Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
ROTIFERA (or ROTATORIA)
Online EncyclopediaEncyclopedia Home :: RON-SAC
del.icio.us it!
|
See also:ROTIFERA (or ROTATORIA) , a small, in many respects well-defined and somewhat isolated, class of the See also:animal See also:kingdom. Now familiarly known as " See also:wheel animalcules," from the wheel-like See also:motion produced by the Tings of See also:cilia which generally occur in the See also:head region, the so-called rotatory See also:organs, they were first discovered by A. See also:Leeuwenhoek, to whom we also owe the See also:discovery of Bacteria and ciliate See also:Infusoria. Leeuwenhock described Rotifer vulgaris in 1702, and he subsequently described Melicerta ringens and other See also:species. A See also:great variety of forms were described by other observers, but they were not separated as a class from the unicellular organisms (See also:Protozoa) with which they usually occur, until the See also:appearance of C. G. See also:Ehrenberg's monograph, which contained a See also:mass of detail regarding their structure. At the See also:present See also:day few See also:groups of the animal kingdom are so well known to the microscopist, few groups present more interesting See also:affinities to the morphologist, and few multicellular animals such a See also:low physiological See also:condition. A rotifer may be regarded as typically a hemisphere or See also:half an oblate See also:spheroid or paraboloid with a mouth somewhere on the See also:flat end (" disk " or " See also:corona "), which bears a usually See also:double ciliated See also:ring, the See also:outer See also:zone the " cingulum," and inner the " trochus ": this ring serves both for progression and for bringing up See also:food. The See also:body-See also:wall, cuticulized outside, is formed by a single layer of See also:ill-defined cells, and surrounds the See also:simple body cavity (archicoele), traversed by simple or branched See also:muscular See also:fibres (" mesenchyme ") (fig. 1, m,m). The mouth opens through a narrow pharynx (p) into a chamber which is (as in See also:Crustacea) at once See also:crop and gizzard, the mastax (ma), whose thickenings are imbedded in the posteroventral wall. A slender ciliated gullet (e) leads into a large See also: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 See also:intestine or rectum into the See also:cloaca. A pair of coiled nephridialtubes (n) formed of a See also:file of perforated " drain-See also:pipe " cells, with ciliated tag-like " See also:flame " cells (f), open into a contractile See also: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 See also:nerve-ganglion or See also:brain (g) lies on the anterodorsal See also:side of the pharynx, and by its position determines the See also: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 See also:wreath, the other distal and usually more or less lateral. The sexes are always See also:separate, the See also:males being of very rare occurrence in most cases. In the See also: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 See also:curve of the gut and the cloacal See also:aperture to See also:form the See also:foot of pseudo-See also:podium which ends in an See also:organ of See also:attachment, a pair of movable toes, each with the opening of a See also:cement-gland (gl) at its tip. Thus for orientation we See also:place the rotifer like the cuttle-See also:fish, head down-wards: the ciliated disk is basal or oral, proximal to the See also:rest of the animal, the foot is apical, and the brain and cloacal aperture are anterodorsal. It is in this position that See also:free-See also: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 See also:case being termed a lorica. Often the head is retractile, and a constriction of flexible cuticle distal to it is termed a See also:neck: in Philodinaceae there are a See also:series of thin flexible rings which permit both distal and proximal ends to be telescoped into the See also:middle; and in Taphrocampa, See also:regular constrictions of the whole bodywall give an appearance of metemeric segmentation to the body. In Philodinacea See also: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 See also:gutter which may be finely ciliated; but in several genera of no See also:close See also:affinity, where it is very oblique to the See also:longitudinal See also:axis of the body, it is represented by a See also:general ciliation of the See also:surface (Taphrocampa, Rattulus, Copeus, Adineta). We may suppose that primitively the mouth was seated in the centre of a See also:funnel-shaped disk, surrounded by a double wreath. The nearest approach to this is found in Microcodon (fig. 2, 1) and its See also:allies, the trochus being See also:oval with two median gaps, the cingulum, more delicate, and See also:complete. In Flosculariaceae the trochus is a horseshoe-shaped See also:ridge deep down in the funnel-shaped disk. The cingulum appears to be represented by the margin, usually produced into See also:long petal-like z 4 5 6 From See also:Comb. Nat. Hist. vol. H. " See also:Worms," &c., by permission of See also:Macmillan & Co. Ltd. 