Which is NOT one of the four layers of the alimentary canal?

• A. Mucosa
• B. Submucosa
• C. Muscular layer
• D. Epithelium

Answer: (D)

The wall of the digestive tract is organized into four tunics: mucosa, submucosa, muscularis externa (the muscular layer), and serosa (or adventitia). The mucosa, the innermost layer, includes epithelium, plus underlying connective tissue and a thin muscle layer called the muscularis mucosae. Epithelium is the tissue that lines the mucosa, but it isn’t counted as a separate tunic. That’s why epithelium is not one of the four layers. The other options are actual tunics: mucosa is the inner lining, submucosa sits beneath it, the muscular layer corresponds to the muscularis externa, and serosa covers the outside.