ROTIFERA (or Rotatoria), a small, in many respects well-defined and somewhat isolated, class of the animal kingdom. Now familiarly known as “wheel animalcules,” from the wheel-like motion produced by the rings of cilia which generally occur in the head region, the so-called rotatory organs, they were first discovered by A. Leeuwenhoek, to whom we also owe the discovery of Bacteria and ciliate Infusoria. Leeuwenhoek described 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 C. G. Ehrenberg's monograph, which contained a mass of detail regarding their structure. At the present day few groups of the animal kingdom are so well known to the microscopist, few groups present more interesting affinities to the morphologist, and few multicellular animals such a low physiological condition.
A rotifer may be regarded as typically a hemisphere or half an oblate spheroid or paraboloid with a mouth somewhere on the flat end (“disk” or “corona”), which bears a usually double ciliated ring, the outer zone the “cingulum,” and inner the “trochus”: this ring serves both for progression and for bringing up food. The body-wall, cuticulized outside, is formed by a single layer of ill defined cells, and surrounds the simple body cavity (archicoele), traversed by simple or branched muscular fibres (“mesenchyme”) (fig. 1, m, m). The mouth opens through a narrow pharynx (p) into a chamber which is (as in Crustacea) at once crop and gizzard, the mastax (ma), whose thickenings are imbedded in the posteroventral wall. A slender ciliated gullet (e) leads into a large stomach (st) whose wall consists of large richly ciliated cells with usually a pair of simple secretory sacs opening into it: it may open through an intestine or rectum into the cloaca. A pair of coiled nephridial tubes (n) formed of a file of perforated “drain-pipe” cells, with ciliated tag-like “flame” cells (f), open into a contractile bladder (bl), which passes by a slender duct into the cloaca. Into this also opens the genital duct from the single or paired gonad (ov). The simple nerve-ganglion or brain (g) lies on the anterodorsal side of the pharynx, and by its position determines the orientation of the animal, the cloacal opening lying on the same side, and the course of the gut being “neural.” The sense organs are a pair of pigmented eyes (oc), and two pairs of antennae, one anterior proximal and near the wreath, the other distal and usually more or less lateral. The sexes are always separate, the males being of very rare occurrence in most cases. In the female the gonad is complex as in flatworms, composed of a germary for the formation of the eggs, and a vitellary, much more conspicuous and alone figured (ov), consisting of a definite number of large nucleated cells for the nourishment of the eggs. The apical end of the rotifer usually narrows suddenly beyond the curve of the gut and the cloacal aperture to form the foot of pseudo podium which ends in an organ of attachment, a pair of movable toes, each with the opening of a cement-gland (gl) at its tip. Thus for orientation we place the rotifer like the cuttle-fish, head downwards: the ciliated disk is basal or oral, proximal to the rest of the animal, the foot is apical, and the brain and cloacal aperture are anterodorsal. It is in this position that free-swimming forms glide over the substratum of organic debris in which they find their food.
The cuticle may be locally or generally hardened, in the latter case being termed a lorica. Often the head is retractile, and a constriction of flexible cuticle distal to it is termed a neck: in Philodinaceae there are a series of thin flexible rings which permit both distal and proximal ends to be telescoped into the middle; and in Taphrocampa, regular constrictions of the whole bodywall give an appearance of metemeric segmentation to the body. In Philodinacea accessory toes are found, unfurnished with cement-glands and distinguished as spurs.
