Threadworm. Coloured scanning electron micrograph (SEM) of the head of a thread worm (Enterobius sp.). This is a nematode worm that is a parasite of the small intestines of numerous animals. It is also known as a roundworm and pinworm. Nematodes are unsegmented worms that may be parasitic or free-living depending upon the species. Magnification: x100 when printed at 10 centimetres wide.