Some similarities occur in the composition of the walls of the digestive tract of various species. The walls include
four layers - epithelium, lamina propria, muscles, and visceral peritoneum. The epithelium is a mucous membrane
that lines the digestive system. It is continuous with the external skin of the animal at the mouth and anus. The lamina
propria is a thin layer of connective tissue supporting the epithelium in the intestines. The muscles of the digestive
system are striated through the esophagus but smooth throughout the remainder of the digestive tract. The visceral
peritoneum covers the digestive organs in the abdomen.