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Open Circulatory System: In this system, blood (or hemolymph) exits blood vessels and bathes organs directly in the open space. This type is characteristic of many invertebrates such as arthropods (insects, spiders) and mollusks (snails).
Closed Circulatory System: Here, blood remains confined within vessels as it circulates. This system is found in vertebrates, including fishes, amphibians, reptiles, mammals, and birds. It allows for more efficient transport of oxygen and nutrients under higher pressure.
Circulation in Animals: Series and Parallel Pathways
In mammals, the circulation involves two main pathways: