Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela.
Although most are less than 20 centimetres (7.9 in) long, one specimen has been estimated at 54 metres (177 ft), which would make it the longest animal ever found.
Most are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration.
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.
A little above the gut is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts (turns inside-out) to emerge just above the mouth and capture the animal's prey with venom.
A very stretchy muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.
The brain is a ring of four ganglia, positioned around the rhynchocoel near its front end. At least a pair of ventral nerve cords connect to the brain and run along the length of the body.
Most nemerteans have various chemoreceptors, and on their heads some species have a number of pigment-cup ocelli.
These ocelli can detect light but can not form an image. Nemerteans respire through the skin. They have at least two lateral vessels which are joined at the ends to form a loop, and these and the rhynchocoel are filled with fluid.
There is no heart, and the flow of fluid depends on contraction of muscles in the vessels and the body wall. To filter out soluble waste products, flame cells are embedded in the front part of the two lateral fluid vessels, and remove the wastes through a network of pipes to the outside.
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.
A few live in the open ocean while the rest find or make hiding places on the bottom. About a dozen species inhabit freshwater, mainly in the tropics and subtropics, and another dozen species live on land in cool, damp places.
Most nemerteans are carnivores, feeding on annelids, clams and crustaceans. A few species are scavengers, and a few species live commensally inside the mantle cavity of molluscs.
Some species have devastated commercial fishing of clams and crabs. Nemerteans have few predators.
Two species are sold as fish bait.
The proboscis is an infolding of the body wall, and sits in the rhynchocoel when inactive. When muscles in the wall of the rhynchocoel compress the fluid in the rhynchocoel, the pressure makes the proboscis jump inside-out to attack the animal's prey along a canal called the rhynchodeum and through an orifice, the proboscis pore.
The proboscis has a muscle which attaches to the back of the rhynchocoel, and which can stretch up to 30 times its inactive length and then retract the proboscis.
Gorgonorhynchus repens, a species within class Anopla,( Anopla are a class of marine worms of the phylum Nemertea, characterized by the absence of stylets on the proboscis, the mouth being below or behind the brain, and by having separate openings for the mouth and proboscis.
The other class of Nemertea are the Enopla. Although Anopla is a paraphyletic grouping, it is used in almost all Scientific classifications.
Anopla is divided into two orders: Palaeonemertea and Heteronemertea.) discharges a sticky branched proboscis.
The proboscis of the class Anopla exits from an orifice which is separate from the mouth, coils around the prey and immobilizes it by sticky, toxic secretions.
The Anopla can attack as soon as they move into the range of the proboscis. Some Anopla have branched proboscises which Ruppert, Fox and Barnes describe as "a mass of sticky spaghetti". The animal then draws its prey into its mouth.
