Pharyngeal slit
Pharyngeal slits are filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth and out the pharyngeal slits. 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. 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. One theory of their origin is the fusion of nephridia which opened both on the outside and the gut, creating openings between the gut and the environment.
Pharyngeal slit-1