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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.

EB1911 Infusoria - Ciliata (2).jpg

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.

EB1911 Infusoria - Changes during Conjugation in Ciliata.jpg
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