Water may collect in the protoplasm rendering it vacuolate. In most Protozoa its excess is got rid of by the formation of drops or vacuoles, which either collapse slowly or are rhythmically pulsatile, appearing at or near the same spot in rapid succession, and arising either by the increase in size of a single drop, or by the fusion of separate droplets, occasionally irregular in outline, but often disposed in a rosette. The pulsation is slower in marine than in freshwater forms, and it is quickened or the size of the vacuole increased by want of oxygen (Fiszer, Journal R. Micr. Soc. (2), vi. p. 463). The pulsation indeed may take place so suddenly and with such force as to propel the animal onwards (Engelmann, Z. A. i. p. 121). The water expelled may contain granules in suspension, and it has been observed that colouring stains, such as Bismarck brown or aniline blue which do not kill the organism, accumulate in it and are thus removed (Ray Lankester, Encyclopaedia Brit. (ed. ix), xix. p. 836; Q. J. M. xxiv. p. 378). A special vacuolar duct exists in a few instances (some Acinetaria, Vorticellidae).

A nucleus is seemingly absent in some Proieomyxa, and occasionally in other Protozoa where it is normally present (Gruber, Biol. Centralblatt, iii. p. 580; cf. infra, p. 821). In structure it presents great variety. It may be homogeneous, or vesicular, i. e. with a membrane, occasionally much specialised, as in Radiolaria, inclosing a nuclear fluid and variously arranged chromatin. Its division may or may not be accompanied by mitosis. There may be one, or several, or many. If more than one, the increase may be normal to the organism (Gruber, Biol. Centralblatt, iv. p. 710), e.g. in some Infusoria, Adinosphaerium, some Amoebae, and be limited, e. g. to two, or unlimited; or it is connected directly or remotely with a reproductive phase. Furthermore the many nuclei may be alike in structure and size, or markedly dissimilar, as in the paranuclei of some Acinetaria, the Infusoria and the Dinoflagellate Polykrikos. Occasionally the Infusorian nucleus undergoes pulverisation, i.e. is disseminated in a minutely divided state through the protoplasm.

See on the forms of nuclei R. Hertwig, M. J. ii. 1876, and on the same with their modes of fission Gruber, Z. W. Z. xxxviii. 1883, and pp. 834-5 infra, on the Infusorian nuclei.

Reproduction takes place by fission, gemmation and spore-formation. The distinction between the two modes first named depends solely on the relative size of the parts: if they are equal or sub-equal then the process is termed fission; if markedly unequal, it is gemmation. Where axes are distinguishable in the organism, fission is termed transverse, longitudinal or oblique with reference to the longest axis. It is usually binary, i. e. the organism divides into two; it is very rarely multiple, i. e. the organism is resolved simultaneously into several parts; but it is frequently repeated without a pause of any length, leading to a more or less rapid diminution of size. Gemmation may be external, i. e. the bud projects freely, or internal, i. e. the bud is developed within a closed or nearly closed depression (some Acinetaria). One bud may be formed at a time or several. The term spore is applied to two different classes of structures: to bodies which are produced by the condensation of the protoplasm accompanied or not by fission, and which pass through a resting phase: or to bodies produced by the progressive or simultaneous total or partial resolution of the protoplasm into ultimately small portions, which are not set free until the process is complete, which do not resemble the parent organism when they are set free, and which as a rule pass through a quiescent period.

The spores, however produced, may be naked, or protected by a special spore-membrane and may then be distinguished as chlamydospores. They may when they become motile be amoeboid or flagellate, and to these two states respectively the terms amoebula, or zoospore s. flagellula may be applied. In fission the nucleus divides before or after the commencement of division in the body of the cell; the same is true of gemmation. Indeed it has been shown experimentally in Infusoria by means of artificial section, that the presence of a part of the nucleus is indispensable to the progress of normal life. Non-nucleated fragments may increase in size and heal a wound, and if an organ such as the peristome in an Infusorian be in process of development at the time of section, it undergoes complete evolution; but such fragments cannot start the formation of an organ themselves; their life is limited, nor are they capable of reproduction. See on this subject Gruber, A. N. H. (5), xvii. 1886, p. 473; Nussbaum, A. M. A. xxvi. 1886, p. 509. As to spore-formation or sporulation, in some instances, e. g.

Radiolaria, Foraminifera, it has been clearly proved that a multiplication of the nucleus precedes the resolution of the protoplasm.

A spontaneous breaking up of the body which is independent of the nucleus occurs in some Infusoria. It is of normal occurrence in the multinucleate Opalina, where it leads to the formation of new individuals. This, however, is probably not invariably the case; and the process is one of destruction. See Gruber, op. cit. supra, p. 481; Saville Kent, Manual of the Infusoria, p. 84; Parker, on Antphi-leptus, A. N. H. (5), xiii. 1884, p. 416.

Inasmuch as a Protozoon multiplies itself solely by some mode of cell-division, and the process continues under normal circumstances in a recurrent manner, it follows that there is no loss of substance at any time similar to the physical destruction or death which inevitably overtakes multicellular animals, i. e. all parts . save the generative products, by which the race is perpetuated. For the discussions to which this point has given rise, see Biitschli, Cholodkowski and Weismann, Z. A. v. 1882: Gotte, 'Uber den Ursprung des Todes,' Hamburg, 1883; Weismann, 'Uber Leben und Tod,' Jena, 1884; Mobius, 'Das Sterben,' etc., Biol. Central-blatt. iv. p. 389.