Gills and lungs are the two structures commonly used by animals for respiration. Both are characterized by large amounts of surface area that function in gas exchange. The difference between them is that gills involve external extensions from the body surface, whereas lungs possess internal foldings. Gills have evolved independently several times in a variety of animal groups.

Among the annelids, certain species of terrestrial worms have long slender, branching gills which extend from the body. Horseshoe crabs possess structures known as book gills, which are actually modified appendages that function in gas exchange. Crustaceans also have gills that have been modified from thoracic or abdominal appendages.

Gill structures are highly diverse among crustaceans. Generally, the more aquatic crustaceans have more elaborate gills, while the more terrestrial species are characterized by simplified gills. That is a consequence of the greater availability of oxygen on land, from the air...