1911 Encyclopædia Britannica/Infusoria

by Marcus Manuel Hartog

INFUSORIA, the name given by Bütschli (following O. F. Ledermüller, 1763) to a group of Protozoa. The name arose from the procedure adopted by the older microscopists to obtain animalcules. Infusions of most varied organic substances were prepared (hay and pepper being perhaps the favourite ones), the method of obtaining them including maceration and decoction, as well as infusion in the strict sense; they were then allowed to decompose in the air, so that various living beings developed therein. As classified by C. G. Ehrenberg in his monumental Infusionstierchen als volkommene Organismen, they included (1) Desmids, Diatoms and Schizomycetes, now regarded as essentially Plant Protista or Protophytes; (2) Sarcodina (excluding Foraminifera, as well as Radiolaria, which were only as yet known by their skeletons, and termed Polycystina), and (3) Rotifers, as well as (4) Flagellates and Infusoria in our present sense. F. Dujardin in his Histoire des zoophytes (1841) gave nearly as liberal an interpretation to the name; while C. T. Van Siebold (1845) narrowed it to its present limits save for the admission of several Flagellate families. O. Bütschli limited the group by removing the Flagellata, Dinoflagellata and Cystoflagellata (q.v.) under the name of “Mastigophora” proposed earlier by R. M. Diesing (1865). We now define it thus:—Protozoa bounded by a permanent plasmic pellicle and consequently of definite form, never using pseudopodia for locomotion or ingestion, provided (at least in the young state) with numerous cilia or organs derived from cilia and equipped with a double nuclear apparatus: the larger (mega-) nucleus usually dividing by constriction, and disappearing during conjugation: the smaller (micro-) nucleus (sometimes multiple) dividing by mitosis, and entering into conjugation and giving rise to the cycle of nuclei both large and small of the race succeeding conjugation.

Fig. i. Ciliata.

1. Opalinopsis sepiolae, Foett.: a parasitic Holotrichous mouthless Ciliate from the liver of the Squid. a, branched meganucleus; b, vacuoles (non-contractile).

2. A similar specimen treated with picrocarmine, showing a remarkably branched and twisted meganucleus (a), in place of several nuclei.

3. Anoplophrya naidos, Duj.; a mouthless Holotrichous Ciliate parasitic in the worm Nais. a, the large axial meganucleus; b, contractile vacuoles.

4. Anoplophrya prolifera, C. and L.; from the intestine of Clitellio. Remarkable for the adhesion of incomplete fission-products in a metameric series. a, meganucleus.

5. Amphileptus gigas, C. and L. (Gymnostomaceae). b, contractile vacuoles; c, trichocysts (see fig. 2); d, meganucleus; e. pharynx.

6, 7. Prorodon niveus, Ehr. (Gymnostomaceae). a, meganucleus; b, contractile vacuole; c, pharynx with horny cuticular lining.

6. The fasciculate cuticle of the pharynx isolated.

8. Trachelius ovum, Ehr. (Gymnostomaceae); showing the reticulate arrangement of the endosarc, b, contractile vacuoles; c, the cuticle-lined pharynx.

9, 10, 11, 12. Icthyophthirius multifilius, Fouquet (Gymnostomaceae). Free individual and successive stages of division to form spores. a, meganucleus; b, contractile vacuoles.

13. Didinium nasutum, Müll. (Gymnostomaceae). The pharynx is everted and has seized a Paramecium as food. a, meganucleus; b, contractile vacuole; c, everted pharynx.

14. Euplotes charon, Müll. (Hypotrichaceae); lateral view of the animal when using its great cirrhi, x, as ambulatory organs.

15. Euplotes harpa, Stein (Hypotrichaceae); h, mouth; x, cirrhi.

16. Nyctotherus cordiformis, Stein (a Heterotriceae), parasitic in the intestine of the Frog; a, meganucleus; b, contractile vacuole; c, food particle; d, anus; e, heterotrichous band of membranelles; f, g, mouth; h, pharynx; i, small cilia.

Thus defined, the Infusoria fall into two groups:—(1) Ciliata, with cilia or organs derived from cilia throughout their lives, provided with a single permanent mouth (absent in the parasitic Opalinopsidae) flush with the body or at the base of an oral depression, and taking in food by active swallowing or by ciliary action: (2) Suctoria, rarely ciliated except in the young state, and taking in their food by suction through protrusible hollow tentacles, usually numerous.

The pellicle of the Infusoria is stronger and more permanent than in many Protozoa, and sometimes assumes the character of a mail of hard plates, closely fitting; but even in this case it undergoes solution soon after death. It is continuous with a firm ectosarc, highly differentiated in the Ciliata, and in both groups free from coarse movable granules. The endosarc is semifluid and rich in granules mostly “reserve” in nature, often showing proteid or fat reactions. One or more contractile vacuoles are present in some of the marine and all the freshwater species, and open to the surface by pores of permanent position: a system of canals in the deeper layers of the ectoplasm is sometimes connected with the vacucle. The body is often provided with not-living external formations “stalk” and “theca” (or “lorica”).

The character of the nuclear apparatus excludes two groups both parasitic and mouthless: (1) the Trichonymphidae, with a single nucleus of Leidy, parasitic in Insects, especially Termites; (2) the Opalinidae, with several (often numerous) uniform nuclei, parasitic in the gut of Batrachia, &c., and producing 1-nuclear zoospores which conjugate. Both these families we unite into a group of Pseudociliata, which may be referred to the Flagellata (q.v.). Lankester in the last edition of this Encyclopaedia called attention to the doubtful position of Opalina, and Delage and Hérouard placed Trichonymphidae among Flagellates.

