can you give me examples of some spineless animals in which this respiratory organ is present?
Spiders, scorpions, coconut crab,  land snails and slugs.
Lung
Some arachnids, such as spiders and scorpions, have structures called book lungs used for atmospheric gas exchange. 
what are the function of lungs in human body Their function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. how is this right organ different from the left organ in humans? The right lung is bigger and heavier than the left. what type of tissue is this respiratory organ made up of? Its connective tissue is made up of elastic and collagen fibres that are interspersed between the capillaries and the alveolar walls. from where does this respiratory organ get its blood supply? Bronchial and pulmonary circulation. what is that second circulation all about? It is the portion of the circulatory system which carries deoxygenated blood away from the right ventricle, to the lungs, and returns oxygenated blood to the left atrium and ventricle of the heart. how does that type of circulation take place in that respiratory organ which we are talking about? The circulatory system of the lungs is the portion of the cardiovascular system in which oxygen-depleted blood is pumped away from the heart, via the pulmonary artery, to the lungs and returned, oxygenated, to the heart via the pulmonary vein. when does that respiratory organ first develop in humans? UNANSWERABLE
wiki:19605796
Lung [SEP] Invertebrates
Some invertebrates have lung-like structures that serve a similar respiratory purpose as, but are not evolutionarily related to, vertebrate lungs. Some arachnids, such as spiders and scorpions, have structures called book lungs used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs. Scorpions have spiracles on their body for the entrance of air to the book lungs. The coconut crab is terrestrial and uses structures called branchiostegal lungs to breathe air. They cannot swim and would drown in water, yet they possess a rudimentary set of gills. They can breathe on land and hold their breath underwater.