Nematomorpha

Definition

A phylum of elongate, unsegmented, pseudocoelomate worms commonly called horsehair worms or Gordian worms. Adults are free-living in freshwater or moist terrestrial habitats, while larvae are obligate parasitoids of arthropods, manipulating host behavior to drive infected insects or crustaceans to water where the adult worm emerges to reproduce. Morphologically convergent with but phylogenetically distinct from Nematoda, with which they share the clade Nematoida within Ecdysozoa.

Etymology

Greek nema (thread) + morphe (form), from resemblance to nematode worms

Example

A cricket infected by a nematomorph larva may seek out and jump into a stream, allowing the 30-cm adult worm to emerge and complete its life cycle.

Synonyms

• Horsehair worms
• Gordian worms

Related Terms

• Nematoda
• Parasitoid
• host manipulation
• Ecdysozoa
• Protostomia
• Gordioidea
• Nectonematoida

Usage Notes

Often confused with nematodes due to superficial similarity, but distinguished by cuticular structure, larval development, and adult habitat. The common name 'Gordian worm' refers to the adult's habit of coiling into tight knots. The phylum contains two orders: Gordioidea (freshwater, terrestrial) and Nectonematoida (marine).