Corona or Disk.—This typically consists of two concentric zones, the trochus and cingulum, often separated by a groove or gutter which may be finely ciliated; but in several genera of no close affinity, where it is very oblique to the longitudinal axis of the body, it is represented by a general ciliation of the surface (Taphrocampa, Rattulus, Copeus, Adineta). We may suppose that primitively the mouth was seated in the centre of a funnel-shaped disk, surrounded by a double wreath. The nearest approach to this is found in Microcodon (fig. 2, 1) and its allies, the trochus being oval with two median gaps, the cingulum, more delicate, and complete. In Flosculariaceae the trochus is a horseshoe-shaped ridge deep down in the funnel-shaped disk. The cingulum appears to be represented by the margin, usually produced into long petal-like lobes, fringed with long stiffish setae, which in Stephanoceros are vibratile at intervals, seemingly at will. In Floscularia they serve to convert the lobed funnel into an efficient casting net or clasp net: in one species (F. pelagica) there is an outer girdle of fine cilia for swimming. In Apsilus and Atrochus (fig. 3, b, d) the cingulum is a mere contractile hood. In most rotifers, on the contrary, the trochus is stronger than the cingulum, often lobed, and with some of its cilia aggregated into vibratile styles homologous with the combplates of Ctenophora (q.v.) and the membranelles of ciliate Infusoria (q.v.). The trochus forms the powerful currents for locomotion, and for the supply of food material, while the cingulum produces a local current round the upper rim of the corona to bring the food articles direct to the mouth, which is displaced through a postero-ventral gap in the trochus to lie behind the disk, just as occurs in the more specialized Ciliata. The current formed by the trochus is a gigantic vortex-ring, the down stroke of the cilia being directly outwards, but the wave beats running round the organ in uniform succession in one direction. Thus the rotifer is, as it were, constantly drawn forward into the centre of this vortex ring. There is a dorsal interruption to the disk, involving both trochus and cingulum and groove in this case the two halves of the disk may be developed in lobes, flower-shaped in Melicerta ringens, but often rounded and projecting like kettledrums. These give a strong impression of two crown wheels revolving in the same sense. This appearance puzzled the older observers, who were led thereby to give the name “wheel-bearers” to the group until the true character of ciliary motion was recognized; for a wheel cannot be in organic continuity with the support on which it rotates. In Conochilus (fig. 2, 5), a Melicertan, the mouth is displaced towards the antero-dorsal side and the gap is postero-ventral.
In Melicerta ringens and M. conifera (fig. 2, 4; fig. 3, e, f) there is a glandular ciliated pit between the mouth and the chin into which the overflow water passes by a pair of gutters, and in which fine particles are aggregated into pellets, which the animal deposits, as formed, on the edge of its tube and so builds it up. M. janus builds up a tube by pellets of its own faeces (fig. 3, g). In most Ploima the dorsal gap is not well marked, and the trochus is broken up into a number of lobes, often furnished with vibratile styles; in front and at the sides, but ventrally passing into the uniformly ciliated oral funnel.
Other ciliated organs to be noticed are the proboscis cup of Bdelloidaceae, and the toes of Pedalion. Besides these Synchaetadae and Notommatidae (fig. 7) possess a pair of aurioles, great eversible ciliated pouches a little above the disk, utilized in swimming. The mouth begins as a funnel, continued into a narrow pharynx, which in Flosculariaceae is prolonged into a slender tube hanging freely down into the crop: this is followed by the crop-gizzard, also ciliated except behind, where it is hardened into a set of articulated sclerites (trophi) to form the gizzard or mastax. Thus the crop-gizzard has the same combination of structures as we find in the stomach of higher Crustacea, with which we may call it homoplastic. The trophi are (1) a median incus or anvil (fig. 4), Y-shaped, with the foot (fulcrum) distal and the arms (rami) apical, often independently jointed; (2) with the outer ends of the rami articulate two lateral pieces (mallei), and again composed of a distal longitudinal piece (manubrium) and an apical transverse piece (the uncus), the whole recalling, as the name implies, a single-clawed hammer. For the varieties and modifications of the trophi we simply refer to Hudson's figure above. The relative size of the crop to the trophi varies greatly; it is small where the trophi are well developed and complex, as well as in Bdelloidea; but in Flosculariaceae it is large, and so it is in Asplanchnaceae. Eversible trophi of the forcipate or virgate type, which can be used for nibbling, are common in Ploima, notably Rattulidae, and are used for attachment to the host in the parasitic Seisonaceae, &c. In Asplanchnaceae also, where the whole crop is strengthened by a framework of bars, the incudate mastax lies in a little postero-ventral pouch which can be everted through the crop and mouth. 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 fibrillate have also been described. Very constantly a pair of simple sack-like 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 a tube the intestine turns round and runs forward, the cloaca being placed so as to debouch over the margin of the tube. 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 faecal masses, by an eversion of the cloaca. Asplanchna, Notommata seiboldii, and certain species of Ascomorpha are devoid of intestine or anus, excrementitious matters being ejected through the mouth. The body cavity (archicoele) contains a fluid in which very minute corpuscles have been detected. There is no trace of a true vascular system. The nephridia (fig. 1, 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 (f) 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 body cavity or not must probably be answered in the negative. At the base these tubes open either into a permanent bladder (fig. 1, bl) which communicates with the cloaca, or directly into the cloaca. They have the same functions as the contractile vacuole of freshwater Protozoa (q.v.).
