The respiratory organ of most aquatic animals that breathe water to obtain oxygen, consisting of a filamentous structure of vascular membranes across which dissolved gases are exchanged. [Free_Online_Dictionary], Anatomical structure consisting of a series of membranes which allow dissolved oxygen from the water to pass into the bloodstream and carbon dioxide to pass out of the bloodstream.[AAO]

uberon

branchia, gills

Gill slits in the pharyngeal region of the intestine, which are also present in (at least) tunicates and acranians, are taken over to the craniote ancestor.[well established][VHOG]

UBERON:0011150