Helminths or worms are animals belonging to the Subkingdom, Metazoa, consisting of many cells. The term helminth, which is derived from the Greek words helmins or helminthos, a worm, is now-a-days usually applied only to the parasitic and non-parasitic species.
In a more restricted sense the name worm, or preferably helminth, is applied to a few phyla of animals, all of which superficially resemble one another in being unquestionably “worm like”, though in life and structure they are widely different and is now-a-days restricted to the members of the phyla Platyhelminthes, Nematoda and Acanthocephala.
Although the phylum Platyhelminthes, incudes the free-living turbellarians, has come to be regarded as being confined to the study of parasitic forms. Typically helminths parasitise vertebrates, although invertebrates, specially arthropods and molluscs, act as secondary (intermediate) hosts. Helminths are divided into two phyla: Phylum— Platyhelminthes (flat worms) and Aschelminthes or Nemathelminthes (round worms).
Phylum: Platyhelminthes (Flat worms):
This phylum includes worms whose body are dorsoventrally flattened, bilaterally symmetrical, without definite arms and body cavity. Organs embedded in specialised connective tissue termed as parenchyma. They are provided with a nervous system and an elaborate excretory system.
The excretory system is formed by flame cells, capillaries and tubules leading to the excretory vesicle. Respiratory and circulatory systems are lacking. They are mostly hermaphrodites. Flatworms parasitic to human belong to classes—Cestoda and Trematoda.
Classification of Helminths:
Phylum: Aschelminthes or Nemathelminthes (Round worms):
The helminths of this phylum are round. Animals that are un-segmented; bilaterally symmetrical, elongated, triploblastic but pseudo-coelomate are included under this phylum. Their alimentary tract is complete, elongated and straight which contains mouth and anal apertures at the two terminal ends.
They are pseudo-coelomate i.e., the body cavity is not lined by mesothelium. Flame cells are absent. Blood vascular system and respiratory system are also lacking. The sexes are separate. They belong to class Nematoda.