The Capillaria Nematode (Capillaria pterophylli) is a freshwater aquatic parasitic roundworm in the Trichosomoididae family of nematodes. It is found in and attached to Goldfish and other freshwater fish. It is a disease in the fish, and eventually kills the fish. It is a capillarid. 

The Capillaria Nematode is very small and difficult to see. It is transparent (clear or see-through). It is long and cyclindrical like a worm (but not flat like a flatworm). It is smooth. It has a thorny head at one end.