1. Gills: This is the most common method for aquatic animals. Gills are specialized, feathery structures that extract oxygen from the water. Fish use their gills to breathe, as do other aquatic creatures like crustaceans and amphibians in their larval stage.
2. Lungs: This is the primary method for terrestrial animals. Lungs are internal organs that allow air to be taken in and carbon dioxide to be released. Mammals, reptiles, birds, and amphibians (as adults) all use lungs for respiration.
3. Skin: Some amphibians, like frogs and salamanders, can absorb oxygen directly through their skin. This is especially important when they are submerged in water or their lungs are inactive.
4. Tracheae: Insects and other invertebrates use a system of tubes called tracheae to transport oxygen throughout their bodies. Tracheae open to the outside world through small openings called spiracles. Air enters the tracheae through these openings and is distributed directly to the cells.
5. Book Lungs: Arachnids, like spiders and scorpions, use book lungs to breathe. Book lungs are folded, leaf-like structures that increase the surface area for gas exchange. Air enters the book lungs through openings called spiracles and then diffuses into the hemolymph (the equivalent of blood in invertebrates).
These are just a few examples of the diverse ways animals have evolved to obtain oxygen. The specific method an animal uses depends on its environment, size, and other factors.