Nematodes

Nematodes, or roundworms, form the phylum Nematoda, a large and diverse group of slender, unsegmented worms. They are typically cylindrical and taper at both ends, with a flexible cuticle secreted by a syncytial epidermis. Most have a simple body plan: a complete digestive system and a pseudocoelom, but they lack both circulatory and respiratory systems.

Nematodes have a roughly cylindrical body with longitudinal muscles that produce thrashing movements. The nervous system includes a nerve ring around the pharynx and dorsal and ventral nerve cords. Gas exchange occurs by diffusion across the cuticle and body walls, and excretion is carried out by specialized excretory structures. The mouth leads to a muscular pharynx, then to a gut that culminates in the anus.

Habitat ranges from soil and freshwater to marine environments. Free-living nematodes are important decomposers and participate in nutrient cycling, while many species are obligate parasites of plants, animals, and humans. Plant-parasitic nematodes damage crops; human and animal infections include pinworms, hookworms, and filarial worms, often with complex life cycles involving larval stages.

Most nematodes reproduce sexually, with separate sexes in many species, though some are hermaphroditic or parthenogenetic. Development typically includes several molts through larval stages (commonly L1 to L4) before reaching reproductive maturity. The model organism Caenorhabditis elegans has been central to studies of genetics, development, and cell biology.

The phylum comprises thousands of described species, with many more believed to exist. Nematodes have profound ecological and economic importance as parasites, biocontrol agents, and model organisms in biology.