2 chamber hearts: Fishes
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Fish have the most simplistic heart out of the three hearts because it only has one ventricle and one atrium. How a fish heart works is the atrium pumps the blood into the ventricle which also pumps the blood into the conus. A conus is is very stretchy and is easily able to squeeze, once through the conus the blood travels t the grills which is where they get a new supply of oxygen which is then pumped back to the atrium. A fish only needs a 2 chamber heart because they have low energy requirements because they are surrounded by water which gives them the oxygen and nutrients they need.