Have you ever imagined a fish that breathes through a pair of lungs instead of gills? The funny thing is, they actually exist!
Lungfish have been around for about 400 million years, inhabiting freshwater swamps, swimming in backwaters, and creeping in small rivers located in the West and South of Africa as well as parts of South America. Throughout their 400 million year existence, they have remained unchanged and unevolved. And yet, their physical adaptations have allowed them to survive long periods of time with no water nor food, an unimaginable notion to consider when thinking of fish who usually die moments after leaving the water.
This leads us to the anatomy of the lungfish. According to Wikipedia, the lungfish have a 'highly specialized respiratory system'. A normal fish usually has a countercurrent system, whereby it takes in oxygen from the water in an opposing direction to the flow of its blood in order to maximize oxygen absorption. A simple diagram can be used to illustrate this:
However, a lungfish's respiratory system does not work in this manner but is more similar to ours. Their lungs are connected to the larynx and pharynx without a trachea. Despite lacking a trachea, their lungs are subdivided into a number of smaller air sacs, like the alveoli in our lungs, which maximizes the surface area for gas exchange to occur.
Most of the lungfish species have two lungs, although the Australian lungfish has only one.
Lungfish tend to live in waters that often dry up, which can threaten their survival. To manage this, they burrow into the mud and secrete a layer of mucus which dries into a cocoon, essentially preserving their body. The lungfish can then live in this cocoon for up to 4 years with no water - respiring through its lungs from the air. It gains energy and food from breaking down the muscle in its tail and consuming it, which explains why their tails are usually very long and tapered at the end, acting as a food store for them.
The Lungfish are often considered as an indicator of evolution, as they are the first 'walking fish' that can walk on land and breathe air, something us humans are able to do.