Which part of an arachnid is responsible for respiration and resembles pages?

The part of an arachnid that is responsible for respiration and resembles pages is the book lungs. Book lungs are specialized structures that facilitate gas exchange in arachnids, such as spiders and scorpions. These organs are named for their layered, flattened structure that resembles the pages of a book. The layers create a large surface area that allows for efficient absorption of oxygen and release of carbon dioxide directly from the air, which is essential for the organism's survival.

In contrast, tracheae are a system of tubes that transport air directly to tissues in some arthropods but are not structured like pages. Gills are used for respiration in aquatic animals, allowing them to extract oxygen from water, which does not apply to terrestrial arachnids. Spiracles are openings that lead to the tracheae in some arthropods, allowing air to enter, but they do not perform respiration themselves. Therefore, book lungs are specifically designed for respiratory functions in arachnids, making them the correct answer.