Defining characteristics of the phylum Annelida include many aspects of their body plan and segmentation.  The class Hirudinea, although retaining most of these defining characteristics, are a very unique class within the phylum due to adaptations made to accommodate a parasitic lifestyle.  This class does not live in the substrate like the other two classes of annelids, and have adapted both their external structure and internal anatomy in response.  The two most noticeable adaptations being the loss of setae, small bristles on the outside of the body wall used for traction in the sand, and the loss of parapodia, appendages on the lateral surfaces of annelids used in locomotion.  Notostomum cyclostoma is a leach characterized by a slender body and both anterior and posterior suckers.  For more detailed explanations these adaptations, along with an overview of the external and internal structures of the animal, click the links below.
External Structure  / Internal Anatomy