A variable, but often complex set of folds, musculature and glands present on the ventral side of the buccal (mouth) opening many polychaetes is usually referred to as the ventral buccal organ. Dales (1962) and Purschke & Tzetlin (1996) have argued that a ventral buccal organ represents the primitive condition for Polychaeta. However, in one of the analyses in Rouse and Fauchald (1997) a ventral buccal organ appears to have evolved between two and four times independently. These results do not support the suggestion by Dales (1962) and Purschke & Tzetlin (1996) that the majority of ventral buccal organs in polychaetes are homologous. However, Rouse and Fauchald (1997) did find ventral buccal organs could represent the primitive condition for Polychaeta and have subsequently transformed into various other forms of polychaete buccal organs.

Among polychaetes there are various ventrally differentiated buccal organs; but two fundamental types are recognised here:

1. An eversible, ventrally muscular, buccal organ. This condition is only found in members of Eunicida and may have evolved from the simple ventral buccal bulb condition. Jaws are present in some taxa.

2. A simple ventral buccal bulb, that may or may not be eversible. This kind of buccal organ is widespread among polychaetes and is possibly the primitive condition for the group and depending on the taxon, is used in a variety of ways, from ingesting sediment to food sorting to tube-building.