Nervous System.—There is a large ganglion lying in close contact with the pharynx, proximal to the crop and on its antero-dorsal side; in Bdelloidaceae at least it is united by short connectives with a smaller postero-ventral ganglion to form a nerve collar. From this simple nerve fibres are given off to the body-wall, especially to the ciliated cells of the corona, to the foot, and also to the muscles and sense organs.
Fig. 5.—Pedalion mira. A, lateral surface view of an adult female: a, median ventral appendage; b, median dorsal appendage; c, distal ventro-lateral appendage; d, dorso-lateral appendage; f, dorsal antenna; g, “chin”; x′, cephalotroch. B, lateral view, showing viscera: oc, eye-spots; n, nephridia; e, ciliated toes; other letters as above. C, ventral view: x′, trochus; x, cingulum; other letters as above. D, ventral view, showing the musculature (cf. text). E, dorsal view of a male: a, lateral appendages; b, dorsal appendage. F, lateral view of a male. G, enlarged view of the antenna f. H, enlarged view of the median ventral appendage. (All after Hudson.)
The sense organs are eyes, antennae, sensory styles and a statocyst in a few species. The eyes are refractive globules set in a cup of red pigment traversed by a nerve fibre, and lie on the proximal side of the body, directly on the postero-dorsal surface of the brain, or at a little distance from it, on the neck, often within the circle on the corona, and usually well within the transparent body. There may be one, a pair, or rarely more, the outer ones being more or less rudimentary. The antennae are short tubular extensions of the body wall, sometimes retractile with a depressed tip from which protrudes a tuft of fine stiff bristles. They are possibly organs of external taste (smell) as well as of touch. Typically there are two pairs-a proximal, more or less approximated on the postero-dorsal surface, and a distal pair, more widely separate. But the proximal pair are often fused into a single median antenna (supplied, however, by two nerves), and in one case at least the distal pair may be similarly fused. Additional paired antennae may occur within the coronal surface, which is the seat of the sensory styles, of less complex structure, which occur in many genera. The statocyst (retro-cerebral organ of P. Marius de Beauchamp) is a sac filled with highly refractive granules soluble in dilute acids, and opening by a slender duct (or a pair) to the surface: its function is doubtless that of an organ of equilibrium, and it resembles in its opening to the surface the primitive internal ear of even Vertebrates, for the duct to the surface persists through life in the sharks.
Locomotor Organs.—Most free rotifers swim by the corona, aided by the ciliated auricles when present. In Bdelloidaceae this may aternate with a leech-like gait; the corona being withdrawn, the cupped end of the proboscis serves as a sucker for attachment alternately with the adherent foot, so that the animal loops its way along. In two families motile articulated rods occur; in Triarthridae they probably simply expand the dimensions of the body in adaptation to life at the surface; or as a protection against being swallowed by their smaller foes. In Polyurthra and Pteroessa they are numerous, pinnated (feathered), and are doubtless used for active swimming by jerks; they can be moved up or down by special muscles attached to their bases, which project into the body. In Pedalion (fig. 5), a remarkable form discovered by Dr C. J. Hudson in 1871 and found in numbers several times since, these appendages have acquired a new and quite special development. They are six in number, median, ventral and dorsal, and two unequal lateral pairs. The largest is placed ventrally at some distance distal to the mouth. Its free extremity is a plumose fan-like expansion (fig. 5, Aa and H). It is, in common with others, a hollow process into which run two pairs of broad, coarsely transversely 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 line. Above this point springs the large median dorsal limb, which terminates in groups of long setae. 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 either side is attached a dorso-lateral and ventro-lateral appendage, each with a fan-like plumose termination consisting of compound hairs or setae, found elsewhere only among arthropods (q.v.); 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. It bears a group of long setose hairs the bases of which are connected with the nerve fibre. There are also two pairs of distal antennae. Pedalion presents a pair of ciliated toes in the posterior region of the body (fig. 5, 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.
