Arrange the following structures from the outer, serosal side toward the GI lumen: mucosa, circular muscle, serosa, submucosa, longitudinal muscle.

From the outer surface inward, the relevant sequence is **serosa → longitudinal muscle → circular muscle → submucosa → mucosa**. The full lecture sequence also inserts the myenteric plexus between the muscle layers and the submucosal plexus and muscularis mucosae between submucosa and mucosa. Ordering the layers gives a spatial framework for understanding where the nerves, glands, and exchange surface are located. **Key takeaway:** The mucosa is innermost and the serosa is outermost.\n\n**Glossary:**\n- **Serosal side**: The external side of the GI wall.\n- **Muscularis mucosae**: Thin muscle layer between the submucosa and mucosa.