One is the presence of villi and microvilli in the small intestines
that increase the surface area for which food is absorbed. Second is the presence
of goblet cells that secrete mucus to prevent self-digestion of the
tract lining. Another is the specialized enzymes in different regions of the digestive tract
that specialize in digesting particular foods.
Because the passage isn't talking about poets in general, it's specifically talking about Paul Lawrence Dunbar. So the topic sentence should have him as the main theme.