Early archaeocetes such as "Pakicetus" had the nasal openings at the end of the snout, but in later species such as "Rodhocetus", the openings had begun to drift toward the top of the skull. This is known as nasal drift. The nostrils of modern cetaceans have become modified into blowholes that allow them to break to the surface, inhale, and submerge with convenience. The ears began to move inward as well, and, in the case of "Basilosaurus", the middle ears began to receive vibrations from the lower jaw. Today's modern toothed whales use their melon organ, a pad of fat, for echolocation.