1. Palaemonetes varians, variety microgenitor, commonly inhabits only saline water around the British coast. It is, however, abundant in water of extremely low salinity in the Cardiff area, where it occurs in the drainage ditches of farm land adjacent to an area of salt-marsh.

2. Animals from areas of both high and low salinity were subjected, in the laboratory, to a range of salinities varying from pure tap water to concentrated sea water. The rate of respiration at each salinity was measured.

3. The population that inhabited the almost fresh water in the ditches differed physiologically from the salt-water form, although morphologically they were identical.

4. The salt-water population showed a minimum respiratory rate when in a medium of 26% NaCl salinity. This was expected, since the animal is isotonic at this salinity and the osmoregulatory work is minimal. The second population respired least when in a medium of 6% NaCl. The significance of this is discussed.

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