<P> Pharyngeal slits are filter - feeding organs found in Invertebrate chordates (lancelets and tunicates) and hemichordates living in aquatic environments . 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's 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 . </P> <P> In vertebrates, the pharyngeal arches are derived from all three germ layers . Neural crest cells enter these arches where they contribute to craniofacial features such as bone and cartilage . However, the existence of pharyngeal structures before neural crest cells evolved is indicated by the existence of neural crest - independent mechanisms of pharyngeal arch development . The first, most anterior pharyngeal arch gives rise to the oral jaw . The second arch becomes the hyoid and jaw support . In fish, the other posterior arches contribute to the brachial skeleton, which support the gills; in tetrapods the anterior arches develop into components of the ear, tonsils, and thymus . The genetic and developmental basis of pharyngeal arch development is well characterized . It has been shown that Hox genes and other developmental genes such as dlx are important for patterning the anterior / posterior and dorsal / ventral axes of the branchial arches . Some fish species have jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation . </P>

What is the function of pharyngeal pouches in aquatic chordates