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Evolution of the aortic arches in phylogenetic context
In this depiction, the viewer is facing the chest (ventral side) of the organism. The birds and mammals are actually very similar to reptiles in their fundamental pattern, except that they have lost the left or right systemic arch, respectively. From: Figure 12-19, p. 461 of: Kardong, Kenneth V. (2006). Vertebrates: comparative anatomy, function, evolution. Boston, McGraw-Hill Higher Education.