鰓 - gills, gill slits

nudibranchs

gill plate; branchial plate; gill lamella; branchial lamella

gill sac; branchial sac; branchial pouch; inner branchial groove; pharyngeal pouch

opisthobranchs

ctenidium; comb gill

gill slit

sea pen (any coelenterate of the family Pennatulidae or related families); sea feather

prosobranchs

gill cover; operculum

gill slit

elasmobranchs

lamellibranches (i.e. bivalves)