Background Paragraph: The lungs of mammals are unique in having alveoli . These are tiny, sac-like structures. Each alveolus is surrounded by a network of very small blood vessels (see Figure below ). Because there are millions of alveoli in each lung, they greatly increase the surface area for gas exchange between the lungs and bloodstream. Human lungs, for example, contain about 300 million alveoli. They give the lungs a total surface area for gas exchange of up to 90 square meters (968 square feet). That’s about as much surface area as one side of a volleyball court!. 
Story: Bob is in a science class, and this week they are doing some dissections. He is going to dissect a fish, and a small rat. His class has been learning about the respiratory system of different animals and will focus on the lungs. 
Question: Which animal that Bob dissects will have alveoli in their lungs?
rat.