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 See also:net or clasp net: in one species (F. pelagica) there is an outer See also:girdle of See also:fine cilia for swimming. In Apsilus and Atrochus (fig. 3, b, d) the cingulum is a See also:mere contractile See also: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 comb-plates of See also:Ctenophora (q.v.) and the membranelles of ciliate Infusoria (q.v.). The trochus forms the powerful currents for locomotion, and for the See also:supply of food material, while the cingulum produces a See also:local current See also:round the upper rim of the corona to bring the food particles See also:direct to the mouth, which From H. S. Jennings in See also:American Naturalist, vol. See also:xxxv., is displaced through by permission of Ginn & Co. a postero-ventral See also:gap coronetta in gelatinous See also:tube, with eggs; specialized See also:Ciliata. d, Apsilus bucinedax, showing lateral The current formed (distal) antennae funnel of mouth See also:hanging by the trochus is a into enormous crop, stomach at apical end gigantic vortex-ring, with gastric glands, anus on postero-ventral the down stroke of surface, large coiled kidneys at proximal the cilia being end, uniting into median duct; e, Melicerta directly outwards, ringens in tube; f, same, proximal end en- but the See also:wave beats larged, showing a pellet in the See also:cup proximal See also:running round the to the paired lateral antennae; g, Melicerta organ in See also:uniform See also:sue- See also:janus, tube formed of faecal pellets. cession in one direc- tion. Thus the rotifer is, as it were, constantly See also:drawn forward into the centre of this vortex ring. There is a dorsal interruption to the disk, in- volving both trochus and cingulum and groove• in this case the two halves of the disk may be See also:developed in lobes, See also:flower-shaped in Melicerta ringens, but often rounded and projecting like kettledrums. These give a strong impression of two See also: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 See also:group. until the true See also:character of cliary motion was recognized; for a wheel cannot be i i 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. conif era (fig. 2, 4; fig. 3, e, f) there is a glandular ciliated See also:pit between the mouth and the See also:chin into which the overflow See also:water d 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 See also:proboscis cup of Bdelloidaceae, and the toes of Pedalion. Besides these Synchaetadae and Notommatidae (fig. 7) possess a pair of aureoles, 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 From C. T. See also:Hudson in Qua': See also:Journal of Microscopical slender tube hanging See also:Science, vol. See also:xxiv., by permission of J. & A. See also:Churchill. freely down into the FIG. 4.—Types of Trophi. a, malleate, with crop: this is followed enlarged view of malleus above—the Y-shaped incus consists of a See also:short median fulcrum bearing two large rami, each of which is in contact with a stout malleus consisting of a toothed uncus carried on a long manubrium; b, sub-malleate, with enlarged view of malleus—the manubria are twice as long as the 3- to 5-toothed unci ; c, virgate—mallei See also:rod-like, manubria and fulcrum very long, unci I- or 2-toothed; d, forcipate—rami large and used as a for. ceps, mallei rod-like, unci pointed or evanescent; e, incudate—stout fulcrum, rami forming a forceps, mallei evanescent; f, uncinate—unci large, 2-toothed, manubria evanescent, incus slender; g, ramate—rami subquadrantic, fulcrum rudimentary, manubria evanescent ; h,malleo-ramate--mallei fastened by their unci to the rami, manubria looped, rami large and fulcrum slender. 3 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 See also:combination of structures as we find in the stomach of higher Crustacea, with which we may See also:call it homoplastic. The trophi are (I) a median incus or See also: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 See also:hammer. For the varieties and modifications of the trophi we simply refer to Hudson's figure above. The relative See also: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 See also:common in Ploima, notably Rattulidae, and are used for attachment to the See also: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 See also:fat globules and yellowish-See also:green or See also: From these are given off at irregular intervals short lateral branches, each of which terminates in a flame-See also:cell (f) precisely similar in structure to the flame-cells found in See also:Planarians, See also: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 See also: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 See also:freshwater Protozoa (q.v.). See also: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 See also:united by short connectives with a smaller postero-ventral ganglion to form a nerve See also:collar. From this simple nerve fibres are given off to the body-wall, especially A oc to the ciliated cells of the corona, to the foot, and also to the muscles and sense organs. 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 See also: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 See also:external See also:taste (See also:smell) as well as of See also: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. See also:Marius de See also:Beauchamp) is a See also:sac filled with highly refractive granules soluble in dilute acids, and opening by a slender duct (or a pair) to the surface : its See also:function is doubtless that of an organ of See also:equilibrium, and it resembles in its opening to the surface the See also:primitive See also:internal See also:ear of even Vertebrates, for the duct to the surface persists through See also:life in the sharks. Locomotor Organs.