Animal Diversity: Chordata

kidney with peritoneal connections is present in many larval amphibians, it does not
persist in the adult stage.
Urodeles have opisthonephric kidneys much like those of elasmobranchs. The kidney
consists of two regions: an anterior portion, which in males is concerned more with
genital than urinary functions, and a posterior expanded urinary region, which makes
up the main part of the opisthonephros. Numerous collecting ducts or tubules leave
the opisthonephros at intervals to join the persisting archinephric duct. The latter, in
both sexes open on either side of the cloaca at the apex of a small papilla. In Necturus
peritoneal connections with some of the kidney tubules persist throughout life.
The opisthonephric kidneys of anurans show a more posterior concentration of
tubules and are confined to the posterior part of the abdominal cavity. The kidneys are
flat, oval, dark-red organs, in the posterior region of the coelom. They are dorsally
located, retroperitoneal, and flattened in a dorso-ventral direction. There is no clearcut
distinction between the anterior and posterior ends, as in urodeles. An adrenal
gland of a yellowish-orange color is located on the ventral side of the kidney. Blood
comes to the kidney from two entirely different sources. The adult frogs have ciliated
nephrostomes on the ventral surfaces of the kidneys. They are usually not connected
with kidney tubules but have become secondarily connected with the renal veins. The
kidneys of female anurans have no relation to the reproductive system, but in males
an intimate connection exists. Certain anterior kidney tubules have become modified
as efferent ductules connecting the testis with the kidney and archinephric duct, the
latter serves to transport spermatozoa as well as urinary wastes. Unlike the condition
in urodeles, the archinephric ducts are located within the kidney along its lateral
margin. They leave the opisthonephros near the posterior near the posterior end and
pass to the cloaca. The structure of a typical renal tubule of an amphibian kidney is
shown in Figure 20. A thin-walled urinary bladder connects with the amphibian
cloaca a short distance beyond the openings of the archinephric ducts. It is bilobed.
There is no direct connection of the ducts with the bladder, so that the urine first
passes into the cloaca.
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