The mouth, pharynx and esophagus have no serosa. What forms their outermost layer instead?

1. A serosa is present only where the alimentary canal lies inside the abdominal cavity. 2. The mouth, pharynx and esophagus sit outside that cavity, so they carry an adventitia instead. 3. The adventitia is a dense sheath of collagen fibers that holds these organs near the vertebral column. 4. Visceral peritoneum is the covering of the serosa itself, so it belongs to the abdominal organs. 5. The oblique smooth muscle layer is an addition to the stomach muscularis, not an outer coat. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 23 "The Digestive System", section 23.1 Overview of the Digestive System_