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Nematodes

The nematodes are also known as roundworms because they are elongated and cylindrical in shape, with bilateral symmetry. Generally, they have a complete digestive tract and a muscular pharynx that is characteristically triradiate. They are provided with separate sexes, although some may be parthenogenetic. There are sensory organs in the anterior and posterior ends of the worm called amphids and phasmids, respectively. The latter are very useful in the grouping of the nematodes. Those roundworms with phasmids are described as phasmid nematodes, while those without them are described as aphasmid worms. Among the nematodes of medical and public health importance, only three are aphasmid worms (Adenophorea). These are Trichuris, Trichinella, and Capillaria. The rest of the nematodes are, therefore, phasmid nematodes (Secernentia). The phasmid worms belong to several orders in the scientific taxonomic classification of the worms. Ascaris belongs to Ascaridida, Parastrongylus and the hookworms to Strongylida, Strongyloides to Rhabditida, Enterobius to Oxyurida, and the filarial worms to Spirurida. -Belizario