ABSTRACT
Nothing is known of the osmotic concentration of the blood of fresh-water phyllopods. In the cladoceran Daphnia magna, Fritzsche (1917) obtained freezing-point depressions for blood varying between—0·2 and—0·67° C. Krogh (1939) did some preliminary experiments on Branchipus (.? grubii) and Apus (Lepidurus) productus and found that these animals can be kept alive without food only for a day or two, whether they are in tap water, distilled water or in Ringer/100. Regular loss of chloride occurred in all experiments, which indicated the absence of active regulation and the necessity of food to make good the constant loss of salts. Artemia, which is closely related to Branchipus, inhabits concentrated salines up to about 35 % salt and shows highly developed powers of regulation involving ability to maintain hypotonicity to the external medium. By Barger’s method, Medwedewa (1927) and Kuenen (1939) found that the osmotic pressure of its blood varied only between 1 · 2 and 2 · 6% NaCl for a change in the external medium from 4 · 7 to 17 · 7% NaCl. Chirocephalus, which belongs to the same tribe of phyllopods as Artemia and Branchipus, is an essentially fresh-water form, though by frequent drying and flooding the pools in which it lives may accumulate slight quantities of salt. Weldon (1909) found them alive in an aquarium where, by evaporation, the chloride content of water had risen up to 0 · 019% NaCl. The following experiments were performed in order to study the osmotic properties of Chirocephalus when a limited supply was available in the summer of 1940.
