The document describes three main types of digestive systems - monogastric, avian, and ruminant. The monogastric system has a single stomach and is found in humans, pigs, horses and rabbits. The avian system in birds has no teeth and includes specialized organs like the crop, proventriculus and muscular gizzard. The ruminant system is characterized by its four chambered stomach (rumen, reticulum, omasum, abomasum) that allows ruminants like cattle to digest roughage. Pseudo-ruminants like horses also digest roughage but do not have a multi-chambered stomach.