A branchiostegal lung is a respiration organ

Respiration organ

Respiratory organs are used by most, or all, animals to exchange the gases necessary for their life function known as respiration...

 used by some air-breathing arthropod

Arthropod

An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s. It is one of the most significant adaptations of some crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

s and hermit crab

Hermit crab

Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...

s such as the coconut crab

Coconut crab

The coconut crab, Birgus latro, is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit of terrestrial animals with exoskeletons in today's atmosphere at a weight of up to...

 to their terrestrial habitats.

The branchiostegal (gill) tissue is supported by folds or other mechanisms to increase surface area

Surface area

Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...

 and are of a similar tissue to that normally found in gills. In this case, the lung is more suited to the absorption of oxygen from air, rather than water.

Instead of branchiostegal lungs, some terrestrial hermit crabs (Coenobita

Coenobita

The genus Coenobita contains the sixteen species of terrestrial hermit crabs. The majority of the species are found in the Indo-Pacific region, with only one species in West Africa, one species occurring along the western Atlantic Ocean, and one species occurring on the Pacific coast of the Americas...

and Birgus

Coconut crab

The coconut crab, Birgus latro, is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit of terrestrial animals with exoskeletons in today's atmosphere at a weight of up to...

) possess multiple gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s and small lung

Lung

The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s, with other varieties of gas diffusion methods supporting the transition from aquatic to terrestrial dwelling.

The developmental shift from water diffusion "gills" to air perfusion "lungs" may have been related to the need for reduce rates of water loss in air.