Nemertea

Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms 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 to emerge just above the mouth to capture the animal's prey with venom. A highly extensible 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 the animal's 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, which 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. Some species of nemerteans are scavengers, and a few live commensally inside the mantle cavity of molluscs.

In most species the sexes are separate, but all the freshwater species are hermaphroditic. Nemerteans often have numerous temporary gonads (ovaries or testes), and build temporary gonoducts (ducts from which the ova or sperm are emitted) opening to a gonopore, one per gonad, when the ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, and others protect their eggs in various ways. 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. However, some form a pilidium larva, in which the developing juvenile has a gut which lies across the larva's body, and usually eats the remains of the larva when it emerges. The bodies of some species fragment readily, and even parts cut off near the tail can grow full bodies (antle ( fragmentation).

Traditional taxonomy divides the phylum in two classes, Anopla ("unarmed" – their proboscises do not have a little dagger) with two orders, and Enopla ("armed" with a dagger) also with two orders. However, it is now accepted that Anopla are paraphyletic, as one order of Anopla is more closely related to Enopla than to the other order of Anopla. The phylum Nemertea itself is monophyletic, its main synapomorphies being the rhynchocoel and eversible proboscis. 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.

History

In 1555 Olaus Magnus wrote of a marine worm which was apparently long ("40 cubits"), about the width of a child's arm, and whose touch made a hand swell. William Borlase wrote in 1758 of a "sea long worm", and in 1770 Gunnerus wrote a formal description of this animal, which he called ''Ascaris longissima''. Its current name, '' Lineus longissimus'', was first used in 1806 by Sowerby. In 1995, a total of 1,149 species had been described and grouped into 250 genera.

Nemertea are named after the Greek sea-nymph Nemertes, one of the daughters of Nereus and Doris. Alternative names for the phylum have included Nemertini, Nemertinea, and Rhynchocoela. The Nemertodermatida are a separate phylum, whose closest relatives appear to be the Acoela.

Description

Body structure and major cavities

The typical nemertean body is very thin in proportion to its length. The smallest are a few millimeters long, most are less than , and several exceed . The longest animal ever found, at long, may be a specimen of '' Lineus longissimus'', Ruppert, Fox and Barnes refer to a '' Lineus longissimus'' long, washed ashore after a storm off St Andrews in Scotland. Other estimates are about . Zoologists find it extremely difficult to measure this species. For comparison:
* The longest recorded blue whale was .
* The dinosaurs ''Argentinosaurus'' and ''Patagotitan'' are estimated at approximately and respectively.
* A specimen of the Arctic giant jellyfish ''Cyanea capillata arctica'' was long.

Although ''L. longissimus'' is usually only a few millimeters wide. The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species, but reduce their length to 50% and increase their width to 300% when disturbed. A few have relatively short but wide bodies, for example ''Malacobdella grossa'' is up to long and wide, and some of these are much less stretchy. Smaller nemerteans are approximately cylindrical, but larger species are flattened dorso-ventrally. Many have visible patterns in various combinations of yellow, orange, red and green.

The outermost layer of the body has no cuticle, but consists of a ciliated and glandular epithelium containing rhabdites, which form the mucus in which the cilia glide. Each ciliated cell has many cilia and microvilli. The outermost layer rests on a thickened basement membrane, the dermis. Next to the dermis are at least three layers of muscles, some circular and some longitudinal. The combinations of muscle types vary between the different classes, but these are not associated with differences in movement. Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species. Inside the concentric tubes of these layers is mesenchyme, a kind of connective tissue. In pelagic species this tissue is gelatinous and buoyant.

The mouth is ventral and a little behind the front of the body. The foregut, stomach and intestine run a little below the midline of the body and the anus is at the tip of the tail. Above the gut and separated from the gut by mesenchyme is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. The rhynchocoel of class Anopla has an orifice a little to the front of the mouth, but still under the front of the body. In the other class, Enopla, the mouth and the front of the rhynchocoel share an orifice. The rhynchocoel is a coelom, as it is lined by epithelium.

Proboscis and feeding

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 inside, the pressure makes the proboscis jump inside-out to 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.

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 can be described as "a mass of sticky spaghetti". The animal then draws its prey into its mouth.

In most of the class Enopla , the proboscis exits from a common orifice of the rhynchocoel and mouth. A typical member of this class has a stylet, a calcareous barb, with which the animal stabs the prey many times to inject toxins and digestive secretions. The prey is then swallowed whole or, after partial digestion, its tissues are sucked into the mouth. The stylet is attached about one-third of distance from the end of the evert