The vertebrate urinary bladder: osmoregulatory and other uses.

The bladder may serve more biological uses than simple storage. The importance of bladder functions can be inferred from its presence among vertebrates, its anatomy and histology. From an evolutionary perspective, bladders have evolved at least twice in the vertebrates. The variability of permeability of the urinary bladder to water and solutes among species is discussed. Finally, the urinary bladder may play an osmoregulatory role.