The theca or shell is present in some pelagic species (fig. iii. 3, 5) and in many of the attached species, notably among the Peritricha (fig. iii. 21, 22, 25, 26) and Suctoria (fig. viii. 11); and is found in some free-swimming forms (fig. iii. 3, 5): it is usually chitinous, and forms a cup into which the animal, protruded when at its utmost elongation, can retract itself. In Metacineta mystacina it has several distinct slits (pylomes) for the passage of tufts of tentacles. In Stentor it is gelatinous; and in the Dictyocystids it is beautifully latticed.

The stalk is usually solid, and expanded at the base into a disk in Suctoria. In Peritrichaceae (fig. iii. 8-22, 25, 26), the only ciliate group with a stalk, it grows for some time after its formation, and on fission two new stalks continue the old one, so as to form a branched colony (fig. iii. 18). In Vorticella (fig. iii. 11, 12, 14, &c.) the stalk is hollow and elastic, and attached to it along a spiral is a prolongation of the ectosarc containing a bundle of myonemes, so that by the contractions of the bundle the stalk is pulled down into a corkscrew spiral, and on the relaxation of the muscle the elasticity of the hollow stalk straightens it out.

On fission the stalk may become branched, as the solid one of Epistylis and Opercularia (fig. iii. 20); and the myoneme also in the tubular stem of Zoothaminum; or the branch-myoneme for the one offspring may be inserted laterally on that for the other in Carchesium (fig. iii. 18). In several tubicolous Peritrichaceae there is some arrangement for closing their tubes. In Thuricola (fig. iii. 25-26) there is a valve which opens by the pressure of the animal on its protrusion, and closes automatically by elasticity on retraction. In Lagenophrys the animal adheres to the cup a little below the opening, so that its withdrawal closes the cup: at the adherent part the body mass is hardened, and so differentiated as to suggest the frame of the mouth of a purse. In Pyxicola (fig. iii. 21-22) the animal bears some way down the body a hardened shield (“operculum”) which closes the mouth of the shell on retraction.

Fig. ii.
1,	Surface view of Paramecium, showing the disposition of the cilia in longitudinal rows.
2,	a, mega-; b, micronucleus; c, junction of ecto- and endosarc; D, pellicle; E, endosarc; f, cilia (much too numerous and crowded); g, trichocysts; g′, same with thread; h, discharged; i, pharynx, its undulating membrane not shown; k, food granules collecting into a bolus; l, m, n, o, food vacuoles, their contents being digested as they pass in the endosarc along the path indicated by the arrows.
3,	Outline showing contractile vacuoles in commencing diastole, surrounded by five afferent canals.
4-7	Successive stages of diastole of contractile vacuole.

The cytoplasm of the Infusoria is very susceptible to injuries; and when cut or torn, unless the pellicle contracts rapidly to enclose the wounded surface, the substance of the body swells up, becoming frothy, with bubbles which rapidly enlarge and finally burst; the cell thus disintegrates, leaving only a few granules to mark where it was. This phenomenon, observed by Dujardin, is called “diffluence.” The contractile vacuole appears to be one of the means by which diffluence is avoided in cells with no strong wall to resist the absorption of water in excess; for after growing in size for some time, its walls contract suddenly, and its contents are expelled to the outside by a pore, which is, like the anus, usually invisible, but permanent in position. The contractile vacuole may be single or multiple; it may receive the contents of a canal, or of a system of canals, which only become visible at the moment of the contraction of the vacuole (fig. ii. 4-7), giving liquid time to accumulate in them, or when the vacuole is acting sluggishly or imperfectly, as in the approach of asphyxia (fig. ii. 3). Besides this function, since the system passes a large quantity of water from without through the substance of the cell, it must needs act as a means of respiration and excretion. In all Peritrichaceae it opens to the vestibule, and in some of them it discharges through an intervening reservoir, curiously recalling the arrangements in the Flagellate Euglenaceae.

The nuclear apparatus consists of two parts, the meganucleus, and the micronucleus or micronuclei (fig. iii. 17d, iv. 1). The meganucleus alone regarded and described as “the nucleus” by older observers is always single, subject to a few reservations. It is most frequently oval, and then is indented by the micronucleus; but it may be lobed, the lobes lying far apart and connected by a slender bridge or moniliform, or horseshoe-shaped (Peritrichaceae). It often contains darker inclusions, like nucleoles.

It has been shown, more especially by Gruber, that many Ciliata are multinucleate, and do not possess merely a single meganucleus and a micronucleus. In Oxytricha the nuclei are large and numerous (about forty), scattered through the protoplasm, whilst in other cases the nucleus is so finely divided as to appear like a powder diffused uniformly through the medullary protoplasm (Trachelocerca). Carmine staining, after treatment with absolute alcohol, has led to this remarkable discovery. The condition described by Foettinger in his Opalinopsis (fig. i. 1, 2) is an example of this pulverization of the nucleus. The condition of pulverization had led in some cases to a total failure to detect any nucleus in the living animal, and it was only by the use of reagents that the actual state of the case was revealed. Before fission, whatever be its habitual character, it condenses, becomes oval, and divides by constriction; and though it usually is then fibrillated, only in a few cases does it approach the typical mitotic condition. The micronucleus described by older writers as the “nucleolus” or “paranucleus” (“endoplastule” of Huxley), may be single or multiple. When the meganucleus is bilobed there are always two micronuclei, and at least one is found next to every enlargement of the moniliform meganucleus. In the fission of the Infusoria, every micronucleus divides by a true mitotic process, during which, however, its wall remains intact. From their relative sizes the meganucleus would appear to discharge during cell-life, exclusively, the functions of the nucleus in ordinary cells. Since in conjugation, however, the meganucleus degenerates and is in great part either digested or excreted as waste matter, while the new nuclear apparatus in both exconjugates arises, as we shall see, from a conjugation-nucleus of exclusively micronuclear origin, we infer that the micronucleus has for its function the carrying on of the nuclear functions of the race from one fission cycle to the next from which the meganucleus is excluded.

