The foregut, stomach and intestine run a little below the midline of the body, the anus is at the tip of the tail, and the mouth is under the front.

All species have a proboscis which lies in the rhynchocoel when inactive but everts to emerge just above the mouth to capture the animal's prey with venom.

A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.

All nemerteans move slowly, using their external cilia to glide on surfaces on a trail of slime, while larger species use muscular waves to crawl, and some swim by dorso-ventral undulations.

The fertilized egg divides by spiral cleavage and grows by determinate development, in which the fate of a cell can usually be predicted from its predecessors in the process of division.

The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or larvae that resemble the planulas of cnidarians.

Traditional taxonomy says that nemerteans are closely related to flatworms, but both phyla are regarded as members of the Lophotrochozoa, a very large clade, sometimes viewed as a superphylum that also includes molluscs, annelids, brachiopods, bryozoa and many other protostomes.

In 1555 Olaus Magnus wrote of a marine worm which was apparently 17.76 metres (58.3 ft) long ("40 cubits"), about the width of a child's arm, and whose touch made a hand swell.

[9] The outermost layer of the body has no cuticle, but consists of a ciliated and glandular epithelium containing rhabdites,[10] which form the mucus in which the cilia glide.

[9] The proboscis of the class Anopla exits from an orifice which is separate from the mouth,[9] coils around the prey and immobilizes it by sticky, toxic secretions.

A typical member of this class has a stylet, a calcareous barb,[9] with which the animal stabs the prey many times to inject toxins and digestive secretions.

[23] Some nemerteans, such as L. longissimus, absorb organic food in solution through their skins, which may make the long, slim bodies an advantage.

The circulatory system consists of the rhynchocoel and peripheral vessels,[27] while their blood is contained in the main body cavity.

[27] Nemertea use organs called protonephridia[27] to excrete soluble waste products, especially nitrogenous by-products of cellular metabolism.

[31] In nemertean protonephridia, flame cells which filter out the wastes are embedded in the front part of the two lateral fluid vessels.

[28][38] The zygote (fertilised egg) divides by spiral cleavage and grows by determinate development,[38] in which the fate of a cell can usually be predicted from its predecessors in the process of division.

The planuliform larva stage may be short-lived and lecithotrophic ("yolky") before becoming a juvenile,[38] or may be planktotrophic, swimming for some time and eating prey larger than microscopic particles.

[33] However, many members of the order Heteronemertea and the palaeonemertean family Hubrechtiidae form a pilidium larva, which can capture unicellular algae and which Maslakova describes as like a deerstalker cap with the ear flaps pulled down.

[17] The terrestrial Argonemertes dendyi is a native of Australia but has been found in the British Isles, in Sao Miguel in the Azores, in Gran Canaria, and in a lava tube at Kaumana on the Island of Hawaii.

[42] Another terrestrial genus, Geonemertes, is mostly found in Australasia but has species in the Seychelles, widely across the Indo-Pacific, in Tristan da Cunha in the South Atlantic, in Frankfurt, in the Canary Islands, in Madeira and in the Azores.

[23] A few species live commensally inside the mantle cavity of molluscs and feed on micro-organisms filtered out by the host.

[44] Near San Francisco the nemertean Carcinonemertes errans has consumed about 55% of the total egg production of its host, the dungeness crab Metacarcinus magister.

[28] The American Cerebratulus lacteus and the South African Polybrachiorhynchus dayi, both called "tapeworms" in their respective localities, are sold as fish bait.

[44][47] Knaust & Desrochers (2019) reported fossils of vermiform organisms with a wide range of morphologies occurring on bedding planes from the Late Ordovician (Katian) Vauréal Formation (Canada).

[52] While Ruppert, Fox & Barnes (2004a) treat the Palaeonemertea as monophyletic,[51] Thollesson & Norenburg (2003) regard them as paraphyletic and basal (contains the ancestors of the more recent clades).

In nemerteans the space between the epidermis and the gut is mainly filled by well-developed muscles embedded in noncellular connective tissue.

This structure is similar to that found in larger flatworms such as polyclads and triclads, but a similar structure of body-wall muscles embedded in noncellular connective tissue is widespread among the Spiralia (animals in which the early cell divisions make a spiral pattern) such as sipunculans, echiurans and many annelids.

Bryozoa Annelida (and phyla merged into them) Mollusca Phoronida Brachiopoda Nemertea Dicyemida Myzostomida Platyhelminthes Gastrotricha (other Platyzoans) Nemerteans' affinities with Annelida (including Echiura, Pogonophora, Vestimentifera and perhaps Sipuncula) and Mollusca make the ribbon-worms members of Lophotrochozoa, which include about half of the extant animal phyla.

[57] Lophotrochozoa groups: those animals that feed using a lophophore (Brachiopoda, Bryozoa, Phoronida, Entoprocta); phyla in which most members' embryos develop into trochophore larvae (for example Annelida and Mollusca); and some other phyla (such as Platyhelminthes, Sipuncula, Gastrotricha, Gnathostomulida, Micrognathozoa, Nemertea, Phoronida, Platyhelminthes, and Rotifera).

[55][57] Most protostome phyla outside the Lophotrochozoa are members of Ecdysozoa ("animals that molt"), which include Arthropoda, Nematoda and Priapulida.