(GARCIA ROMEU and MAETZ, 1964; MAETZ and GARCIA ROMEU, 1964). It is not
clear what proportions of the total ion transfers are coupled in this manner.
Sodium transfer across the gills of freshwater chondrichthyeans has not been investigated .
Output. KEYS in 1931 showed that the perfused gills of the eel could secrete
chloride into an external solution of sea-water. Active sodium extrusion has also
been demonstrated in the isolated gills of these eels (BELLAMY, 1961). Experiments
on intact teleost fish also show that this commonly occurs in marine species. An
active secretion of sodium or chloride has not been shown among the Chondrichthyes though, as shown by MAETZ and LAHLOU (1966) and suggested by BURGER and TOSTESON (1966), considerable efflux of sodium and chloride takes place
from the gills of dogfish. Whether or not this involves active transport is unknown.
Extrarenal salt excretion through the gills of the marine myxinoid agnathans is not
considered likely (McFARLAND and MUNZ, 1965; MORRIS, 1965) but has not been
adequately investigated.
The detailed mechanisms of active ion transport across the gill epithelia of fish
are not known, but an overall common denominator of this process in vertebrates,
the enzyme N a-K activated ATPase, has been found in the gills of the killifish,
Fundulus heteroclitus (EpSTEIN et al., 1967) and eels (UTIDA et al., 1966; MOTAIS,
1970). The activity of this enzyme in these fish was also shown to increase when
the y were adapted to sea-water, a situation in which they may be expected to secrete
more sodium. The cellular site for ion transport in fish gills was suggested by KEYS
and WILLMER (1932) to be a specialized acidophil cell, termed a 'chloride secreting
cell', which is rich in mitochondria (MORRIS, 1957). The evidence for such a site
is equivocal and contentious (see PARRY, 1966), but nevertheless may be correct .
The ro le of the gills in osmoregulation of larval and aquatic Amphibia is not
known. In the mudpuppy, Necturus maculosus, the external gills are not an important avenue for ammonia excretion (FANELLI and GOLDSTEIN, 1964). The tadpoles of the marine frog, Rana cancrivora, excrete salt extrarenally, and circumstantial evidence suggests that the gills may be involved (GORDON and TUCKER,
1965).
f) Gut
The gut is the somewhat fastidious barrier between th e dietary food and water and
the body fluids. Additional endogenous fluids are continually secreted into the gut
as bile, the secretions of salivary glands, exocrine pancreas and the glands in the
wall of the alimentary tract. Intake is largel y related to ' hunger' and ' app etite' . The
secretions are also largely related to the digestive requirements of the animal. The
volume of such secretions is considerable ; a man in one day secretes into his digestive tract 1600 to 9700 ml. of water, 650 to 1040 m-moles of sodium and 50
to 82 m-moles of potassium (KRUHOFFER and THAYSEN, 1960). The gut regulates
the composition of the body fluids with the aid of thirst and 'salt' appetites while
also conserving water and solutes by reabsorbing the secretions. In reptiles, birds
and mammals the gut represents the only avenue for accumulation of water and
salts , but in the Amphibia and fishes these may also be taken up through the skin
and gills.
23
clear what proportions of the total ion transfers are coupled in this manner.
Sodium transfer across the gills of freshwater chondrichthyeans has not been investigated .
Output. KEYS in 1931 showed that the perfused gills of the eel could secrete
chloride into an external solution of sea-water. Active sodium extrusion has also
been demonstrated in the isolated gills of these eels (BELLAMY, 1961). Experiments
on intact teleost fish also show that this commonly occurs in marine species. An
active secretion of sodium or chloride has not been shown among the Chondrichthyes though, as shown by MAETZ and LAHLOU (1966) and suggested by BURGER and TOSTESON (1966), considerable efflux of sodium and chloride takes place
from the gills of dogfish. Whether or not this involves active transport is unknown.
Extrarenal salt excretion through the gills of the marine myxinoid agnathans is not
considered likely (McFARLAND and MUNZ, 1965; MORRIS, 1965) but has not been
adequately investigated.
The detailed mechanisms of active ion transport across the gill epithelia of fish
are not known, but an overall common denominator of this process in vertebrates,
the enzyme N a-K activated ATPase, has been found in the gills of the killifish,
Fundulus heteroclitus (EpSTEIN et al., 1967) and eels (UTIDA et al., 1966; MOTAIS,
1970). The activity of this enzyme in these fish was also shown to increase when
the y were adapted to sea-water, a situation in which they may be expected to secrete
more sodium. The cellular site for ion transport in fish gills was suggested by KEYS
and WILLMER (1932) to be a specialized acidophil cell, termed a 'chloride secreting
cell', which is rich in mitochondria (MORRIS, 1957). The evidence for such a site
is equivocal and contentious (see PARRY, 1966), but nevertheless may be correct .
The ro le of the gills in osmoregulation of larval and aquatic Amphibia is not
known. In the mudpuppy, Necturus maculosus, the external gills are not an important avenue for ammonia excretion (FANELLI and GOLDSTEIN, 1964). The tadpoles of the marine frog, Rana cancrivora, excrete salt extrarenally, and circumstantial evidence suggests that the gills may be involved (GORDON and TUCKER,
1965).
f) Gut
The gut is the somewhat fastidious barrier between th e dietary food and water and
the body fluids. Additional endogenous fluids are continually secreted into the gut
as bile, the secretions of salivary glands, exocrine pancreas and the glands in the
wall of the alimentary tract. Intake is largel y related to ' hunger' and ' app etite' . The
secretions are also largely related to the digestive requirements of the animal. The
volume of such secretions is considerable ; a man in one day secretes into his digestive tract 1600 to 9700 ml. of water, 650 to 1040 m-moles of sodium and 50
to 82 m-moles of potassium (KRUHOFFER and THAYSEN, 1960). The gut regulates
the composition of the body fluids with the aid of thirst and 'salt' appetites while
also conserving water and solutes by reabsorbing the secretions. In reptiles, birds
and mammals the gut represents the only avenue for accumulation of water and
salts , but in the Amphibia and fishes these may also be taken up through the skin
and gills.
23