What are book lungs primarily composed of?

Book lungs are primarily composed of flat layers stacked like pages. This unique structure resembles a book, which is where they get their name. The layers are made of thin, highly vascularized tissue that allows for efficient gas exchange, facilitating the transfer of oxygen and carbon dioxide between the air and the hemolymph in arachnids, such as spiders and scorpions.

The layered design increases the surface area available for gas exchange, optimizing the lungs' function despite being an internal gas exchange system. Each layer acts much like a page in a book, allowing air to flow between them. This is essential for the respiratory efficiency that these organisms require, especially when they are active.

The other options, while related to breathing mechanisms, do not accurately describe the structure of book lungs. Elastic membranes or liquid-filled sacs do not convey the specific layered organization critical to the function of book lungs, nor do spiracles themselves provide the respiratory surface area needed for gas exchange; they serve more as openings that connect the external environment to the internal respiratory structures.