Etymology:- From the Greek Nema for Thread and Eidos for
form.
1)Bilaterally symmetrical, and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a pseudocoel, body fluid under high
pressure.
4)Body possesses a through gut with a subterminal anus.
5)Body covered in a complex cuticle.
6)Has a nervous system with pharyngeal nerve ring.
7)Has no circulatory system (no blood system)
8)Reproduction normally sexual and gonochoristic.
9)Feed on just about everything.
10)Live just about everywhere, many species are
endoparasites.
Nematodes are the most speciose phylum after the arthropods, they occur
in nearly every habitat including as parasites in all
sorts of plants and animals, (they don't like dry places
however). One species is known that can live in old vinegar (Turbatrix
aceti)and another that as only been found in German beer
mats. Though only about 80 000 species have been described some
scientists estimate there may be as many as a million
species all told. They can occur in very dense numbers in the soil and rotting
vegetation, as many as 90 000 have been found in a single
rotting apple, while millions occur in the top 3cm (1 inch) of a square
metre of good quality soil. While there are a huge number
of free living Nematodes there are also a large number of parasitic
species, many of which cause diseases to man and other
animals as well as to plants, nearly every living organism has been found
to be parasitised by one species of nematode or another.
Most nematodes are reasonably small, they range in size from 100
micrometres in length (1/10th of a mm or 1/250th of an in)
to the female Giant Nematode Dioctophyme renale which may be up
to 1 metre, or 3 ft long
Free living nematodes are long thin worms with transparent and typically
curled bodies, parasitic species have a variety of
less streamline shapes relating to their degenerate
parasitic life styles, one unifying characteristic that makes the phylum unique
is
the lack of cilia or flagella, even the sperm of nematodes
are amoeboid. Nematodes as parasites have been known for a long time
and the earliest recorded literary mention of them is an
Egyptian papyrus from 1500 BC, they are also mentioned by the ancient
Greeks Aristotle and Hippocratis the father of scientific
medicine.
Nematodes live in a vast variety of habitats, ecologically they can be
divided into free living forms and parasitic forms. Free
living forms have a simple life cycle involving 4 juvenile
instars on the path from egg to adult. Parasitic species have developed a
wide range of variations on this basic theme. The
variations involve whether there is a secondary host and the amount of time
spent in one or either hosts. There is also considerable
variability in the way that they move from one host species to another. thus
while many species lay eggs that pass out of the primary
host with the feces where they are eaten by the secondary host which
then gets eaten in turn by the primary host after the
Nematodes have developed. Because it is not always totally reliable that the
secondary host will be eaten just as the Nematode larvae
have developed into the infective stage many species have the ability to
encyst themselves in the muscle or cuticle of their
secondary hosts.
Some species use another animal to transport them from one host to
another thus Wuchereria bancrofti releases minute
live young called 'microfilaria' into the primary hosts
blood stream rather than eggs into the digestive tract. These microfilaria get
ingested by mosquitoes when they feed on an infected
person. Inside the mosquito they live in the mosquitoes gut where they
develop until the Larva 3 stage wait for the mosquito to
bite another host whereupon they enter the host via the mosquitoes
proboscis sheath and the wound it makes in the hosts skin.
Human beings, along with all other living things are host
to numerous Nematode parasites. The commonest of these is Ascaris
lumbricoides with an estimated 700 million people effected
globally, this Nematode is not normally fatal and in low numbers may
have very little effect on adults, however in heavy doses
it can be quite debilitating, especially for children. The Nematodes
infecting mankind include several species of filarial
worms, the most important of these are Wuchereria bancrofti and Brugia
malayi which are very similar and cause lymphatic
filariasis, Onchocerca volvulus which causes River Blindness and Loa loa
which causes Loiasis. Other species are Dranunculus
medinensis known as Guinea Worm, Trichinella spiralis causing
Trichinosis, Necator americanus and Ancylostoma duodenale
causing Hookworm, Enterobius vermicularis causing Pinworms
and Trichuris trichuria causing Whipworm or Trichuriasis.
Basically a Nematode is a long hollow tube within which is another tube,
the alimentary canal and the reproductive organs.
