Baleen whale (suborder Mysticeti), also called toothless whale, any cetacean possessing unique epidermal modifications of the mouth called baleen, which is used to filter food from water. Baleen whales seek out concentrations of small planktonic animals. The whales then open their mouth and take in enormous quantities of water. When the mouth is closed, they squeeze the water out through the sides, catching the tiny prey on the baleen’s bristles. (See also cetacean: Feeding adaptations).