Fission is the ordinary mode of reproduction in the Infusoria, and is usually transverse, but oblique in Stentor, &., as in Flagellata, longitudinal in Peritrichaceae; in some cases it is always more or less unequal owing to the differentiation of the body, and consequently it must be followed by a regeneration of the missing organs in either daughter-cell. In some cases it becomes very uneven, affording every transition to budding, which process assumes especial importance in the Suctoria. Multiple fission (brood-formation or sporulation) is exceptional in Infusoria, and when it occurs the broods rarely exceed four or eight—another difference from Flagellata. The nuclear processes during conjugation suggest the phylogenetic loss of a process of multiple fission into active gametes. As noted, in fission the meganucleus divides by direct constriction; each micronucleus by a mode of mitosis. The process of fission is subject in its activity to the influences of nutrition and temperature, slackening as the food supply becomes inadequate or as the temperature recedes from the optimum for the process. Moreover, if the descendants of a single animal be raised, it is found that the rapidity of fission, other conditions being the same, varies periodically, undergoing periods of depression, which may be followed by either (1) spontaneous recovery, (2) recovery under stimulating food, (3) recovery through conjugation, or (4) the death of the cycle, which would have ensued if 2 or 3 had been omitted at an earlier stage, but which ultimately seems inevitable, even the induction of conjugation failing to restore it. These physiological conditions were first studied by E. Maupas, librarian to the city of Algiers, in his pioneering work in the later ’eighties, and have been confirmed and extended by later observers, among whom we may especially cite G. N. Calkins.

Syngamy, usually termed conjugation or “karyogamy,” is of exceptional character in the majority of this group—the Peritrichaceae alone evincing an approximation to the usual typical process of the permanent fusion of two cells (pairing-cells or gametes), cytoplasm to cytoplasm, nucleus to nucleus, to form a new cell (coupled cell, zygote).

This process was elucidated by E. Maupas in 1889, and his results, eagerly questioned and repeatedly tested, have been confirmed in every fact and in every generalization of importance.

Previously all that had been definitely made out was that under certain undetermined conditions a fit of pairing two and two occurred among the animals of the same species in a culture or in a locality in the open; that after a union prolonged over hours, and sometimes even days, the mates separated; that during the union the meganucleus underwent changes of a degenerative character; and that the micronucleus underwent repeated divisions, and that from the offspring of the micronuclei the new nuclear apparatus was evolved for each mate. Maupas discovered the biological conditions leading to conjugation: (1) the presence of individuals belonging to distinct stocks; (2) their belonging to a generation sufficiently removed from previous conjugation, but not too far removed therefrom; (3) a deficiency of food. He also showed that during conjugation a “migratory” nucleus, the offspring of the divisions of the micronucleus, passes from either mate to the other, while its sister nucleus remains “stationary”; and that reciprocal fusion of the migratory nucleus of the one mate with the stationary nucleus of the other takes place to form a zygote nucleus in either mate; and that from these zygote nuclei in each by division, at least two nuclei are formed, the one of which enlarges to form a meganucleus, while the other remains small as the first micronucleus of the new reorganized animal, which now separates as an “exconjugate” (fig. iv). Moreover, if pairing be prevented, or be not induced, the individuals produced by successive fissions become gradually weaker, their nuclear apparatus degenerates, and finally they cannot be induced under suitable conditions to pair normally, so that the cycle becomes extinct by senile decay. In Peritrichaceae the gametes are of unequal sizes (fig. iii. 11, 12), the smaller being formed by brood fissions (4 or 8); syngamy is here permanent, not temporary, the smaller (male) being absorbed into the body of the larger (female); and there are only two nuclei that pair. Thus we have a derived binary sexual process, comparable to that of ordinary bisexual organisms.

Fig. iii.— Ciliata: 1, 2, Heterotrichaceae; 3-7, 23-24, Oligotrichaceae; 8-22, 25, 26, Peritrichaceae.

1, Spirostomum ambiguum, Ehr.; on its left side oral groove and wreath of membranellae; a, moniliform meganucleus; b, position of contractile vacuole.

2, Group of Stentor polymorphus, O. F. Müller; the twisted end of the peristome indicating the position of the mouth.

3, Tintinnus lagenula, Cl. and L., in free shell.

4, Strombidium claparedii, S. Kent.

5, Shell of Codonella campanella, Haeck.

6, 7, Torquatella typica, Lank. (＝Strombidium according to Bütschli); p, oral tube seen through peristomial wreath of apparently coalescent membranellae.

8. Basal, and 9, side (inverted) views of Trichodina pediculus, Ehr.; a, meganucleus; c, basal collar and ring of hooks; d, mouth; contractile vacuole and oral tube seen by transparency in 8.

10, Spirochona gammipara, Stein; a, meganucleus; g, bud.

11, 12, Vorticella microstoma, Ehr.; d, formation of a brood of 8 microgametes c by multiple fission; b, contr. vacuole.

13, Same sp. in binary fission; a, meganucleus.

14, V. nebulifera, Ehr.; bud swimming away by posterior wreath, peristome contracted; e, peristomial disk; f, oral tube.

15, V. microstoma; b, contr. vacuole; c, d, two microgametes seeking to conjugate.

16, V. nebulifera, contracted, with body encysted.

17, Same sp. enlarged; c, myonemes converging posteriorly to muscle of stalk; d, micronucleus.

18, Carchesium spectabile, Ehr.; (×50).

