Nematomorpha

Nematomorpha, commonly known as horsehair worms or gordian worms, are a phylum of unsegmented, pseudocoelomate worms. They are parasitic in their larval stage and free-living as adults. Adults are found in freshwater habitats such as ponds, streams, and temporary pools, though some species are marine. Nematomorphs are characterized by their extremely long and slender bodies, which can measure from a few centimeters to over a meter in length. Their name derives from the resemblance of their bodies to strands of horsehair.

The life cycle of a nematomorph involves an aquatic or terrestrial arthropod host, such as a cricket, grasshopper, or beetle. Larvae are released from the adult female into water and are ingested by an intermediate aquatic host, often a freshwater crustacean. The larva then develops within the crustacean until it is consumed by a definitive terrestrial host. Once inside the final host, the juvenile worm grows and manipulates the host's behavior, often causing it to seek out water where the adult worm can emerge and reproduce. The adult worms do not feed and their sole purpose is reproduction. They are often found entangled in knots, leading to the term "gordian worm."