| THEY
HAVE/ARE |
THEY DON'T
HAVE |
| Circular in
cross-section |
A circulatory
system |
| Bilaterally symmetrical |
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| Longitudinal muscles only,
arranged in four zones |
|
| A nervous system with four
longitudinal nerve cords |
|
| Complex cuticle |
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| Triangular arrangement of
mouth |
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| Pseudocoelomate with body
fluid always under high pressure (hydrostatic skeleton). |
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| Anterior mouth, muscular
pharynx and gut, and anus |
|
| Fertilisation is
internal |
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| Parasitic and
free-living |
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They are everywhere! About 80 000 species of
Nematode have been described so far, but some authorities estimate that there
may be as many as 500 000 to 100 million species in all! They live in all
environments and can parasitize nearly all animals and plants. In fact there is
scarcely an animal alive that does not harbour a population of parasitic
nematodes at some stage in its life, and that does include you! Around 50 different species are known parasites of humans (see below for more details). Luckily most of
the time they are so harmless that we don't even know they are there. Parasitic
species may be up to nine metres long, but most species are less than five
centimetres long. The longest known nematode is found in the placenta of sperm whales. To give you an idea of just how ubiquitous they are, you
could expect to find around 10 million in one square yard of soil. One apple left on the ground to decompose had 90 000 individual nematodes living on and in it. One fig had 50 000 individuals belonging to over 8 species on and in it. In 1914
Nathan Cobb famously wrote "If all the matter in the universe except
nematodes were swept away, our world would still be recognizable, its
mountains, hills, vales, rivers, lakes, and oceans represented by a film of
nematodes."
Compared to other phyla the
external structure of the Nematoda is very uniform. They all have slender,
elongated bodies with tapered ends. Some have specialised mouthparts, and the
only other projecting parts are concerned with reproduction. The cuticle can
have as many as nine layers, with cross fibres forming a spiral network. The
body is a high pressure pseudocoel within a cuticle covered body wall with only
longitudinal muscles. Locomotion is basically wriggling or thrashing with the
longitudinal muscles on one side contracting, while the other side expands. The
cuticle prohibits radial expansion, so serves the purpose of circular muscles.
Males are usually smaller than females and the posterior is curled into a hook.
This can be easily seen in the drawing below right. This shows a male and
female Ascaris sp. in the gut of a pig.
On the right is a cross-section of a female Ascaris sp. The muscle arm extends to
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either the ventral or dorsal nerve cord. The pseudocoel functions as a hydrostatic skeleton. The cuticle is non-cellular and secreted by the epidermis, and is mainly collagen laid down in three criss-crossing layers. There are no circular muscles, and it is the body movement which moves food from the mouth to the pharynx, intestine, rectum and anus.
Ascaris life cycle. Ascaris sp. adults
can range from 2 - 30 cm long, with the male slightly smaller. The female can
lay 200 000 eggs a day, and these have thick resistant shells. The eggs pass
out of the host in the faeces. If the egg is swallowed by another host, the
host's digestive juices dissolve the thick egg shell and liberate the young
worm. The young worm then burrows through the intestinal wall into the veins, and is carried to the heart and lungs. It then breaks into the alveoli, climbs the bronchial tubes, and up into the trachea. A heavy infestation can cause pneumonia at this stage. Finally the juvenile worm is swallowed, passes down into the stomach and returns to the alimentary canal again where
it matures and spends the rest of its life. From ingestion of eggs to maturity takes around two months. A small number of worms in an otherwise healthy human won't do too much harm, but they can cause an allergic reaction. Large numbers can cause blockages in the intestine, perforations, anaemia and lack
of energy; severe infestations can result in mental retardation. Wandering worms can be passed with faeces, can be coughed up, and can even enter the middle ear.
Usually the first indication of infection in areas where hygiene and sanitation are good, and the infected person is healthy is finding a worm in the toilet as you are about to flush. Many people ignore this thinking the worm is an earthworm that somehow made its way up through pipes. It is too much of a shock to them to believe that it came out of their bodies. Ascaris infections are easily cleared up with modern medicine if treated promptly. The most common cause of infection in the affluent world is eating contaminated food while on exotic vacations.
