Odd-toed ungulates are browsing and grazing mammals which make up the order Perissodactyla (Latin: "odd-toed"). This group includes horses, tapirs, and rhinos. The name of the order is derived from the fact that their middle toe is larger than the others, and the plane of symmetry of the foot passes through it, a condition called mesaxonic. Most species have three digits on the hindfoot and three or four on the forefoot, but in some only a single digit, the third, remains. Some species also have horns. Perissodactyls have a simple stomach, in contrast to the chambered structure of most artiodactyls. Their cecum is enlarged and in it some bacterial digestion of cellulose takes place. Most species are herbivorous.