ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

BY

MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.)

Professor of Natural History in the Queen's College, Cork.

CHAPTER VIII

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

ROTIFERA—HISTORY—EXTERNAL FEATURES—MOVEMENT—ANATOMY—REPRODUCTION—EMBRYOLOGY—CLASSIFICATION—DISTRIBUTION—AFFINITIES—GASTROTRICHA—KINORHYNCHA

The Rotifera are microscopic animals, the largest not exceeding one-eighth of an inch in length. According to Hudson and Gosse,^[233] they are first recorded in an observation of the Rev. John Harris, in 1696, of "an Animal like a large Maggot which could contract itself into a Spherical Figure, and then stretch itself out again; the end of its Tail appeared with a Forceps like that of an Ear-wig."^[234] This was certainly a Bdelloid Rotifer.

In 1703 Leeuwenhoek^[235] gave a fuller description of a tubicolous form, probably Limnias, and noted the peculiar appearance of the ciliary wreath as "two wheels thickset with teeth as the wheel of a watch." He also noted a little later^[236] the way in which Melicerta (see p. 206) builds its tube, and was the first to observe the revivification of certain species after drying.^[237] Joblot, a French professor of mathematics, in 1718 figured and described a large number of new genera and species with more or less fantastic details. Baker's figures^[238] are a considerable advance on Joblot's, and his descriptions of habits are still fresh and accurate. Eichhorn found a number of new and interesting forms; and O. F. Müller, influenced by the new discipline of Linnaeus, not only figured many species, but gave good short diagnoses of their characters. Ehrenberg in 1838 brought out his magnificent Infusionsthierchen, which contains descriptions and figures of what are now divided into Protophyta, Protozoa, Rotifera, and Gastrotricha. Dujardin's monograph on the "Infusoires," in the Suites à Buffon,^[239] was in several respects an advance on Ehrenberg, whose power of observation was so great as to render his mistakes the more inexplicable. But Ehrenberg ever adhered to his errors as firmly as to his facts.

The occurrence of Rotifers among microscopic plants induced the botanists Cohn and Williamson^[240] to work at their structure; the group has been studied by men engrossed in other professional cares, such as Gosse, Bedwell, Moxon, Rousselet, and Maupas. Huxley,^[241] Leydig,^[242] and Cohn^[243] studied Rotifers in the '50's and early '60's with a precision the more remarkable when we remember the imperfect methods then available. This period was closed by the valuable monograph published in Arlidge's (4th) edition of Pritchard's Infusoria,^[244] under the supervision of W. C. Williamson. Leidy began the study of the American Rotifers. Eckstein^[245] gave a careful and interesting account of the species about Giessen in a richly illustrated paper. In recent times the modern methods of histological and embryological research have been applied by Vallentin,^[246] Plate,^[247] Tessin,^[248] and Zelinka,^[249] the three Studien ueber Rotatorien of the last author being indispensable to every student, and containing a full bibliography.

Hudson and Gosse's Monograph (1886-89) contains a history of the class to which, as to the whole book, we are deeply indebted; and a full systematic account of all published species.^[250] C. Rousselet has introduced a method^[251] of preparation of Rotifers in microscopic slides which enables workers to preserve the types they figure and describe for future identification and comparison. Gunson Thorpe has collected and studied Rotifera in China and Australia. It would be unfair not to record here the invaluable services of the late Thomas Bolton, and his son of the same name, both of Birmingham, and of J. Hood of Dundee, who have found and widely distributed living specimens of new, rare, and interesting species.

Definition of the Class.—We may define Rotifera as a class of minute bilaterally symmetrical animals, with a chitinous integument, a soft terminal "disc" fringed by a complex ciliary "wreath," an anterior or subventral mouth, and a dorsal cloacal aperture, beyond which the body is usually prolonged into the "foot" or process bearing cement glands, and serving for attachment, temporary or permanent. The body-cavity has no epithelial lining, and is traversed by nerves and muscles. The alimentary canal possesses a chitinous gizzard or mastax of peculiar arrangement, and it usually opens into a cloaca. The nervous centre consists of a ganglion on the dorsal side of the pharynx, to which a second one on the ventral side is sometimes connected to form a complete ring; eyes and bristle-bearing feelers are usually present as sense-organs. A paired system of renal tubes serves for excretion, opening through a median contractile bladder into the ventral side of the cloaca. The sexes are distinct; but the males (Fig. 107), which mostly lack digestive organs, occur rarely, and the females are usually viviparous, or carry about the eggs till they are hatched; while, owing to the rarity of the males, parthenogenesis is habitual. Fission and budding are alike unknown. The fertilised eggs are of the kind termed "winter" or "resting" eggs, and resist conditions adverse to life.

