Flat, flexible plates with frayed edges, arranged in two parallel rows attached to the upper jaw of a whale. Although whalers called baleen "whalebone," baleen is not bone, but is composed of keratin, the same substance as hair, horn, claws and nails. Whales use these combs for filter feeding. A baleen whale opens its mouth wide and scoops in dense shoals of prey (such as krill), together with large volumes of water. It then partly shuts its mouth and presses its tongue against its upper jaw, forcing the water to pass out sideways through the baleen while its prey remains inside, and is then swallowed.