Final answer:
Pharyngeal slits in early chordates functioned as a filtering mechanism during feeding in aquatic environments. As chordates evolved, these structures adapted to become gill supports in fish and components of the ear and throat in tetrapods. In many chordates, they are present only in the embryonic stage, later developing into various structures.
Step-by-step explanation:
One characteristic found only in chordates is the presence of pharyngeal slits. Early in the history of chordates, the primary function of pharyngeal slits was to filter food particles from the water that entered the mouth during feeding. These slits are openings in the pharynx just posterior to the mouth which extend to the outside environment. In aquatic environments, they allow for the exit of water, while trapping food particles in a layer of mucus produced by the endostyle, which is part of the floor of the pharynx.
In different groups of chordates, pharyngeal slits evolve to serve other functions. For example, in fish, they are modified into gill supports and eventually into jaw supports in jawed fishes. In tetrapods, which evolved from aquatic ancestors to live on land, these slits become highly modified into components of the ear, tonsils, and thymus glands. In many chordates, including humans, pharyngeal slits are present only during embryonic development, where they later develop into different parts of the inner ear and other structures in the neck and throat.