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The word ‘helminth’ is a general term meaning ‘worm’, but there are many different types of worms.
Prefixes are therefore used to designate types: platy-helminths for flat-worms and nemat-helminths for round-worms.
All helminths are multicellular eukaryotic invertebrates with tube-like or flattened bodies exhibiting bilateral symmetry.
In contrast, segmented annelids (such as earthworms) are coelomate (with body cavities enclosed by mesoderm).
Many helminths are free-living organisms in aquatic and terrestrial environments whereas others occur as parasites in most animals and some plants.
Parasitic helminths are an almost universal feature of vertebrate animals; most species have worms in them somewhere.
Three major assemblages of parasitic helminths are recognized: the Nemathelminthes (nematodes) and the Platyhelminthes (flatworms), the latter being subdivided into the Cestoda (tapeworms) and the Trematoda (flukes): (roundworms) have long thin unsegmented tube-like bodies with anterior mouths and longitudinal digestive tracts.
They have a fluid-filled internal body cavity (pseudocoelum) which acts as a hydrostatic skeleton providing rigidity (so-called ‘tubes under pressure’).
Worms use longitudinal muscles to produce a sideways thrashing motion.
Adult worms form separate sexes with well-developed reproductive systems.(tapeworms) have long flat ribbon-like bodies with a single anterior holdfast organ (scolex) and numerous segments.
They do not have a gut and all nutrients are taken up through the tegument.
They do not have a body cavity (acoelomate) and are flattened to facilitate perfusion to all tissues.
Segments exhibit slow body flexion produced by longitudinal and transverse muscles.
All tapeworms are hermaphroditic and each segment contains both male and female organs.