A branchiostegal lung is a

respiration organ The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gr ...

used by some air-breathing

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

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

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

s and

hermit crab Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an a ...

s such as the

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 terrestrial arthropod in the world, with a weight of up to . It can grow to up to in width from the tip ...

to their terrestrial habitats. The branchiostegal (gill) tissue is supported by folds or other mechanisms to increase

surface area The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...

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 :''The junior homonym ''Coenobita'' Gistl, 1848 is now the moth genus '' Ectropis''. The genus ''Coenobita'' contains 17 species of terrestrial hermit crabs. Several species in this genus are kept as pets. Ecology ''Coenobita'' species carry wa ...

'') possess multiple

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s and small

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

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 reduced rates of water loss in air.

References

Organs (anatomy) Invertebrate respiratory system Crustacean anatomy {{crab-stub