Book lungs are respiratory organs of

• A. Mollusca
• B. Arachnida
• C. Mammals
• D. Aves

Book lungs are specialized respiratory organs found in certain arachnids, such as spiders and scorpions. They are not present in mollusks, mammals, or birds. Instead, book lungs consist of stacked, leaf-like structures that allow for gas exchange.

Key Characteristics of Book Lungs

• Structure: Composed of thin, flat layers that resemble the pages of a book.
• Function: Facilitate the exchange of oxygen and carbon dioxide directly with the environment.
• Location: Typically located in the abdomen of arachnids.

Comparison with Other Respiratory Systems

Unlike book lungs, mammals have lungs that operate through a diaphragm, while birds possess a unique air sac system for efficient respiration. Mollusks may use gills or a simple lung-like structure depending on their habitat.

In summary, the correct answer is B. Arachnida, as book lungs are exclusive to this group of animals.