19, Nematocysts of Epistylis flavicans. Ehr. (after Greeff).

20, Opercularia stenostoma, St.; (×200); a small colony showing upstanding (“opercular”) peristomial disk, protruded oral undulating membranejand cilia in oral tube.

21, 22, Pyxicola affinis, S.K., with stalk and theca; x, chitinous disk, or true “operculum” closing theca in retracted state.

23, 24, Caenomorpha medusula, Perty, (×250), with spiral peristomial wreath.

25, 26, Thuricola valvata, Str. Wright, in sessile theca, with internal valve (v) to close tube, as in gastropod Clausilia; owing to recent fission two animals occupy one tube.

From Lankester’s Treatise on Zoology.

Fig. iv.—Diagrammatic Sketch of Changes during Conjugation in Ciliata. (From Hickson after Delage and Maupas.)

1, Two individuals at commencement of conjugation showing meganucleus (dotted) and micronucleus; successive stages of the disintegration of the meganucleus shown in all figures up to 9.

2, 3, First mitotic division of micronuclei.

4, 5, Second ditto.

6, One of the four nuclei resulting from the second division again dividing to form the pairing-nuclei in either mate, while the other 3 nuclei degenerate.

7, Migration of the migratory nuclei.

8, 9, Fusion of the incoming migratory with the stationary nucleus in either mate.

10, Fission of Zygote nucleus into two, the new mega- and micronucleus whose differentiation is shown in 11, 12. The vertical dotted line indicates the separation of the mates.

Ciliata.—The Ciliate Infusoria represent the highest type of Protozoa. They are distinctly animal in function, and the Gymnostomaceae are active predaceous beings preying on other Infusoria or Flagellates. Some possess shells (fig. iii. 3, 5, 21, 22, 25, 26), most have a distinct swallowing apparatus, and in Dysteria there is a complex jaw—or tooth-apparatus, which needs new investigation. In the active Ciliata we find locomotive organs of most varied kinds: tail-springs, cirrhi for crawling and darting, cilia and membranellae for continuous swimming in the open or gliding over surfaces or waltzing on the substratum (Trichodina, fig. iii. 8) or for eddying in wild turns through the water (Strombidium, Tintinnus, Halteria). Their forms offer a most interesting variety, and the flexibility of many adds to their easy grace of movement, especially where the front of the body is produced and elongated like the neck of a swan (Amphileptus, fig. iii. 5; Lacrymaria).

From Lankester’s Treatise on Zoology.

Fig. v.—Diagram 1 illustrating changes during conjugation of Colpidium colpoda. (From Hickson, after Maupas.)

M, Old meganucleus undergoing disintegration.

m, Micronucleus.

N, migratory, and

S, Stationary pairing-nucleus.

M′, M′, the new meganuclei, and

m′, The new micronuclei in the products of the first fission of each of the exconjugates; the continuous vertical line indicates period of fusion, its cessation, separation; dotted lines indicate fission; the spaces lettered 1-7 successive stages in the process; the clear circles indicate functionless nuclei which degenerate.

The cytoplasm is very highly differentiated: especially the ectoplasm or ectosarc. This has always a distinct elastic “pellicle” or limiting layer, in a few cases hard, or even with local hardenings that affect the disposition of a coat of mail (Coleps) or a pair of valves (Dysteria); but is usually only marked into a rhomboidal network by intersecting depressions, with the cilia occupying the centres of the areas or meshes defined. The cytoplasm within is distinctly alveolated, and frequently contains tubular alveoli running along the length of the animal. Between these are dense fibrous thickenings, which from their double refraction, from their arrangement, and from their shortening in contracted animals are regarded as of muscular function and termed “myonemes.” Other threads running alongside of these, and not shortening but becoming wavy in the general contraction have been described in a few species as “neuronemes” and as possessing a nervous, conducting character. On this level, too, lie the dot-like granules at the bases of the cilia, which form definite groups in the case of such organs as are composed of fused cilia; in the deeper part of the ectoplasm the vacuoles or alveoli are more numerous, and reserve granules are also found; here too exist the canals, sometimes developed into a complex network, which open into the contractile vacuole.

The cilia themselves have a stiffer basal part, probably strengthened by an axial rod, and a distal flexible lash; when cilia are united by the outer plasmatic layer, they form (1) “Cirrhi,” stiff and either hook-like and pointed at the end, or brush-like, with a frayed apex; (2) membranelles, flattened organs composed of a number of cilia fused side by side, sometimes on a single row, sometimes on two rows approximated at either end so as to form a narrow oval, the membranelle thus being hollow; (3) the oral “undulating membrane,” merely a very elongated membranelle whose base may extend over a length nearly equal to the length of the animal; such membranes are present in the mouth oral depression and pharynx of all but Gymnostomaceae, and aid in ingestion; a second or third may be present, and behave like active lips; (4) in Peritrichaceae the cilia of the peristomial wreath are united below into a continuous undulating membrane, forming a spiral of more than one turn, and fray out distally into a fringe; (5) the dorsal cilia of Hypotrichaceae are slender and motionless, probably sensory.

Embedded in the ectosarc of many Ciliates are trichocysts, little elongated sacs at right angles to the surface, with a fine hair-like process projecting. On irritation these elongate into strong prominent threads, often with a more or less barb-like head, and may be ejected altogether from the body. Those over the surface of the body appear to be protective; but in the Gymnostomaceae specially strong ones surround the mouth. They can be injected into the prey pursued, and appear to have a distinctly poisonous effect on it. They are combined also into defensive batteries in the Gymnostome Loxophyllum. They are absent from most Heterotrichaceae and Hypotrichaceae, and from Peritrichaceae, except for a zone round the collar of the peristome.

