There are about 900 known species in the phylum Nemertini
(also spelled Nemertina or Nemertea by different authors).
Nemertines are known as "ribbon worms" because of the great length of
many species; the European nemertine Lineus longissimus has been
known to reach 30 meters (nearly 100 ft) in length, although most are much
shorter. Most nemertines are marine, but there are
a few freshwater species, and even a few species that live in moist
tropical habitats on land. The beautifully banded marine nemertine
shown to the left, Basiodiscus mexicanus, was photographed at Los Arcos,
near Puerto Vallarta, Mexico.
The most distinctive feature of nemertines is a large proboscis, which you can clearly see in the image to the right. In one group of nemertines is tipped with a piercing barb known as a stylet. In other nemertines, the proboscis is unarmed, but often secretes sticky fluid. Normally, this proboscis is retained in a specialized sac in the animal's body, the rhynchocoel. To capture prey, the proboscis is rapidly everted (turned inside-out) and shot out of the rhynchocoel. It wraps around the prey, and toxic secretions immobilize the prey; nemertines with stylets use them to stab the prey repeatedly, introducing toxins into the body. Generally, nemertines are carnivorous; most feed on small invertebrates like crustaceans and annelids, but some feed on the eggs of other invertebrates, and a few live inside the mantle cavity of molluscs and feed on microbes filtered out by the host.
Nemertines were once classified close to the flatworms, which they superficially resemble. Like flatworms, nemertines are soft and unsegmented. However, nemertines have major features that flatworms lack, notably a complete gut with an anus, and a system of blood vessels. This vessel system may in fact be homologous with the coelom, or fluid-filled lined body cavity, found in many other invertebrates. Flatworms lack a coelom entirely, and this suggests that nemertines are not close relatives of flatworms. Recent molecular studies have tended to confirm this view, placing nemertines among the trochozoan coelomates such as annelids and molluscs than to flatworms.
The fossil record of nemertines is extremely sparse, as would be expected for this completely soft-bodied group. The stylets of nemertines would be expected to survive as fossils, since they are made of the mineral calcium phosphate, but none have been reported so far. The only fossil that may be a nemertine is Archisymplectes from the Pennsylvanian-age Mazon Creek biota of northern and central Illinois. Even this fossil only preserves the outline of the worm, and it is not absolutely certain that it represents a nemertine. The Cambrian fossil Amiskwia from the Burgess Shale has been classed as a nemertine, based on a resemblance to certain aberrant deep-sea swimming forms. This is not currently accepted by many paleontologists.
Some information on common nemertines of the eastern United States is available from the Marine Biological Laboratory at Woods Hole, Massachusetts. MBL has also made available the classic 19th-century zoological wall charts created by Rudolph Leuckart; you can view the anatomy and embryology of a nemertine.
Thanks to Chris Meyer and Allen Collins for providing the photos.