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PRECAMBRIAN PERIOD

555-545 million years ago

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digestive tract in hemichordate

Ancestral hemichordates evolved the precursor of gills although these pharyngeal slits were used primarily for feeding rather than gas exchange. The mucociliary mechanism which once functioned in food capture would be used by higher vertebrates to remove debris from the respiratory tract. Although ancestral hemichordates lacked a postanal tail, their digestive tract shared some features with that of vertebrates such as the presence of goblet cells and microvilli.

Ciliated pseudostratified epithelia (which lines the human trachea) exists in the pharynx of hemichordates. (Benito, form Harrison 1997, p. 72.)
Deuterostomes originally seem to have used their pharyngeal slits for filter feeding. These structures cannot be considered gills in hemichordates given that they possess too few blood vessels servicing the pharyngeal slits to perform much gas exchange (Harris; Benito, form Harrison 1997, p. 59). In hemichordates (acorn worms) the pharynx includes pouches which open to the exterior through slits and solid endoskeletal bars, just as in the gill-bearing pharynx of fish (Benito, form Harrison 1997, p. 20).

From the mouth, food passes through the pharynx to the intestine. The ability to pass water through pharyngeal slits is involved in feeding in primitive filter-feeding chordates and hemichordates: food from the water becomes stuck in the mucus lining the pharynx and is carried to the intestine by ciliary action. Pharyngeal slits became involved with respiration secondarily and higher vertebrates use this mucociliary mechanism as a way of directing microbes and dirt to the stomach and away from the lungs. Most filter feeding animals make use of mucus to capture the food and cilia to transport it. The development of a perforated, muscular pharynx in chordates allowed the change from microphagy (feeding on small material suspended in water) to macrophagy (feeding on larger material)(Barrington).
In hemichordates, food travels from the pharynx into the esophagus where peristalsis moves it through the remainder of the gastrointestinal tract (Burighel, from Harrison, 1997, p. 255). In hemichordates and tunicates, as in higher chordates, microvilli in the GI tract form brush border extending columnar epithelial cells to increase surface area for secretion and absorption (Benito, form Harrison 1997, p. 88). Unlike hemichordates and protostomes, the digestive tract of chordates does not proceed to the tip of the abdomen. One of the major traits which all chordates share is that of a muscular, postanal tail.