Other species of hookworm are parasites of man, see the list below, and infection is fairly common in the US, in fact it is said that as much as one sixth of the world's human population is infected by Ascaris sp. alone. The eggs are usually eaten along with food.
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Above is a piece of a false-killer whale's gut which has a heavy infestation of Bolbosoma capitatum, the thorny-headed worm. There have been cases where infestations reach up to 600 worms per square metre of gut.
Far right is part of a whale's kidney infected with several Crassicauda boopis, the fin whale round worm. The head of the worm is found in the blood vessels in the liver, the middle in the whale's kidney, and the tail end in the reproductive and excretory system. The entire length of the worm can be as long as 8 m. |
Common parasitic nematodes of humans
Hookworm, Ancylostoma duodenale and Necator americanus. Juveniles in the soil can burrow into human skin. Common in southern USA.
Pinworm, Enterobius vermicularis. The female pinworm creeps out of the human anus ans lays her eggs around the margin of the anus at night. This action causes itching and scratching by the human. The eggs are caught under the fingernails, and easily spread to other humans or back to the original host. Eggs can also be inhaled with dust or licked off fingers which have touched dusty surfaces. The most common worm parasite in the modern affluent areas of the world. 30% of US children are estimated to be infected, however infection causes little harm in a healthy human.
Intestinal roundworm, Ascaris lumbricoides. Eggs are eaten with contaminated food. Infection is common in some rural areas of the US with up to 60% of the population infected. For more details see above.
Trichina worm, Trichinella spiralis. Infection occurs when eating infected meat.
Whipworm, Trichuris thrichiura. Infection occurs when eating contaminated food, or through unhygienic habits. This parasite tends to be found in areas where Ascaris. sp. is also common. Adults live in the intestine,
but the larva burrow through the gut wall into the muscles causing cysts which
can cause great pain and even death.
Filarial worms, Wucheria bancrofti and Brugia malayi etc. Infection comes from infected female mosquitoes taking a blood meal. Repeated and long exposure can lead to elephantiasis. Another filarial worm carried by the blackfly causes onchocerciasis, river blindness.
Loa-loa, the African eye worm lives in the subcutaneous tissues of man. The worm migrates and if it passes across the carnea of the eye it can be seen - hence its common name. Some believe that this is origin of lovers gaze into each other's eyes as an infected partener would not make a good mate.
Guinea worm, Dracunulus medinensis, occurs mainly in Asia and Africa. In humans infection occurs from drinking water containing Cyclops which have been infected by the worm. The worm larvae in the cyclops hatch out and penetrate the human intestine. Eggs are passed out of the human when a gravid female migrates to the skin and causes an ulcer. When this ulcer comes into contact with water the free-swimming larvae are released. The larvae swim until they either die or are eaten by a cyclops. A mature female is 1 mm in diameter but can reach 120 cm in length.
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One nematode, Caenorhabditis
elegans, has become very well known as it was the very first multicellular
organism to have its full genome sequenced. It is unusual in that it has a fixed number of cells (959) in its body.
Sphaerularia
bombi (right),a nematode infecting bumblebee queens. This parasite is only found in
queens and affects her behaviour. The bumblebee queen is infected by an adult
female worm while she hibernates. It is believed that the nematode enters through her mouthparts. Around 12% of queens are infected, and this can rise to as much as 50% for late emerging queens. In the spring when the queen emerges from
hibernation the worm begins to grow, then it turns its whole reproductive
system inside out. The uterus grows and grows till it is between 1-2
centimetres long, while the rest of the worm is only a thin thing of a few
millimetres. In a normal queen a hormone would be released and her ovaries
would start to develop stimulating her to start building a nest, but somehow
this does not happen in an infected queen. Meanwhile the worm releases up to
100 000 eggs into the blood of the queen, these eggs hatch and develop, moving
into the gut and reproductive system. During this time the queen feeds only for
herself, she makes no attempt to find a nest site, and her ovaries do not
develop. Often she returns to her hibernation site, here the worm larvae are
discharged with faeces into the soil. The mature worms mate, and wait for
another queen to use the site to hibernate. |
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