The Rotifera are of cosmopolitan distribution; most of the species inhabit fresh water, whilst some are brackish, and a few are marine; 84 genera and about 700 species have been described.

External Features.^[253]—The body is divided into three regions: (1) the head, ending in the disc, which bears the ciliary wreath; (2) the trunk, containing the viscera; (3) the foot, which only contains muscles, nerves, and cement-glands. The general form of the BODY varies greatly: it is spherical in Trochosphaera, ovoid in Asplanchnidae, conical in Scirtopoda, Triarthridae, and Synchaeta; moderately elongated in the majority of the Ploima, among which some forms are very flat, like Pterodina, Metopidia, and Brachionus; shortly elongated and cylindrical in Hydatina (Fig. 106), Notommatidae, and many others. In Taphrocampa it is cylindrical and segmented, while the segments are telescopic in the Bdelloida, both ends being retractile into the middle segment. In most attached, tube-dwelling forms the body is ovate, tapering behind into the elongated stalk-like foot.

The FOOT at the hinder end of the body is usually more or less jointed; in Pterodina and Brachionus it is long, transversely wrinkled, and retractile. Usually it terminates in a couple of acute, mobile toes, perforated at the tips by the ducts of the pedal glands (Fig. 106, fg), whose viscid secretion serves to anchor the animal. In Rotifer there are three of these toes, which are retractile, and in addition there are in this genus, as in most of the Bdelloida, toe-like pointed spurs in pairs on the more proximal joints of the foot. In Callidina the spurs are often perforated, and the toes are replaced by numerous openings on the last joint of the foot (Fig. 109, A); while in Discopus the end of the foot expands into a large disc, with numerous pores for the exudation of the pedal cement, and there are no spurs. In Pedalion mirum the foot is represented by two tubular processes ciliated at the apex and at the outer side near the base (Fig. 117, f). These are inconstant in size and form, that of one side being sometimes reduced or absent, while both are absent in the closely allied species P. fennicum.

In Melicertidae and Flosculariidae the long foot ends in an expanded disc, which is cupped and ciliated in the larva (Fig. 112, B) and in the larva-like male (Fig. 107); but in two species it is prolonged into a long flexible thread which is not contractile. The foot is also elongated in the Bdelloid genus Actinurus and the Ploimal genus Scaridium. It forms a mere ventral disc in Apsilus (and Atrochus?), and is absent in Asplanchnidae (except Asplanchnopus), Triarthridae, and Anuraeidae, and in the genera Trochosphaera (Melicertaceae) and Pompholyx (Pterodinidae).

The fringed spines of Triarthridae are jointed appendages moved by powerful muscles; in Triarthra one is median and ventral, the others being attached to the shoulders. In Polyarthra, there are twelve flattened and serrated spines, a bunch of three being attached to the dorsal and ventral faces of either shoulder. An easy transition leads to the hollow appendages of Scirtopoda, which end in a fringe of bristly hairs, themselves feathered with finer hairs (Fig. 117). These processes are in Pedalion six in number, two median (respectively dorsal and ventral), two antero-lateral, and two postero-lateral. As they contain proper muscles, and the postero-lateral pair contain part of the nephridia and bear the lateral antennae, they are true outgrowths of the body, and are not homologous with the spines of Triarthridae.

