How did early agnathans probably feed on tiny organisms?

Early agnathans likely fed on tiny organisms through filter feeding, a process similar to that observed in the larvae of modern lampreys.1

These early vertebrates possessed a large gill cavity that served both respiratory and feeding functions. Small organisms were likely ingested via a nibbling or sucking action through the mouth and passed into the gill cavity along with water. Once inside, the gill apparatus strained the organisms from the water and directed them toward the food canal. Some extinct forms, such as certain Osteostraci, also exhibited mouth shapes and sizes that suggest they were specialized filter feeders.12