Vertebrate digestive tracts tend to be the shortest and simplest in carnivores, and longer in omnivores. Due to the high levels of cellulose in the walls of plant cells, the digestive tracts of herbivores are more complex, and include a system for the additional digestion of plant material. The midgut, which includes the small intestine, is the principal site of digestion and nutrient absorption in all vertebrates. It is separated from the large intestine, or hindgut, by a valve. The hindgut is site of reabsorption of water. The cecum is a blind sac located at the junction between the midgut and hindgut. It is the structure utilized for additional digestion of plant material.