marine chondrichthyeans and probably myxinoid agnathans. Precise measurements of the osmotic permeability of the gills are difficult, as their complete isolation from the rest of the body surface is onerous in vivo. Nevertheless, experiments with perfused and isolated gills indicate that they are permeable to water.
KROGH in 1939 made a careful assessment of the evidence available, and concluded that in freshwater teleost fish a major part of the osmotic water uptake
occurs through the gill epithelium. More recent experiments on the gills of the eel
(BELLAMY, 1961; MOTAlS et al., 1969) indicate that they can readily take up water
osmotically from hypoosmotic solutions. Furthermore, experiments on the perfused gills of a marine chondrichthyean, Squalus acanthias, show that they take
up water osmotically, but are 10 to 15 times less permeable than membranes like
the anuran skin and urinary bladder (BOYLAN, 1967). The overall contribution
of the gills to osmotic water uptake by fish probably varies due to differences both
in the permeability of the gills of various species and also in the gill surface area
in relation to the rest of the body surface . In this latter respect, the gill area in some
species has been calculated to be 60 times as great as that of the skin (see PARRY,
1966). Calculations made from experiments on isolated skin and the total water uptake of the agnathan, Lampetra fluviatilis, indicate that the gills and the skin have
a similar osmotic permeability; about 2.5 .ullcm2h (osmotic gradient 200 mOsm)
(BENTLEY, 1962a), compared to 3 to 30.ullcm 2h in amphibian skin . The contribution
of the gills of fishes to their osmotic water uptake, is a substantial one.
Osmotic water loss across the gills is also difficult to determine concisely but
SCHLIEPER (see KROGH, 1939) suggested that the perfused eel gill bathed with seawater has a very low permeability to water. MoTAlS et al. (1969), using a cannulated
gill preparation in eels found it to be less osmotically permeable in sea-water than
fresh water. Substantial amounts of water nevertheless leave the fish by this route.
Both BELLAMY (1961) and KAMlYA (1967) concluded from experiments on isolated gills of the eel, that sodium transfer by diffusion takes place readily in either
direction; earlier experiments on intact teleost fish are consistent with this (KROGH,
1939). Sodium and urea diffuse across the perfused gills of the marine
chondrichthyean, Squalus acanthias , but the permeability is very restricted compared to amphibian membranes (BOYLAN, 1967). Recent experiments on intactfish
support these conclusions. BURGER and TOSTESON (1966) measured the influx of
sod ium through the anterior end of the spiny dogfish, Squalus ; their results indicate
that sodium is accumulated by diffusion through this channel. The rate of sodium
influx in the dogfish, Scyliorhinus caniculus, is similar to Squalus (MAETZ and LAHLOU, 1966) but 30 to 50 times less than in marine teleosts (MOTAlS and MAETZ,
1965).
In summary : it appears that water and sodium chloride move by diffusion
across the gills of fish and that this occurs far more rapidly in marine teleosts than
chondrichthyeans.
ii. Active transport of sodium and chloride . Uptake . Sodium and chloride may
be accumulated against their concentration gradients by the gills of the agnathan
Lampetra fluviatilis (WlLKGREN, 1953), by a variety of freshwater teleosrs, and possibly by the freshwater chondrichthyean, Pristis microdon (KROGH, 1939). In the
goldfish, Carassius auratus, the transport of sodium and chloride may be independent, and takes place in exchange for ammonium and bicarbonate ions respectively
22
KROGH in 1939 made a careful assessment of the evidence available, and concluded that in freshwater teleost fish a major part of the osmotic water uptake
occurs through the gill epithelium. More recent experiments on the gills of the eel
(BELLAMY, 1961; MOTAlS et al., 1969) indicate that they can readily take up water
osmotically from hypoosmotic solutions. Furthermore, experiments on the perfused gills of a marine chondrichthyean, Squalus acanthias, show that they take
up water osmotically, but are 10 to 15 times less permeable than membranes like
the anuran skin and urinary bladder (BOYLAN, 1967). The overall contribution
of the gills to osmotic water uptake by fish probably varies due to differences both
in the permeability of the gills of various species and also in the gill surface area
in relation to the rest of the body surface . In this latter respect, the gill area in some
species has been calculated to be 60 times as great as that of the skin (see PARRY,
1966). Calculations made from experiments on isolated skin and the total water uptake of the agnathan, Lampetra fluviatilis, indicate that the gills and the skin have
a similar osmotic permeability; about 2.5 .ullcm2h (osmotic gradient 200 mOsm)
(BENTLEY, 1962a), compared to 3 to 30.ullcm 2h in amphibian skin . The contribution
of the gills of fishes to their osmotic water uptake, is a substantial one.
Osmotic water loss across the gills is also difficult to determine concisely but
SCHLIEPER (see KROGH, 1939) suggested that the perfused eel gill bathed with seawater has a very low permeability to water. MoTAlS et al. (1969), using a cannulated
gill preparation in eels found it to be less osmotically permeable in sea-water than
fresh water. Substantial amounts of water nevertheless leave the fish by this route.
Both BELLAMY (1961) and KAMlYA (1967) concluded from experiments on isolated gills of the eel, that sodium transfer by diffusion takes place readily in either
direction; earlier experiments on intact teleost fish are consistent with this (KROGH,
1939). Sodium and urea diffuse across the perfused gills of the marine
chondrichthyean, Squalus acanthias , but the permeability is very restricted compared to amphibian membranes (BOYLAN, 1967). Recent experiments on intactfish
support these conclusions. BURGER and TOSTESON (1966) measured the influx of
sod ium through the anterior end of the spiny dogfish, Squalus ; their results indicate
that sodium is accumulated by diffusion through this channel. The rate of sodium
influx in the dogfish, Scyliorhinus caniculus, is similar to Squalus (MAETZ and LAHLOU, 1966) but 30 to 50 times less than in marine teleosts (MOTAlS and MAETZ,
1965).
In summary : it appears that water and sodium chloride move by diffusion
across the gills of fish and that this occurs far more rapidly in marine teleosts than
chondrichthyeans.
ii. Active transport of sodium and chloride . Uptake . Sodium and chloride may
be accumulated against their concentration gradients by the gills of the agnathan
Lampetra fluviatilis (WlLKGREN, 1953), by a variety of freshwater teleosrs, and possibly by the freshwater chondrichthyean, Pristis microdon (KROGH, 1939). In the
goldfish, Carassius auratus, the transport of sodium and chloride may be independent, and takes place in exchange for ammonium and bicarbonate ions respectively
22