no evidence to suggest that such transfer plays a significant role in osmoregulation.
In reptiles and the Amphibia, however, the urinary bladder may act as a useful
storage organ for water, and further conservation of urinary sodium may take place
by active transport from the urine back into the blood (see BENTLEY, 1966a).
CHARLES DARWIN in his account of the voyage of the 'Beagle' (1839) had the
following comments to make about the bladders of frogs and the tortoises of the
Galapagos.
"I believe it is well ascertained, that the bladder of the frog acts as a reservoir
for the moisture necessary to its existence; such seems to be the case with the tortoise. For some time after a visit to the springs, their urinary bladders are distended
with fluid, which is said to decrease gradually in volume and to become less pure.
The inhabitants (men) when walking in the lower district, and overcome with thirst,
often take advantage of this circumstance, and drink the contents of the bladder
if full; in one I saw killed, the fluid was quite limpid, and had only a very slightl y
bitter taste ."
Australian aborigines, who inhabit the arid interior regions of the continent,
on occasions drink water stored in the urinary bladders of desert frogs. The volume
of fluid held in the bladders of such amphibians may be equivalent to as much as
50% of the normal body weight of species such as Cyclorana platycephalus and
Notaden nichollsi, while in a turtle, Malaclemys centrata, I have found urine
amounting to 20% of the body weight stored in the bladder.
h) Salt Glands
Extrarenal salt excretion plays a major role in the osmoregulation of some species
of reptiles, birds and fishes. Apart from some epithelial cells in the gills of fish,
certain glandular tissues also may have this function. The role of such tissues in
birds and reptiles was first described by KNUT SCHMIDT-NIELSEN and his collaborators and has been admirably reviewed by him (SCHMIDT-NIELSEN, 1960 and
1965).
'Salt' glands are so named because of their remarkable ability to secrete solutions
containing high concentrations of sodium, potassium and chloride. The volumes
of such secretions may be large. SCHMIDT-NIELSEN, for instance, has shown that
the salt gland of the herring gull secretes at about twice the rate of the human kidney
on a unit body weight basis and 20 times the rate in relation to the relative weights
of the glandular tissues . The secretions are hyperosmotic to the body fluids and
in most instances are even more concentrated than sea-water. They thus afford a
channel for ion excretion which is economical to accompanying obligatory water
loss, and may even allow the extra ction of osmotically-free water from imbibed
sea-water (Table 1.4).
Several distinct types of glands may act as 'salt' glands. In the head , these may
open into the nasal cavity, termed nasal glands, or they may be modified orbital,
Harderian or lachrymal glands. Nasal salt glands have been described among the
birds and reptiles while orbital glands have this function in marine turtles. In the
chondrichthyean, Squalus acanthias, (the spiny dogfish), the rectal gland, which
opens into the distal part of the gut , functions as a salt gland (BURGER and HESS,
26
In reptiles and the Amphibia, however, the urinary bladder may act as a useful
storage organ for water, and further conservation of urinary sodium may take place
by active transport from the urine back into the blood (see BENTLEY, 1966a).
CHARLES DARWIN in his account of the voyage of the 'Beagle' (1839) had the
following comments to make about the bladders of frogs and the tortoises of the
Galapagos.
"I believe it is well ascertained, that the bladder of the frog acts as a reservoir
for the moisture necessary to its existence; such seems to be the case with the tortoise. For some time after a visit to the springs, their urinary bladders are distended
with fluid, which is said to decrease gradually in volume and to become less pure.
The inhabitants (men) when walking in the lower district, and overcome with thirst,
often take advantage of this circumstance, and drink the contents of the bladder
if full; in one I saw killed, the fluid was quite limpid, and had only a very slightl y
bitter taste ."
Australian aborigines, who inhabit the arid interior regions of the continent,
on occasions drink water stored in the urinary bladders of desert frogs. The volume
of fluid held in the bladders of such amphibians may be equivalent to as much as
50% of the normal body weight of species such as Cyclorana platycephalus and
Notaden nichollsi, while in a turtle, Malaclemys centrata, I have found urine
amounting to 20% of the body weight stored in the bladder.
h) Salt Glands
Extrarenal salt excretion plays a major role in the osmoregulation of some species
of reptiles, birds and fishes. Apart from some epithelial cells in the gills of fish,
certain glandular tissues also may have this function. The role of such tissues in
birds and reptiles was first described by KNUT SCHMIDT-NIELSEN and his collaborators and has been admirably reviewed by him (SCHMIDT-NIELSEN, 1960 and
1965).
'Salt' glands are so named because of their remarkable ability to secrete solutions
containing high concentrations of sodium, potassium and chloride. The volumes
of such secretions may be large. SCHMIDT-NIELSEN, for instance, has shown that
the salt gland of the herring gull secretes at about twice the rate of the human kidney
on a unit body weight basis and 20 times the rate in relation to the relative weights
of the glandular tissues . The secretions are hyperosmotic to the body fluids and
in most instances are even more concentrated than sea-water. They thus afford a
channel for ion excretion which is economical to accompanying obligatory water
loss, and may even allow the extra ction of osmotically-free water from imbibed
sea-water (Table 1.4).
Several distinct types of glands may act as 'salt' glands. In the head , these may
open into the nasal cavity, termed nasal glands, or they may be modified orbital,
Harderian or lachrymal glands. Nasal salt glands have been described among the
birds and reptiles while orbital glands have this function in marine turtles. In the
chondrichthyean, Squalus acanthias, (the spiny dogfish), the rectal gland, which
opens into the distal part of the gut , functions as a salt gland (BURGER and HESS,
26