncus? forming the malleus. (After Hudson.) may become very slender, as in Diglena forcipata (fig. 5, B) ; the mallei may be absent, as in Asplanchna (fig. 5, c), the rami being highly developed into curved forceps and movable one on the other ; or, the manubria being absent and the fulcrum rudimentary, the rami may become massive and subquadratic, as in Philodina (fig. 5, D). All the true Rotifers possess a mastax. Ehrenberg's group of the Agomphia consisted of a heterogeneous collection of forms, —Ichthydium and Chxtonotus being Gastrotricha, and Cyphonautes


a Polyzoan larva, while Entcroplea is probably a male Rotifer, and, like the other males, in a reduced condition. There is no reason for considering this mastax as the homologue of either the gastric mill of Crustaceans on the one hand or the teeth in the Choctopods' pharynx on the other; it is merely homoplastic with these structures, but has attained a specialized degree of development. Both the pharynx and the oesophagus which follows it are lined with chitin. The oesophagus varies in length and in some genera is absent (Philo-dinadm), the stomach following immediately upon the pharynx. The stomach is generally large ; its wall consists of a layer of very large ciliated cells, which often contain fat globules and yellowish-green or brown particles, and outside these a connective tissue membrane; muscular fibrillae have also been described. Very constantly a pair of glands open into the stomach, and probably represent the hepato-pancreatic glands of other Invertebrates.

Following upon the stomach there is a longer or shorter intestine, which ends in the cloaca. The intestine is lined by ciliated cells. In forms living in an urceolus the intestine turns round and runs forward, the cloaca being placed so as to debouch over the margin of the urceolus. The cloaca is often very large ; the nephridia and oviducts may open into it, and the eggs lodge there on their way outwards ; they are thrown out, as are the fsecal masses, by an eversion of the cloaca. Asplanchna, Notommata sieboldii, and certain species of Ascomorplia are said to be devoid of intestine or anus, excreinentitious matters being ejected through the mouth (11).

Nephridia.—The ccelom contains a fluid in which very minute corpuscles have been detected. There is no trace of a true vascular system. The nephridia (fig. 2, B, n) present a very interesting stage of development. They consist of a pair of tubules with an intracellular lumen running up the sides of the body, at times merely sinuous, at others considerably convoluted. From these are given off at irregular intervals short lateral branches, each of which terminates in a flame-cell precisely similar in structure to the flame-cells found in Planarians, Trematodes, and Cestodes ; here as there the question whether they are open to the ccelom or not must remain at present undecided. At the base these tubes open either into a permanent bladder which communicates with the cloaca or into a structure presenting apparently no advance in its development upon the contractile vacuole of a ciliate Infusorian.

Nervous System and Sense-Organs. —Various structures have been spoken of as nervous which are now acknowledged to have been erroneously so described (18). There is a supra-cesophageal gang-lion which often attains considerable dimensions, and presents a lobed appearance (fig. 2, A and B, g). Connected with this are the eye-spots, which are seldom absent. Where these are most highly developed a lens-like structure is present, produced by a thicken-ing of the cuticle. In the genus Rotifer and other forms these are placed upon the protrusible portion of the head, and so appear to have different positions at different moments. The number of eye-spots varies from one to twelve or more. They are usually red, red-dish-brown, violet, or black in colour. Other structures are found which doubtless act as sense-organs. The calcar above-mentioned generally bears at its extremity stiff hairs which have been demon-strated to be in connexion with a nerve fibril. On the ventral sur-face of the body just below the mouth a somewhat similar structure is often developed—the chin. There are besides at times special organs, like the two lateral organs in Microcodon (fig. 1, n, s), which no doubt in common with the calcar and chin have a tactile function.

Reproductive Organs and Development.—The Rotifera were formerly considered to be hermaphrodite, but, while the ovary was always clear and distinct, there was always some difficulty about the testis, and various structures were put forward as representing that organ. One by one, however, small organisms have been dis-covered and described as the males of certain species of Rotifers, until at the present time degenerated males are known to occur in all the families except that of the Philodinadae. The male Rotifers are provided with a single circlet of cilia (a peritroch), a nerve ganglion, eye-spots, muscles, and nephridial tubules all in a some-what reduced condition, but there is usually no trace of mouth or stomach, the main portion of the body being occupied by the testi-cular sac. There is an aperture corresponding with the cloaca of the female, where the testis opens into the base of an eversible penis. The males of Floscularia are shown in fig. 1. The male of Pedalion mira possesses rudimentary appendages. The ovary is usually a large gland lying beside the stomach connected with a short oviduct which opens into the cloaca. The ova often present a reddish hue (Philodina roseola, Brachionus rubens), due doubtless, like the red colour of many Crustacean ova, to the presence of tetronerythrin.

