Animals from California (Marina) and from Italy (Cagliari, Sardinia) were reared in the laboratory from eggs, and fed on a unicellular alga Dunaliella viridis Teod., which abounds in most natural brines in which Artemia occurs. In the course of the main experiments (described in Chapter III-V) several observations and small experiments on other subjects from the biology of the animal were made, and the results of these are laid down in Chapter II. In 1910 DADAY brought all animals living in salt water from the genus Artemia, which had hitherto been described, under one species: Artemia salina (L.). From a study of the data now available in literature and from hybridising experiments, which are here described, the conclusion is drawn that there are at least two distinct species in the genus. The animals from America then have to beare the name Artemia gracilis Verrill, but as further experiments may show further differentiation within this species the solution can only be considered as preliminary. Artemia lives constantly in a medium which has a higher osmotic pressure than the blood. Consequently the osmoregulatory activity will necessarily reduce the concentration of the blood. The refractive index of the blood was determined as a function of the concentration of the brine (Table I, fig. 6). The changes in the osmotic pressure of the blood were determined as a function of the concentration of the brine (fig. 7). Both these series of determinations showed an increase of the concentration of the blood with that of the brine, but to a much smaller extent. The amounts of water excreted by the animals when transferred to brines of higher concentration were measured. The change in size of the animals was measured directly when they had been transferred to brines of different concentration (Table II, fig. 8). When comparing the results of these different methods it appeared that the amounts of water excreted from the body were less than the amounts necessary to increase the concentration of the brine to the measured level (and vice versa). This leads to the conclusion that water can be stored in the animal. The intestine may act as such a storage chamber. This follows from the following observations. The rate of defecation is reduced after the animal is transferred to a different concentration. The diameter of the intestine can be shown to change considerably. Crystals have been shown to occur in the intestine sometimes, which also indicates an activity in the water regulation. Finally the histology showed a marked activity of the wall of the intestine after transfer of the animals to other concentrations. When aquatic animals are transferred to media of different concentration, a marked change in the rate of respiration can often be shown. This may, according to different theories, be either due to the change in the water content of the tissues, or to a change in the intensity of the osmoregulatory activity. Oxygen consumption was measured in brines of ½, 1 and 2 molar NaCl and it was shown that it was greater in the higher concentrations. This shows that the osmoregulation influences the rate of respiration to a higher degree than the water content of the tissues, as the first increases in higher concentrations while the last becomes less.