This is an inadequately described species. Up to about 20 mm long and 0.6-0.8 mm wide. The head is typically oval in shape, flattened and wider than the adjacent body regions, with two pairs of cephalic furrows. The general body colour is dull whitish, salmon, pink, pale orange or, less frequently, a rich apricot yellow. The intestinal region occasionally appears pale green. On either side of the head a dark brown longitudinal pigment streak, characteristic for the species, extends between the anterior and the posterior eyes (T. vermiculus-detail). The streaks may be of uniform width or be broader near the front. A single mid-dorsal whitish stripe is sometimes present, commencing between the posterior pair of eyes. This stripe may be quite short or extends some distance down the body, although it is not always evident.
Tetrastemma vermicularis appears to have a prolonged reproductive period, as ripe individuals have been found from late autumn through to early summer.
Often locally quite abundant, this lively and restless species is found on the lower shore and sublittorally to depths of about 40 m. Occurs under rocks and stones, on small algae or bryozoans, or in laminarian holdfasts.
Extends from the British Isles to Scandinavia, the Mediterranean, Madeira, and the eastern coast of North America from the Bay of Fundy southwards to the Gulf of Mexico.