Pharyngeal slit

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).

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.[1] These repeated segments are controlled by similar developmental mechanisms. Some hemichordate species can have as many as 200 gill slits.[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.[3] However, it is now accepted[who?] that it is the vertebrate pharyngeal pouches and not the neck slits that are homologous to the pharyngeal slits of invertebrate chordates.[citation needed] Pharyngeal arches, pouches, and clefts 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.[4]

  1. ^ Kardong KV (2014-02-14). Vertebrates : comparative anatomy, function, evolution (Seventh ed.). New York, NY. ISBN 9780078023026. OCLC 862149184.{{cite book}}: CS1 maint: location missing publisher (link)
  2. ^ Gerhart J, Lowe C, Kirschner M (2005). "Hemichordates and the origin of chordates". Current Opinion in Genetics & Development. 15 (4): 461–467. doi:10.1016/j.gde.2005.06.004. hdl:2060/20020085372. PMID 15964754.
  3. ^ Gould, S.J. (1977). Ontogeny and Phylogeny. Cambridge, Massachusetts: The Belknap Press of Harvard University Press. pp. vii–viii. ISBN 978-0-674-63940-9.. Also ISBN 0-674-63941-3 (paperback)
  4. ^ Ezhova, Olga V.; Malakhov, Vladimir V. (2015). "The nephridial hypothesis of the gill slit origin". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 324 (8): 647–652. Bibcode:2015JEZB..324..647E. doi:10.1002/jez.b.22645. PMID 26227807.