what is the distinctive feature of the feet of a frog?
Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water
Frog
Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water. 
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Frog [SEP] Feet and legs
However, in modern frogs, almost all muscles have been modified to contribute to the action of jumping, with only a few small muscles remaining to bring the limb back to the starting position and maintain posture. The muscles have also been greatly enlarged, with the main leg muscles accounting for over 17% of the total mass of frogs. Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water. The completely aquatic African dwarf frog ("Hymenochirus" sp.) has fully webbed toes, whereas those of White's tree frog ("Litoria caerulea"), an arboreal species, are only a quarter or half webbed.