—Most free rotifers swim by the corona, aided by the ciliated auricles when present. In Bdelloidaceae this may alternate with a See also: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 See also:adaptation to life at the surface; or as a See also: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 See also: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 See also: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 See also:fan-like expansion (fig. 5, Aa and H). It is, in common with others, a hollow See also: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 See also: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 See also: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 dorsolateral and ventro-lateral appendage, each with a fan-like plumose termination consisting of See also: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 See also: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. See also:Reproduction Organs.—Rotifera are unisexual, with the sexes dimorphic. The ovary is, as in many Platyhelminthes, duplex; one See also:part, the germary, being an organ for the See also:production by cell multiplication of the germ-cells or eggs proper, the other, the vitellarium, much more conspicuous and usu- ally consisting of a de- finite number of large cells, producing yolk material for the growth of the See also:egg. The whole ovary is unilateral and unpaired in most rotifers; symmetrical in Asplanchnaceae, Philodinaceae and Seisonaceae. In Asplanchnaceae the germary is median, See also:col.. tinuous 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 See also:rule, the wall of the ovary is continued into a uterine tube opening into the cloaca; but in Philodinaceae this is absent, and the See also:young are free in the body cavity and See also: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 See also:females in most species; only in Eosphora digitata, Rhinops vitrea, Proales werneckii, and the Seisonaceae a complete See also: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 See also:Bright-well 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 See also: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 See also:record of " See also: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 See also:state has physiologically replaced the need for resistent fertilized eggs. Insemination takes place either by the introduction of the penis into the cloaca of the female, or by the puncture of the body-wall of the female by the penis, and the injection of the sperm intothe 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 See also:mother, and these pass into a state of rest, for which they are furnished with a dense See also: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 See also:Entomostraca, q.v.). But the appearance of males seems to be as much associated with those of summer drought as of winter See also: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 See also:plankton dwellers. Not only is the appearance of males regular, but the forms of the females at different times of the See also: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. Un- like the molluscs and "p ap annelids, however, the 'r+ cloacal invagination lies A g bz,\g, outside this region, and the foot is formed by an t• '" I~ ~-1 ~,{ See also:elongation of the end of the body between the two apertures. The nerve ganglion is formed by an ingrowth of epiblast, a.,, Da and so are the pedal glands. The body cavity is the primitive C ~~-9 at blastocoele. Relationships and Mor- phology.—Passing over mti •s the earlier authors who regarded this group as p' br ~` t allied to Infusoria, a view first contested by Dujar- din, T. H. See also:Huxley viewed F nar them as See also:equivalent to . and on a level with the larvae of Echinoderms, (^ k kT r and of such other trocho- phore larvae as resembled these, a view gener- ally adopted. But it sn f i tt became more and more From See also:Cambridge Natural See also:History, vol. ii., "Worms, apparent that the larvae &c.," by permission of Macmillan & Co. Ltd. of this See also:category de- FIG. 8.—See also:Diagram of morphological rela- veloped mouth, gut and tions of Rotifera. A, pilidium larva anus by the See also:closure in of nemertine; B, Asplanchnapus the middle of such a slit- schematized; C, a ploimal rotifer; like blastopore opening D, trochosphaera female (schematized into a sack-like stomach from See also:Semper) ; E, veliger larva of mol- as is seen in the larvae lusc; F, trochophore larva of annelid. of Turbellaria and Ne- a, anus; ap, apical organ, correspond- mertina. The extra- See also:ing to foot of rotifers; at, median blastoporic opening of antenna, united by a nerve to br, brain the cloaca leads us to a (See also:letter omitted in B) ; bl, bladder, re- very different view, which ceiving ramified See also:kidney in B, C, D ; finds negative support f, foot, and f.g, its cement-gland ; in the failure of previous g, ovary ; k, kidney; m, mouth ; n, supra- morphologists to adapt oesophageal ganglion; nr, nerve ring in the details of develop- See also:section. ment and of the struc- See also:ture of the disk to their See also: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 See also: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 See also:note that it was long since shown that the apical organ (at first assumed to De 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 See also:ordinary bipolar From H. S Jennings in American Naturalist, vol. xxxv., by permission of Ginn & Co. 