The body reaches a length of up to 50 cm but is only 3-4 mm in maximum width. The head is rounded, wider than the adjacent trunk, rather dorsoventrally flattened and bears 20-30 or more small eyes on each side arranged into antero- and posterolateral groups of about equal numbers (E. gracile). In small and young individuals there are generally fewer eyes and their distribution is irregular. The body is long and slender, very contractile and of a more or less uniform width throughout most of its length, only narrowing posteriorly to end in a blunt rounded tail. The dorsal colouration varies from a dull olive-green to greyish-green or a dark blue-green, occasionally greenish-brown. Often irregularly shaped and distributed small dark green or black pigment flecks are scattered over the dorsal and dorsolateral surfaces. A blue-grey iridescence, due to the flickering of the epidermal cilia, is sometimes apparent. At the rear of the head the cerebral ganglia show as two dull red lobes. Ventrally the body is a pale greyish-yellow, greyish-green or dirty yellowish-white colour, through which mature gonads show white or yellowish. Usually the ventral colouration appears as a distinct broad longitudinal stripe clearly bordered by ventrolateral extensions of the dorsal pigmentation. Cerebral sensory organs are small, located some distance in front of cerebral ganglia; cephalic glands are voluminous, posteriorly extending into foregut regions. Reproduces during the months of April, May and June.
A fairly common species on rocky shores. Occurs intertidally beneath stones and boulders on coarse muddy silt, shelly gravel or silty sand, in rock crevices and clefts, in cavities in boulders, in mussel beds or among the holdfasts of laminarians. Mostly occupying the mid- to lower-shore levels, the species may occasionally be found crawling between barnacles high up the shore when the tide is out. Copious amounts of a thick and sticky mucus are secreted when the animal is disturbed, and fine particles of mud or silt may adhere to the mucus to form a tube in which the animal rests. Among algal holdfasts or beneath boulders Emplectonema gracile is most frequently found with its body in a knotted tangle which is extraordinarily difficult to unravel without rupturing it. Occasionally it is dredged from coarser sediments in shallow water and in other parts of the world has been found at depths down to 100 m.
The species possesses a wide geographic range and is recorded from the west coast of North America, Chile, the Aleutian Islands, the northern coasts of Europe, the Mediterranean, Madeira, the Kamchatka Peninsula of Russia and Japan.