Chordate pharyngeal slits appear to have functioned first as

suspension-feeding devices.

Explanation:

Chordates are animals that are characterized with the presence of pharyngeal slits, notchord,  a dorsal hollow nerve cord and another tail feature.

The pharyngeal slits of chordates develops in the internal pharynx  as a cleft, which further extends into the external pharynx. The pharyngeal slits in aquatic chordates functioned first as suspension-feeding device, which acts by straining foods from water to be ingested by such animals.

The pharyngeal slit is now commonly known as a filter feeding device

it takes hundreds of years to become stable.

if you have another question lmk.