The filtering rate of four species of marine planktonic copepods was measured by estimating the rate at which they consumed cultures of Chlamydomonas.The filtering rate was independent of the concentration of the food organism and it is concluded that the copepods were acting as filter feeders.The filtering rates were much greater than those reported by Fuller & Clarke (1936) and Fuller (1937), but agreed with those reported by Harvey (1937).Some evidence was obtained that grazing was restricted to some only of the 24 hr., most probably to the hours of darkness.The filtering rates were approximately proportional to the square of the linear dimensions of the copepod.The purely mechanical selection suggested by Fuller (1937) to account for differences in filtering rates obtained with different species of diatoms cannot account for all the differences which have been observed.It is probable that the copepods could obtain sufficient particulate food in the sea by filtering a daily volume of water corresponding to the filtering rates found.

" Wimpenny (1938) has shown that the percentage of copepods taken in the sea with food in the gut tends to be higher at night than by day. Most planktonic animals perform diurnal vertical migrations, of greater or less extent.' Wimpenny's observations may well be a consequence ,of such migrations. , On the other hand, during investigations. into grazing rate of planktonic copepods (Fuller, 1937;Gauld, 1951) some of the measurements have suggested that there is a diurnal variation in the rate at which the copepods ' collect their food, even under laboratory conditions when migration is impossible, and that they filter the water more quickly at night than by day. The variation in the percentage of copepods containing food may be caused simply by changes in the rate of feeding, this being controlled either by the amount of light in their environment or by some internal rhythm affecting their behaviour.An investigation of diurnal variations in the feeding behaviour of Calanus finmarchicus'(Gunn.) was undertaken to investigate (i) its behaviour in nature:, and (ii) the possible existence of alternations of feeding and resting periods under laboratory conditions. This species was chosen because it is the dominant copepod species in the Clyde sea area, is readily obtainable ill reasonable numbers at all times of the year, and has already been the subject of investigation of feeding rate. . FIELD OBSERVATIONS MethodsCalanus were obtained from la-min. hauls of 50-em. coarse tow-nets (26 meshes to the inch) taken at 4-hourly intervals through 24 hr. Three nets were fished at once, 25 fathoms apart on a trawl warp, to the end of which was attached a cable depressor (Barnes, 1951). Rigged in this way, the uppermost net fished 2-3 m., the second 35-40 m. and the lowest c. 75-80 m. from the surface. There was no means of closing the nets before hauling, but the percentage of the fishing time spent by the lower nets in. the upper strata' was small and unlikely to have interfered greatly with the results. The hauls were taken at a station just outside Tarbert, Loch Fyne. At this station deep water

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The respiratory rates of four species of planktonic copepods were measured. In two of the species measurements were made with two copepodite stages, and with adult male and female specimens.The rate for Calanus finmarchicus Stage V at 17°C. was found to be 0·52 μl./cop./hr., a figure which agreed with previous measurements.For all the species and stages studied the respiratory rate was found to be closely proportional to the length to the power of 2·2.The relevance of such measurements to conditions in the sea is briefly discussed.