What features of annelids have allowed them to populate most of the planet?

Ringed worms have the highest organization of all worms. They are characterized by such progressive features as:

– metamerism, or segmentation;

– the presence of bifurcated parapodia, the first limbs (in polychaetes);

– secondary body cavity;

– periopharyngeal nerve ring and abdominal nerve cord;

– relatively developed sense organs;

– great differentiation of the digestive system;

– isolation with metanephridia;

– a closed circulatory system;

– breathing through the skin or gills.