Sharks were among the first vertebrates to develop teeth. The teeth are not set into the jaw, but rather sit on top of it—thus, they are not firmly anchored and are easily lost. The teeth are arranged in six to twenty rows, with the ones in front doing the biting and cutting. Behind these teeth, others grow. When a tooth breaks or is worn down, a replacement moves forward. In fact, one shark may eventually develop and use more than 20,000 teeth in its lifetime.