The structure composed of a sheet of simple squamous epithelial cells folded onto itself to form a double layer is called a mesothelium.

Here's a breakdown:

* Simple squamous epithelium: This is a single layer of thin, flat cells.

* Folded onto itself: This means the single layer folds over, creating two layers.

* Double layer: The two layers are in direct contact with each other.

Mesothelium is a specialized type of simple squamous epithelium that lines the body cavities and covers organs. It acts as a protective and lubricating layer.

Here are some examples of mesothelium:

* Pleura: Lines the chest cavity and covers the lungs.

* Pericardium: Surrounds the heart.

* Peritoneum: Lines the abdominal cavity and covers abdominal organs.

Let me know if you have any other questions!