Mammals are the only animals which have milk producing glands. They are the only ones that nourish their young with milk. They also possess hair on their body.
Marine mammals have several independent lines of evolution. But, they offer an excellent example of convergent adaptations in different groups, resulting in a similar mode of life.
The main group are: the Cetacea, Sirenia and Pinnipedia. Of these the last group is not found in Indian waters. The whales, dolphins, and porpoises belongs to the first one, while seals, sealions and walruses are of the second one. As such we have in our waters whales and dolphins. But, if we take the whole list, porpoises, seals, sealions and walruses will also be there among the marine mammals.
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