Stylaria lacustris exhibits an elongated, vermiform body. It features a prostomium that elongates to form a proboscis, and eyes are present. The ventral bundles of setae consist of slender crotchets, with a proximal nodulus and a distal prong that is large and highly curved, while the proximal prong is very small, almost rudimentary. Dorsal bundles of setae begin in segment VI and comprise long hair setae alternating with very short, straight needles, which are single-pointed and lack a nodulus. The stomach is located in segments VIII or VII and VIII.