[[1]] are specialized respiratory structures found in aquatic animals such as fish, amphibians, and some invertebrates like crustaceans and molluscs that allow them to extract oxygen from water and expel carbon dioxide.

Gills are specialized respiratory structures found in aquatic animals such as fish, amphibians, and some invertebrates like crustaceans and molluscs that allow them to extract oxygen from water and expel carbon dioxide.

Gills are made up of thin, highly vascularized filaments that are covered with a slew of small, feather-like projections known as lamellae. 

The gills' large surface area allows for a large area for gas exchange between water and blood.

Water diffuses oxygen into the bloodstream and carbon dioxide into the water as it flows over the gills. 

This enables the aquatic animal to obtain the oxygen it requires to support its metabolic processes while also eliminating the waste product, carbon dioxide.