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A method for the estimation of the amount of oxygen dissolved in water is described. The method requires only 1-2 c.c. of water and is accurate to 2%, even at low oxygen concentrations. The apparatus is portable.

1. The oxygen consumption of normal and gill-less nymphs of the mayflies Baetis sp., Cloeon dipterum and Ephemera vulgata has been measured at various oxygen concentrations. 2. It has been found that over the complete range of oxygen concentrations studied, the tracheal gills do not aid oxygen consumption in Baetis sp. In Cloeon dipterum, at all oxygen concentrations tested, no gaseous exchange takes place through the gills; at low oxygen concentrations, however, the gills function as an accessory respiratory mechanism in ventilating the respiratory surface of the body and so aid oxygen consumption. In Ephemera Vulgata the gills aid oxygen consumption even at high oxygen concentrations. In this species the gills may function both as true respiratory organs and as a ventilating mechanism. 3. It is shown that the differences in gill function can be related to the oxygen content of the habitat of each species.

Summary. Whereas in some genera of marine decapod crustaceans the scaphognathite and heart rates of species from English waters are no faster than the rates for other species of the same genera living in arctic waters, each measured at their natural temperatures, in other genera the rates for the English species are faster than those for the arctic species. The prawn Pandulus montagui attains a larger size in arctic than in English waters. Taken from the former habitat, it supports a lower maximum aquarium temperature than from the latter. The curves relating oxygen consumption to temperature for the prawns Pandalus borealis from arctic waters and P. montagui from Plymouth are parallel to the corresponding curves for the isolated muscles of these animals. This bears out the hypothesis that the greater oxygen consumption of warm water animals is due to a greater non‐locomotory metabolism. Larger individuals of Pandalus montagui have a slower heart rate than smaller individuals, but there is no such relation between their rates of oxygen consumption.

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