Nemertines are characteristically elongate, vermiform animals with soft bodies, often capable of extreme contraction and elongation, which range in length from a few millimetres up to several metres. The length, width and shape of the body depend on the degree of contraction or extension.
Nemertines do not possess a distinct head. Anteriorly they are mostly pointed, rounded or blunted, although in some genera there is an anterior cephalic lobe, which is lanceolate, spatulate, semi-circular or heart-shaped. The cephalic region of many species bears transverse, oblique or longitudinal furrows, the cephalic grooves or slits (Cephalic furrow). The anterior region also contains the eyes, although many species, at least as adults, lack them. Eye number varies both inter- and intraspecifically, ranging from two to many. The eyes are arranged more or less bilaterally. In species with 2-6 eyes the number is usually constant, whereas in other forms, eye number often increases with age and size. Frequently the eyes are partially or almost completely obscured by the cephalic pigmentation.
The proboscis pore opens at or just below the anterior tip in most nemertines, but is located further back in some genera, or lies close to the cerebral ganglia in others. The mouth and proboscis are separate in the class Anopla and usually open via a common anterior aperture in the class Enopla. In members of the Enopla with separate oral and proboscis apertures, the mouth is always located close to the proboscis pore and anterior to the cerebral ganglia.
The posterior end of the body usually either tapers gradually to a sharp or blunt tip, or terminates in a slender tail or caudal cirrus (Caudal cirrus). The anus opens at or just dorsal to the posterior end or at the base of the caudal cirrus when one is present. In Malacobdella grossa the anus is terminal, but above the sucker.
Many species are more or less uniformly white or cream coloured or tinted in shades of grey, brown, red, orange, pink, green or yellow. In pale coloured forms, or in those in which the body is translucent, internal organs are often discernible through the body wall. In some species colour changes apparent during the reproductive period can be attributed to pigments contained within the mature gonads, and differences between the sexes can lead to a colour dimorphism. Colour variations in the intestinal regions may be due to the gut contents. There are many nemertines that possess striking and distinctive colour patterns. These patterns are often species specific and are frequently more clearly marked on the dorsal surface or are confined to the cephalic regions.
Source: Gibson, 1982.