Helminths are a group of organisms which share a similar form but are not necessarily related as part of evolution. The term "helminth" is an artificial term. There is no real consensus on the taxonomy (or groupings) of the helminths, particularly within the nematodes. The term "helminth" contains a number of phyla, many of which are completely unrelated. However, for practical considerations the term is currently used to describe four phyla with superficial similarities: Annelida (ringed or segmented worms), Platyhelminthes (flatworms), Nematoda (roundworms), and Acanthocephala (thorny-headed worms). The phylum Platyhelminthes includes two classes of worms of particular medical significance: the cestodes (tapeworms) and the trematodes (flukes and blood flukes), depending on whether or not they have segmented bodies.