Gastrointestinal systems differ largely with respect to the presence of a meaningful symbiotic microbial population and its location. Simple-stomached animals (Figs. 1A and B) do not have an extensive microbial population to greatly alter nutrient recovery, whereas ruminants (Fig. 1C) and nonruminant herbivores (Fig. 1D) support symbiotic populations prior to and after formal digestion by the small intestine, respectively. All GI systems accomplish the same sequence of events but are anatomically and functionally modified to accommodate predominating food and microbial populations.
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