Reproduction Organs.—Rotifera are unisexual, with the sexes dimorphic. The ovary is, as in many Platyhelminthes, duplex; one part, the germary, being an organ for the production by cell multiplication of the germ-cells or eggs proper, the other, the vitellarium, much more conspicuous and usually consisting of a definite number of large cells, producing yolk material for the growth of the egg. The whole ovary is unilateral and unpaired in most rotifers; symmetrical in Asplanchnaceae, Philodinaceae and Seisonaceae. In Asplanchnaceae the germary is median, continuous at the distal end with the middle of the transverse horseshoe-shaped vitellary. In Bdelloidaceae and Seisonaceae the whole organ is paired, the germary proximal, the vitellary next the cloaca. As a rule, the wall of the ovary is continued into a uterine tube opening into the cloaca; but in Philodinaceae this is absent, and the young are free in the body cavity and escape by perforating the cloacal walls. The male organs are usually a testis, a large seminal bladder and a protrusible penis. The males are unlike the females in most species; only in Eosphora digitata, Rhinops vitrea, Proales werneckii, and the Seisonaceae a complete digestive system is present. Frequently the foot is ciliated at the tip, as in the young of tubicolous forms.
The males of rotifers are of relatively rare occurrence, except in the genus Asplanchna, where they were first recognized as such by Brightwell in 1841; though those of Hydatina had long since been seen and described as a distinct genus. Despite their rare occurrence, the males of over one hundred and twenty species have now been recognized, and we may well believe that all species will be found to present males. This statement may seem to need qualification; for the male of no Bdelloid has been seen, and there is but a doubtful record of “winter-eggs” in this group. But possibly, as in Seisonaceae, the males resemble the females, and have escaped recognition. It may, however, well be that the capacity for wintering in the dry state has physiologically replaced the need for resistant fertilized eggs. Insemination takes place either by the introduction of the penis into the cloaca of the female, or by the puncture of the bodywall of the female by the penis, and the injection of the sperm into the body cavity, whence the single spermatozoa must make their way to the eggs. The females habitually produce eggs without impregnation, which again habitually develop into females, more rarely into males. These unfertilized eggs develop directly, often in the uterus. In other cases the eggs are liberated earlier and adhere to the foot, or are hatched within the tube (fig. 3, b, c). The impregnated eggs undergo a very partial development in the mother, and these pass into a state of rest, for which they are furnished with a dense shell. They always give rise to parthenogenetic females (see Reproduction). The thin-walled eggs are often termed “summer-eggs,” the fertilized ones “winter” or “ephippial” eggs (by parity with the phyllopod Entomostraca, q.v.). But the appearance of males seems to be as much associated with those of summer drought as of winter cold. No adequate knowledge of the conditions under which males arise has been established. The phenomenon of seasonal dimorphism is of especial moment for the plankton dwellers. Not only is the appearance of males regular, but the forms of the females at different times of the year may be so distinct as to have led them to be classed as distinct species.
Development.—The egg is holoblastic, but the segmentation is very unequal, recalling that of marine annelids and of molluscs. Gastrulation takes place by epiboly, and the stomodaeum (oral invagination—mastax pharynx) takes place in two stages of the region of the closed blastopore. Unlike the molluscs and annelids, however, the cloacal invagination lies outside this region, and the foot is formed by an elongation of the end of the body between the two apertures. The nerve ganglion is formed by an ingrowth of epiblast, and so are the pedal glands. The body cavity is the primitive blastocoele.
