Part 24

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.

[Illustration: 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.

[Illustration: 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.]

[Illustration: 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_).

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.

[Illustration: 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 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.

[Illustration: 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.