The typical nemertean body is very slim in proportion to its length. The smallest are a few millimeters long, most are less than 20 centimetres (7.9 in), and several exceed 1 metre (3.3 ft). The longest animal ever found, at 54 metres (177 ft) long, may be a specimen of Lineus longissimus, although L. longissimus is usually only a few millimeters wide. The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species, but reduce their length to 50% and increase their width to 300% when disturbed.[ A few have relatively short but wide bodies, for example Malacobdella grossa is up to 3.5 centimetres (1.4 in) long and 1 centimetre (0.39 in) wide, and some of these are much less stretchy. Smaller nemerteans are approximately cylindrical, but larger species are flattened dorso-ventrally. Many have visible patterns in various combinations of yellow, orange, red and green.
The outermost layer of the body has no cuticle but consists of a ciliated and glandular epithelium containing rhabdites, which form the mucus in which the cilia glide. Each ciliated cell has many cilia and microvilli. The outermost layer rests on a thickened basement membrane, the dermis. Next to the dermis are at least three layers of muscles, some circular and some longitudinal. The combinations of muscle types vary between the different classes, but these are not associated with differences in movement. Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species. Inside the concentric tubes of these layers is mesenchyme, a kind of connective tissue. In pelagic species this tissue is gelatinous and buoyant.
The mouth is ventral and a little behind the front of the body. The foregut, stomach and intestine run a little below the midline of the body and the anus is at the tip of the tail. Above the gut and separated from the gut by mesenchyme is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. The rhynchocoel of class Anopla has an orifice a little to the front of the mouth, but still under the front of the body. In the other class, Enopla, the mouth and the front of the rhynchocoel share an orifice. The rhynchocoel is a coelom, as it is lined by epithelium.