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.

No abstract

No abstract

An account is given of a commonly accepted method for measurement of the gill area of fishes. A number of practical hints are included together with some warning about possible sources of error. In particular, these include sampling methods and reduction of errors due to shrinkage. Emphasis is given to the importance of weighting in the calculation of gill area. Problems which arise in comparisons of gills from different species are discussed and attention is drawn to possible errors that can be introduced by the reduction of all data to a given body weight offish. Such dangers are especially present where comparisons are made between fishes which may differ in weight by as much as four orders of magnitude.

No abstract