With more than 47,000 described species, the spiders (Araneae) represent one of the largest monophyletic taxa within the Arachnida. Their outer anatomy is largely uniform despite their diverse lifestyles. The circulatory system of Araneae is also built up quite uniform and is the subject of this work. The circulatory system of 13 spider species was investigated morphologically by use of classical and modern methods. After describing, the features of the circulatory system were compared inner- and interspecifically. In addition to intraspecific variability, the most striking differences are in the number of ostia and cardiac arteries which is reduced interspecifically within the Araneae. The reduction of ostia, from five to two pairs, and the cardiac arteries, from four to two pairs, took place several times independently. To resolve these different reduction steps, possible homologies of these structures are discussed. Based on the differences resulting from the comparison, feature complexes were conceptualized and applied to existing current phylogenetic hypotheses by means of character mapping. Subsequently, several evolutionary scenarios of some circulatory structures were discussed.