Up to the present very few authors have directed their attention to factors influencing the emergence of larval trematodes from their snail hosts. Cort (1922) has made a study of the escape of cercariae from their hosts and the writer (1931) has examined the effect of light and temperature on the emergence of three species of larval trematodes from three species of fresh-water molluscs. More recently, Giovannola (1936) has investigated the effect of a reversal of light and darkness on the periodicity of emission of cercariae from three species of host snails. All these studies have been made on freshwater molluscs. In the case of molluscs living between tide-marks on the shore, in addition to variation in temperature there exists another variable factor, namely, the salinity of the water. Many rock-pools situated in the intertidal zone are small and will therefore be subject to considerable variations in temperature and salinity at different times of the year and even for short periods of up to 12 hr. during individual days. In the summer months the temperature of the water in these pools may rise to a high degree and the high temperature would be maintained for a longer period near high-water mark than near low-water mark, where the period of exposure is less. Similarly, the decrease in temperature would be very marked during the cold winter months. As the water in the exposed pools evaporates the salinity increases and high up on the shore, between successive tides on hot summer days, it may become twice that of normal sea water. The pools are also exposed to rain and in the same period the salinity of the water may decrease considerably if the precipitation is heavy. The light factor would be the same for molluscs living in fresh and in salt water, but the reaction of various species of larval trematodes to the presence or absence of light varies. Some emerge from their hosts in larger numbers during the daytime, others during the night and some indiscriminately during periods of light and darkness.