NEREIS CULTRIFERA)

NUMEROUS studies have been published on the extent to which aquatic animals can maintain internal environments differing in osmotic pressure from their surround-ings. In the case of polychaete worms, it has been shown that some species, especially Nereis diversicolor, can maintain a considerable degree of hypertonicity, even in very dilute brackish water, while in others, such as Arenicola marina., the internal and external fluids are always isotonic (Schlieper, 1929; Beadle, 1937; Zenkewitch, 1938). These facts have been correlated with the extent to which the various species can invade brackish water. In one respect, however, our know-ledge is at present very deficient. Remarkably little—in the case of polychaete worms, nothing—is known about the extent to which the tissues of animals can tolerate dilution of their bathing media. Information on this question is evidently necessary if the significance of the body-fluid data is to be properly understood. The writers were studying the action of ions on an isolated rhythmic preparation from A. marina, and, for the reasons stated above, thought it worth while to make some additional experiments on the behaviour of the preparations in hypotonic, but balanced, solutions. Comparative experiments were also undertaken on prepara-tions from two other species: Nereis diversicolor, because of its power of osmotic regulation, and Perinereis cultrifera, a species closely related to the last, but much less able to survive under brackish conditions. The results are described below, and discussed as they arise, with reference to the relations between the different species and their environments. MATERIAL AND METHODS The "isolated extrovert " of Arenicola marina has already been fully described (Wells, 1937; Wells & Ledingham, 1940). It consists of the proboscis with a certain amount of oesophageal tissue attached, and gives a characteristic behaviour pattern, consisting of alternating periods of vigorous rhythmic activity and rest