The dimensions of fish gills in relation to their function.

It is known from the extensive measurements of Gray (1954) that the gills of marine fishes of the Atlantic Coast of North America show a wide variation in the extent of their gill area. This is true whether the area is based on the unit of body surface area or on a unit of body weight. Little data are available, however, for fishes from European waters (Byczkowska-Smyk,* 1957, 1958, 1959)The present investigation was undertaken because earlier work (Hughes & Shelton, 1958) suggested that the nature of the gill resistance is an important variable in the ventilating mechanism. Therefore, as part of a comparative study of the respiratory mechanism c. marine fishes (Hughes, 1960) the opportunity was taken to make certain measurements of the gills which might give some indication of the variations in gill resistance of the species studied. From these measurements it was also possible to calculate the gill area. Because of the extremely fine dimensions of the gill sieve some have doubted whether the respiratory current flowed between the secondary lamellae. This is especially relevant as the mean differential pressure across the gills of many fish has been shown to be under 0-5 cm. water (Hughes, i960). In addition to the British marine fishes studied, data are also presented from two freshwater fishes and from the gills of two specimens of the Antarctic icefish, Chaenocephalus, which is of great interest because haemoglobin is absent from its blood. Consequently its oxygen-carrying capacity is only 0-67 vol. %, whereas the blood of other fish in these regions have O2 capacities of 6-7 vol. % (Ruud, 1954). Some of the data have already been published (Hughes & Shelton, 1962).