Relationships and Morphology.—Passing over the earlier authors who regarded this group as allied to Infusoria, a view first contested by Dujardin, T. H. Huxley viewed them as equivalent to and on a level with the larvae of Echinoderms, and of such other trochophore larvae as resembled these, a view generally adopted. But it became more and more apparent that the larvae of this category developed mouth, gut and anus by the closure in the middle of such a slit-like blastopore opening into a sack-like stomach as is seen in the larvae of Turbellaria and Nemertina. The extra-blastoporic opening of the cloaca leads us to a very different view, which finds negative support in the failure of previous morphologists to adapt the details of development and of the structure of the disk to their identification of “trochus” and “cingulum” with the preoral and postoral wreaths of the trochophore larva. We homologize the rotifer with the Turbellarian larva (fig. 8, A), and with the preoral or upper part of the trochopore (fig. 8, E, F). Its adhesive foot is paralleled by a cup-shaped ciliated depression, possibly nervous, found in all the larvae cited, except some Echinoderms, and which in Asterids and Crinoids actually serves as an organ of attachment. This view obviates the deed for assuming the complicated flexures of the wreath which has to be done on other assumptions (see Rotifera, Encycl. Brit. ed. 9). Thus Trochosphaera (fig. 8, D) (which has a male of the same type as Melicerta, &c.) is an extremely modified type, and its resemblance to the trochophore larva of Lepadorhynchus or Polygordius is only superficial. We may note that it was long since shown that the apical organ (at first assumed to be the brain) of these larvae was innervated from an anterior thickening of the circular nerve ring, corresponding with the brain of Rotifers; the nerve cells immediately below the pit are the ordinary bipolar ganglion cells below invertebrate sense-organs. Moreover, the body cavity of the Rotifers is a primitive archicoele; the persistent or accrescent cleft between epiblast and hypoblast, traversed by mesenchymal muscular bands. Thus we regard Rotifers as an independent stem branching off at the outset of the rise from the Platode type to higher Invertebrata. The Polyzoa (q.v.) which in many ways recall Rotifers, appear to be equally independent.
The following classification of Rotifers is our modification of that of Hudson and Gosse, further altered through considerations put forward by C. Wesenberg-Lund, which, however, we do not consider wholly convincing. He notably regards an oblique disk with uniform ciliation as primitive, a view which we cannot adopt.
(A.) Disk usually with well-marked strong trochus, ciliated groove and more delicate cingulua interrupted by an antero-dorsal median gap, usually more or less bilobed.
(i.) Trophi incudate:
1. Asplanehnaceae; trochus circular; foot absent or minute; trophi incudate; stomach blind; males frequent, not very dissimilar to females. Asplanchna Gosse (fig. 9, g-i); Asplanchnopus Deguerne (fig. 9, f); Ascomorpha Perty (fig. 9, j).
(ii.) Trophi malleoramal:
2. Melicertaceae; females tubicolous, usually attached, or forming spherical floating social aggregates; males free swimming. Melicerta Schranck (fig. 3, e, f); Oecistes Ehrenberg; Lacinularia Schweigger; Conochilus Ehrenberg, with gap postero-ventral and mouth antero-dorsal (fig. 2, 5).
3. Trochosphaeraceae; female footless; sub spherical, the corona bulging into a hemisphere which may equal the hemispherical body; anus apical; male as in Melicertaceae, Trochosphaere Semper (fig. 8, D).
4. Ploimoidaceae; subconical; corona bilobed; retractile foot absent or ciliated; motile appendages present in two families.
(a) Pterodinidea; foot a ciliated cup; cuticle forming flat lorica. Pterodina Ehr. (fig. 7, d).
(b) Triarthridae; body with a pair of long cervical spines pointing distally and serving for leaping movements or to extend the body and make it too big for small enemies to swallow; Pedetes Gosse (no median spines); Triarthra Ehr., one postero-ventral spine; Tetramastix Zacharias, two unequal median spines.
(c) Pedalionidae, foot represented by two styles, sometimes ciliated; body provided with six hollow-jointed muscular fins for swimming and leaping. Pedalion Hudson (fig. 5).
