Describe the evolutionary change in the pattern of heart among the vertebrates.
All vertebrates possess a muscular chambered heart. The evolutionary pattern shows a progressive increase in the number of chambers, correlating with improved efficiency of blood circulation.
Fishes have a 2-chambered heart with one atrium and one ventricle. The heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to body parts, after which deoxygenated blood returns to the heart. This represents single circulation.
Amphibians and reptiles (except crocodiles) possess a 3-chambered heart with two atria and a single ventricle. The left atrium receives oxygenated blood from gills/lungs/skin, while the right atrium receives deoxygenated blood from body parts. However, these get mixed in the single ventricle which pumps out mixed blood, resulting in incomplete double circulation.
Crocodiles, birds and mammals have a 4-chambered heart with two atria and two ventricles. Oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. This ensures complete separation of blood types.
Thus, the evolutionary trend shows progression from 2-chambered to 3-chambered to 4-chambered heart, improving circulatory efficiency to meet higher metabolic demands.
Explanation
This answer traces the evolutionary development of the heart across vertebrate groups as described in the NCERT textbook. The key progression is: Fishes (2 chambers, single circulation) → Amphibians/Reptiles (3 chambers, incomplete double circulation) → Crocodiles/Birds/Mammals (4 chambers, complete double circulation). The answer uses only facts from the provided context and follows the word limit and formatting requirements for a 5-mark long answer question.