Up to the present our embryological knowledge of the group is very incomplete. Many Rotifers are known to lay winter and summer eggs of different character. The winter eggs are provided with a thick shell and probably require fertilization. Two or three of them are often carried about attached to the parent (Brachionus, Notommata), but they are usually laid and fall into the mud, there to remain till the following spring. The summer eggs are of two kinds, the so-called male and female ova, both of which are stated to develop parthenogenetically. They may be carried about in large numbers in the cloaca or oviduct or attached to the body of the parent. The female ova give rise to female and the male ova to male individuals. Male individuals are only formed in the autumn in time to fertilize the winter ova.

Habitat and Mode of Life.—The Rotifera are distri-buted all over the earth's surface, inhabiting both fresh and salt water. The greater number of species inhabit fresh water, occurring in pools, ditches, and streams. A few species will appear in countless numbers in infusions of leaves, &c, but their appearance is generally delayed until the putrefaction is nearly over. Species of Rotifer and Philodina appear in this way. A few marine forms only have been described—Brachionus miilleri, B. hepta-tonus, Synchxta baltica, and others.

A few forms are parasitic. Albertia lives in the intestine of the earthworm; a form has been described as occurring in the body-cavity of Synapta; a small form was also observed to constantly occur in the velar and radial canals of the freshwater jelly-fish, Limnocodium. Notommata parasitica leads a parasitic existence within the hollow spheres of Volvox globator, sufficient oxygen being given off by the Volvox for its respiration.

Many Rotifers exhibit an extraordinary power of resist-ing drought. Various observers have dried certain species upon the slide, kept them dry for a certain length of time, and then watched them come to life very shortly after the addition of a drop of water. The animal draws itself to-gether, so that the cuticle completely protects all the softer parts and prevents the animal itself from being thoroughly dried. This process is not without parallel in higher groups ; e.g., many land snails will draw themselves far into the shell, and secrete a complete operculum, and can remain in this condition for an almost indefinite amount of time. The eggs are also able to withstand drying, and are pro-bably blown about from place to place. The Rotifera can bear great variations of temperature without injury.

Since their removal from among the Protozoa various attempts have been made to associate the Rotifera with one or other large phylum of the animal kingdom Huxley, insisting upon the importance of the trochal disk, put forward the view that they were " permanent Echino-derm larvae," and formed the connecting link between the Nemertidse and the Nematoid worms. Ray Lankester proposed to associate them with the Chsetopoda and Arthropoda in a group Appendiculala, the peculiarities in the structure of Pedalion forming the chief reason for such a classification. There is, however, no proof that we thus express any genetic relationship. The well-developed ccelom, absence of metameric segmentation, persistence of the trochal disk in varying stages of development, and the structure of the nephridia are all characters which point to the Rotifera as very near representatives of the common ancestors of at any rate the Mollusca, Arthropoda, and Chxtopoda. But the high development of the mastax, the specialized character of the lorica in many forms, the movable spines of Polyarthra, the limbs of Pedalion, and the lateral appendages of Asplanchna, the existence of a diminutive male, the formation of two varieties of ova, all point to a specialization in the direction of one or other of the above mentioned groups. Such specialization is at most a slight one, and does not justify the definite associa-tion of the Rotifera in a single phylum with any of them.

Classification.—The following classification has been recently put forward by Dr C. T. Hudson (19).

CLASS ROTIFERA. Order I.—Rhizota. Fixed forms ; foot attached, transversely wrinkled, non-retractile truncate.

Fam. 1. FLOSCULARIAD^I. Floscularia, Stephanoceros. Fam. 2. MELICERTAD.E. Melicerta, Gephalosiphon, Megalo-trocha, Limnias, AJcistes, Lacinularia, Conochilus.

