They have a complex 4 part stomach for digesting cellulose. It consists of 4 chambers the first three of which are storage, digestion and fermentation chambers. They have to eat large quantities of plants and this way they can fill the stomach and regurgitate the material and chew later while under cover, and out of sight of predators. These are horses, zebras, rabbits, hares, elephants, and several rodents. Small intestines - are long and coiled and where most absorption takes place.
Herbivores have longer small intestines than carnivores.
The 4-chambered heart evolved twice independently in 2 lineages of reptiles. This is called convergent evolution . The birds retained the right aorta and mammals retained the left aorta . We see more connections with reptiles all the time. Lets look at the unique blood flow in a fetus as compared to an adult mammal.