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Evolution of the Circulatory System
The simplest kinds of hearts, present in some invertebrates such as some types of worms, consist of a muscular tube which squeezes rhythmically and moves blood-like liquid by peristaltic contraction
Some mollusk hearts have quite a complex structure, which may include four atria and one ventricle(Nautilus), or be composed from multiple (seven and more) individual hearts as in annelid worms.
Such diversity in the structure and number of invertebrate hearts can be explained by the relatively small size and low metabolic activity of these animals as well by the fact that the role of invertebrate circulatory system is not necessarily respiratory exchange, but rather nutrient transport (which does not require as rigid and systematic circulation as does respiratory exchange).
In vertebrates, however, one of the primary roles of the circulatory system is transporting oxygen to the cells and a certain generalization about circulation in the different phyla and stages of evolution of the heart are possible.
The warm-blooded animals such as birds and mammals have a high metabolic rate and an efficient double-circuit circulation is a necessity for these species. The hearts of birds and mammals are four-chambered organs, in which blood flowing through pulmonary circulation is completely separated from that of systemic circulation, and vice versa. This allows for no mixing of oxygenated and deoxygenated blood and, as a result, is much more efficient.
Evolution of the heart over time. (n.d.). Retrieved from http://library.thinkquest.org/C003758/Development/heart_evolution.htm