Gaseous Exchange in Lower Animals

 Sponges and coelenterates All cells in the bodies of sponges and coelenterates are in contact with the water in which they live. Oxygen diffuses into each cell and carbon dioxide diffuses out simultaneously. Soon, however, the water bathing these cells would become deficient in oxygen and saturated with carbon dioxide, bringing gaseous diffusion to a stand still.

      To maintain a high diffusion rate, flagellated cells in the body walls of these animals beat rhythmically to create water currents. The water bathing the cells is continuously replaced by a fresh supply of water rich in oxygen and poor in carbon dioxide. As a result , the diffusion gradient for the gases remains high, enabling gaseous exchange to proceed at a sufficiently high rate.

 Annelids

 In the terrestrial earthworms, gaseous exchange occurs by diffusion through their moist skins. There is no special mechanism to circulate air over their skins. Instead , a rich blood supply to the skin rapidly removes the oxygen that diffuses into the epidermal cells, thus maintaining a sufficiently high diffusion gradient. Simultaneously, the carbon dioxide in the blood capillaries diffuses out of them, to enter the epidermal cells. From here, the gas diffuses out of the body into the external environment.

     When the oxygen rich blood from the skin reaches the various body tissues oxygen diffuses out of the capillaries and enters the individual body cells. At the same time, carbon dioxide waste diffuses out of the body cells and enter the blood capillaries to be transported to the skin , where they can be got rid of to the external environment. 

Note : in annelids, the removal and transport of oxygen from the skin is enhanced by the presence of the oxygen carrying pigment, haemoglobin, in their blood. This type of blood transports oxygen more efficiently than one without such a pigment.

Insects

 Air enters and leaves the body of an insect through its tracheal system. This flow of air is controlled by adjusting the size of the spiracular openings. The spiracles open fully when the carbon dioxide concentration in the body tissues is high. This occurs when the insect is active. When the insects is at rest, the carbon dioxide Concentration in the tissues drops. This triggers the valves guarding the spiracular openings to behave like tiny doors and decreases the size of the openings.

     Usually, oxygen diffuses in and carbon dioxide diffuses out passively through the tracheal system. In a large and active insects, like the locust, air is actively pumped in and out of the tracheae by ventilation movements, also referred to as breathing movements. In this mechanism , dorso ventral muscles (vertical muscles connecting the roof and floor of the body segments) contract the flatten the body. This reduces the volume of the tracheal system and forces air out of the body (expiration). When the muscles relax, the body returns to its normal size. The tracheal system, too, returns to its original (larger) size. This causes air to flow into the body (inspiration). In co ordination with these ventilation movements, the spiracles in the anterior and posterior parts of the body open and close alternately. This causes a one way flow of air through the tracheal system, with air being sucked in through the anterior spiracles and expelled through the posterior ones.

     The oxygen in the air that enters the tracheal system dissolves in the tissue fluid in the fine tracheoles. From here, the oxygen diffuse in the body cells. At the same time, carbon dioxide wastes in the cells diffuse out in to the tissue fluid. In the fine tracheoles, the carbon dioxide in the tissue fluid escapes as a gas that leaves the body through the spiracular openings.