User:Philcha/Sandbox/ANematode2

09, 16 April 2012

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Description
Nematodes resemble annelids and flatworms, but are more robust and elongate than most flatworms, and lack annelids' segmentation. In a few species of nematodes, the epidermis is annulated, and internal organs such as gonads and nephridia make serials. On the other hand muscles, nerves and other internal structures do not form serials.

The body wall is composed of a dense ciliated, a layer of connective tissue and a thick musculature. It has no cuticle, and each ciliated cell has many cilia and microvilli.

There are 1150 nematodes species, or ribbon worms.

Nematodes, like arthropods and tardigrades, lack motile cilia.

Most species are less than 20 cm long, some are only a few millimeters long, but the genera Cerebrat and Lineus may be a meter long. In some species, nematodes' long, slender are longer than 1 m. One from St. Andrews, Scotland, was a 54 m boot-lace worm, the longest animal on Earth.

The smallest nematodes are circular or only slightly flattened in cross section, while species which are larger-bodied are flattened and ribbon-like to conserve resources.

Beneath the epidermis is usually at least layers of well-developed muscles. The outermost is circular, the innermost is usually longitudinal and often very layers, and crisscrossing helical muscles cross between the other layers. The helical muscles allot the animal to twist and and coil, and a series of dorsoverntran muscles flatting the body.

In some species a connective tissue over a less or greater extent overlayes the muscule and gut. In pelagic nematodes, this is gelatinous and bouyant.

While some species are pale and nondescript, many, including some that live in darkness, have patterns and pigments of yellow, orange, red and green.

Senses
The central nervous system of a brain and paired long nerve cords.The brain is a ring of four anterior ganglia around the rhynchodeum or anterior rhynchocoel. The two lateral nerve cords are large and nonganglionated. Additional longitudinal cords are often present, including a dorsal cord. The nerve cords and especially the brain are commonly pink or red because they contain a noncirculating haemoglobin (neuroglobin). This respiratory protein stores oxygen, thus extending the time period of peak neuromuscular functioning, pr even normal functioning, under conditions of environmental anoxia (such as burrowing in oxygen-free sediments). Haemoglobin is absent in muscles and only rarely occurs in the circulatory system. Perhaps for these reasons, most nemerteans are limited to short bursts of vigorous movement.

Sense organs consist of sensory epidermal pits, pigment-cup ocelli, ciliated cephalic slits and grooves, cerebral organs, and eversible frontal organs. The last three of these are probably chemoreceptors. Cephalic slits and grooves are shallow, ciliated furrows underlaid by neurons. The cerebral organs are a pair of blind sacs lined by nerve and neuroendocrine cells and associated with the cerebral ganglia. A ciliated canal leads from each sac to the exterior. The external openings of the canals are in the cephalic slits or grooves, or in a pair of pits over the brain area. Bidirectional water currents created by cilia in the canals appear to be activated by the presence of food. The cerebral organs also may have a neuroendocrine role in osmoregulation. the cerebral organs of worms placed in low-salinity water release a substance into the blood that may increase the pumping rate of the nephridia.

Gas exchange, internal transport, and excretion
Nemerteans lack specialized gills, and gas exchange occurs across the surface of the long, sometimes flattened body. Nemerteans, like other large animals with thick body walls, use fluid circulation rather than simple diffusion to transport substances throughout their bodies. Nemerteans have a coelomic circulatory system consisting of two components: the central rhynchocoel and peripheral vessels. The rhynchocoelic fluid transportssubstances to and from the proboscis, as well as performing a fluid-skeletal role in proboscis eversion and burrowing. Fluid in the vessels substances throughout the body, including to and from the rhynchocoel. The vessels provide whole-body circulation and the rhynchocoel, specialised local circulation.

In basic design, the vessels form a simple loop consisting of two lateral vessels, one on each side of the gut, joined together anteriorly and posteriorly. In many nemerteans, additional longitudinal and transverse vessels have seen added to the circulatory loop. Commonly, a branch from each laterat vessel extends into the rhynchocoel wall. The vessels are lines by a mesothelium, and some cells are epitheliomuscuscuar and bear a cilium.

