Up to 20 cm or more long and 2-3 mm wide, with a slightly flattened body, which gradually tapers posteriorly to end in delicate and slender caudal cirrus. The head is bluntly rounded, lacks eyes and is not distinctly marked off from the remainder of the body.
The colour is a dark rich purplish-brown on the dorsal surface, in life often appearing iridescent when light is reflected by the beating epidermal cilia. The head is generally darker, with the cerebral ganglia visible through the body wall as a pair of reddish lobes, but is anteriorly marked by a transverse band of brilliant yellow or yellowish-white which is characteristic of the species. On occasion this band may be divided into two yellow spots, rarely it is completely absent. In front of the band the snout is whitish or translucent, behind it there are usually a few white granules. The ventral body surface is a similar colour to the dorsal, although it is frequently somewhat paler in hue. The caudal cirrus too is pigmented, usually a pale brown colour (M. purpurea). Dermis with a distinct connective tissue layer. Proboscis with two muscle layers and two muscle crosses. Thick dorsoventral muscles developed between the intestinal diverticula. Sexually mature individuals are found during the April to June. In captivity Micrura purpurea demonstrates voracious feeding habits and readily devours worms of various types; it is sometimes cannibalistic.
Micrura purpurea is often found associated with the related species, Micrura fasciolata , and occurs in an equally wide range of habitat types. It is most commonly obtained by dredging muddy, shelly, sandy or gravelly sediments at depths of 10-40 m, but may dwell intertidally near low water level in rocky clefts, among laminarian holdfasts, beneath stones and boulders, or between the shells of bivalve molluscs.
The geographic distribution of Micrura purpurea parallels that of Micrura fasciolata.