gill slit

Noun

Meaning

one of a series of slit openings in the pharynxes of fishes and aquatic amphibians through which water passes

Source: WordNet

Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. Source: Internet

Cod have a distinct white lateral line running from the gill slit above the pectoral fin, to the base of the caudal or tail fin. Source: Internet