The structure of the excretory system of fish and amphibians is generally similar: paired trunk kidneys, the ureters and bladder extending from them. Unlike fish, in which urine from the bladder flows through the urethra to the excretory opening and is poured out, in amphibians, urine from the ureters enters the cloaca, where water is sucked out and returned to the body. Further, from the cloaca, urine enters the bladder, where it is concentrated, and from it again into the cloaca, from where it is removed through the opening of the cloaca to the outside. Both in the cloaca and in the bladder, water is sucked out and returned to the body. Such structural features of the excretory system of amphibians are associated with their terrestrial lifestyle and the need to conserve water under these conditions.
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