The front of the body constitutes the HEAD, which is scarcely distinct, though usually separated by a slight neck-like constriction. The DISC, which terminates the head, varies greatly in shape and in the arrangement of its parts. Imagine a circular funnel, finely ciliated within, and with the mouth at the bottom, the prominent rim bearing two zones of cilia, the inner or anterior being the coarser, and termed the "trochus" or hoop; the outer finer, and termed the "cingulum" or girdle, while a very finely ciliated groove lies between the two zones. Either or both of these zones may be interrupted on the dorsal or ventral median line, or both; and the funnel-shaped mouth may be shifted—usually ventrally, so that it forms only a dilatation of the ciliated groove. Again, the wreath as a whole may be festooned or lobed; or the lobing may be confined to the area between the cingulum and trochus, as in most Ploima (Figs. 106 and 108, 3). Very frequently on these lobes adjacent cilia are fused together during life, producing "vibratile styles," whose true nature is only revealed after death. In Microcodonidae the structure of the disc (Fig. 108, 1) nearly conforms to the primitive type; but the ciliated groove is absent, and the "trochus" is in two separate half-elliptical bands. In the Flosculariaceae (Fig. 108, 6) the mouth is also central, the disc is funnel-shaped, and the trochus is a horseshoe-shaped ridge, with its ends dorsal and raised into prominent knobs. The margin of the funnel is in Flosculariidae (Fig. 115) usually lobed, and furnished either with exceptionally strong cilia, or else with very long bristles which are usually passive. However, by the retraction of the lobes that bear them they are clasped together like casting-nets to enclose prey brought into the funnel by the action of the trochal cilia. An external ring of cilia in Floscularia mutabilis and F. pelagica serves for swimming. In Apsilidae the margin of the disc bears neither cilia nor bristles, but is either simple and ring-like, or is produced into tentacles (Fig. 112, C). The oral funnel is probably represented in Flosculariaceae by the continuation of the small central mouth into a ciliated tube (Fig. 115, C, tf), open below, and hanging freely down into the crop.

In all other cases the mouth is displaced, and lies in the groove and on its ventral side (except in Conochilus, where it is dorsal, Fig. 108, 5). In the Bdelloida the disc is prolonged into two great lobes like kettle-drums, round the posterior, external, and ventral edges of which run the trochus, cingulum, and ciliated groove (Fig. 108, 2). All three are interrupted behind in the median line; ventrally the groove widens into the oral funnel, the cingulum is continued into a sort of spout-like lower lip (Fig. 109, C, D, l), and the trochus is absent. The body is prolonged dorsally above the lobes into a two-jointed proboscis, ending in a ciliated cup overhung by two dorsal flaps: this we regard as a detached portion of the wreath.

This "Bdelloid" type of wreath occurs also in Scirtopoda (Fig. 117), and in the Ploimal genera Triarthra, Pterodina, and Pompholyx. A simpler wreath of essentially the same type occurs in Asplanchnaceae and Melicertaceae; the disc is not prolonged into drum-shaped lobes, but is thin at the rim, where it bears the triple ciliated zone, interrupted on the dorsal median line and depressed ventrally into the oral funnel. In the Melicertidae, moreover, the disc is widened into a great plate-like extension, often beautifully lobed; and in many of the species a ciliated cup lies ventral to the lips, and is connected with the groove by a short ciliated channel on either side (Figs. 108, 4, and 116). Even the simpler wreath of Asplanchnidae is complicated by stronger lobes on either side bearing vibratile styles.

The most complex discs are found in Ploima, especially in Brachionus, Hydatina, and Synchaeta, since the groove is replaced by a zone of lappets, as above mentioned. In Proales the whole face of the disc is strongly ciliated. The wreath is reduced in the parasitic genera Drilophagus, Albertia, Balatro, and the Seisonaceae; in Adineta and Taphrocampa it is only represented by a general but scanty ciliation of the disc.

The head is very frequently retractile, as a whole, by strong muscles. In Bdelloida the disc proper is retracted when the animal crawls, while the proboscis is exserted (Fig. 109). Ciliated patches occurring outside the region of the disc point to a primitive condition when the whole surface of the body was ciliated, as does the partial ciliation of the foot in certain groups. Synchaeta and many Notommatidae possess a pair of lateral, hollow, ciliated pits on the body, which can be everted to serve as additional swimming organs; these are termed "auricles."

