When littoral animals are exposed by the receding tide they are subjected to the environmental factors of what is essentially a terrestrial environment. Of these factors desiccation (see Davies, 1969) and temperature are of paramount importance. In winter the animals may be subject to a rapid change from the relatively high temperature of the sea to a very much lower air temperature. In summertime the opposite is true and the animals will spend the dry phase in air temperatures often far in excess of sea-water temperatures. The most important temperatures from an ecological point of view, however, are the body temperatures of the animals themselves. As shown by Southward (1958) this cannot be deduced from measurements of air temperatures, since the animals are subject to heating by absorption of solar radiant energy and this in turn may be mitigated by other environmental factors.