It was originally thought they arose spontaneously from hairs of a horse's tail. They appear to be non-specific parasites, and all but one marine genus live in fresh water or damp soil. The body of the hairworm is extremely long and threadlike with length of 1 foot or more being typical.
Food is adsorbed through the body wall as their digestive tract is vestigial.
The adults do not feed. The hairworms can be confused with the mermithid nematodes, which also parasitize insects. The latter have pointed ends to their tapered bodies. Locomotion consists of bending and writhing movements. Males can swim in a crude, undulatory manner.