What type of respiration does earthworm has? In many complex animals, the cells of internal organs are distant from the outside environment; respiratory systems provide the passage of gases to or from internal tissues. In such kind of systems, there is a difference in pressure of a specific gas on opposite sides of a membrane, the gas diffuses from the side of greater pressure to side of lesser pressure, or each gas is transported independently of other gases. For i.e., in tissues a carbon dioxide concentration is high or oxygen concentration is low as a product of active metabolism, oxygen diffuses into the carbon dioxide and tissue diffuses out. 

In many lower animals, gas diffusion takes place through a moist surface membrane, as in flatworms; through the thin body wall, as in earthworms; through air ducts, or tracheae, as in insects; or through specialized tracheal gills, as in aquatic insect larvae. In the gills of fish the blood vessels are exposed directly to the external aquatic environment. Oxygen–carbon dioxide swap occurs between the surrounding water & the blood within the vessels; the blood carries gases to and from tissues.