Gaseous Exchange Body Surfaces in Some Simple Animals

 Sponges and coelenterates

 In these simple animals, all their body cells are in direct contact with the surrounding aquatic medium. Thus, each cell can get its own supply of oxygen. 

Flatworms

 In flatworms, the body is extremely flattened and elongated. In the free living planaria, the body is 10 mm long and only 0.6 mm thick. This body form that

 • Increases the surface area to volume ratio, and

 • Brings the innermost cell very close to the body surface.

     It is, therefore, possible for these worms to meet their oxygen requirements through diffusion over their body surface. In addition, free living flatworms are found in well aerated flowing waters so that the oxygen concentration in the water surrounding them is high.

 Note : Parasitic flatworms are anaerobes as the oxygen concentration in their environment is very low. Annelids In these more advanced worms, gaseous exchange occurs by diffusionnovr teor moist body surface. This is possible because

 • Their cylindrical shape gives a high surface area to volume ratio so that the rate at which oxygen diffuses is sufficient to meet the worms' requirements; and

 • The cells in their body surface (epidermis) have a rich supply of blood capillaries. 

Amphibianshe

These more advanced group of animals live both on land and in water. They have lungs whicbbare not properly developed to meet their oxygen demands. As a result, many amphibians, like the frog, obtain much of their oxygen through their moist skins. This is knonn as cutaneous respiration. It is possible for the skin of a frog to carry the gaseous exchange because the skin is

 • Richly supplied with blood capillaries, and

 • Kept moist by mucus secreted by glands.