Annelids are similar to Nematodes. They are bilaterally symmetrical, cylindrical in shape and triploblastic. Still, they are placed in a separate category. Why?
The difference in the type of coelom categorises Annelids and Nematodes. Annelids are the first organism to show true body cavity whereas Nematodes have false coelom (pseudocoelom). Annelids are different from nematodes in that they have a segmented body. Annelids show a well developed circulatory system which is absent in nematodes. Thus annelids are little more complex in the organisation than the nematodes.