With the development of the flukes there came a change in the whale’s fore-limbs, which were flattened and covered with connective tissue and blubber. The excellent paddles thus formed, while probably of little use in forward motion, assist in rapid turning and act as balancing organs to keep the animal upright in the water. In some species an adipose dorsal fin has also developed as a further balancing aid. During the development of the flippers and flukes the hind-limbs, which were no longer of use to the whale, became small and weak, sunk into the blubber and finally disappeared altogether, the greatly modified pelvic elements and nodules of bone or cartilage representing the femur alone remaining. The heads of most cetaceans are long and pointed, acting as a “cut-water,” but one of the most remarkable aquatic adaptations is the position of the nostrils, or blowholes, which open upon the very summit of the head, in either a single or double aperture, instead of at the end of the snout. The cause of this migration of the nostrils is obvious, for in this position the blowholes first appear at the surface and the whale can begin to breathe while the rest of its body is yet under water. In all cetaceans the facial portion of the skull is greatly elongated, and especially in the Mystacoceti the mouth is exceedingly large to accommodate the baleen, which hangs in two parallel rows from the upper jaw. Probably no mammalian adaptation for the securing of food is more remarkable than the whale’s baleen. It is almost unbelievable that an animal which once had teeth could, as its food changed, replace them by a complicated straining apparatus such as the whalebone. The baleen is an epidermal growth and is in reality merely an exaggeration of the transverse ridges present in the mouths of land mammals. We know that the Mystacoceti at one time had teeth, for in fœtal whales two sets of minute teeth are present under the skin, corresponding to the “milk” and “permanent” dentition of ordinary mammals, but these disappear before the baleen begins to develop. Another interesting feeding adaptation is present in the throat of the whale. The nostrils, instead of opening into the back of the mouth, as in land mammals, are directly connected with the lungs by a prolongation of the “windpipe” called the epiglottis, which entirely shuts off the whale’s breathing passage from the mouth. Thus the animal can swallow its food beneath the surface without danger of strangulation through getting water into its lungs. When whales lived upon the land external ears were necessary, but as they became completely aquatic such “sound collectors” were not only of no more use but highly undesirable, because, like the useless hind-limbs, they offered additional resistance to the water; therefore the external ears were lost, but their muscles still remain about the minute ear-orifices of the present-day Cetacea.