fl) Role of Cloaca and Urinary Bladder. Urine does not pass from the reptiles kidney directly to the outside of the animal, but is first accumulated in a cloaca and
in some species may pass from here into a urinary bladder. A urinary bladder is
absent in Crocodilia, Ophidia and some Lacertilia, but is present in many of the
latter, as well as in the Chelonia. As the urinary and faecal pellets in many reptiles
contain little water, its reabsorption presumably occurs across the cloaca and large
intestine, as in birds .
The Brazilian snake, Xenodon sp., suffers an increased rate of water loss when
the openings of its ureters are cannulated so as to allow water to by-pass the cloaca
(JUNQUEIRA et al., 1966). The increased fluid loss was equivalent to 2.6% of the
body weight in a day, and this may represent the amount which is normally reabsorbed in the region of the cloaca and large intestine of these snakes.
Direct measurements of fluid (water and sodium) absorption from the cloaca
of a large Australian lizard, Varanus gouldii, have been made (BRAYSHER and
GREEN, 1970). The coprodaeum of this lizard was isolated in vivo from the rest
of the cloaca with the aid of rubber bulbs. When isotonic saline was introduced
into this segment it was absorbed at the rate of 8 mllkg of the lizards body weight
each hour. This was increased to 21 mllkg h following the injection of vasotocin.
Sodium absorption was facilitated, but the relative roles of this and differences in
colloid osmotic pressure (MURRISH and SCHMIDT-NIELSEN, 1970) in promoting
the water movement are not clear. The large intestine of the tortoise, Testudo
graeca, is osmotically permeable to water in vitro, but high concentrations of vasotocin have no effect on water movement (BENTLEY, 1962 b) . The urinary bladder
of chelonians is also permeable to water and in Pseudemys scripta water movement
may take place both along an osmotic gradient and as an accompaniment to active
sodium transport from the mucosal to serosal surface (BRODSKY and SCHILB, 1965).
The permeability of the bladder to water is, in contrast to frogs and toads, unaffected by vasotocin (CANDIA, GONZALES, GENTILE, and BENTLEY, unpublished observations). The urinary bladder of the terrestrial tortoise, Testudo graeca, is also
osmotically permeable in vitro, and is also unaffected by vasotocin (BENTLEY, 1962
b). DANTZLER and SCHMIDT-NIELSEN (1966) examined the permeability of the
bladder in the desert tortoise, Gopherus agassizii, to water (in vivo), and found
that when water was introduced into the bladder, it was reabsorbed at the rate of
about 20 mllh in either hydrated or non-hydrated animals . The urinary bladder
of turtles may hold large amounts of fluid ; I have observed volumes equivalent
to 20 % of the body weight in the bladder of the diamondback terrapin, Malaclemys
centrata. Such stores of water could facilitate survival in circumstances in which
water supplies are restricted. As reptiles onl y lose water at a slow rate. rapid increases in the osmotic permeability of the bladder, like those mediated by vasotocin
in frogs and toads, are probably unnecessary, so that the absence of such an action
of this hormone is not surprising. Many lizards also store fluid in a urinary bladder
and an investigation of the functioning of this organ, particularly in species from
desert areas, would be interesting.
148
in some species may pass from here into a urinary bladder. A urinary bladder is
absent in Crocodilia, Ophidia and some Lacertilia, but is present in many of the
latter, as well as in the Chelonia. As the urinary and faecal pellets in many reptiles
contain little water, its reabsorption presumably occurs across the cloaca and large
intestine, as in birds .
The Brazilian snake, Xenodon sp., suffers an increased rate of water loss when
the openings of its ureters are cannulated so as to allow water to by-pass the cloaca
(JUNQUEIRA et al., 1966). The increased fluid loss was equivalent to 2.6% of the
body weight in a day, and this may represent the amount which is normally reabsorbed in the region of the cloaca and large intestine of these snakes.
Direct measurements of fluid (water and sodium) absorption from the cloaca
of a large Australian lizard, Varanus gouldii, have been made (BRAYSHER and
GREEN, 1970). The coprodaeum of this lizard was isolated in vivo from the rest
of the cloaca with the aid of rubber bulbs. When isotonic saline was introduced
into this segment it was absorbed at the rate of 8 mllkg of the lizards body weight
each hour. This was increased to 21 mllkg h following the injection of vasotocin.
Sodium absorption was facilitated, but the relative roles of this and differences in
colloid osmotic pressure (MURRISH and SCHMIDT-NIELSEN, 1970) in promoting
the water movement are not clear. The large intestine of the tortoise, Testudo
graeca, is osmotically permeable to water in vitro, but high concentrations of vasotocin have no effect on water movement (BENTLEY, 1962 b) . The urinary bladder
of chelonians is also permeable to water and in Pseudemys scripta water movement
may take place both along an osmotic gradient and as an accompaniment to active
sodium transport from the mucosal to serosal surface (BRODSKY and SCHILB, 1965).
The permeability of the bladder to water is, in contrast to frogs and toads, unaffected by vasotocin (CANDIA, GONZALES, GENTILE, and BENTLEY, unpublished observations). The urinary bladder of the terrestrial tortoise, Testudo graeca, is also
osmotically permeable in vitro, and is also unaffected by vasotocin (BENTLEY, 1962
b). DANTZLER and SCHMIDT-NIELSEN (1966) examined the permeability of the
bladder in the desert tortoise, Gopherus agassizii, to water (in vivo), and found
that when water was introduced into the bladder, it was reabsorbed at the rate of
about 20 mllh in either hydrated or non-hydrated animals . The urinary bladder
of turtles may hold large amounts of fluid ; I have observed volumes equivalent
to 20 % of the body weight in the bladder of the diamondback terrapin, Malaclemys
centrata. Such stores of water could facilitate survival in circumstances in which
water supplies are restricted. As reptiles onl y lose water at a slow rate. rapid increases in the osmotic permeability of the bladder, like those mediated by vasotocin
in frogs and toads, are probably unnecessary, so that the absence of such an action
of this hormone is not surprising. Many lizards also store fluid in a urinary bladder
and an investigation of the functioning of this organ, particularly in species from
desert areas, would be interesting.
148