Cetaceans also have rudiments of the hind-limbs. These consist of the pelvis, which is fairly well developed, and small nodules of bone representing the femur and sometimes the tibia; the latter is cartilaginous except in rare cases. These rudiments are, of course, entirely concealed within the body and can only be found by carefully cutting away the flesh surrounding the sexual organs. [Illustration: The flipper of a humpback whale. “The flippers of all cetaceans are merely the fore-limbs of ordinary land mammals, which have become overlaid with blubber to form a paddle.” ] One of the most striking things about the blue whale, and indeed all its relatives, are the folds which extend longitudinally from the lower jaw backward over the throat, breast, and abdomen. In different species of whales the folds vary in number and width, the furrows between them being about an inch in depth and the skin capable of great extension. The use of the folds has been a subject of disagreement among naturalists, but my own belief is that they are an adaptation to increase the mouth capacity and to give greater power of expansion to the lungs. [Illustration: After the humpback’s flipper has been stripped of blubber. The forearm, wrist and fingers are shown. In this species the digits have been reduced to four and are greatly elongated. ] The folds are not composed of flesh but entirely of blubber, the layer of fibrous fat which covers the bodies of all whales, porpoises, and dolphins and lies between the skin and the flesh. Since cetaceans are warm-blooded animals (fish and reptiles are cold-blooded) it is necessary for them to have some protection from the cold. Hair is not sufficient for this purpose as in land mammals; consequently the layer of blubber, which acts as a non-conductor and prevents the heat of the animal’s body from being absorbed by the water, has been developed. It is from this that the whale oil of commerce is boiled or tried out. The blubber may be easily peeled off the body in strips called “blanket pieces,” which are cut into blocks and after being sliced are put into the trying out kettles. [Illustration: The folds on the throat of a finback whale. Probably the folds are an adaptation to increase the mouth capacity and to give greater power of expansion to the lungs. ]