Animals obtain oxygen in the following five ways :

1. From water or air through moist body surface directly into the body as in Amoeba.

2. From air or water through the thin body wall in blood vessels e.g. Earthworm.

3. From air through spiracles to a system of air tubes-tracheae to tissues and tissue cells or from water through tracheal gills e.g. insects.

4. From water through gill surface to blood vessels and then to tissue cell e.g. fishes and amphibians.

5. From air through moist lung surfaces to blood vessels and from there to tissue cells e.g. animals.