The slug, Arion ater, at all times, and the snail, Helix pomatia, when fully extended, maintain a body temperature well below that of the surrounding air unless it is fully saturated, and slightly, if at all, above that of the wet-bulb thermometer. By withdrawal into the microclimate of the shell the snail can appreciably reduce loss of water by evaporation; and in such circumstances its body temperature tallies more nearly with that of the surrounding atmosphere. After the formation of the epiphragm the body temperature of H. pomatia is identical with that of the atmosphere outside and varies accordingly. Since the slime of the slug loses water in air unless the R.H. is very near saturation point the water-binding power of the mucus is not an effective check to loss of water by evaporation. The body temperature of an earthworm after relatively short periods of exposure to fairly dry air diverges increasingly from the wet-bulb reading. This appears to be due to rapid desiccation of the surface. Since the upper thermal death-point of the earthworm is relatively low, this means that earthworms are not adapted to long survival at ground level in sunlight. To this extent their equipment for maintaining body temperature below the danger point accords both with their habits, and with what views may plausibly be entertained about their ancestry.