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In page Dolphin:

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Dolphins have two pectoral flippers, each containing four digits, a boneless dorsal fin for stability, and a fluke for propulsion. Although dolphins do not possess external hind limbs, some possess discrete rudimentary appendages, which may contain feet and digits. Orcas are fast swimmers in comparison to seals which typically cruise at 9–28 km/h (5.6–17.4 mph); the orca, in comparison, can travel at speeds up to 55.5 km/h (34.5 mph).[citation needed] A study of a Pacific white-sided dolphin in an aquarium found fast burst acceleration, with the individual being able with 5 strokes (2.5 fluke beats) to go from 5.0 m s-1 to 8.7 m s-1 in 0.7 seconds.[1]