The gill is an organ found in many aquatic animals. You need it to breathe. They can use it to get oxygen from the water, just like mammals with their lungs. Small aquatic animals do not need gills. It is enough for them to be able to breathe through their skin.
The name “gill” comes from the old word for “nick” or “notch”. Under the microscope, you can see fine plates from a special skin and the incisions in between. This skin must always be moist because it is a mucous membrane. When exposed to air, gills die off very quickly and the animal suffocates.
All fish have gills, including most mollusks. These include snails, mussels, and many others. Many worms and crabs also breathe with gills. The amphibians only breathe with gills as long as they are still larvae. Later on land, most amphibians breathe with their lungs.