What are pharyngeal slits or clefts?
In all chordate embryos, a series of pouches separated by grooves froms along the side of pharynx.
In most chrodates, these grooves (known as pharyngeal clefts) develop into slits that open to the outside of the body. These pharyngeal slits allow water entering the mouth to exit the body without passing through the entire digestive tract.
In vertebrates (except tetrapods), these slits and the structures that support them have been modified for gas exchange and are known as gill slits.
In tetrapods, the pharyngeal clefts do not develop into slits. Instead, they play an important role in the development of parts of the ear and other structures in the head and neck.