Recently the work of Collander (1949), ofJacobs (1952) and of Ussing and Zeuthen (this symposium) has shown that very small molecules, such as methyl alcohol, Koch, H.J. (1938). The absorption of chloride ions by the anal papillae of Diptera larvae. J. exp. Biol. 15, 156-160. Koch, H.J. (1954). L'intervention de cholinesterases dans l'absorption et le transport actif de matieres minerales par les branchies du crabe, Eriocheir sinensis