(iii.) Trophi ramate:
5. Bdelloidaceae; foot with two toes and accessory spurs or a simple perforated disk; body telescopic at either end, with an antero-dorsal proboscis ending in a ciliate cup and bearing the proximal antenna; corona usually bilobed, very wheel-like. Males if present probably like the females. Germary and ovary paired; oviduct absent; young viviparous. Rotifer Schrank (fig. 9, d, e); Philodina Ehr. (fig. 9, c); Callidina Ehr. (eyeless); Adineta Hudson is eyeless with the corona uniformly ciliated, and proboscis adnate, hooked.
(iv.) Trophi uncinate:
Flosculariaceae; disk a contractile cup, often lobed, the cingulum of long vibratile cilia, of very long motionless bristles or absent, rarely with an outer zone of fine cilia. Trochus a pair of ridges or horseshoe open in front. Oral funnel roduced into a fine tube hanging freely into a pharyngeal cup, containing the uncinate trophi. Body-wall usually traversed by a network of canals serving by their contraction to expand the disk. Males and larvae with a ciliated pedal cup and a simple ciliated disk.
(a) Floscularidae; tubicolous, with a lobed disk, bearing stiff or vibratile setae. Floscularia Oken (fig. 3, b); Stephanoceros Ehr. (fig. 3, a).
(b) Acyclidae. Disk entire or tentaculate, not setiferous; Acyclus Leidz (fig. 3, c). Foot represented by a button-like disk, carried far from the posterior surface; Apsilus Metchnikoff (fig. 3, d); Atrochus Wierzezski (fig. 3, c).
(B.) Ploimaeae; disk variable, often circular, sometimes with a lobed trochus bearing membranelles (vibratile styles); trophi complete, malleate, submalleate, virgate, or forcipate; anus subapical; foot usually short, and usually bearing two toes which may be much elongated.
Illoricata, cuticle soft; ciliated exsertile auricles above the disk sometimes present. Albertia Dujardin; Drilophagus Vejdovsky; Microcodon Ehr. (fig. 9, a); Rhinops Hudson (fig. 9, b); Synchaeta Ehr. (fig. 7, c); Hydatina Ehr. has no eye; Notommata Ehr. (restricted by Gosse); Copens Gosse; Notops Hudson (fig. 6, 3); Proales Gosse; Gastroschiza; Diglena Ehr. (fig. 6, 4).
Loricata, cuticle hardened armour-like, often sculptured; Polyarthra Ehr.; Pedetes Gosse; Euchlanis Ehr. (fig. 6, 1); Anuraea, Ehr. (fig. 7, b); Notholca Gosse (fig. 7, a); Distylis Eckstein (fig. 7, e); Rattulus Ehr. (fig. 7, f); Colurus Ehr. (fig. 6, 2); Taphrocampa Gosse.
(C.) Seisonaceae. Body elongated with a narrow neck above the disk; foot ending in a terminal perforated disk. Trophi virgate exsertile; germary paired; genito-urinary cloaca opening above the neck in the male, subapically in the female. Gut blind (Paraseison), or opening into cloaca (Seison). Males resembling females, common. All known species are parasitic on the Crustacean Nebalia; Seison Claus; Paraseison Plate.
Habitat and Habits.—The Rotifera are all aquatic, the majority dwelling in fresh water with Protozoa and Protophyta, as well as Entomostracous Crustacea. This association with Protophyta accounts for their study by many distinguished botanists, such as W. C. Williamson and F. Cohn. Some are moss-dwellers, inhabiting the surface film of water that bathes these plants: such especially are the Bdelloids, with their exceptional capacity for resisting desiccation. Others—the majority—live among weeds, the tubicolous ones mostly upon them. A few are sapropelic, haunting the looser debris that forms the uppermost layer of the bottom ooze of quiet waters: we may cite the aberrant Floscularian Atrochus. Widely different are the habits of the plankton forms, which float or swim near the surface, and are often provided with long cuticular extensions for this purpose (fig. 7, a, b). Asplanchnaceae, plankton, dwellers in small pools, are, however, ovoid, and Trochosphaera is spherical and must owe its floating powers to the low density of the liquid in its enormously dilated body-cavity. Lacinularia racemovata and Conochilus form free floating aggregates, the eggs, as laid, hatching and the young settling among the approximated gelatinous tubes of the parents. Some species only frequent the clearest waters; but the lovely transparent Hydatina senta (fig. 2, 3) likes water contaminated by the visits of cattle or the trainings of manure. Drilophagus and Albertia are parasitic on the surface or within the gut of Naid Oligochaete worms: Seisonaceae are ectoparasitic on the Crustacean Nebalia, Proales werneckii forms galls within the Conferva Vaucheria, and P. parasita infests the central jelly of the Phytoflagellate Volvox; P. pelromyzon is a frequent commensal in the gill cavity of some Cladoceran Crustacean Eurycereus lamellatus.
