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.
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| 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 grooveand wreath of membranellae;a, moniliform meganucleus;b, position of contractilevacuole. 2, Group of Stentor polymorphus,O. F. Müller; thetwisted end of the peristomeindicating the positionof the mouth. 3, Tintinnus lagenula, Cl. andL., in free shell. 4, Strombidium claparedii, S.Kent. 5, Shell of Codonella campanella,Haeck. 6, 7, Torquatella typica, Lank.(= Strombidium accordingto Bütschli); p, oral tubeseen through peristomialwreath of apparently coalescentmembranellae. 8. Basal, and 9, side (inverted)views of Trichodina pediculus,Ehr.; a, meganucleus;c, basal collarand ring of hooks; d,mouth; contractile vacuoleand oral tube seen bytransparency in 8. 10, Spirochona gammipara,Stein; a, meganucleus; g,bud. 11, 12, Vorticella microstoma,Ehr.; d, formation of abrood of 8 microgametes cby multiple fission; b,contr. vacuole. 13, Same sp. in binary fission;a, meganucleus. | 14, V. nebulifera, Ehr.; budswimming away byposterior 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 tomuscle of stalk; d, micronucleus. 18, Carchesium spectabile, Ehr.; (×50). 19, Nematocysts of Epistylis flavicans. Ehr. (afterGreeff). 20, Opercularia stenostoma, St.; (×200); a smallcolony 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 inretracted state. 23, 24, Caenomorpha medusula, Perty, (×250), with spiralperistomial wreath. 25, 26, Thuricola valvata, Str. Wright, in sessile theca, withinternal valve (v) to close tube, as in gastropod Clausilia;owing to recent fission two animals occupy one tube. |
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| From Lankester’s Treatise on Zoology. | |
| Fig. iv.—Diagrammatic Sketch of Changes during Conjugation inCiliata. (From Hickson after Delage and Maupas.) | |
1, Two individuals at commencementof conjugationshowing meganucleus(dotted) and micronucleus;successive stages of thedisintegration of the meganucleusshown in all figuresup to 9. 2, 3, First mitotic division ofmicronuclei. 4, 5, Second ditto. 6, One of the four nuclei resultingfrom the second divisionagain dividing to form thepairing-nuclei in eithermate, while the other 3nuclei degenerate. | 7, Migration of the migratorynuclei. 8, 9, Fusion of the incomingmigratory with the stationarynucleus in either mate. 10, Fission of Zygote nucleusinto two, the new mega- andmicronucleus whosedifferentiation is shown in11, 12. The vertical dottedline indicates the separationof 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).