1. Male Ligia oceanica were used in an investigation of the relationship of body size to rate of oxygen consumption and pleopod beat.

2. Animals varied in weight from 0.04 to 1.03 g. and from 0.95 to 3.1 cm. in length.

3. Body shape does not change significantly over the size range, for length and breadth both increase at the same rate, and pleopod dimensions bear a constant relation to body length.

4. Specific gravity also is constant, for the relation of body weight to the cube of body length shows no trend with increasing size.

5. Oxygen consumption per gram decreases with increasing size and is proportional to the -0.274 Power of body weight. Total oxygen consumption is therefore proportional to the 0.726 power of body weight; but this value does not differ significantly from two-thirds.

6. As shape is constant, surface area is proportional to the square of a linear dimension. It is shown that oxygen consumption per unit of length2 is constant over the size range. Although body length was measured far less accurately than body weight it is shown that it assesses ‘body size’ more accurately.

7. Rate of pleopod beat was measured at 15 and 25°C.; it decreases with the size of the animal. At 15°C. time per beat varies as the 0.66 power of body length, and at 25°C. as the 0.59 power; neither of these values differs significantly from 0.5. Despite the fact that pleopod movement is heavily damped, the rate therefore varies like that of a pendulum.

9. The workof Fox (1936-9) and Fox et al. (1937a)on the rate of oxygen consumption of animals from cold and warmer seas and from different habitats is considered. It is suggested that many of their comparisons are invalidated by differences in body size of the animals concerned, and that, in relation to environment, no basis, theoretical or experimental, has been established for a distinction between ‘nonlocomotory’ and ‘activity’ metabolism.

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