Summary 1. The general structure of the gills of different fishes is compared and it is concluded that, though essentially the same, there are certain differences by which they can be recognized. Possible ways in which they may have evolved from one another are considered. 2. A detailed account is given of the structure of the secondary lamellae, where gaseous exchange takes place, and it is shown that two epithelial sheets are separated by a vascular axis mainly composed of pillar cells overlain by a basement membrane on each side. Blood pathways through the gills are discussed in relation to their respiratory function. 3. The embryonic development of gills is described and evidence regarding homo‐logies of different structures, particularly the pillar cells, is reviewed. 4. The gills of fish having different modes of life show variations in (a) the number of arches, (b) the number and length of the gill filaments, and (c) the size and frequency of the secondary lamellae. Ways in which measurements of gill area may be carried out and some of the complications involved are reviewed and a summary given of measurements made for a wide variety of species. Measurements of the thickness of the water‐blood barrier are also discussed; the more active fish generally have thinner water‐blood barriers and larger gill areas. 5. The different mechanisms of gill ventilation are summarized and characteristics of gill resistance in elasmobranchs and teleosts are compared. Gas exchange is discussed in relation to available techniques and the current terminology and symbols, and to indicate the value of analogies between gill exchangers and systems studied by engineers. 6. It is outlined how studies of the functioning of gills during coughing, parasitic infection, and in polluted waters add to knowledge of their role in respiration.

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Rodlet cells in various stages of development were found in large numbers in the bass gill and pseudobranch. In the gill, rodlet cells were found in the epithelium at the base of the secondary lamellae and on the filament between adjacent lamellae, whilst in the pseudobranch they were found over the whole area of the secondary lamellae as well as in the filament epithelium.During early development, rodlet cells are characterised by their amorphous cell inclusions, prominent supranuclear Golgi complex and network of granular endoplasmic reticulum. Later, with formation of a fibrous border the arrangement of the cell organelles undergoes reorganisation; the endoplasmic reticulum becomes distended, numerous vesicles appear and the mitochondria aggregate in the apical region of the cell. One of the most striking features is the development of club-shaped sacs containing electron dense cores, which are orientated towards the open apex of the cell.Various staining properties of rodlet cells for light and electron microscopy were compared with those of mucous cells found in the same tissues. Possible functions of the cell are discussed.

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