Fluids are transported in different ways through animal bodies, but several animals possess a specific system for this purpose: the circulatory system. This can be an open system of clefts and interstities, a closed system of tubelike structures (vessels), or a combination of the two. In most cases, circulatory vessels are limited and formed by coelom epithelia of adjacent coelomic compartments. They are lined by extracellular matrix and represent the primary body cavity (the blood vascular system). In only a few cases do circulatory vessels have an epithelium, and are then coelomic circulatory systems. The occurrence, shape, and extension of circulatory systems among animals are very diverse and this chapter concludes that their multiple parallel evolution has to be assumed.
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