I'm reading an essay on the creating of the Mammalia zoological classification (Londa Schiebinger, The American Historical Review, Vol. 98, No. 2 (Apr., 1993), pp. 382-411).
It contains the statement (page 386)
All mammals (including the whale) have hair, and it is still today considered a distinguishing characteristic of mammals.
Is that statement correct? Do all known species of whale and dolphin have hair? If so, where is it? You can't see it on the surface of the animal, is it internal like we have nostril hair?