• All terrestrial vertebrates, with the exception of birds, utilize a tidal ventilation mechanism for breathing. Air moves in and out by the same route. Air entering mixes with used air in the lungs Lungs do not completely empty or fill during each breathing cycle. Decreases gas-exchange efficiency • In the simple lungs of snails and spiders, air movement takes place by diffusion only. • Amphibian (兩棲類) and related animals push air into their lungs via positive pressure breathing (正壓呼吸). • Humans and other mammals pull air into their lungs via negative pressure breathing (負壓呼吸).