Animals that do not have lungs are generally referred to as "breath through the skin" or "skin-breathing" animals. These animals rely on their skin or other specialized structures for gas exchange, rather than using lungs. Some examples of such animals include:
Fish: Most fish have gills, which are specialized respiratory organs that extract oxygen from water. They absorb dissolved oxygen from the water and release carbon dioxide through their gills.
Amphibians: Adult amphibians (e.g., frogs, toads, salamanders) typically have lungs for breathing air. However, during their larval stage (e.g., tadpoles), they may rely on gills for respiration before undergoing metamorphosis.
Some Invertebrates: Some invertebrates, such as certain species of mollusks (e.g., snails, slugs) and some arthropods (e.g., spiders, insects), have specialized structures for gas exchange. For example, insects have a system of tubes called tracheae, which allow for direct gas exchange with their surroundings.
Jellyfish: These simple, gelatinous marine animals absorb oxygen and release carbon dioxide through their thin, permeable body walls.
Flatworms: Certain flatworms obtain oxygen through diffusion across their thin, flat bodies.
Nematodes: Some nematode species, also known as roundworms, have a simple diffusion-based respiratory system.
It's important to note that while some animals lack lungs, they still have specialized structures or adaptations for gas exchange that suit their specific environments and evolutionary adaptations. The efficiency of their respiratory systems can vary widely, and some may be limited to living in aquatic environments or damp habitats where gas exchange through the skin is more effective.