[General] Any extension of the body wall which aids respiration.

[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.

[Pteropoda] In Gymnosomata, lateral and posterior appendages function as gills, real ctenidia are absent in Pteropoda (Pneumodermopsis in ventral view).

[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.

[Amphipoda] Hollow sacs inserted into the medial surfaces of the coxae. Typically there are 6 pairs of gills in gammarids, 5 in hyperiids, and 3 in caprellids, but some pairs may be reduced. Gills are always lacking on the first pereopods.

Alternative form for Gills : Gill.