A wrapper for organs
Visceral means having to do with your organs. These cells cover the outside of your lungs, heart, stomach, and intestines.
They are flat cells in a single layer, like a very thin, shiny wrapping. A second lining sits on the inside of your body walls. Between the two is a thin film of fluid.
The two linings are really one sheet that folds back on itself, like a balloon pushed in by a fist. The organ is the fist!