The openings of the body are the mouth, absent in a few parasital species (Opalinopsis, fig. i. 1, 2), the anus and the pore of the contractile vacuole. The mouth is easily recognizable; in the most primitive forms of the Gymnostomaceae and some other groups, it is terminal, but it passes further and further back in more modified species, thereby defining a ventral, and correspondingly a dorsal surface; it usually lies on the left side. The anus is usually only visible during excretion, though its position is permanent; in a few genera it is always visible (e.g. Nyctotherus, fig. i. 16). The pore of the contractile vacuole might be described in the same terms.

The endoplasm has also an alveolar structure, and contains besides large food-vacuoles or digestive vacuoles, and shows movements of rotation within the ectoplasm, from which, however, it is not usually distinctly bounded. In Ophryoscolex and Didinium (fig. i. 13) a permanent cavity traverses it from mouth to anus.

From Calkins’ Protozoa, by permission of the Macmillan Company, N. Y.

Fig. vi.—Diagrammatic view of behaviour of the motile reaction of Paramecium after meeting a mechanical obstruction at A. (From G. N. Calkins after H. S. Jennings.) For clearness and simplicity the normal motion is supposed to be straight instead of spiral.

Ingestion of food is of the same character in all the Hymenostomata. The ciliary current drives a powerful stream into the mouth, which impinges against the endosarc, carrying with it the food particles; these adhere and accumulate to form a pellet, which ultimately is pushed by an apparently sudden action into the substance of the endosarc which closes behind it (fig. ii. 2). In some of the Aspirotrichaceae accessory undulating membranes play the part of lips, and there is a closer approximation to true deglutition. The mouth is rarely terminal, more frequently at the bottom of a depression, the “vestibule,” which may be prolonged into a slender canal, sometimes called the “pharynx” or “oral tube,” ciliated as well as provided with a membrane, and extending deep down into the body in many Peritrichaceae.

In Spirostomaceae the “adoral wreath” of membranelles encloses more or less completely an anterior part of the body, the “peristome,” within which lies the vestibule. This area may be depressed, truncate, convex or produced into a short obconical disk or into one or more lobes, or finally form a funnel, or a twisted spiral like a paper cone. In most Peritrichaceae a collar-like rim surrounds the peristome, and marks out a gutter from which the vestibule opens; the peristome can be retracted, and the collar close over it. This rim forms a deep permanent spiral funnel in Spirochona (fig. iii. 10).

Movements of Ciliata.—H. S. Jennings has made a very detailed study of these movements, which resemble those of most minute free-swimming organisms. The following account applies practically to all active “Infusoria” in the widest sense.

Fig. vii.—Diagram of a mode of progression of a Ciliate like Paramecium; m, mouth and pharynx; the straight line A, B, represents the axis of progression described by the posterior end, and the spiral line the curve described by the anterior end; the clear circles are the contractile vacuoles on the dorsal side.

The position of the free-swimming Infusoria, like that of Rotifers and other small swimming animals, is with the front end of the body inclined outward to the axis of advance, constantly changing its azimuth while preserving its angle constant or nearly so; if advance were ignored the body would thus rotate so as to trace out a cone, with the hinder end at the apex, and the front describing the base. On any irritation, (1) the motion is arrested, (2) the animal reverses its cilia and swims backwards, (3) it swerves outwards away from the axis so as to make a larger angle with it, and (4) then swims forwards along a new axis of progression, to which it is inclined at the same angle as to the previous axis (figs. vi., vii.). In this way it alters its axis of progression when it finds itself under conditions of stimulation. Thus a Paramecium coming into a region relatively too cold, too hot, or too poor in CO₂ or in nutriment, alters its direction of swimming; in this way individuals come to assemble in crowds where food is abundant, or even where there is a slight excess of CO₂. This reaction may lead to fatal results; if a solution of corrosive sublimate (Mercuric chloride) diffuses towards the hinder end of the animal faster than it progresses, the stimulus affecting the hinder end first, the axis of progression is altered so as to bring the animal after a few changes into a region where the solution is strong enough to kill it. This “motile reaction,” first noted by H. S. Jennings, is the explanation of the general reactions of minute swimming animals to most stimuli of whatever character, including light; the practical working out is, as he terms it, a method of “trial and error.” The action, however, of a current of electricity is distinctly and immediately directive; but such a stimulus is not to be found in nature. The motile reaction in the Hypotrichaceae which crawl or dart in a straight line is somewhat different, the swerve being a simple turn to the right hand—i.e. away from the mouth.

Parasitism in the Infusoria is by no means so important as among Flagellates. Ichthyophthirius alone causes epidemics among Fishes, and Balantidium coli has been observed in intestinal disease in Man. The Isotricheae, among Aspirotrichaceae and the Ophryoscolecidae among Heterotrichaceae are found in abundance in the stomachs of Ruminants, and are believed to play a part in the digestion of cellulose, and thus to be rather commensals than parasites. A large number of attached species are epizoic commensals, some very indifferent in choice of their host, others particular not only in the species they infest, but also in the special organs to which they adhere. This is notably the case with the shelled Peritrichaceae. Lichnophora and Trichodina (fig. iii. 8, 9) among Peritrichaceae are capable of locomotion by their permanent posterior wreath or of attaching themselves by the sucker which surrounds it; Kerona polyporum glides habitually over the body of Hydra, as does Trichodina pediculus.

Several Suctoria are endoparasitic in Ciliata, and their occurrence led to the view that they represented stages in the life-history of these. Again, we find in the endosarc of certain Ciliates green nucleated cells, which have a cellulose envelope and multiply by fission inside or outside the animal. They are symbiotic Algae, or possibly the resting state of a Chlamydomonadine Flagellate (Carteria?), and have received the name Zoochlorella. They are of constant occurrence in Paramecium bursaria, frequent in Stentor polymorphus and S. igneus, and Ophrydium versatile, and a few other species, which become infected by swallowing them.

