Dero dorsalis are medium-sized worms. They are brownish in color and have a blunt prostomium. Segmentation is clear. The dorsal bundle starts from segment IV, with one hair and one needle per bundle, although some specimens may have two hairs and two needles in segment VII. Ventral setae range from 2 to 4 per bundle. They possess five pairs of ciliated gills. The pharynx is located in segments II-III, while the esophagus is in segments IV-VII, continuing into a straight intestine in segment IX. They lack a stomach, and their blood is red. The first nephridium is found in segment VII.