The geographical distribution is cosmopolitan, as is the case with Protozoa and Protophyta of similar habits. A curious fact is that when a new and striking form is found first in one place it is shortly after collected from widely separated areas. In the case of one genus, Gastroschiza, this led to the creation of no less than six generic names.
History and Bibliography.—As rotifers are common in ponds, the first workers with the microscope observed them repeatedly, the first record being that of John Harris in 1696, who found a Bdelloid in a gallipot that had been standing in his window. Leeuwenhoek found and described some tubicolous species; and during the 18th century a fair number of species were observed, figured and described with names. During this time the illusion of a wheel or wheels produced by the ciliary action of the disk had puzzled all observers. C. E. Ehrenberg included the Rotifers in his Infusionsthiere, and described and figured with fair precision many of the genera and species. Dujardin gave a less detailed but more accurate account under the name of Zoophytes Systolides. The next full work was a valuable compilation by W. C. Williamson (best known as a botanist) in Pritchard's Infusoria, in 1861. Much work was done with the gradual introduction of improved methods during the last forty years of the century. The discovery and recognition of the males was made, however, at the close of the fifties. P. H. Gosse collected and described many species, and elucidated the structure of the mastax in 1856. Zoologists of the standing of Huxley, Claus and Leydig added to our knowledge of the anatomy and to the theory of their relations. But the monumental monograph of C. T. Hudson and Gosse containing a new classification, an illustrated description of all the then known species and much information on habits and structure, provided students with an easy access to the domain and stimulated many to work hard at the group. Of these new-comers we may cite C. F. Rousselet, who has found many new species and many unknown males of known species, elucidated habits and faithfully kept record of the publications on the class in the Journal of the Royal Microscopical Society. He has moreover elaborated a method for preserving Rotifera for microscopic observation, so that the types of each observer are now as readily available for comparison as the plant-specimens of the botanist's herbarium. C. Zelinka has given us the most detailed anatomical accounts we possess for several Bdelloidaceae, and was the first to utilize modern methods of microscopic technique on a complete scale.
C. G. Ehrenberg, Die Infusionsthiere als vollkommenere Organismen (1838); F. Dujardin, Histoire naturelle des zoophytes (1841); T. H. Huxley, “Lacinularia socialis,” Trans. Micr. Soc. i. (1853); P. H. Gosse, “Manducatory Organs in Class Rotifera,” Phil. Trans. (1856); W. C. Williamson, “The Rotifera” in A. Pritchard's History of the Infusoria (1861); C. T. Hudson and P. H. Gosse, The Rotifera (1886), and supplement (1889); Marcus Hartog, “Rotifera,” in Cambridge Natural History, vol. ii., reprinted 1901; H. S. Jennings, Synopses of North American Invertebrates, xvii., “The Rotifera,” Amer. Nat. xxxv. (1901); C. F. Rousselet, numerous papers in Journ. Micr. Soc. and Journ. Quekett Club; C. Wesenberg-Lund, “Danmarks Rotifera,” in Vid. Meddel. Nat. For. Kjöbenhavn (1899); C. Zelinka, “Studien über Rotiferen,” in Zeit. Wiss. Zool. xliv. (1886), xlvii. (1888), liii. (1891).
- (M. Ha.)