Two small posterior segments are produced simultaneously in Anoplophrya nodulata: a posterior region is marked off in A. prolifera and Benedenia, and is then divided into a chain of segments. Opalina itself breaks up into a number of portions by repeated transverse or oblique divisions: and the fragments thus formed encyst and are devoured by the tadpole of the Frog with its food3. The peristome of the new individual in Stentor and some other Heterotricha, and probably in most Hypotricha begins to develope before separation is completed. The products of fission may grow in size before the process is repeated; if they do not do so, the result is a rapid diminution in size of the individual remedied eventually by conjugation. The products of fission may remain living in societies (p. 830): or if they are organically connected by a stalk or pedicle they constitute colonies, as in the Vorticellina. New colonies are founded by individuals which retract or lose the peristome, develope a posterior ciliary wreath, are detached and swim away to settle in a new spot. In some species of Zoothamnium, e.g. Z. arbuscula, this duty falls upon special large spheroidal individuals.

The function of the slender elongate zooids of Z. alter nans and Epistylis ophioidea is not known.

1Butschli says the cilia persist, M. J. xi. p. 561.

2 Binary fission of one of two individuals just produced by fission has been seen in Vorticella microstoma, and an instance of the formation of microzooids recorded; see D'Udekem, Mem. Acad. Roy. Belg. xxxiv. p. 8, Pl. I. fig. 5a. In the genus Vorticella microzooids are perhaps formed as a rule by gemmation.

8 See on this subject, Gruber, Biol. Centralblatt, pp. iv. 712-13, 715-16.

Gemmation is rare. It is the sole mode of reproduction observed in Spirochona gemmipara, and may be exhaustive of the parent. It occurs also in Stylochona nebalina, Lagenophrys ampulla, and in some species of Vorticella, e.g. V. microstoma. The free-swimming bud is unlike its parent; in Lagenophrys it is Hypotrichan in its ciliation (Stein), or provided with a terminal ciliary wreath (Plate); in Vorticella it resembles a rosette microzooid (supra), and has a similar destiny.

The term spore-formation is applied to the production by fission of small individuals within a cyst. The number produced varies from two in Prorodon teres and Amphileptus to 16 in Chilodon cucullulus, or even 1000 and upwards in Ichthyophthirius multifiliis, an ectoparasite on some freshwater fish. Encysted Vorticellids with many nuclei have been observed. In Vorticella nebulifera Everts saw emerge from the cyst 6-10 minute globular bodies each with a ciliary girdle and in its centre a mouth. They multiplied by fission, but eventually affixed themselves by the oral end, developed a peduncle and ciliary disc. The young Ichthyophthirius is unlike its progenitor. Small Infusoria differing from the adult form have been observed, but whether produced from cysts or not is not known 1.

Conjugation is universal and for the most part temporary. The conjugating individuals are as a rule small in size, produced by rapidly repeated fission. They unite by their oral aspects, e. g. in Paramecium, Stentor, or by their lateral as in Hypotricha. The union brings about changes; the nucleus is broken up; the paranuclei divide with mitosis; protoplasm may be interchanged as well as paranuclei (p. 835 and note 2, ante), the cilia may be withdrawn, the peristome obliterated. Separation brings about a reconstitution of all these organs, followed by growth or rejuvenescence. Permanent conjugation ( = copulation of Engelmann) is met with in Stylonychia and some other Hypotricha, and in some Vorticellina. In Vorticella and Epistylis it takes place between a microzooid, whether a fission-product or a bud, and an ordinary individual 2. It is followed by a fragmentation of the nuclei and the formation of a new nucleus. Lateral union between two or three ordinary individuals has been witnessed in Vorticella microstoma, Epistylis brevipes and Carchesium polypinum. The united zooids develope posterior ciliary wreaths, and are detached from their peduncles. Similar union may take place between zooids of V. campanula, already free.

The united animals may either settle down again or encyst, the latter being perhaps the normal result 1.

1 Everts looked on the reproduction of V. nebulifera as an instance of Alternations of Generations. Dimorphism may affect the ordinary fission-product, e. g. Folliculina (Freja) ampulla, according to Mobius, undergoes unequal longitudinal fission, and the part set free is uniformly ciliated, but when settled developes its bilobed peristome and Heterotrichan ciliation (Biol. Centrablatt. vi. p. 540). The so-called internal germs are probably always due to parasites, or to diseased nuclear products, caused by Bacteria. See on the whole subjects, Engelmann, M.J. i.pp. 584-602; Butschli, Abhandl. Senck. Ges. x. pp. 343-55; van Rees, Z. W. Z. xxxi. 1878.

2 See Engelmann, M. J. i. pp. 582, 622, 624, and summary, p. 628 et seqq. In Epistylis flavicans he states that a zooid A. gives rise by fission to two zooids B.B. Of these, one is resolved into microzooids. The detached microzooid always conjugates with an individual B, never with A. Hence he concludes there is a sexual distinction; the microzooid is male, the zooid B. female. Engelmann's statement that the paranucleus is absent in Vorticellina appears to be wrong: nor has it the sexual value which he and other authorities of the time supposed.

All Infusoria appear to encyst; and the Hypotrichan Gastrostyla vorax has been preserved alive in this condition for two years. Cilia, peristome, all organs save the nucleus and contractile vacuole are lost. The cyst is spheroidal, as a rule smooth, occasionally ridged or papillate; flask-shaped in Stentor coeruleus, and closed by an operculum. Its membrane may be single or double. The formation of a gelatinous envelope has been observed in Zoothamnium mucedo inclosing 1-9 individuals in situ on their stalks 2.