nematod

Nematod, more commonly spelled nematode, refers to members of the phylum Nematoda, a large and diverse group of slender, cylindrical worms known as roundworms. They occur in almost every environment, from soil and freshwater to marine habitats and the bodies of plants and animals. Sizes range from microscopic to several centimeters, and many free-living species play important roles in decomposition and nutrient cycling, while numerous others are parasitic.

Anatomy and physiology: Nematodes are unsegmented and covered by a flexible cuticle that they molt as they grow. They lack specialized circulatory and respiratory systems; gas exchange and waste removal occur mainly by diffusion. The body cavity is a pseudocoel, which functions as a hydrostatic skeleton. The nervous system typically includes a nerve ring and longitudinal nerve cords. Most species have separate sexes and reproduce sexually, though some reproduce asexually or show complex life cycles. The digestive tract runs from mouth to anus, forming a complete internal tube.

Ecology and life cycles: Free-living nematodes inhabit soil, freshwater, and marine environments and contribute to soil structure and nutrient cycling. Plant-parasitic nematodes feed on roots and can cause substantial crop losses, with groups such as root-knot nematodes (Meloidogyne) and cyst nematodes (Heterodera, Globodera). Animal parasites include Ascaris lumbricoides and hookworms. Lifecycle stages typically include eggs and several larval molts; many species have direct life cycles, though some require intermediate hosts or exhibit diapause under unfavorable conditions.

Significance: Caenorhabditis elegans is a widely used model organism in developmental biology and neuroscience. Nematodes are studied for their ecological roles, in agriculture as biological control agents (for example, certain entomopathogenic nematodes), and as agents of human and animal disease.