Nematodes are round in cross section, this is because
unlike the other worms that below them in the phyla table they maintain their
body fluids under great pressure (on average internal
pressure in a nematode equals 70mm of mercury or 1.49 PSI, with a
maximum recorded value of 125mm of mercury or 2.41 PSI). To
contain this high pressure nematodes have an extremely tough,
yet elastic and flexible cuticle. This cuticle consists of
up to 9 layers of proteinaceous fibres, with 3 layers being easily discerned,
these are called, from the outside in, the cortex, the
matrix layer and the fibre layer. Despite its complexity the Nematode cuticle is
permeable to both water and gases, so respiration occurs
through it. Beneath the cuticle is a hypodermis and a layer of longitudinal
muscle. The combination of the flexure of these muscles
with the high pressure of the system produces a characteristic whip-like
wriggle that Nematodes use to swim. Scientifically this is
called undulatory propulsion with sinusoidal waves passing back along
the body.
At the anterior (head) end there is a mouth which has 3 lips behind which
predatory species possess a few teeth, this leads
to a pharynx which is triangular in cross section. Because
of the high pressure within the body unsupported organs such as the
intestines tend to collapse in much the same way that an
uninflated bicycle tube tends to become oval or flat in cross section when
laid flat on the table. The pharynx of Nematodes is an
efficient pump and forces food into the intestines, there is a one way valve
between the intestines and the pharynx. The pharynx can,
when this valve is closed, be used to suck liquid food into the mouth.
Digestion is rapid and feces are expelled under pressure.
This pressure is so great that the parasitic nematode Ascaris
lumbricoides which is about 12cm to 18cm long (5 to 7
inches) may shoot its feces 60cm or 2 feet into the air.
Nematodes, especially free living forms generally have a reasonably well
developed nervous system. This is comprised of a
circum-pharyngeal nerve ring made up from 4 nerve ganglia
from which 6 longitudinal nerves extend down through the body to the
various parts of the gut and the reproductive organs. There
are also 6 shorter nerves which extend forwards from the
circum-pharyngeal ganglia towards the mouth. Nematodes have
no circulatory or respiratory organs and the excretion of
metabolic waste is via two simple ducts or tubules which
have no nephridia or flame cells.
Nematodes are copiously reproductive and most of their body cavity, which
is a pseudocoelom is filled with paired sets of
reproductive organs, either ovaries or testes. Males and
females copulate and the male introduces sperm to the females vagina
with the help of 2 stiff horny spicules that are a part of
his cloaca. Fertilisation is internal and females lay eggs over a prolonged
time period, thus a female Ascaris lumbricoides may lay her
eggs at the rate of 200,000 per day and have had a total 27 million
eggs within her at the start of her reproductive career.
Young nematodes hatch from these eggs and go through 4 moults before
they become adults.
Phylum
Class
Order
Nematoda
Adenophorea
Enoplida
Isolaimida
Mononchida
Dorylaimida
Trichocephalida
Mermithida
Muspiceida
Araeolaimida
Chromadorida
Desmoscolecida
Desmodorida
Monhysterida
Secernentea
Rhabdita
Strongylida
Ascaridida
Spirurida
Camallanida
Diplogasterida
Tylenchida
Below are two lists of of organisms and the Nematodes that infect them,
this is only a minute example covering only two
groups of organisms Commercial Plants and Domestic Animals,
in the plant list many of the species listed for one plant group also
infest other plant groups.
Some Nematode
Parasites of
Domestic Animals
Animal Group
Nematode Species
Rodents
Angiostrongylus cantonensis
Nippostrongylus brasiliensis
Syphacia obvelata
Capillaria hepatica
Cattle
Dictyocaulus viviparus
Oesophagostomum radiatum
Onchocerca gutterosa
Horses
Strongylus edentatus
Parascaris equorum
Oxyuris equi
Pigs
Stephanurus dentatus
Ascaris suum
Sheep
Haemonchus contortus
Ostertagia ostertagi
Dogs
Driofillaria spp.
Dioctophyma renale
Chickens
Syngamus trachea
Ascaridia galli
Heterakis gallinarum
Some Nematode
Parasites of
Commercial
Plants
Plant Group
Nematode Species
Chrysanthemums
Aphelenchoides ritzemabosi
Buddleja
Aphelenchoides ritzemabosi
Scabious
Aphelenchoides blastophthorus
Lillies
Aphelenchoides fragaria
Ferns
Aphelenchoides fragaria
Cacti
Heterodera cacti
Wallflowers
Heterodera cruciferae
Figs
Heterodera fici
Hops
Heterodera humuli
Roses
Xiphinema spp.
Narcissus
Pratylenchus penetrans
Potatoes
Ditylenchus destructor
Pot Plants
Meloidogyne javanica
Herbaceous Plants
Ditylenchus dipsaci
Many plants
Longidorus elongatus
Many Plants
Longidorus Macrosoma