The body part that sharks have but whales lack is the gills.
Mammals may be defined as any member of the class of vertebrate animals in which the young are nourished with milk from the special mammary glands of the mother.
Sharks have gills which involve in the process of gas exchange inside the water, while whales have blowholes instead of gills that also perform the same function. But whales respire out of the water so blowholes have limited functions.
Therefore, the body part that sharks have but whales lack is the gills.
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