It is well known that the aggregation of red blood cells (RBC) as discussed in this book is a mammalian feature. Mammalian RBC do not contain a nucleus or other organelles, while all nonmammalian species, including avian, reptile, and fish, are characterized by nucleated RBC (see frog RBC in Figure 9.1e as an example). Primarily due to an extensive degree of evolution, RBC in mammalian species are usually biconcave discs (Figure  9.1) with only a few exceptions (e.g., camelids, Figure 9.1c). This special geometry of RBC is common for all sub and infraclasses of the Mammalian class, including placental mammals, marsupials, and monotremes (Baskurt et al. 2010) that represent the first diversification of Mammalia dating back ~200 million years (Grutzner et al. 2003). It has been suggested that this special geometry evolved to optimize blood flow in the circulatory system (Uzoigwe 2006).