The evolution of polychaete buccal organs according to Dales (1962). Dales suggested that a simple ventral buccal organ was the primitive condition and that the muscular form seen in Eunicida evolved from this. However as can be seen in the lower part of the diagram he was uncertain as gto how to relate the polychaetes with simple axial proboscis and muscular axial proboscis to each other and to those with ventral buccal organs.