Horsehair worms can be discovered in damp areas, such as watering troughs, swimming pools, streams, puddles, and cisterns.

[8] The adults are mostly free-living in freshwater or marine environments, and males and females aggregate into tight balls (Gordian knots) during mating.

Absence of nematomorphs from riparian communities can thus lead to char predating more heavily on other aquatic invertebrates, potentially causing more widespread physiological effects.

During the larval stage, the animals show a resemblance to adult kinorhyncha and some species of Loricifera and Priapulida, all members of the group Scalidophora.

[20] The earliest Nematomorph could be Maotianshania, from the Lower Cambrian; this organism is, however, very different from extant species;[21] fossilized worms resembling the modern forms have been reported from mid Cretaceous Burmese amber dated to 100 million years ago.

[23] This order is monotypic containing the genus Nectonema Verrill, 1879: adults are planktonic and the larvae parasitise decapod crustaceans, especially crabs.

[23] They are characterized by a double row of natotory setae along each side of the body, dorsal and ventral longitudinal epidermal cords, a spacious and fluid-filled blastocoelom and singular gonads.

[23] Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled with mesenchyme in young animals but become spacious in older individuals.