Types of Respiratory Organs

Respiratory organs may be of the following types:

• Those that have respiratory surface turned out forming an evagination. These are called gills.
• Those that have respiratory surface turned in forming an invagination. These are called lungs. Our own lungs are good examples.

Figure:  Some aquatic animals and animals that maintain moist skins, can exchange gases directly through their integuments

Insects have a special respiratory system. Small openings on the insect's body surface connected to small tubes or trachea that branch and spread throughout the body. The gases diffuse through these branches directly into the cells. Generally gills are for aquatic breathing and lungs for breathing air. We shall now consider specific kinds of respiratory organs employed by animals that live in water and on land.