book gill

A book gill is a type of respiratory organ found in horseshoe crabs. It consists of membranous folds arranged similarly to the leaves of a book. These structures are a form of gill, which are respiratory organs in many aquatic animals that facilitate the exchange of oxygen and carbon dioxide between the animal and the surrounding water. Gills are typically composed of branched or feathery tissue rich in blood vessels, allowing for efficient gas exchange. In fish, for example, water flows over gill filaments, enabling oxygen to enter the blood and carbon dioxide to exit.123