Some years ago it was shown that adult limpets, Patella vulgata (L.), could give a graded response to water of diminished salinity, the precise extent of which varied according to the tide level from which the individual was drawn (Arnold, 1957). This response, an upward movement of the front edge of the shell, accompanied by protrusion of the cephalic and mantle tentacles, was elicited by dilutions of sea water with a salinity as low as 17‰ in the case of limpets living near H.W.N.T., but was not shown to salinities below about 27‰ by those settled near L.W.S.T. Such a difference in tolerances and behaviour could be expected to play an important part in the lives of intertidal organisms, especially those of a sessile or sedentary habit, since it would permit populations inhabiting the higher levels of the shore to maintain activity despite the generally disadvantageous nature of prolonged exposure to the air. Moreover, those species capable of such adaptations might, under estuarine conditions, evolve physiologically different populations specially fitted for life under conditions of low salinity. Such studies are thus of value not only in regard to the biology of the particular species concerned, but also for fuller understanding of the general conditions of intertidal and estuarine life and their practicability as routes toward establishment of a terrestrial mode of life. Subsequent studies have revealed the salinity tolerances of a number of intertidal organisms, including those of the larvae of both the commoner barnacles and several families of polychaetes (Bhatnagar & Crisp, 1965; Lyster, 1965). Investigation with media of various compositions suggests that in both limpets and barnacles the response is essentially to the presence of chloride ion, modulated by the presence of calcium ion (Arnold, 1959; Barnes & Barnes, 1958).