A few Holotricha e. g. some species of Opalina, and Hypotricha e. g. some species of Holosticha, have a number of similar globular nuclei, which multiply with mitosis in Opalina, Irregularly shaped nuclear fragments are found as well in some species of Holosticha; and in the Holotrichans Choenia teres and Trachelocerca Phoenicopterus the nucleus is reduced to chromatin granules scattered irregularly, but the condition may not be a permanent one. The Opalinids Benedenia and Opalinopsis, which are endoparasitic in Cephalopoda, vary at different times. The nucleus of the former is a long convoluted band, which branches and segments into globular or irregular fragments; that of the latter is reticulate, but may break up into fragments. The Hetero-trichan Plagiotoma Lumbrici is said to resemble Benedenia. During fission or gemmation the round or ovate nucleus and the paranuclei divide with mitosis: the band-like nucleus is said not to do so. In many multinucleate forms fission does not affect the nuclei1, but in Holosticha scutellum and Opalinopsis their previous fusion has been observed. The small encysted fragments of Opalina Ranarum, O. obtrigona, and O. dimidiata become uni-nucleate just before or just after their exclusion from the cyst.

During conjugation, the nucleus is resolved into fragments, the paranuclei divide with mitosis. Interchange of a paranucleus, or of a portion of one, and fusion of the interchanged structure, whichever it may be, with a corresponding one in situ has been observed (Gruber, Maupas). A new nucleus and paranucleus are then formed from the old paranuclei. It is not certain, however, how far the fragments of the old nucleus are excluded from any share in the process; whether they are extruded or absorbed2.

Physiologie, xxxii. 1883; Ryder, Proc. U. S. National Museum, vii. 1884. Stokes states (Journal New York Micr. Soc. i. 1885) that Leucophrys emarginata, though coloured by chlorophyl bodies is exceedingly voracious. On the other hand, Gruber's Strombidium oculatum has no mouth (Nova Acta, 46, p. 514). Stokes' Vorticella smaragdina is green throughout, but is perhaps not tinted with chlorophyl (Amer. Naturalist, xix. p. J9). The marine Vorticellid Spastostyla (Rhabdo-styld) Sertnlariamm harbours symbiotic Zooxanthellae (Entz, Mitth. Zool. Stat. Naples, v. p. 4i6).

1See Maupas, A. Z. Expt. (2), i. p. 611, and Entz, op. cit. supra, p. 383. It is generally asserted that the filament is coiled within a sac and everted, as in the nematocysts of e. g. Hydroids. Greeff maintains that the freshwater Peritrichan, Epistylis jlavicans, has Hydroid-like nematocysts, but other observers have failed to find them.

2 See Maupas, A. Z. Expt. (2) i. pp. 527, 660-1.

The contractile vacuole is rarely absent, as in the Opalinid genera Opalina, Opalinopsis, and Benedenia, and in a few Hypotricha3. It is nearly always superficial in position, and is usually single, but there may be more, e.g. 2 in Paramecium, and Vorticellids with a well developed cuticle, several in Chilodon, 50 in Trachelitis ovum, or even 100 in Prorodon margaritifer. Or small secondary vacuoles, irregular in position and appearance, may be present, e. g. in Prorodon teres. When distended it is usually globular, rarely linear as in some Opalinids. It is discharged by a pore, which is sometimes permanent if the cuticle is thick, e. g. in Paramecium Aurelia, or in the Vorticellina by a canal leading into the oral vestibule. The new vacuole may arise as a simple expanding drop, by the coalescence of a number of droplets disposed irregularly or in a rosette, sometimes, however, taking the shape of converging linear or branched canals.

1Maupas, op. cit. p. 654.

2Joseph has observed a natural fragmentation of the nucleus in Stylonychia, one induced by confinement in the dark in Pa?-amecium caudatum. An exchange of portions of the nucleus has been seen by Schneider in Anoplophrya circulans, of portions of the paranucleus by Joseph, Gruber, and Maupas. The first-named of the three has recorded movements of the chlorophyl bodies in Ophrydium versatile indicating an interchange of protoplasm. Plate observed the resolution and re-constitution of the nucleus in Lagenophrys ampulla when about to quit its tube, or after fission (Z. W. Z. xliii. pp. 213-14), or during gemmation (Ibid. pp. 214-15). But in the permanent conjugation of two young Spirochona gemmipara, he saw both nuclei and paranuclei fuse without previous change (loc. cit. pp. 206-9). See the authorities quoted p. 840, post.

3 See Maupas, A. Z. Expt. (2), i. p. 633. Maupas thinks (op. et p. citt.) that Strombidium urceolare and S. sulcatum have also no vacuole. Saville Kent assigns one to them. So does Geza Entz to Actinotricha, which has not got one according to Maupas, with the remark that there is a long pause between the discharge and formation de novo of the vacuole.

Reproduction is by fission, gemmation, and spore-formation. Fission is usually binary and transverse; oblique in Stentor, Lagenophrys, and Vaginicola; longitudinal in Vorticella and most of its allies. The ciliary disc and peristome of Vorticella are retracted or obliterated during the process to be reproduced at its close1. One of the two individuals retracts its disc, developes a posterior girdle of cilia on the ciliary ring (p. 834, ante), breaks away and swims off to affix itself elsewhere. So too in Spirochona tintinnabulum the funnel and peristome may be retracted, a median ciliary wreath be evolved, and transverse fission then takes place. The anterior part swims away, the posterior developes a new funnel and peristome. In the Vorticellids Epistylis, Zoothamnhtm, and Carchesium (?) repeated fission gives rise to 4-8-10 microzooids disposed in a rosette2. They are detached. Each individual has a rudimentary ciliary disc, and a well developed posterior ciliary circlet: it is probably always destined to conjugate with an ordinary individual or macrozooid (p. 837, infra). In the Opalinidae variations occur. Haptophrya gigante a is resolved by repeated binary fission into a chain of eight individuals.