Data on the duration of egg development at 5, 10, 15 and 20°C are given for eight cladoceran and one cyclopoid copepod species found associated with the Yellow Water Lily (Nuphar lutea) in the River Thames. A decrease in temperature caused a marked increase in the time taken to complete development for all nine species. The data are compared with published data and a number of conclusions drawn. In general, cladoceran eggs tend to take longer to complete development than copepod eggs, and within the Cladocera the eggs of epiphytic species tend to take longer to develop than those of planktonic species, and the eggs of large species longer than the eggs of small species.The relationship of development time to temperature was examined, and the linear reciprocal relationship, Krogh's curve and the vant' Hoff-Arrhenius function were found to be inadequate. Statistical analysis of the data required a logarithmic transformation, and this gave a significantly curvilinear relationship for all nine species. A quadratic of the form: logD = log a + b log T + c(logT) gave the best fit for eight of the species, but this could be reduced to log D = log a +c (logT) with only a slight decrease in the predictive value of the model. A comparison of regression lines showed that the data for each species could only be described properly by individual regression lines for each species: common slope or common intercept lines, or a single line were not valid. A survey of the literature suggests that the analyses in many previous investigations are statistically inadequate. A critical discussion of temperature functions is given.

Data on generation time, length of life, instar duration and frequency of moulting at a range of constant temperatures are given for eight species of epiphytic Cladocera from the River Thames. For each species each of the above events was inversely related to temperature. The number of juvenile instars was constant for a particular species, but varied between species; large species tending to have more juvenile instars than small species. A similar size relationship was found for generation time. Juveniles moulted more frequently than adults. The duration of an adult instar was the same as the duration of egg development, except at low temperatures. The data are compared with published data on daphniids, and possible reasons for the basic differences discussed.Regression analysis of the relationships between the various dependent variables and temperature required a logarithmic transformation. A reciprocal transformation was statistically inadequate for describing the relationship.

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The ephippia of Ceriodaphnia megalops and Ceriodaphnia pulchella are described. Their ephippia may be identified by their size and shape, supplemented by measurement of the length of the resting egg. The ephippia are shed free in the water and float by means of a special area of gas-filled tissue. Each ephippium contained one ellipsoid resting egg. The length, width and shape of the resting eggs of both species were investigated, and comparisons made with published data of their parthenogenetic eggs. For both species the volume of a resting egg was virtually the same as the volume of a parthenogenetic egg, but the resting egg was more elongated. The shape of resting eggs was linearly related to egg volume on logarithmic axes in a manner similar to that reported for the parthenogeneticeggs of these two species. The linear relationship between total parthenogenetic egg volume and body length reported for these two species did not apply to their resting eggs.