In scorpion and spiders the respiratory organs are book lungs. They are named so because their folds resemble the leaves in a book. In this the. exchange of gases takes place between the air ofinterlamelar spaces and the venous blood through the thin-membranous walls of he lamellae. Gills are the organs for branchial respiration in aquatic animals where they help in the exchange of gases. They can be external (axolotal larva, early tadpole) or internal (prawn, unio, fish). Gill book or book gill is an aquatic respiratory organ found in Limulus (king crab). It consists of a number of leaf-like structures in which water circulates by taking oxygen in and releasing CO2 into water. Lungs are a pair of conical spongy, pinkish bodies occuring in thoracic cavity of mammae and higher verterbrates.