Morphologically the gut may be a simple tubular structure, as in the Agnatha,
or it ma y be anatomically divided into functional regions such as the stomach and
small and large intestine (or colon). A cloaca into which the urinogenital ducts also
open may be present. The small intestine, colon and poss ibly the cloaca are th e
segments from which the water and salts are mainly absorbed.
Drinking, associated with 'thirst', is a common animal habit but is not universally apparent in all verte brates. Most reptiles , birds and mamm als drink, though
even in thes e groups, as in marine mammals and some desert species , there are exceptions. Animals that eat very succulent food may also not require water from
drinking. The Amphibia take up water osmotically through their skin and even
when dried out do not drink, though the y ma y do so when placed in hyperosmotic
salt solutions (KROGH, 1939; BENTLEY and SCHMIDT-NIELSEN, unpublished observations). Freshwater teleost fish and marine chondrichthyeans do not drink, but
marine teleosts are dependent on this custom and die of deh ydration if prevented
from doing so. The sea-water that these teleosts drink is hyperosmotic to their body
fluids, but they excrete most of the salt extrarenally by way of the gills (H. SMITH,
1930b; 1931; KEYS, 1931).
The volume of sea-water imbibed varies, depending on such factors as body
size and gill surface; in the eel HOMER SMITH found drinking to be about 40 mllkg
body weight in a day, while in the flounder, Piatich thys fle sus, it is about 240 mllkg
in the same period (MOTAIS and MAETZ, 1965). The latter investigators found that
th e sodium uptake through the gut of the flounder amounted to about 144 mequi v/kg day, which represents less than on e-quarter of the total sodium excreted
through the gills in that time. The marine myxinoids, Eptatretus stouti and Myxine glutinosa, also swallow substant ial amounts of sea-water, but this do es not
appear to be appreciably absorbed (McFARLAND and MUNZ, 1965; MORRIS, 1965).
Ingested water and solutes are principally absorbed from the intestinal region
of the gut. Water and man y solutes may move down their diffusion gradients in
either direction across th e intestinal wall but sodium, and probably chloride, can
be transported actively across the intestine from the lumen to the blood. Water
can also move against an osmotic concentration gradient but this is linked to the
transport of sodium and probably results from local osmotic and hydrostatic
gradients set up in the gut wall (CURRAN, 1965). While a detailed knowledge of
such processes has been mainly obtained on mammals, sodium transport has been
indicated in the intestine of the Amphibia(UssING and ANDERSEN,1955), reptiles
(BAILLIEN and SCHOFFENIELS, 1961) and marine and freshwater teleosts (HOUSE
and GREEN, 1963; M. SMITH, 1964).
While in most vertebrates sodium absorption from the intestine is essential for
the maintenance of a positive sodium balance, in the marine teleosts it is primarily
related to th e animals' water balance. HOMER SMITH (1930b) found that while monovalent ions, like sodium and chloride, were readily absorbed from the intestine
of marine teleosts, considerable amount s of magnesium, calcium and sulphate remained and were excreted in the faeces.
Differences between the permeability of the intestines of freshwater and marine
teleosts to water and sodium have been observed. The rates of passive water mo vement are greater in eels from sea-water than those adapted to fresh water (SHARRAIT
et al., 1964b; UTIDA, ISONO, and HIRANO, 1967). The rate of sodium transfer across
24
or it ma y be anatomically divided into functional regions such as the stomach and
small and large intestine (or colon). A cloaca into which the urinogenital ducts also
open may be present. The small intestine, colon and poss ibly the cloaca are th e
segments from which the water and salts are mainly absorbed.
Drinking, associated with 'thirst', is a common animal habit but is not universally apparent in all verte brates. Most reptiles , birds and mamm als drink, though
even in thes e groups, as in marine mammals and some desert species , there are exceptions. Animals that eat very succulent food may also not require water from
drinking. The Amphibia take up water osmotically through their skin and even
when dried out do not drink, though the y ma y do so when placed in hyperosmotic
salt solutions (KROGH, 1939; BENTLEY and SCHMIDT-NIELSEN, unpublished observations). Freshwater teleost fish and marine chondrichthyeans do not drink, but
marine teleosts are dependent on this custom and die of deh ydration if prevented
from doing so. The sea-water that these teleosts drink is hyperosmotic to their body
fluids, but they excrete most of the salt extrarenally by way of the gills (H. SMITH,
1930b; 1931; KEYS, 1931).
The volume of sea-water imbibed varies, depending on such factors as body
size and gill surface; in the eel HOMER SMITH found drinking to be about 40 mllkg
body weight in a day, while in the flounder, Piatich thys fle sus, it is about 240 mllkg
in the same period (MOTAIS and MAETZ, 1965). The latter investigators found that
th e sodium uptake through the gut of the flounder amounted to about 144 mequi v/kg day, which represents less than on e-quarter of the total sodium excreted
through the gills in that time. The marine myxinoids, Eptatretus stouti and Myxine glutinosa, also swallow substant ial amounts of sea-water, but this do es not
appear to be appreciably absorbed (McFARLAND and MUNZ, 1965; MORRIS, 1965).
Ingested water and solutes are principally absorbed from the intestinal region
of the gut. Water and man y solutes may move down their diffusion gradients in
either direction across th e intestinal wall but sodium, and probably chloride, can
be transported actively across the intestine from the lumen to the blood. Water
can also move against an osmotic concentration gradient but this is linked to the
transport of sodium and probably results from local osmotic and hydrostatic
gradients set up in the gut wall (CURRAN, 1965). While a detailed knowledge of
such processes has been mainly obtained on mammals, sodium transport has been
indicated in the intestine of the Amphibia(UssING and ANDERSEN,1955), reptiles
(BAILLIEN and SCHOFFENIELS, 1961) and marine and freshwater teleosts (HOUSE
and GREEN, 1963; M. SMITH, 1964).
While in most vertebrates sodium absorption from the intestine is essential for
the maintenance of a positive sodium balance, in the marine teleosts it is primarily
related to th e animals' water balance. HOMER SMITH (1930b) found that while monovalent ions, like sodium and chloride, were readily absorbed from the intestine
of marine teleosts, considerable amount s of magnesium, calcium and sulphate remained and were excreted in the faeces.
Differences between the permeability of the intestines of freshwater and marine
teleosts to water and sodium have been observed. The rates of passive water mo vement are greater in eels from sea-water than those adapted to fresh water (SHARRAIT
et al., 1964b; UTIDA, ISONO, and HIRANO, 1967). The rate of sodium transfer across
24