Protula tubularia forms a white, calcareous tube lacking distinct growth rings in which it lives. It has a small, elongated, segmented body that tapers towards the posterior and grows to 0.3-0.5 cm in length. The body is divided into a head, a short, green, anterior region of eight segments and a much longer reddish-orange, posterior region of up to 90-120 segments. The head is reduced and has two gill lobes each bearing 20-45 white or pinkish, slightly spiralled tentacles that are pinnate in two rows and marked with paired red blotches and numerous eyespots. The head is surrounded by a collar that bears finely toothed chaetae and extends dorsally to a distinctive, large, lobed membrane either side of the thorax. Chaetae are present on all segments except the first. Chaetae in the thoracic region are pale golden, long and slender and display narrow wings. Those in the posterior region are longer but lack narrow wings.
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