The vessels are contractile, although fluid flow depends on contraction of both vesssels and body-wall musculature. Circulation is intermittent in some species, and fluid ebbs and flows in the two lateral vessels. In other species, such as Amphiporus cruentataus, which has the basic loop plus a dorsal longitudinal vessel, fluid flows anteriorly in the dorsal vessel and posteriorly in the lateral vessel.

The vessel fluid is usually colourless, but in many species it contains cells that are yellow, orange, green or occasionally red (hemogologbin). The hemogolOgbin-containing cells bind and transport oxygen, but the function of the other pigments is unknown. In addition to the pigmented corpuscles, the vessel and rhynchocoeic fluid also contain colourless amebocytes.

Communication between the vessel and rhynchocoelic coeloms occurs across specialised exchange site called vascular plugs. A vascular plug occurs at the blind end of vessel that distends the rhynchocoel wall and bulges into the rhynchocoel. In at one species, the plug is overlaid with podocytes, across which exchange of fluid, ions, nutrients, and other substaces may occur between the two communications.

The excretory system consists of two or more protonephridia, each bearing many terminal cells. The protonephria geneally are restricted to the anterior foregut region of the body, not scattered throughout as in most flatworms. The protonephridia generally are to the anterior foreguts region of the body, not scattered thoughout as in most flatworms. The terminal cells project onto the wall of the lateral vessels and modify the fluid delivered to them by the circulatory system. In a few cases, the vessel lining is interrupted at the sites of contact so that the basal lamina around the terminal cells is bathed directly in fluid. The many nephridial tubules unite to form larger collecting ducts before opening to the exterior. a nephridiopore, or pores, is located on each side of the body at the level of the foregut. Protonephridia play a role in osmoregulation because semiterrestrial and freshwater species have many more terminal cellls, sometimes thousands more, than do their marine counterparts.

Movement
Most nematodes use their epidermal cilia muscles to glide over the substrate on a trail of slime, of which some is secreted on the head by cephalic glans.

Cerebratulus and other large species use muscular waves to crawl over surfaces, and most powerful near the front, where the fluid-filled rhynchocoel of the proboscis seems to functions as a hydrostat. In fact burrowing nematodes often have very strong muscular body walls, and branches from the mesodermas muscles enter the epidermis to form as extra muscular layers. Other ribbon worms, such as Cerebratulus, swim by particularly well developed dorsovertral muscales, which flatten the body.

... a few annelids, most nematodes are ... extensible and some, such as Tubulanuss, Micrura and Lineus, can exten to at least 10 times the resting body. The muscales of palaeo-nematodes are smooth, ... the rest, mesodermal myocytes, are obliquely stria.

!!!!!!!!!! and these worms' dorsoventral musculature swim

Nematodes lasso or harpoon their prey with a sticks, penetrating or venomous proboscis.

Feeding and excreting
Like flatworms, nematodes transport oxygen across the body wall.

Many burrow in sediments, in crevices or the roots of algae and sessile animals, and some speices make gelatinous lairs in deep water. A few species live as ectosymbionts in the mantles of bivalves, in the atrium of tunicates or on crabs.

About 12 species live in fresh water, and about 15 primarily live in humid tropics and subtropics.

A nematodes' proposcis is a long, extensible and muscles tube lying in a fluid-filled coelomic cavity called the rhynchocoel, and is used to capture prey and sometimes to burrow. The inner end of the of the proboscis has no opening, but is attached to the rear of the rhynchocoel by a large extensible retractor muscle, and may stretch up to 30 times its resting length. Anteriorly the lumen of the proboscis join a short canal known as the rhynchodeum, which begins at approximately the level of the brain and extends forward to a proboscis pore on the anterior tip of the nematodes. The proboscis everts through the proboscis pore as muscles in the retractor muscles compress the rhynchocoel - the retractor muscle pulls the proboscis back into the rhynchocoel. Both the rhynchocoel and the proboscis develop as body wall invaginations, and their walls are similar to the body wall.

