Gill slits are present not just in vertebrates, but in hemichordates where they are used for filter-feeding instead of respiration. The only extand deuterostomic lineage that does not have them are echinoderms. However I came upon this when reading Wikipedia:
Intriguingly, extant echinoderms lack pharyngeal structures, but fossil records reveal that ancestral forms of echinoderms had gill-like structures
and this:
With the placement of hemichordates and echinoderms as a sister group to chordates, a new hypothesis has emerged-suggesting that pharyngeal gill slits were present in the deuterostome ancestor
Is it possible or likely that the deuterostome last common ancestor had gill slits?