The cuticle varies much in texture. It may be smooth and flexible, dotted or shagreened, or in the Loricata firm and of definite shape, constituting a lorica, which may be more or less distinctly divided up into areas or separated into distinct pieces. In this case it resists decomposition, and several species are only known by this "skeleton." In Ploesoma it is much thickened and looks like a honeycomb. A regular alternation of harder and softer zones effects the annulation of the body in certain genera.

The hypoderm or protoplasmic layer of the skin has no cellular boundaries, though it contains large and distinct nuclei; it is usually somewhat granular. It forms the wall of the body-cavity, which contains a transparent liquid without corpuscles.

The principal external glands are the pedal or cement-glands, which secrete a viscid substance that sets in water and serves to anchor the animal. They are formed from an ingrowth of the hypoderm, are usually paired, and open by fine ducts on or near the apex of the toes, when these processes of the foot are present (Fig. 106, fg). These glands are mostly absent when there is no foot, as in most Asplanchnidae and in Anuraeidae, but in Asplanchna herrickii a small gland on the ventral side of the cloacal aperture appears to represent the last rudiment of the foot.

In addition to these, the ciliated ventral cup below the disc of many Melicertidae secretes a viscid substance (Fig. 116, p); and possibly the whole surface of the body is secretory in those species of this group, and of the Flosculariidae, whose tube (Fig. 115, A) is uniform and not made of pellets. In several other species belonging to Bdelloida and Ploima-Illoricata a viscid secretion of the surface of the body renders it "sordid" with adherent particles of dirt.

When the secretion takes the form of a tube, the body can be wholly withdrawn into it by the contraction of the foot. In Floscularia, Stephanoceros, and Conochilus the tube is hyaline and thin-walled; in Oecistes and Cephalosiphon it is more or less floccose; and in Limnias it is thin, firm, and annulated. In Melicerta and some species of Oecistes the tube thus secreted by the body is only formed in a very young state. In M. janus and M. pilula it is increased by the successive deposition of ovoid faecal pellets on to the rim. In M. ringens (Fig. 116) and M. conifera pellets are formed of the excess of the food particles brought to the disc by the ciliary current; they are carried through the gutters on either side of the projecting ventral lip or "chin" into the ciliated glandular cup on that side of the head. Here, as they revolve, they are cemented together into a pellet which is spheroidal in the former species, cylindro-conoidal with a basal hollow like a rifle-bullet in the latter. After a pellet is completed the animal stoops down and deposits it on the edge of the tube. This may easily be verified by furnishing a young Melicerta with water containing solid particles of carmine. M. tubicolaria forms a thick tube which is laminated, the laminae being directed upwards and outwards, and having diatom shells, etc., between the layers. In this case we have observed that the faeces are pellucid, and sometimes are so ejected as to lie in a sheet against the funnel-shaped mouth of the tube, and we are inclined to believe that the tube itself is formed altogether in this way. A similar process probably occurs in Oecistes crystallinus and Oe. umbella.

The muscles are simple elongated fibres, usually having near the middle a mass of granular protoplasm containing a nucleus; they may be smooth or striated. The principal muscles of the body are conspicuously striated in many active free-swimming forms (Pedalion, Synchaeta, Pterodina, Triarthra).

The muscles of the body-wall are transverse and longitudinal. They are best seen in Bdelloida. The principal muscles of the body-cavity are longitudinal; the most conspicuous and constant are the retractors of the disc and of the foot, protraction of these organs being usually accomplished by the contraction of the transverse muscles. Special muscles effect the vigorous springing of the Triarthridae and Scirtopoda; in the former group the muscles only raise the spines, and their elastic recoil is the actual mechanism of progression; but in the latter (Fig. 117) special flexor muscles of the limbs are the effective agents of the leaping movements.