Classification.

Order I.—Section A.—Gymnostomaceae. Mouth habitually closed; swallowing an active process; cilia (or membranelles) uniform, usually distributed evenly over the body; form variable, sometimes of circular transverse section.

Section B.—Trichostomata. Mouth permanently open against the endosarc, provided with 1 or 2 undulating membranes often prolonged into an inturned pharynx; ingestion by action of oral ciliary apparatus.

Order 2.—Subsection (a).—Aspirotrichaceae. Cilia nearly uniform, not associated with cirrhi or membranelles, nor forming a peristomial wreath. Form usually flattened, mouth unilateral. (N.B.—Orders 1, 2 are sometimes united into the single order Holotrichaceae.)

Subsection (b).—Spirotricha. Wreath of distinct membranelles—or of cilia fused at the base—enclosing a peristomial area and leading into the mouth.

§§ i.—Wreath of separate membranelles.

Order 3.—Heterotrichaceae; body covered with fine uniform cilia, usually circular in transverse section.

Order 4.—Oligotrichaceae; body covering partial or wholly absent; transverse section usually circular.

Order 5.—Hypotrichaceae; body flattened; body cilia represented chiefly by stiff cirrhi in ventral rows, and fine motionless dorsal sensory hairs.

Order 6.—§§ ii.—Peritrichaceae. Peristomial ciliary wreath, spiral, of cilia united at the base; posterior wreath circular of long membranelles; body circular in section, cylindrical, taper, or bell-shaped.

Illustrative Genera (selected).

1. Gymnostomaceae. (a) Ciliation general or not confined to one surface. Coleps Ehr., with pellicle locally hardened into mailed plates; Trachelocerca Ehr.; Prorodon Ehr. (fig. i. 6, 7); Trachelius Ehr., with branching endosarc (fig. i. 8); Lacrymaria Ehr. (fig. i. 5), body produced into a long neck with terminal mouth surrounded by offensive trichocysts; Dileptus Duj., of similar form, but anterior process, blind, preoral; Ichthyophthirius Fouquet (fig. i. 9-12), cilia represented by two girdles of membranellae; Didinium St. (fig. i. 13), cilia in tufts, surface with numerous tentacles each with a strong terminal trichocyst; Actinobolus Stein, body with one adoral tentacle; Ileonema Stokes. (b) Cilia confined to dorsal surface. Chilodon Ehr.; Loxodes Ehr., body flattened, ciliated on one side only, endosarc as in Trachelius; Dysteria Huxley, with the dorsal surface hardened and hinged along the median line into a bivalve shell, ciliated only on ventral surface, with a protrusible foot-like process, and a complex pharyngeal armature. (c) Cilia restricted to a single equatorial girdle, strong (probably membranelles); Mesodinium, mouth 4-lobed.

2. Aspirotrichaceae. Paramecium Hill (fig. ii. 1-3); Ophryoglena Ehr.; Colpoda O. F. Müller; Colpidium St.; Lembus Cohn, with posterior strong cilium for springing; Leucophrys St.; Urocentrum Nitsch, bare, with polar and equatorial zones and a posterior tuft of long cilia; Opalinopsis Foetlinger (fig. i. 1, 2); Anoplophyra St. (fig. i. 3, 4). (The last two parasitic mouthless genera are placed here doubtfully.)

3. Heterotrichaceae. (a) Wreath spiral; Stentor Oken. (fig. iii. 2), oval when free, trumpet-shaped when attached by pseudopods at apex, and then often secreting a gelatinous tube; Blepharisma Perty, sometimes parasitic in Heliozoa; Spirostomum Ehr., cylindrical, up to 1′ in length; (b) Wreath straight, often oblique; Nyctotherus Leidy, parasitic anus always visible; Balantidium Cl. and L., parasitic (B. coli in man); Bursaria, O.F.M., hollowed into an oval pouch, with the wreath inside.

4. Oligotrichaeceae. Tintinnus Schranck (fig. iii. 3); Trichodinopsis Cl. and L.; Codonella Haeck. (fig. iii. 5); Strombidium Cl. and L. (fig. iii. 4), including Torquatella Lank. (fig. iii. 6, 7), according to Bütschli; Halteria Duj., with an equatorial girdle of stiff bristle-like cilia; Caenomorpha Perty (fig. iii. 23, 24); Ophryoscolex St., with straight digestive cavity, and visible anus, parasitic in Ruminants.

5. Hypotrichaceae. Stylonychia Ehr.; Oxytricha Ehr.; Euplotes Ehr. (fig. i. 14, 15); Kerona Ehr. (epizoic on Hydra).

6. Peritrichaceae. 1. Peristomial wreath projecting when expanded above a circular contractile collar-like rim.

(a) Fam. Urceolaridae: posterior wreath permanently present around sucker-like base. Trichodina Ehr. (fig. iii. 8, 9), epizoic on Hydra; Lichnophora Cl. and L.; Cyclochaeta Hatchett Jackson; Gerda Cl. and L.; Scyphidia Duj.

(b) Fam. Vorticellidae = Bell Animalcules: posterior wreath temporarily present, shed after fixation.

