The superficial layer of cells lining the alimentary
   canal and all its appendages, all glands and their ducts, blood vessels
   and lymphatics, serous cavities, etc. It often includes the epidermis
   (i. e., keratin-producing epithelial cells), and it is sometimes
   restricted to the alimentary canal, the glands and their appendages, --
   the term endothelium being applied to the lining membrane of the blood
   vessels, lymphatics, and serous cavities.