Movements.—The Rotifera vary very greatly in their movements. The cilia of the disc, and especially of the trochus, are the principal organs of prehension of food, and also of swimming when the animal is not fixed by its foot. In some cases, as in Bdelloida, the cilia lash downwards successively in the longitudinal plane of the body (Fig. 109, C, D); this motion during fixation produces a hollow vortex ring, like the rings of a skilled cigarette-smoker, but when the animal is free it determines a simple forward progression through the water. In other cases the animal rotates on its long axis, or may even turn somersaults (Synchaeta). The appearance of the spokes of a wheel is a pure illusion due to the greater visibility of the cilia in their slow recovery than in their instantaneous down-lash. The finer cilia of the groove and cingulum play a very minor part in the act of swimming, and in the production of the great vortices at the edge of the disc when the animal is fixed; they serve to direct the particles brought by the vortices to the edge of the disc onwards towards the mouth. It is easy to see that the stream must be in opposite directions on opposite sides of the groove; its prolongation across the dorsal median line would be useless, which explains the existence of the dorsal median gap. At the ventral side we usually find a prominent ciliated lip, whose cilia work outwards, and carry off the excess of food particles as by an overflow spout. In many cases among the Notommatidae, Coluridae, etc., the disc serves as much for creeping over organic débris as for swimming.

We have already noticed the springing bristles and limbs of the Triarthridae and Scirtopoda respectively; the great foot of Scaridium is also used for leaping. The Bdelloida have the power of retracting their disc and progressing in loops like a leech or looper (Geometrid) caterpillar.

Baker, in a letter addressed to Martin Folkes, Esq., President of the Royal Society, dated London, 16th January 1744-5,^[254] gives the following lively account of the aspect and movements of Philodina roseola belonging to this group, with figures, some of which we reproduce from the original copper-plate engraving:—"I call it a Water Animal, because its Appearance as a living Creature is only in that Element. I give it also for Distinction Sake the Name of Wheeler, Wheel Insect or Animal; from its being furnished with a Pair of Instruments, which in Figure and Motion appear much to resemble Wheels. It can, however, continue many Months out of Water, and dry as Dust; in which Condition its Shape is globular, its Bigness exceeds not a Grain of Sand, and no Signs of Life appear. Notwithstanding, being put into Water, in the Space of Half an Hour a languid Motion begins, the Globule turns itself about, lengthens by slow Degrees, becomes in the Form of a lively Maggot, and most commonly in a few Minutes afterwards puts out its Wheels, and swims vigorously through the Water in Search of Food; or else, fixing by its Tail, works them in such a Manner as to bring its Food to it. But sometimes it will remain a long While in the Maggot Form and not shew its Wheels at all....

"If the Water standing in Gutters of Lead, or the slimy Sediment it leaves behind, has any Thing of a red Colour, one may be almost certain of finding them therein,^[255] and, if in Summer, when all the Water is dried away, and nothing but Dust remains, that Dust appears red, or of a dark brown, one shall seldom fail, on putting it into Water, to discover Multitudes of minute reddish Globules, which are indeed the Animals, and will soon change their Appearance, in the Manner just now mentioned....

"A Couple of circular Bodies, armed with small Teeth like those of the Balance-Wheel of a Watch, appear projecting forwards beyond the Head, and extending sideways somewhat wider than the Diameter thereof. They have very much the Similitude of Wheels, and seem to turn round with a considerable Degree of Velocity, by which Means a pretty rapid Current of Water is brought from a great Distance to the very Mouth of the Creature, who is thereby supplied with many little Animalcules and various Particles of Matter that the Waters are furnished with.

"As these Wheels (for so from their Appearance I shall beg Leave to call them) are every where excessively transparent, except about their circular Rim or Edge on which the Cogs or Teeth appear, it is very difficult to determine by what Contrivance they are turned about, or what their real Figure is, though they seem exactly to resemble Wheels moving round upon an Axis....

"As the Animal is capable of thrusting these Parts out, or drawing them in, somewhat in the Way that Snails do their Horns, the Figure of them is different in their several Degrees of Extension and Contraction, or according to their Position to the Eye of the Observer, whereby they not only appear in all the various Forms before represented, but seem at certain Times as if the circular Rim of the Wheel or Funnel were of some Thickness, and had two Rows of Cogs or Teeth, one above and the other below that Rim."

