The animals collect at that zone where the conditions of aeration are most suitable, usually just within the edge of the cover, and when well supplied with food are rather sluggish, not swimming far, so that they are easily studied and counted. When well supplied with appropriate food they undergo binary fission at frequent intervals, dividing as often as five times in the twenty-four hours at a temperature of 65-69° F. (_Glaucoma scintillans)_, so that in this period a single individual has resolved itself into a posterity of 32; but such a rapid increase is exceptional. At a minimum and a maximum temperature multiplication is arrested, the optimum lying midway. If the food-supply is cut off, encystment occurs in those species capable of the process; but when there is a mixture of members of different broods of the same species, subject to the limitations that we shall learn, conjugation ensues. Under the conditions of Maupas' investigations he found a limit to the possibilities of continuous fissions, even when interrupted by occasional encystment. The individuals of a series ultimately dwindle in size, their ciliary apparatus is reduced, and their nuclear apparatus degenerates. Thus the ultimate members of a fission-cycle show a progressive decay, notably in the nuclear apparatus, which Maupas has aptly compared to "senility" or "old age" in the Metazoan. If by the _timely_ mixture of broods conjugation be induced, these senile degenerations do not occur.[165] In _Stylonychia {149}mytilus_ the produce of a being after conjugation died of senility after 336 fissions; in _Leucophrys_ after 660. Save in the Peritrichaceae (p. 151) CONJUGATION takes place between similar mates, either of the general character and size of the species, or reduced by fissions, in rapid succession, induced by the same conditions as those of mating. The two mates approach, lying parallel and with their oral faces or their sides (_Stentor_) together, and partially fuse thereby; though no passage of cytoplasm is seen it is probable that there is some interchange or mixture.[166] [Illustration: 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.] {150}[Illustration: FIG. 54.—Four individuals of _Coleps hirtus_ (Gymnostomaceae) swarming about and ingesting a Vorticella (?) (From Verworn.)]