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Summary.1. The two series of observations on Cladocera showing (a) that a state of depression produced by unfavourable external conditions in Daphnia females kept in cultures causes a change from parthenogenetic to gamogenetic propagation, and (b) that gamogenesis in nature is accompanied by a state of depression in the populations in question, go to prove the correctness of the hypothesis that in nature the transition from parthenogenesis to gamogenesis is caused by the influence of unfavourable external conditions. Unfavourable external conditions cause states of depression in the females and thereby the change in the mode of reproduction and so also a change in sex determination. (Section II.)2. A number of earlier and more recent laboratory experiments have given results which support this depression hypothesis. (Section III.)3. Investigations in nature on parthenogenesis and gamogenesis in the Cladocera and simultaneous observations on other functions in these animals likewise afford support for the depression hypothesis. (Section IV.)4. The reproductive cycle of the Rotifera is compared with that of the Cladocera. There is a good deal of evidence which would seem to indicate that in the Rotifera transition from parthenogenesis to gamogenesis—and so also the sex determination associated with it—is likewise caused by a state of depression, and that this state of depression may be induced by the influence of various external factors. (Section V.)5. A general survey is taken of some of the biological peculiarities characterising the state of depression associated with the change from parthenogenesis to gamogenesis. (Section VI.)

1. The oxygen consumption of some Danish freshwater snails was studied in relation to varying periods of starvation, varying temperatures, weight of animals and oxygen content of the water. The observed respiration is a moderately active metabolism, not a basal one. 2. In the case of Lymnaea palustris and Bithynia leachi a distinct decrease of oxygen consumption has been found in the period 1-24 hr. after collecting; the decrease is supposed to be caused by starvation. In similar experiments Lymnaea pereger, Myxas giutinosa, Bithynia tentaculata, Valvata piscinalis and possibly Physafontinalis and Lymnaea auricularia show only a small decrease (or no decrease) of oxygen consumption. 3. During a gradual increase of the temperature (c. I° C. per hr.) the snails increase their oxygen consumption by 65-90% of the increase expected from Krogh's curve. In the case of Myxas glutinosa and Physa fontinalis the increase of respiration was nearly the same as that found by Krogh for other animals. 4. The relation of oxygen consumption to body size (live weight) is not a fixed, unchangeable quantity characteristic of every species, but may vary seasonally. A tentative explanation of this variation is given. 5. The oxygen consumption in relation to body size has also an interspecific variation. In prosobranchs the slopes b of the regression lines in a logarithmic co-ordinate system have in some cases nearly the magnitude 0.67 required by the surface law, but others are higher, e.g. c. 0.95. In pulmonates the relation varies as much as from b=c. 0.45 to b=c. 1.00, i.e. between less than proportional to surface and proportional to weight. 6. The oxygen consumption of the freshwater snails in relation to the sizes of the standard individuals is depicted in a logarithmic co-ordinate system as a belt showing only a slight deviation (Fig. 4, p. 697), i.e. the snails regarded as a whole have a fairly uniform respiration. The regression line of oxygen consumption to sizes of the standard individuals seems to be expressed by a regression line with a slope just under 1.0. 7. Experiments on oxygen consumption in relation to oxygen content of the water have shown that some species (Lymnaea auricularia, Myxas glutinosa, Physafontinalis, Valvata piscinalis and Bithynia leachi) are able to maintain their consumption with decreasing oxygen content of the water to a critical point of oxygen supply. But in some other species (Lymnaea pereger, L. palustris and Bithynia tentaculata) oxygen consumption decreases immediately in response to a declining oxygen supply. 8. In some freshwater snails (Myxas glutinosa, Lymnaea pereger, Physa fontinalis) the decrease in oxygen consumption in response to a decreasing oxygen supply is not gradual, but shows a steep fall below certain low values of the oxygen content. The only species able to maintain a comparatively high oxygen consumption at low oxygen supply is Bithynia leachi.