Calanus which had been rendered practically sterile by a system of washing failed to moult when placed in sterile Berkefeld-filtered sea water but remained alive for about two weeks. The introduction of bacteria even in high concentration did not enable the copepods to moult. Rough calculations indicate that the concentration of bacteria in the sea is much too low to satisfy the nutritional needs of Calanus.Calanus was found to live and moult in sea water from which the larger planktonic organisms had been removed by a paper filter, although it throve better in unfiltered water. Whether this difference was caused by qualitative or quantitative differences in the food supply has not yet been determined.Preliminary measurements on the rate of filtration of Calanus gave a result less than that calculated to be necessary for the copepod to obtain sufficient food if diatoms and peridinians are the sole source of supply. Since bacteria cannot be regarded as an important source of food, the other small and littl...

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