Subfam. 1. Vorticellinae animals naked. (i.) Solitary; Vorticella Linn. (fig. iii. 11-17), stalk hollow with spiral muscle; Pyxidium S. Kent, stalk non-contractile. (ii.) Forming colonies by budding on a branched stalk: Carchesium Ehr., hollow branches and muscles discontinuous; Zoothamnium. Ehr., branched hollow stem and muscle continuous through colony; Epistylis Ehr., stalk rigid—(the animal body in these three genera has the same characters as Vorticella)—Campanella Goldf., stalked like Epistylis, wreath of many turns (nematocysts sometimes present) (fig. iii. 19); Opercularia, stalk of Epistylis, disk supporting wreath obconical, collar very high (fig. iii. 20).

Subfam. 2. Vaginicolinae; body enclosed in a firm theca: Vaginicola Lam., shell simple, sessile; Thuricola St. Wright, shell sessile, with a valve opening inwards (fig. iii. 25-26); Cothurnia Ehr., shell stalked, simple; Pyxicola S. Kent, shell stalked, closed by an infraperistomial opercular thickening on the body (fig. iii. 21-22).

Subfam. 3. Shells gelatinous; those of the colony aggregated into a floating spheroidal mass several inches in diameter Ophrydium Bory, O. versatile contains Zoochlorella, which secretes oxygen, and the gas-bubbles float the colonies like green lumps of jelly.

2. Peristomial wreath, not protrusible, surrounded by a very high usually spiral collar.

Fam. Spirochonina. Spirochona St. (fig. iii. 10); Kentrochona Rompel; both genera epizoic on gills, &c., of small Crustacea.

Suctoria.—These are distinguished from Ciliata by their possession of hollow tentacles (one only in Rhyncheta, fig. viii. 1, and Urnula) through which they ingest food, and by not possessing cilia, except in the young stage. Fission approximately equal is very rare. Usually it is unequal, or if nearly equal one of the halves remains attached, and the other, as an embryo or gemmule, develops cilia and swims off to attach itself elsewhere; Sphaerophrya (fig. viii. 2-6) alone, often occurring as an endoparasite in Ciliata, may be free, tentaculate and unattached.

The ectosarc is usually provided with a firm pellicle which shows a peculiar radiate “milling” in optical section, so fine that its true nature is difficult to make out; it may be due to radial rods, regularly imbedded, or may be the expression of radial vacuoles. The tentacles vary in many respects, but are always retractile. They are tubes covered by an extension of the pellicle; this is invaginated into the body round the base of the tentacle as a sheath, and then evaginated to form the outer layer of the tentacle itself, over which it is frequently raised into a spiral ridge, which may be traced down into the part sunk and ensheathed within the body: in Choanophrya, where the tentacles are largest, the pellicle is further continued into the interior of the tentacle. The tentacles are always pierced by a central canal opening at the apex, which may be (1) enlarged into a terminal capitate sucker, (2) slightly flared, (3) truncate and closed in the resting state to become widely opened into a funnel, or (4) pointed. The tentacles are always capable of being waved from side to side, or turned in a definite direction for the reception or prehension of food; in Rhyncheta, the movements of the long single tentacle recall those of an elephant’s trunk, only they are more extensive and more varied. In the majority of cases the food consists of Ciliata; and the contents of the prey may be seen passing down the canal of the sucker beyond where it becomes free from the general surface. In Choanophrya the food appears to consist of the débris of the prey of the carnivorous host (Cyclops), which is sucked into the wide funnel-shaped mouths of the tentacles—by what mechanism is unknown. The endosarc is full of food-granules and reserve-granules (oil, colouring matter and proteid).

Fig. viii.—Suctoria (in all a, meganucleus; b, contractile vacuole).

 1, Rhyncheta cyclopum, Zenker; only a single tentacle and that suctorial; epizoic on Cyclops.

 2, Sphaerophrya urostylae, Maupas; normal adult; parasitic in Ciliate Urostyla.

 3, The same dividing by transverse fission, the anterior moiety with temporarily developed cilia.

4, 5, 6, Sphaerophrya stentorea, Maupas. Parasitic in Stentor, and at one time mistaken for its young.

 7, Trichophrya epistylidis, Cl. and L.

 8, Hemiophrya gemmipara, Hertwig. Example with six buds, into each of which a branch of the meganucleus a is extended.

 9, The same species, showing the two kinds of tentacles (the suctorial and the pointed), and two contractile vacuoles b.

10, Ciliated embryo of Podophrya steinii, Cl. and L.

11, Acineta grandis, Saville Kent; showing pedunculated cup, and animal with two bunches of entirely suctorial tentacles.

12, Sphaerophrya magna, Maupas. It has seized with its tentacles, and is in the act of sucking out the juices of six examples of the Ciliate Colpoda parvifrons.

13, Podophrya elongata, Cl. and L.

14, Hemiophrya benedenii, Fraip.; the suctorial tentacles retracted.

15, Dendrocometes paradoxus, Stein. Parasitic on Gammarus pulex; captured prey.

16, A single tentacle of Podophrya. R. Hertwig.

17-20, Dendrosoma radians, Ehr.:—17, free-swimming ciliated embryo. 18, Earliest fixed condition of the embryo. 19, Later stage, a single tentaculiferous process now developed. 20, Adult colony; c, enclosed ciliated embryos; d, branching stolon; e, more minute reproductive (?) bodies.

21, Ophryodendron pedicellatum, Hincks.

The meganucleus and the micronucleus are both usually single, but in Dendrosoma (fig. viii. 20), of which the body is branched, and the meganucleus with it, there are numerous micronuclei. In most cases the micronucleus has not been recorded, though from the similarity of conjugation, and its presence in most cases of fission and budding that have been accurately described, we may infer that it is always present. In unequal fission the meganucleus sends a process into the bud, while the micronucleus divides as in Ciliata. The bud may be nearly equal to the remains of the original animal, or much smaller, and in that case a depression surrounds it which may deepen so as to form a brood-cavity, either communicating by a mere “birth-pore” with the outside or entirely closed. In some cases the budding is multiple (fig. viii. 8), and a large number of buds are formed and liberated at the same time. In all cases the bud escapes without tentacles, and possesses a characteristic supply of cilia, whose arrangement is constant for the species.

