Phylum Annelida : Morphology, Physiology, Classification & Evolutionary Significance | UPSC GS III Annelida, one of the most important phyla in animal kingdom, introduces true coelom, segmentation, closed circulatory system, and advanced excretory organs (nephridia) for the first time in evolution. From the humble earthworm (Pheretima) improving soil fertility, to the leech (Hirudinaria) as a blood-sucking parasite, and the marine polychaetes with parapodia, Annelids display remarkable diversity