Nematodes are unsegmented cylindrical worms belonging to the phylum Nematoda, found in nearly every habitat and many are important parasites of plants and animals.
Structure and diversity
Nmatodes, also called roundworms, have a simple body plan consisting of a fluid-filled pseudocoelom enclosed by a tough, flexible cuticle. The cylindrical body tapers at both ends and is surrounded by longitudinal muscles that generate a characteristic sinusoidal movement. Unlike segmented annelids, nematodes lack true segmentation and appendages. The digestive tract extends from a muscular stoma through a tubular intestine to an anus near the posterior end. A dorsal and ventral nerve cord and a rudimentary excretory system run along the body. Most species are dioecious with separate male and female individuals; males often have copulatory spicules, while females are larger and produce large numbers of eggs. Over 25,000 species have been described, from tiny free-living soil dwellers less than a millimeter long to parasitic forms that can reach meters in length in the intestines of whales. Morphological diversity reflects adaptation to a wide range of habitats including marine sediments, fresh water, soil, and the tissues of plants and animals.
Ecology and importance
Nematodes are among the most abundant multicellular organisms on Earth, playing pivotal roles in ecosystems. Free-living species feed on bacteria, fungi and small invertebrates, contributing to nutrient cycling and soil structure. Plant-parasitic nematodes such as root-knot nematodes (Meloidogyne) and cyst nematodes (Heterodera) cause significant crop losses by feeding on roots and facilitating secondary infections. Animal parasites include intestinal roundworms (Ascaris lumbricoides), whipworms (Trichuris trichiura), hookworms (Necator americanus and Ancylostoma duodenale) and filarial worms (Wuchereria bancrofti, Brugia malayi) that cause lymphatic filariasis and onchocerciasis. These parasites have complex life cycles involving larval migration and often rely on intermediate hosts or vectors. On the positive side, the free-living nematode Caenorhabditis elegans has become a key model organism in developmental biology and neuroscience because of its defined cell lineage, transparent body and ease of genetic manipulation. Understanding nematode biology informs agriculture, medicine and basic science.
Nematodes illustrate how a simple body plan can adapt to diverse lifestyles ranging from free-living soil organisms to highly specialised parasites. Their ecological and medical impacts make them important subjects of research and management.
Related Terms: Helminth, Roundworm, Parasitology, Caenorhabditis elegans, Soil ecology