In some cases an adult may withdraw its tentacles, moult its pellicle and develop an equipment of cilia and swim away: this is the case with Dendrocometes, parasitic on Gammarus, when its host moults.

The numerous species of Suctoria, often so abundant on various species of Cyclops, are not found on the other freshwater Copepoda, Diaptomus and Canthocamptus, belonging indeed to other families. Again, these Suctoria affect different positions, those found on the antennae not being present on the mouth parts; the ventral part of the thorax has another set; and the inside of the pleural fold another. Rhyncheta occupies the front of the “couplers” or median downgrowths uniting the coxopodites of the swimming legs, and Choanophrya settles in the immediate neighbourhood of the mouth, preferably on the epistoma, labrum and metastomatic region, but also on the adoral appendages and in rare cases extends, when the settlement is extensive, to the bases of the two pairs of antennae; while distinct species of Podophrya settle on the antennae, the front of the thorax and the inside of the pleural folds. Dendrocometes is common on the gills of the freshwater shrimp (Amphipod) Gammarus and Stylocometes on the gills and gill-covers of the Isopod Asellus, the water-slater. The independence of the Acinetaria was threatened by the erroneous view of Stein that they were phases in the life-history of Vorticellidae. Small parasitic forms (Sphaerophrya) were also regarded erroneously as the “acinetiform young” of Ciliata. They now must be regarded as an extreme modification of the Protozoon series, in which the differentiation of organs in a unicellular animal reaches its highest point.

Principal Genera.

1. Unstalked simple forms. Urnula Cl. and L., permanently ciliate; Rhyncheta Zenker (fig. viii. 1), on the limb couplers of Cyclops; Sphaerophrya Cl. and L. (fig. viii. 2-6, 12), endoparasitic in Ciliata and formerly taken for embryos thereof, never attached; Trichophrya Cl. and L. (fig. viii. 7), of similar habits, but temporarily attached, sessile.

2. Stalked simple forms; Podophrya Ehr. (fig. viii. 10, 13, 16), tentacles all knobbed or flared; Ephelota Strethill Wright, tentacles all pointed; Hemiophrya S. Kent (fig. viii. 8, 9, 14), tentacles of both kinds; Choanophrya Hartog, tentacles thick, truncate, very retractile, when expanded opening into funnels for aspiration of floating prey, never for attachment—epizoic on antero-ventral parts of Cyclops.

3. Cupped forms; Solenophrya Cl. and L., cup sessile; Acineta Ehr., cup stalked; Acinetopsis Bütschli, like Acineta, but the cup flattened, closed distally with only slit-like apertures (“pylomes”) for the bundles of tentacles; Podocyathus, like Acineta, but with pointed as well as knobbed tentacles.

4. Tentacles in bundles at the tips of one or more processes or branches of the body. Ophryodendron Cl. and L., tentaculiferous process single (fig. viii. 21); Dendrocometes Stein (fig. viii. 15), body rounded, processes repeatedly branched, epizoic on gills of Gammarus pulex; Dendrosoma Ehr. (fig. viii. 17-20), body freely branched from a basal attached stolon, meganucleus branching with the body.

Bibliography.—(a) Infusoria in the widest sense: C. E. Ehrenberg. Die Infusionstierchen als vollkommene Organismen (1838); F. Dujardin, Zoophytes infusoires (1841). (b) Infusoria, including Mastigophora: M. Perty, Zur Kenntniss Kleinster Lebensformen (1852); E. Claparède and J. Lachmann, Études sur les infusoires et les Rhizopodes (1858–1861); F. von Stein, Der Organismus der Infusionstiere (1859–1883); W. Saville Kent, A Manual of the Infusoria, including a description of all known Flagellate, Ciliate and Tentaculiferous Protozoa (1880–1882). (c) Infusoria, as limited by Bütschli. O. Bütschli, Bronn’s Tierreich, vol. i. Protozoa, pt. 3 Infusoria (1887–1889), the most complete work existing, but without specific diagnoses; S. J. Hickson, “The Infusoria” in Lankester’s Treatise on Zoology, vol. i. fasc. 2 (1903), a general account, well illustrated, with a diagnosis of all genera. See also Delage and Hérouard, Traité de Zoologie concrète, vol. i. “La Cellule et les Protozoaires” (1896), with an illustrated conspectus of the genera; E. Maupas, “Recherches expérimentales sur la multiplication des Infusoires ciliés,” Arch. zool. exp. vi. (1888); and “Le Rajeunissement karyogomique chez les Ciliés,” ib. vii. (1889); R. Sand, Étude monographique sur le groupe des Infusoires tentaculifères (Suctoria), (1899), with diagnoses of species; A. Lang, Lehrb. der vergleich, Anatomie der wirbellosen Tiere, vol. i. “Protozoa” (1901) (a view of comparative anatomy, physiology and bionomics); Marcus Hartog, “Protozoa,” in Cambridge Natural History, i. (1906); H. S. Jennings, Contributions to the Study of the Behaviour of Lower Organisms (1904); G. N. Calkins, “Studies on the Life History of Protozoa” (Life cycle of Paramecium), I. Arch. Entw. xv. (1902), II. Arch. Prot. i. (1902), III. Biol. Bull. iii. (1902), IV. J. Exp. Zool. i. (1904). Numerous papers dealing especially with advances in structural knowledge have appeared in the Archiv für Protistenkunde, founded by F. Schaudinn in 1902.  (M. Ha.)