Introduction to the Nemertini
Tied up in knots. . .
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.
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.
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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.
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
