Lung fish (dipnoi) i.e. salamander fish are well known because while they first appear in Devonian period they still live and multiply in the warm seas of Earth today. Biologists can study them in nature and put them under the microscope at will. The hallmark of these fishes are the lungs that they use to breath air reaching them through their gills.
We will discuss Devonian lung fish later more.
For now, read more about these wonders of Nature in wikipedia.