Nematomorpha also is known as hairworms. About 300 freshwater abd 5 marine horsehair worms (Nematomorpha) species have been described to date. They are parasitic in arthropods during their juvenile stage. When they become adult they live in an aquatic environment as free-living organisms. Most aspects of their systematics and biology of Gordius aquaticus (Nematomorpha) found in Sarıyer, Istanbul. After observing  the morphology, the anatomy of the specimen was investigated under light microscopy to confirm the species as Gordius aquaticus.