Helminths (/ˈhɛlmɪnθs/), also commonly known as parasitic worms, are large multicellular organisms, which can generally be seen with the naked eye when they are mature. They are often referred to as intestinal worms even though not all helminths reside in the intestines. For example, schistosomes are not intestinal worms, but rather reside in blood vessels. The word helminth comes from ancient Greek ἕλμινς, hélmins, an intestinal worm.