Summary.  The haemoglobin content of Daphnia in the ponds studied was found to vary inversely with the oxygen concentration in the water, but not to be proportional to the iron content of the water.  In six ponds Daphnia was found to have a greater haemoglobin concentration in winter than in summer, whereas in one body of water the haemoglobin concentration was greater in summer than in winter.  Haemoglobin may be more concentrated in the blood of one pond‐dwelling species of Daphnia than of another species in the same water.

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