Pharyngeal slit

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Pharyngeal slits, characteristic of both hemichordata and chordata, are used by organisms in feeding. The wall of the pharynx is perforated by up to 200 vertical slits, which are separated by stiffening rods.

Rows of beating cilia cause currents of water to flow through the mouth, through the pharyngeal slits and out of the body through a hole in the body wall called the atriopore. Small particles in the water are trapped by the cilia in different parts of the mouth chamber and separated into materials that the organism can eat.

In primitive chordates the pharyngeal slits are used to strain water and filter out food particles; in fish they are modified for respiration. In tetrapods, they occur only in the embryo and disappear as development progresses. All vertebrates possess pharyngeal gill slits (and tails) in early embryonic stages, lending weight to the theory of common ancestry.

This is one of five hallmark characteristics that all chordates possess at some point in their life, along with the notochord, endostyle, post-anal tail and dorsal hollow nerve cord.

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