The Anopla proboscis proboscis is a simple unbranched or branched tube. In Enopla (armed nematodes) the proboscis usually have one calcareous barb called a style and some species have many stylets. The stylet(s) are attached to the proboscis by a bulbous, secreted structure called the basis. The point of stylet atttachment is not at the at the end of the proboscis but approximatly two-thirds of the distance from the anterior end of the body. Reserve stylets are help on each side of the active central style to provide replacements as the animal increases or when the main style is lost during feeding. In armed nematodes, the proboscis everts only enough to expose the style. The everts part of the proboscis is called the anterior proboscis and the uneverted part behind the style is the posterior proboscis.

Nematodes are cannivores that fee chiely on annelids and crustaceans, but a few may scavene. The discharegs proboscis of unarmed nematodes coils around the prey while nematodes coils around the prey while sticky, toxic secretions aid in holing and immobilising it. In the armed nematodes, the proboscis is everted to expose the style at at the proboscis tip. The style stabs the prey repeatedly, which allows the neurotoxic and cytolytic secretions to enter the prey's body. Some additional toxin may be pumped into the wound from the posterior proboscis. The immobilised prey is either swallowed whole or, after partial digestion, its tissues are sucked directly into the mouth.

Nematodes feed on a variety of prey, including clams, crustaceans and worms. Some large, bivalve-feeding can be devastates clam beds. The large, unarmed Cerebratulus lacteus of the eastern coast of the United States enter the burrows of the razor clam Ensis directus from below and swallows the clam as it withdraws downward. Paranemertes peregrina, an armed intertidal nematode found on the Pacific coast of the United States, feeds on polychaete annelids. This nematode leaves its burrow to feed and can follow the mucous trails of prey, but it must touch the prey to initiate the feeding response. Once contact occurs, the everted poboscis wras around and stabs the prey repeatedly. The paralysed prey is swallowed as the retracting proboscis drags it into the mouth.After feeding, the nematode finds its home burrow by backtracking along its own mucous trail. Other armed nematodeans feed on small crustaceans such as as amphipods. The kill the prey with a piercing stylet strike to the ventral exoskeleton and then wedge their head into the breech. The esophagus is everted and the contents of the prey are suck out and digested. Species of Carcinonemertes are economically important predator on the external egg "sponge" of brooding crabs. The short proboscis everts to just beyond the proboscis pore, where its stylet punctures the eggshell to allow the contents to be sucked out.

The digestive system consists of mouths, foregut, stomach, intestine and anus. The ventral mouth is at the anterioir end of the body near the level of the brain. It opens into a foregut, which is often subdivided into buccal cavity, pharynx, and glandular stomach. The foregut joins a long intestine with lateral intestine diverticula. In some species, another intestinal diverticula, the cecum, extends anteriorly past the foregut junction. The intestine opens at the anus, located at the tip of the tail.

In many armed nematodeans, the mouth has disappeared and the pharynx unites with the rhynchodeum so that pharynx and proboscis share the proboscis pore. A similar union occurs in the commensal bdellonemerteans but a rhynchodeum is absent. In all other nemerteans the digestive system is completely separate from the proboscis apparatus.

Digestion is initially extracellular in the intestinal lumen, including the diverticula. Partially digested particulates are phagocytosed by gastrodermal cells and digestion is completed intracellularly. Nutrient storage also occurs in these cells.

fragments to == Reproduction and development ==

Nemerteans readily regenerate, and reproduce both clonally and sexually. Fragmentation is common in large-body species, especially when irritated. Because of this tendency, collecting large, intact specimens is other difficult. Sometimes, too, the proboscis become detached when everted. The worm soon regenerates a lost proboscis, but the ability of both regenerates new worms varies greatly, depending on species.

Reproduction and development
Nemerteans readily regenerate, and reproduce both clonally and sexually.

Diverstity of nemerteans
Bottom-feeding fish, some shore-birds, and other invertebrates such as horseshoe crabs, and also other species of nemerteans eat other nemerteans. Nemerteans' edidermis secretes a sticky, toxic mucus to discourage predators, and nemerteans' bright and contrasting colours advertise their bad taste.

The North American Cerebratulus lacteus and the South African Polybrachiorhynchus dayi are sold as fish bait. These species are not related to true tapeworms, and are not parasites.

Phylogeny


Most of the characters shared by nemerteans and flatworms are ...