Adaptations of Amphibia to Arid Environments

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Abstract
Amphibia utilize the skin to collect water from damp surfaces and to assist rapid rehydration. Extra water can be stored in and subsequently reabsorbed from a large urinary bladder, a structure which has no homologue in their phyletic forebears, the fishes, but persists in other terrestrial tetrapods. Water transfer, as well as urine formation by the kidney, is controlled by hormones from the neurohypophysis, a mechanism which likewise is not present in the fishes. Amphibian tissues can continue to function in the presence of high osmotic concentrations of solutes, especially urea, so that the amounts of water loss or metabolic solute accumulation consistent with life are great, compared with those for other terrestrial tetrapods such as the mammals. Most important, the Amphibia have literally "used their heads" to find microenvironments where conditions are favorable for survival and reproduction. Much of this had indeed been said previously, for Townson in 1799 (33) described how frogs and toads "have power of absorbing the fluids necessary for their support . . . through the external skin ... a large part of them appearing to be retained in the so called urinary bladder, though gradually thrown off again by the skin.".