1. Fish: Gills are the primary respiratory organs for most fish species. Fish have paired gills located on both sides of their heads, covered by a protective flap called the operculum.
2. Amphibians: Certain larval amphibians, such as tadpoles, use external gills during their aquatic stage. As they metamorphose into adults, they develop lungs for terrestrial respiration.
3. Aquatic Insects: Some aquatic insects, including mayflies, dragonflies, and damselflies, have larval stages with external gills, allowing them to breathe underwater.
4. Crustaceans: Many crustaceans, including crabs, shrimps, lobsters, and barnacles, possess gills for oxygen uptake from water.
5. Mollusks: Aquatic mollusks, such as clams, mussels, and octopuses, utilize gills for respiration.
6. Echinoderms: Certain echinoderms, like sea cucumbers and sea urchins, have specialized structures called "tube feet" that contain gills for respiration.
It's important to note that aquatic animals' gill structures can vary depending on their specific species and environment. Gills have evolved in various forms, allowing different organisms to extract oxygen efficiently from water for respiration.