AQUATIC AND AERIAL RESPIRATION IN ANIMALS

Summary 1. The physical conditions governing respiratory exchange across epithelia exposed to water and air are considered, and the efficiency of aerial and aquatic types of respiration are compared. The effects of the known differences in the dissociation curves of the various respiratory pigments which occur in animals are discussed. 2. The changes which have taken place in the respiratory organs of animals possessing aerial respiration, but belonging to groups of which the majority of the species are aquatic, are reviewed. 3. From the previous discussion of the physical conditions of respiration, it is concluded that respiration in water should be more efficient than in air, if the structure of the respiratory epithelia are similar. It is found that a comparison between the area of respiratory epithelium in organs of aerial and aquatic respiration in the vertebrates supports this conclusion. 4. The suggestion is made that the relative ease with which invertebrates, such as the Molluscs and Crustaceans, can pass from water to land as compared with the vertebrates is due, at least in part, to the value of the tension of loading of haemocyanin, which is lower than that of the types of haemoglobin contained in corpuscles, which are found in the organs of aerial respiration of the vertebrates. 5. The apparently greater efficiency of some forms of cutaneous respiration in air than in water is considered, and is found to be due to secondary factors. It is suggested that one condition which has permitted the large size of some animals, possessing no restricted organ of respiration, such as some of the Oligochaetes, is the presence of haemoglobin in solution. The tension of loading of this pigment is much lower than that of any other respiratory pigment. 6. The reason why the majority of terrestrial animals have reached the land through the fresh waters rather than directly from the sea is discussed. The suggestion is made that this has been partly caused by the favourable series of inter‐mediate environments between the fresh waters and the land, and that the evolution of many of the numerous adaptations necessary for terrestrial life has been induced in these environments serially, and therefore with much greater ease. The much greater variability of the fresh‐water environments has also undoubtedly been important in producing this result. The importance of the tide in maintaining the constancy of conditions on the marine littoral is emphasised.