page_title: Nematode
section_title: Taxonomy and systematics
hierarchical_section_title: Nematode / Taxonomy and systematics
caption_attribution_description: English: A gravid adult female Nippostrongylus brasiliensis worm was collected from the small intestine of an infected BALB/c mouse and photographed with a Nikon light microscope and mounted digital camera. The picture shows a hooked anterior end containing eggs. See Nieuwenhuizen et al. doi:10.1371/image.pntd.v07.i08.g001
context_page_description: The nematodes or roundworms constitute the phylum Nematoda, with plant-parasitic nematodes being known as eelworms. They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with insects and other moulting animals in the clade Ecdysozoa, and unlike flatworms, have tubular digestive systems with openings at both ends. Like tardigrades they have a reduced number of Hox genes, but as their sister phylum Nematomorpha has kept the ancestral protostome Hox genotype, it shows that the reduction has occurred within the nematode phylum.
  Nematode species can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species described to date vary by author and may change rapidly over time. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode, a claim which has since been repeated in numerous publications.
caption: Nippostrongylus brasiliensis
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