State the major difference between fish and mammalian hearts.

The major difference between fish and mammalian hearts is that fish have a two-chambered heart, consisting of one atrium and one ventricle, while mammals have a four-chambered heart, consisting of two atria and two ventricles. This difference allows mammals to have a more efficient circulation system, as oxygenated and deoxygenated blood are kept separate in the heart.

The major difference between fish and mammalian hearts is the number of circuits present in the circulatory system. Fish have a single circuit circulatory system, meaning that blood travels through the heart and gills only once before being pumped to the rest of the body. Mammals, on the other hand, have a double circuit circulatory system, meaning that blood travels through the heart and lungs twice before being pumped to the rest of the body. This double circuit system allows mammals to maintain a higher body temperature and to deliver oxygen more efficiently to their tissues.