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 See also:independent See also:stem branching off at the outset of the rise trom the Platode type to higher Invertebrata- The See also:Polyzoa (q v ), which in many ways recall Rotifers, appear to be equally independent. The following See also:classification of Rotifers is our modification of that of Hudson and See also:Gosse, further altered through considerations put From H. S Jennings in American Naturalist, vol. xxxv., by permission of Ginn & Co. forward by C. Wesenberg-See also: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. Classification: (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. Asplanchnaceae; trochus circular; foot absent or minute; trophi incudate; stomach See also:blind; males frequent, not very dissimilar to females. Asplanchna Gosse (fig. 9, g—i) ; Asplanchnopus Deguerne (fig. 9, f) ; Ascomorpha Perty (fig. 9, j). 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 anterodorsal (fig. 2, 5). 3_ Trochosphaeraceae; female footless; subspherical, 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 See also:swallow; Pedetes Gosse (no median spines) ; Triarthra Ehr., one postero-ventral spine; Tetramastix See also: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 produced 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 See also:Oken (fig. 3, b) ; Stephanoceros Ehr. (fig. 3, a). (b) Acyclidae. Disk entire or tentaculate, not seti- ferous; Acyclus Leidz (fig. 3, c). Foot repre- sented by a See also:button-like disk, carried far from the posterior surface; Apsilus Metchnikoff (fig. 3, d) ; Atrochus Wierzerski (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 See also:eye; Notommata Ehr. (restricted by Gosse) ; Co pens Gosse; Notops Hudson (fig. 6, 3); Proales Gosse; Gastroschiza; Diglena Ehr. (fig. 6, 4). Loricata, cuticle hardened See also:armour-like, often sculptured; Polyarthra Ehr.; Pedetes Gosse; Euchlanis Ehr. (fig. 6, I) ; 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) ; Taphrocam pa 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, subapicall 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 See also:Plate. See also:Habitat and Habits.—The Rotif era are all aquatic, the See also: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. See also:Williamson and F. See also:Cohn. Some are See also:moss-dwellers, inhabiting the surface film of water that bathes these See also:plants: such especially are the Bdelloids, with their exceptional capacity for resisting See also: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 See also:ooze of quiet See also:waters: we may cite the aberrant Floscularian Atrochus. Widely different are the habits of the plankton forms, which See also: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 See also:powers to the low See also: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 See also:cattle or the drainings 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 See also:galls within the Conferva Vaucheria, and P. parasita infests the central jelly of the Phytoflagellate Volvox; P. petromyzon is a frequent commensal in the gill cavity of some Cladoceran Crustacean Eurycereus lamellatus.
The See also:geographical See also:distribution is See also: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 See also:microscope observed them repeatedly, the first record being that of See also: C. Williamson (best known as a botanist) in See also:Pritchard's Infusoria, in 1861. Much work was done with the See also:gradual introduction of improved methods during the last See also: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 See also: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 See also:information on habits and structure, provided students with an easy See also:access to the domain and stimulated many to work hard at the group. Of these new-corners 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 See also:herbarium. C. Zelinka has given us the most detailed anatomical accounts we possess for several Bdelloidaceae, and was the first to utilize See also:modern methods of microscopic technique on a complete See also:scale. C. G. Ehrenberg, See also:Die Infusionsthiere als vollkommenere Organismen (1838); F. Dujardin, istoire naturelle See also:des zoophytes (1841); T. H. Huxley, " Lacinularia socialis," Trans. Micr. See also: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) ; See also:Marcus Hartog, "Rotifera," in Cambridge Natural History, vol. ii., reprinted 1901; H. S. tJennings, Synopses of See also:North American Invertebrates, xvii., " The otifera," Amer. Nat. xxxv. (1901); C. F. Rousselet, numerous apers in Journ. Micr. Soc. and Journ. Quekett See also:Club; C. Wesenergg-Lund, " Danmarks Rotifera," n Vid. Meddel. Nat. For. Kjbbenhavn (1899) ; C. Zelinka, " Studien fiber Rotiferen," in Zeit. Wyss. Zool. xliv. (1886), xlvii. (1888), liii. (1891). (M. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] ROTHWELL |
[next] ROTORUA |