These are commonly
known as the ribbon worms or proboscis worms. They range in length from less than 0.5 cm to over
50 m. This is more than twice the average length of an adult blue whale. So at 50 m the ribbon worm is the world's longest animal.
There are about 1200 known species. They are found world wide, but most commonly in the temperate zones, along the European and mediterranean coasts. They are found mainly in shallow waters, beneath shells, stones, amongst algae, and burrowing in sand and mud.
They are acoelomate carnivores
using their eversible proboscis to grasp prey. The proboscis lies in a cavity
that runs almost the entire body length (see Prostoma rubrum right), as well as catching prey
it can be used in locomotion and in defence.
Contraction of the muscles exerts pressure in the
proboscis cavity causing the proboscis to evert; it is retracted by the action
of a longitudinal muscle. The blood flow system is driven by body movements and
the contractions of the blood vessel walls.
Nemerteans can have from two to 250
pigment-cup eyes. The one below has two eyes, and on the right six eyes.
There are two Classes; the Anopla with a
simple proboscis; and the Enopla with a more complicated proboscis armed
with stylets (see Prostoma rubrum above right, which in some can deliver toxic secretions), and Tetrasemma sp. a mud-dwelling species below right.