Digestive Organs.—The pharynx is usually a narrow ciliated tube, which varies in length from genus to genus, but in no other important point, save in Flosculariidae, where it assumes the form of a crop, into which the mouth hangs freely down as a narrow ciliated tube. At its lower end is an enlargement, the mastax or gizzard.^[256] This is a strong muscular sac containing the trophi or hard chitinous chewing organs, with an antero-ventral inlet from the pharynx, and a postero-dorsal outlet through which the food passes into the stomach either directly or through a slender gullet (Fig. 106, oe). In the ventral wall of the gizzard of most Ploima is a median piece, the fulcrum, from which run forwards and upwards two pieces, the rami, which are hinged on the fulcrum. The Y-shaped structure formed of these three pieces is called the incus (anvil). At either side of the gizzard and at a higher level is a paired piece, the malleus, so called from its resemblance to a hammer, of which the manubrium (handle) looks backwards, and is embedded in the side walls of the mastax, while the toothed claw or uncus looks forwards and inwards, and is hinged at its inner side with the tip of the ramus. As the unci and rami are usually strongly toothed, this gizzard forms a very efficient apparatus for chewing. In some cases, when the pharynx is short and dilatable, the points of the unci and rami may be protruded for biting, for clinging to the host (in the parasitic genera Albertia and Drilophagus), or for the prehension of food (Rattulidae, etc.).

The type we have just described is termed the "malleate" type (Fig. 111, A). If all the trophi are slender and scarcely toothed, we have the "virgate" type (C), which is frequently asymmetrical. In the "submalleate" type (B) the mallei only are slender; in the "forcipate" type (D) both the unci and rami are slender and sharply pointed.^[257] In the "malleoramate" type (E) the manubrium is a curious looped structure, while the uncus is formed of a number of parallel slender elongated teeth; this characterises the family Melicertidae, and the genera Triarthra, Pterodina, and Pedalion. In the "uncinate" type (G) the mallei are simply incurved hooks with a few teeth at the free end, the rami are simple or absent, and there is no fulcrum; this type occurs in Flosculariaceae only. In Asplanchnidae the rami are large and hooked, constituting the "incudate" mastax (F); but here reduced mallei are often present, and in Asplanchnopus they are almost as well developed as in Melicertidae, affording a transition to the malleoramate type. In this group too the mastax has a very peculiar form; it is divided into two chambers, dorsal and ventral. The dorsal chamber forms a great purse-like sac or crop, with a framework of four longitudinal bars: into this the gullet and pharynx open. The ventral pouch is much smaller, and in its base the large rami are inserted, so that they can be protruded into the crop. This ventral sac with the rami may even be everted through the crop and the mouth, to swallow the small Rotifers and Entomostraca which form the food of this group, or to eject the undigested remains of the food. Two lateral sacs open at the junction of the ventral pouch and the crop, but whether they play a part in the deglutition of food or in the disgorging of faeces is uncertain. The fact that the whole of this apparatus is lined by a non-ciliated chitinous cuticle justifies our view that it is simply an enlargement and specialisation of the mastax.

The trophi in Bdelloids also are only represented by the rami, which have the form of segments of a sphere, excavated on the curved sides for the attachment of muscles, and transversely ridged on the two flat sides; the gizzard is here called "ramate" (H).

It will be seen that the characters of the gizzard are very useful for classification, only breaking down indeed in the Ploima; for though the majority of these present one or other of the four varieties of the malleate type, Triarthra and Pterodina (but not the other genera of their respective families) have the gizzard malleoramate.

The oesophagus is, when present, a contractile ciliated tube in which the food makes no sojourn on its way to the stomach.

The stomach may be nearly spherical, ovoid, or elongated and cylindrical. Its walls are formed of large cells, often granular and sometimes brownish, whence a hepatic function has been assigned to them. Its apertures are both surrounded by constricting muscular fibres. The intestine may be simple or divided by a similar constriction into intestine proper and rectum. The whole of the alimentary tract, with the exception of the mastax, is richly ciliated within. The rectum opens into the slender non-ciliated cloaca. The intestine is sharply bent upwards and towards the back in the tubicolous forms, but is nearly straight elsewhere; in Trochosphaera and Apsilus it is bent ventrally. In Asplanchnaceae and in Paraseison there is no rectum, the stomach being a blind sac.

