Invertebrates are the organisms in which the vertebral column is absent. Through the course of evolution, these organisms came first and then from them the vertebrates were developed. So a very simple type of respiratory system is present in the lower classes and as soon as we go upward in the chart of evolution, the system starts to become more complex. In the coelenterates, the gaseous exchange takes place from the surface of the cells. This is because, in these organisms, the cells are in direct contact with the environment. The respiratory system is absent in some of the lower organisms and in such cases transport of gases is feasible with the help of blood. By the process of diffusion, this oxygen and other gases are passed from one place to another. The moist cuticle of the earthworms is used as a respiratory organ by them. Their body is covered with cuticles and this can be used as a method to exchange gases. It is thin, moist, and vascular in nature and thus it becomes easy for the transport of gases to take place from here. The cockroaches have a network of tracheal systems that is just a network of tubes. Spiracles are some small openings that are present in them and these are the spaces from which the oxygen enters the body of insects and then flows through them. Also by this same mechanism, carbon dioxide is carried out from the body also.