|MadSci Network: Zoology|
First, a little info on right versus left. When referencing these directions, it is the specimen which is always used as a model. For example, if you are looking at a human from the chest/abdominal surface, the heart is considered to be on the left, which would be to the observer's (your) right. I hope this is not too confusing.
In mammals, the aorta (the major artery carrying blood to the body) bends to the specimen's left side. In birds and reptiles, the aorta bends to the specimenšs right side. This alteration reflects a difference in the evolution of these two types of vertebrates from a common, more fish-like ancestor. That is the difference you are wondering about.
HERE IS THE COMPLICATED EXPLANATION:
Generally, the heart of vertebrates lies in the approximate center of the thoracic (chest) cavity. For fish this is generally perfect because the pattern of circulation is symmetrical about the heart (the gills are exactly the same on both sides). In other vertebrates in which the primitive (reflected in the early embryo) pattern is reorganized to accommodate the lungs, the altered pattern of major vessels tends to give the appearance of the heart being set to the right or left in the thorax (chest cavity).
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Try the links in the MadSci Library for more information on Zoology.