Fig. 53.—Diagram of conjugation in Colpidium colpoda. Horizontal line means degeneration; parallel vertical lines, separation of gametes; broken lines (above), boundary between pairing animals; (below), first fission; single vertical line, continuity or enlargement. M, Meganucleus; µ, micronucleus; Z, zygote-nucleus.
Fig. 54.—Four individuals of Coleps hirtus (Gymnostomaceae) swarming about and ingesting a Vorticella (?) (From Verworn.)
The meganucleus lengthens, becomes irregularly constricted, and breaks up into fragments, which are ultimately extruded or partially digested. The micronucleus enlarges (Fig. 52, A) and undergoes three successive divisions, or, strictly speaking, two fissions producing four nuclei, of which one only undergoes the third. The other three nuclei of the second fission degenerate like the meganucleus.[[167]] Of the two micronuclei of this last division one remains where it is as a "stationary" pairing nucleus, while its sister passes as a "migratory" pairing-nucleus into the other mate, and fuses with its stationary pairing-nucleus. Thus in either mate is formed a "zygote-nucleus," or "fusion-nucleus." All these processes are simultaneous in the two mates; and the migratory nuclei cross one another on the bridge of junction of the two mates (Fig. 52, C). Each mate now has its original cytoplasm (subject to the qualification above), but its old nuclear apparatus is replaced by the fusion-nucleus. This new nucleus undergoes repeated fissions; its offspring enlarge unequally, the larger being differentiated as mega-, the smaller as micro-nuclei. The mates now separate (Fig. 52, F, G), and by the first (or subsequent) fission of each, the new mega- and micro-nuclei are distributed to the offspring. Colpidium colpoda offers the simplest case, on which we have founded our diagram showing the nuclear relations. During conjugation the oral apparatus often atrophies, and is regenerated; and in some cases the pellicle and ciliary apparatus are also "made over."
Fig. 55.—Paramecium caudatum (Aspirotrichaceae). A, The living animal from the ventral aspect; B, the same in optical section, the arrows show the course taken by food-particles. buc.gr, Buccal groove; cort, cortex; cu, cuticle; c.vac, contractile vacuole; f.vac, food vacuole; gul, gullet; med, medulla; mth, mouth; nu, meganucleus; pa.nu, micronucleus; trch, trichocysts discharged. (From Parker's Biology.)
In the Peritrichaceae the mates are unequal; the larger is the normal cell, and is fixed; the smaller, mobile, is derived from an ordinary individual by brood-divisions, which only occur under the conditions that induce conjugation (Fig. 60). Here, though the two pairs of nuclei are formed, it is only the migratory nuclei that unite, the stationary ones aborting in both mates. During the final processes of conjugation the smaller mate is absorbed into the body of the larger, and so plays the part of male there. But this process, though one of true binary sex, is clearly derived from the peculiar type of equal reciprocal conjugation of the other Infusoria.
The Ciliata are almost all free-swimming animals with the exception of most of the Peritrichaceae, and of the genera we now cite. Folliculina forms a sessile tube open at either end; and Schizotricha socialis inhabits the open mouths of a branching gelatinous tubular stem, obviously secreted by the hinder end of the animal, and forking at each fission to receive the produce. A similar habit to the latter characterises Maryna socialis; all three species are marine, and were described by Gruber.[[168]] Stentor habitually attaches itself by processes recalling pseudopodia, and often forms a gelatinous sheath.
The majority of the Oligotrichaceous Tintinnidae inhabit free chitinous tests often beautifully fenestrated, as in Dictyocystis.
Many genera are parasitic in the alimentary canal of various Metazoa, but none appear to be seriously harmful except Ichthyophtheirius, which causes an epidemic in fresh-water fish. Quite a peculiar fauna inhabit the paunch of Ruminants. Nyctotherus and Balantidium are occasionally found in the alimentary canal of Man.[[169]]