Pharyngeal slit

It is postulated that this is how pharyngeal slits first assisted in filter-feeding, and later, with the addition of gills along their walls, aided in respiration of aquatic chordates.

[2] Pharyngeal clefts resembling gill slits are transiently present during the embryonic stages of tetrapod development.

The presence of pharyngeal arches and clefts in the neck of the developing human embryo famously led Ernst Haeckel to postulate that "ontogeny recapitulates phylogeny"; this hypothesis, while false, contains elements of truth, as explored by Stephen Jay Gould in Ontogeny and Phylogeny.

[10] 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 .

[12] Comparative developmental and genetic studies of these pharyngeal structures between hemichordates and urochordates have brought about important insights regarding the evolution of the deuterostome body plan.

image showing gill slits in acorn Worm and tunicate
The presence of gill slits (in blue) in an acorn worm (left) and a tunicate (right).
phylogeny
A phylogeny showing when gill slits may have arisen. It is thought that gill slits were subsequently lost in echinoderms .