Nematoda

Nematoda, commonly known as roundworms, is a diverse phylum of mostly microscopic to several-centimeter-long worms. They are unsegmented, cylindrical pseudocoelomates with a moist, flexible cuticle that is molted during development. The body has a simple architecture: a longitudinal muscle layer operating inside a hydrostatic skeleton, and there is no true circulatory or respiratory system. The digestive system is a tube with a mouth at one end and an anus at the other, and the nervous system includes a nerve ring around the pharynx with longitudinal nerve cords.

Reproduction is typically sexual, with separate males and females, though some species are hermaphroditic or undergo parthenogenesis. Males often bear copulatory spicules. Development usually includes several larval stages (L1–L4) before adulthood; many free-living species display direct development, while some parasitic species have complex life cycles with intermediate hosts or specialized stages.

Ecology and significance: Nematodes inhabit virtually every environment, including soil, freshwater, and marine ecosystems. They feed on bacteria, fungi, or decaying organic matter, while many are plant parasites or animal parasites, including several notable human pathogens. Examples of human-infecting nematodes include Ascaris lumbricoides, Enterobius vermicularis, Trichuris trichiura, and hookworms. The model organism Caenorhabditis elegans has been extensively used in biology to study development, neuroscience, and genetics. Some nematodes are employed in biological control of pests.

Taxonomy and impact: Nematoda is a phylum within the group Ecdysozoa, which molts its cuticle (ecdysis). They are among the most abundant animals on Earth by individual numbers, contributing to soil fertility, nutrient cycling, and interactions with microbial communities, as well as playing roles as parasites of plants and animals.