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gill slit An opening leading from the pharynx to the exterior in aquatic vertebrates and lancelets. In lancelets they function in filter feeding. In fish they contain the gills and are usually in the form of a series of long slits. They are absent in adult tetrapod vertebrates (except for some amphibians) but their presence in some form in the embryos of all vertebrates is a characteristic of the phylum Chordata.