Pharyngeal slits are filter-feeding organs found in
Invertebrate chordates (
lancelets and
tunicates) and
hemichordates living in aquatic environments. These repeated segments are controlled by similar developmental mechanisms. Some hemichordate species can have as many as 200
gill slits.
Pharyngeal slits resembling gill slits are transiently present during the embryonic stages of
tetrapod development. The presence of gill-like slits 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. However, it is now accepted that it is the vertebrate pharyngeal pouches and not the neck slits that are
homologous to the pharyngeal slits of invertebrate chordates. Gill slits are, at some stage of life, found in all chordates.