the isolated int estine of the freshwater teleost, Carassius auratus, (M. SMITH, 1964)
is much less than in the marine teleost, Cottus scorpius(HOUSE and GREEN, 1963).
It is also very interesting that the intestine of Japanese eels adapted to sea-water
has a much higher concentration of Na-K activated ATPase than the same species
which has been living in fresh water (OIDE, 1967). Hormones may be concerned
with such changes .
The cloaca of the reptile, Caiman sclerops, (BENTLEY and SCHMIDT-NIELSEN,
1965) and the domestic fowl (SKADHAUGE, 1967) is concerned with active transport
of sodium from the luminal side of the gut to the blood, but its role in osmoregulation is still somewhat enigmatical. In domestic fowl only 4 % of the
urinary sodium is reabsorbed from the gut of sodium-loaded birds, but this
proportion may be greater in normal animals. The sodium concentration of the
cloacal fluid of the caiman is reduced by about 50% after storage for three hours
in this region, suggesting a relativel y greater role for the cloaca in this species . KNUT
SCHMIDT-NIELSEN and his group (SCHMIDT-NIELSEN et al., 1963) have suggested
that the cloaca and the nasal ' salt ' gland in some reptiles and birds may act to gether
to conserve water. Thus sodium and accompanying water may be reabsorbed from
the cloaca, and the salt subsequently excreted as a very concentrated solution by
the nasal gland.
Fluid reabsorption (water and small solutes and ions) may occur from cloacal
solutions which have a similar crystalloid osmotic pressure to the blood plasma.
Such water absorption is not necessaril y linked to active sodium transport. In a
series of very ingenious experiments MURRISH and SCHMIDT-NIELSEN (1970) have
shown that differences in the colloid osmotic pressure across the cloaca of the desert
iguana, Dipsosaurus dorsalis, are sufficient to account for fluid reabsorption. The y
demonstrated the presence of such gradients (measured intracloacally as an equivalent hydrostatic pressure) and these could be increased by deh ydrating the lizards
or abolished by placing protein solutions in the cloaca . The magnitude of the
gradients in colloid osmotic pressure corresponds to the forces holding water in
cloacal pellets from the lizards.
Potassium is an important ion in osmoregulation and is absorbed from the alimentary tract. Little is known about the mechanism by which this takes place and
it is not clear whether it represents another example of active transport in this tissue
(USSING, 1960).
The role of the gut in osmoregulation is not as dramatically apparent as that
of the kidney and ' salt' glands (see later), but nevertheless usually is the site for
the initial processes involved in accumulation of water and salt.
g) Urinary Bladder
The urine in many species is stored for a period of time before being finally voided.
Some fishes have a urinary bladder which morphologically represents an expansion
of the mesonephric ducts. The typical tetrapod urinary bladder originates as a ven -
tral diverticulum of the cloaca. Such a bladder is present in most species with the
exception of the birds and some reptiles, like the snakes and crocodiles. Exchanges
of water and solutes can take place across the bladder, but in mammals there is
25
is much less than in the marine teleost, Cottus scorpius(HOUSE and GREEN, 1963).
It is also very interesting that the intestine of Japanese eels adapted to sea-water
has a much higher concentration of Na-K activated ATPase than the same species
which has been living in fresh water (OIDE, 1967). Hormones may be concerned
with such changes .
The cloaca of the reptile, Caiman sclerops, (BENTLEY and SCHMIDT-NIELSEN,
1965) and the domestic fowl (SKADHAUGE, 1967) is concerned with active transport
of sodium from the luminal side of the gut to the blood, but its role in osmoregulation is still somewhat enigmatical. In domestic fowl only 4 % of the
urinary sodium is reabsorbed from the gut of sodium-loaded birds, but this
proportion may be greater in normal animals. The sodium concentration of the
cloacal fluid of the caiman is reduced by about 50% after storage for three hours
in this region, suggesting a relativel y greater role for the cloaca in this species . KNUT
SCHMIDT-NIELSEN and his group (SCHMIDT-NIELSEN et al., 1963) have suggested
that the cloaca and the nasal ' salt ' gland in some reptiles and birds may act to gether
to conserve water. Thus sodium and accompanying water may be reabsorbed from
the cloaca, and the salt subsequently excreted as a very concentrated solution by
the nasal gland.
Fluid reabsorption (water and small solutes and ions) may occur from cloacal
solutions which have a similar crystalloid osmotic pressure to the blood plasma.
Such water absorption is not necessaril y linked to active sodium transport. In a
series of very ingenious experiments MURRISH and SCHMIDT-NIELSEN (1970) have
shown that differences in the colloid osmotic pressure across the cloaca of the desert
iguana, Dipsosaurus dorsalis, are sufficient to account for fluid reabsorption. The y
demonstrated the presence of such gradients (measured intracloacally as an equivalent hydrostatic pressure) and these could be increased by deh ydrating the lizards
or abolished by placing protein solutions in the cloaca . The magnitude of the
gradients in colloid osmotic pressure corresponds to the forces holding water in
cloacal pellets from the lizards.
Potassium is an important ion in osmoregulation and is absorbed from the alimentary tract. Little is known about the mechanism by which this takes place and
it is not clear whether it represents another example of active transport in this tissue
(USSING, 1960).
The role of the gut in osmoregulation is not as dramatically apparent as that
of the kidney and ' salt' glands (see later), but nevertheless usually is the site for
the initial processes involved in accumulation of water and salt.
g) Urinary Bladder
The urine in many species is stored for a period of time before being finally voided.
Some fishes have a urinary bladder which morphologically represents an expansion
of the mesonephric ducts. The typical tetrapod urinary bladder originates as a ven -
tral diverticulum of the cloaca. Such a bladder is present in most species with the
exception of the birds and some reptiles, like the snakes and crocodiles. Exchanges
of water and solutes can take place across the bladder, but in mammals there is
25