The nereis is also known as clam worm or rag worm. They occur in the sea. They are unisexual and has a reproductive phase called as heteronereis. They fertilize in sea water and form a larva. Their head is made up of prostomium and peristomium. The prostomium has couple of tentacles and palps. On the dorsal side of prostomium they have a couple of eyes. The multiple cirri are present on peristomium. They show metameric segmentation in which segments bear parapodia which helps in swimming. The anal segment has no parapodia but has elongated anal cirri. They belong to class polychaeta.