The Evaporation of Water from Helix Aspersa

It is known that the activity of terrestrial pulmonates is connected in some way to the extensive and often rapid fluctuations in water content which are characteristic of these animals (Howes & Wells, 1934; Wells, 1944). The work of Hughes & Kerkut (1956) and Kerkut & Taylor (1956) has further shown that the activity of the pedal ganglion of slugs changes with the osmotic pressure of the bathing solution. The experiments described in this series of papers are concerned with evaporative water loss, which can cause marked changes in blood concentration. The common garden snail, Helix aspersa, was chosen as the experimental animal. Although important advances have been made in the field of evaporation from arthropods by Beament (1958,1959,1961), detailed studies of evaporation from moistskinned animals have apparently never been made. Strictly speaking the term 'evaporation' describes two processes, evasion and diffusion. Evasion refers to the vapourization of molecules at the evaporating surface and diffusion to their movement through the adjacent layers of air (Ramsay, 1935). The work presented here is relevant to the vapourization of water molecules at the outer surface of the snail. Since the skin is covered by a thin layer of mucus, it is necessary to consider the properties of this mucous layer and the means by which it is maintained.