Generally, any extension of the body wall which aids respiration.

[Cephalopoda] In decapods and octopods (other than cirromorphs) the gills are not flattened; the inner and outer demibranchs are attached to a narrow central axis and typically are arranged vertically in two diverging rows or are oriented parallel to the gill axis (lamellae perpendicular to axis). In sepioids the gills have free lamellae (not attached at tip) and have no branchial canal.

[Gastropoda] Real gills, the so called ctenidia, are present in heteropods; in Atlantidae they are in the mantle cavity, in the other families they hang out of the mantle in the surrounding water; real ctenidium are absent in Pteropoda, the appendages lateral and posterior to the body function like, and are named, gills.

[Crustacea, Eumalacostraca] The highly-branched respiratory structures arising from the outer surface of the first segment (coxa) of the 2nd-8th thoracic legs.