○○○ oar ⪢⪢
Besides its streamline body, its most obvious adaptation into underwater life is a pair of feathered rear legs that have evolved into oars and help it maneuver underwater.
http://qindex.info/i.php?x=15858

• seal & sea lion
Their rear flippers can be rotated forward and their front flippers are really strong so they can stand up and walk on all four flippers on land.
http://qindex.info/i.php?x=6929

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