The so-called salivary glands, usually two in number, open into the pharynx or mastax; and the paired gastric glands (Fig. 106, gg) open into the oesophagus or stomach. While the prehension of food is usually accomplished by the ciliary current of the disc and pharynx, we have seen that a more active swallowing action takes place in Flosculariaceae and Asplanchnidae, which devour whole Algae, Infusoria, and even other Rotifers, the long spines of Triarthra not availing as a protection. Many Ploima put out the tips of their trophi to nibble at débris, or, in the case of Diglena and Distemma, to attack Desmids, or the Infusorian Stentor. But this use of the trophi is most efficient in Ploesoma. Bilfinger^[258] writes: "It has the courage to attack larger Rotifers; thus I was able to observe under the microscope how it fell upon a Rattulus but little smaller than itself and destroyed it. First it plunged the sharp prongs of its mastax deep into the tender frontal area of its unhappy victim; then followed a pumping action of the gizzard, and stroke by stroke the whole contents of the victim's body passed into the brigand's stomach." From this it is an easy transition to the ectoparasitism of Drilophagus, Balatro, and some species of Albertia, which cling to their host by the exserted trophi.

Renal Organs.—The kidneys consist of a pair of convoluted tubes, formed of a succession of perforated, so-called "drainpipe" cells (Fig. 106, k); they open directly or indirectly into the cloaca. Their walls are thin in the straight parts, but thick and glandular in the coils which occur at intervals. These tubes bear little tag-like appendages, hanging freely into the body-cavity, often widening towards the free end, and flattened or circular in section (Fig. 106, ns). They show during life a peculiar flickering motion in their interior, like the equivalent "flame-cells" of many Platyhelminthes (see p. 25), and are in function the representatives of the multicellular renal funnels of Annelids. On one side, especially on the edge of the flattened tags, the appearance is as of a tapering whip-like lash, attached by its base to the free end of the tag and waving in its cavity; but the side view of the flattened tags shows an appearance of successive transverse or oblique waves. In many if not all cases the free end of the tag is closed by a vacuolated plug of protoplasm, which sometimes at least bears two flagella waving freely in the body-cavity. The probable explanation of the two distinct wave appearances within the tag is that the protoplasmic plug bears on its inner face a row or tuft of long cilia hanging down into the cavity of the tag. The tags probably keep up a current of liquid through the kidneys, while the contents of the body-cavity are constantly replenished by osmosis.

The two renal tubes may end blindly below the disc, or else join by a short transverse dorsal communication in front of the brain, as in Stephanoceros, Atrochus (Fig. 112, C), and Apsilus among Flosculariaceae, Lacinularia among Melicertidae, and Hydatina among the Illoricate Ploima (Fig. 106, rc). In some species of Asplanchna, if not all, a recurrent branch occurs opening at either end into the main tube of its own side.

The kidneys unite to discharge into the cloaca near its orifice, and on its distal (primitively ventral) side in many Melicertidae. In Bdelloida the common duct formed by their fusion opens into the ventral side of a dilated bladder-like section of the cloaca (Fig. 109, A, bl), which contracts rhythmically to discharge the liquid; while in the majority of the class they open singly or by a common duct into a separate contractile vesicle or bladder, which also discharges at regular intervals into the cloaca on its ventral or distal side (Figs. 106, bl and 112).

This bladder may reach when expanded one-third the diameter of the whole animal, and contract as often as three times per minute; so that in a period of nine minutes a bulk of water equal to that of the animal must have diffused through the body-wall, to be removed by the kidneys. It is obvious that while the function of the kidneys is primitively excretory, the passage of the water through the body must bring in the oxygen dissolved in the external medium, and carry off the carbonic acid formed in the tissues, and so fulfil the act of respiration. This mechanism is physiologically comparable with that of the contractile vacuole of fresh-water Protozoa. In a few genera (Conochilus, Lacinularia, Pterodina) the kidneys open separately after a slight dilatation into the cloaca.

Nervous System.—The nervous centre of the Rotifera is the brain (Fig. 112, C, br), a ganglion lying dorsal to the pharynx; and when this is short it may be immediately below the surface of the disc (Microcodon). In Bdelloida a second ganglion is present below the pharynx, and is connected with the former by lateral cords which contain ganglion cells. From the brain, nerves are given off to the disc, to the muscles, and to the integument of the body, as well as to the sense organs. The largest nerves are two given off from the sides of the brain, each of which divides into a lateral and a ventral trunk, which run nearly the whole length of the animal.