Order II.—Bdelloida.
Forms which swim and creep like a leech ; foot retractile, jointed, telescopic, termination furcate.
Fam. 3. PHILODINAD^;. Philodina, Rotifer, Oallidina.
Order III.— Ploi'ma. Forms which swim only.
Grade A. ILLOEICATA. Fam. 4. HYDATINAD2E. Sydatina, Rhinops. Fam. 5. SYNOH^TAD^;. Synchasta, Polyarthra. Fam. 6. NoTOMMATADiB. Notommata, Diglena, Furcularia,
Scaridium, Pleurotrocha, Distemma. Fam. 7. TKIAKTHKAD^:. Triarthra. Fam. 8. ASPLANOHNAD^;. Asplanchna.
Grade B. LORICATA. Fam. 9. BRACHIONIM:. Brachionus, Noteus, Anursea, Sac-culus.
Fam. 10. PTBRODINAD^:. Pterodina, Pompholyx.
Fam. 11. EUCHLANIDJE. EucManis, Salpina, Diplax, Monostyla, Oolurus, Moiiura, Metopodia, Stephanops, Monocerca, Mastigocerca, Dinocharis.
Order IV.—Scirtopoda.
Forms which swim with their ciliary wreath, and skip by means of hollow limbs with internal locomotor muscles. Fam. 12. PEDALIONID.S. Pedalion.

The above list includes only the principal genera. There are, however, a number of forms which could not be placed in any of the above families.

ABERRANT FORMS. Trochosphsera mquatorialis (fig. 6, G), found by Semper in the Philippine Islands, closely resembles a monotrochal polychaetous

FIG. 6.—Various aberrant forms. A, Balatro calvus (after Claparede): a, mastax. B, Seison nebalix (after Clans) : m. mouth ; vd, position of the aperture of the vas deferens. C, Lindia torulosa : a, ciliated processes at the sides of the head representing cephalotroch; oc, eye-spots. D, E, and F, Apsilus lentiformis (after Mccznikow). D, adult female with expanded proboscis : m, position of the mouth ; s, lateral sense-organs. E, young free-swimming female. F, adult male. G, Trochosphxra sequatorialis (after Semper) : m, mouth; g, ganglion ; a, anus ; b, velum; oc, eye-spot; c, muscles. larva while possessing undoubtedly Rotifeial characters. Mecznikow has described a remarkable form, Apsilus lentiformis (fig. 6, D, E, and F), the adult female of which is entirely devoid of cilia but possesses a sort of retractile hood ; the young female and the males are not thus modified. Claparede discovered fixed to the bodies of small Oligochcetes a curious non-ciliated form, Balatro calvus (fig. 6, A), which has a worm-like very contractile body and a well-developed mastax. As mentioned above, the ciliation is reduced to a minimum in the curious worm-like form Lindia (fig. 6, c). Seison nebaliie (fig. 6, B), living on the surface of Nebaliae, which was described originally by Grube, is the same form as the Saccdbdella nebalim, which was supposed by Van Beneden and Hesse to be a leech. It has been shown by Claus to be merely an aberrant Rotifer.

Of the curious aquatic forms Icthydium, Chxtonotus, Turbanella, Dasyditis, Gephalidium, Ghxtura, and Hemidasys, which Mecznikow and Claparede included under the name Gastrotricha, no further account can be given here. They are possibly allied to the Rotifera, but are devoid of mastax and trochal disk.

The following are some of the more important memoirs, tec, on the Rotifera. (1) Leeuwenhoek, Phil. Trans., 1701-1704. (2) Ehrenherg. Die Infusionsthierchen als vollkommene Organismen, 18-38. (3) M. F. Dujardin, Hist. Nat. des Zoophytes: Infusoires, 1841. (4) W. C. Williamson, "On Melicerta ringens," Quart. Jour. Micr. Sci., 1853. (5) Ph. H. Gosse, " On Melicerta ringens," Quart. Jour. Micr. Sci., 1853. (6) T. H. Huxley, " On Lacinularia socialis," Trans. Micr. Soc, 1853. (7) Fr. Leydig, " Ueber den Ban und die systematische Stellung der Raderthiere," Zeit.f. w. Zool., vi., 1854. (8) Ph. H. Gosse, Phil. Trans., 185G. (9) F. Cohn, Zeit f. w. Zool,, vii., ix., and xii. (10) Ph. H. Gosse, Phil. Trans., 1858. (11) Pritchard, Infusoria, 1801. (12, 13. 14) C. T. Hudson, "On Pedalion," Quart. Jour. Micr. Sci , 1R72. and Monthly Mia: Jour., 1S71 and 1872. (15) E. Ray lankester, " On Pedalionf Quart. Jour. Micr. Sci., 1872. (16) E'. Mecznikow, " On Apsilus lenti-formis," Zeit. f. w. Zool., 1872. (17) C. Semper, "On Trochosphsera," Zeit. f. w. Zool., xxii.. 1872. (18) K. Eckstein, " Die Rotatorien der Umgegend von Giessen, " Zeit.f. w. Zool., 1883. (19) C. T. Hudson, 11 On an Attempt to reclassify Rotifers,"
Quart. Jour. Micr. Sci., 1S84. (A. G. B.)