Horsehair (Gordian) worms (phylum Nematomorpha) are extremely elongated cylindrical worms often exceeding 20 cm in length. The diameter of their unsegmented body is chiefly less than 1 mm. Their colour is light brown or reddish. Body surface is covered with thick, more-or-less hardened cuticle. Muscular system consists only of longitudinal muscle elements. The contraction of the muscles of such arrangement makes the slowly squirming motion that often results in intricate knots on their body.