The brain of several Notommatidae has a curious appendage, white by reflected light and very opaque; it is a sac full of chalky mineral matter, which dissolves readily in dilute acids.

Sense Organs.—The most widely diffused sense organs are the antennae or feelers, which may serve for touch or smell, or possibly both. Each antenna is a conical or tubular outgrowth of the skin; from its apex projects a fine pencil of sense hairs borne on a protoplasmic cushion, which receives a nerve. Often the antenna is elongated, and may then contain a muscle by which it is retractile (lateral antennae of Melicerta); sometimes it is reduced to a slight prominence bearing the setae (dorsal antenna of this genus). There are usually three antennae—a median dorsal (Figs. 109, B, a, and 112, C, am) and two lateral (Figs. 106, 112, C, and 115, A, al), often approximated towards the ventral surface, and sometimes all but fused on the middle line, or completely united (Conochilus dossuarius, Copeus caudatus).^[259]

Most Rotifers possess an organ of sight. This in its simplest form is a refractive globule seated in a red pigmented cup through which the nerve passes; in other cases it lies directly on the brain. Very frequently the eye is paired (Figs. 112, B, and 115, A); and these paired eyes may lie on the brain, and then are so close together that the pigment-cups have the shape of an x, or else they are seated in the dorsal region of the head behind the disc. In some cases they lie just under the ciliary wreath, or even within the region of the disc, and pass towards its ventral side in Pedalion (Fig. 117, A, e). In Rotifer they lie just under the dorsal side of the proboscis just below its apex. The median and two lateral eyes often exist together, as in Eosphora; and sometimes additional paired eyes exist. In Furcularia longiseta, var. grandis a pair of pigment spots (eyes?) occurs at the hinder end of the body just in front of the foot.

The active Ploima show a spontaneity of movement and marked power of avoiding obstacles, etc. This is still more marked in the very active Pedalion, which, as Rousselet notes, clearly avoids capture by the dropping tube, aided by its sense of sight, as he suggests, or by the tactile or olfactory powers of the antennae. They must rank as psychically high in the scale of creatures of simple organisation.

Reproductive Organs and Reproduction.—The most conspicuous organ in the female is the large yolk-gland or vitellarium (Figs. 106 and 109, A, vm), which was regarded as the ovary by all the older observers. It consists usually of eight cells, with conspicuous nuclei, lying on the ventral side of the stomach, and frequently displaced to one side; but in most Asplanchnidae it forms a broad transverse band of numerous cells. In Pterodina it is horseshoe-shaped, while in Seisonaceae and Bdelloida it is paired, either gland containing four or eight cells. The true ovary or germarium (Fig. 106, gm) lies more or less hidden between the yolk-gland and the stomach; it is composed of numerous minute rounded cells, of which the hindmost for the time being enlarges by nutrition from the yolk-gland, and finally receives a membranous shell. This true ovary is somewhat lateral in most Rotifers, but is median in Asplanchnidae, and paired in Pterodina, Bdelloida, and Seisonaceae. A membranous covering is common to the ovary and yolk-gland (paired when these are paired); it is continued into a thin-walled tube or oviduct, which opens into the cloaca on its ventral side beyond the bladder or common renal duct. In the viviparous species the mature ovum (Fig. 112, em) usually lies in the oviduct, dilating it into a sort of "uterus" until the birth of the young. The ordinary eggs or "summer eggs" are formed without any fertilisation, and develop immediately; they are often hatched within the tube of the tubicolous species.

Under certain conditions the unfertilised females produce exclusively smaller eggs, which develop into males. Maupas^[260] has demonstrated that a rise in temperature to a minimum of 26° C. (79° F.) is the efficient factor. But as Bergendal points out,^[261] the critical temperature probably varies with the antecedent conditions of the race, since males occur in Greenland at a very much lower temperature; and it would seem probable that a temperature approaching that at which the pools habitually dry up is what is necessary for the production of males, as a provision for those fertilised eggs, which, having a hard shell often adorned with prickly prominences, and usually remaining for some time before development, are capable of withstanding drought; such eggs are termed "winter eggs," but a better term would be "resting eggs" (German, "Dauereier").^[262]