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Invertebrate
Zoology Kingdom: Animalia, Phylum: Nematoda (Contact)
CLICK on underlined file names and
included illustrations to enlarge: The Nematoda,
formally known as the Nemathelminthes,
are unsegmented, cylindrical
worms. Although over 1,200 species
have been identified, it is widely believed that these are just a small
fraction of the total species in existence.
Taxonomy of nematodes has been very difficult due to the relatively few
diagnostic characters available. Most
species are free-living, and there are also many parasitic species, which are
the best known. Nematodes occur in
marine and freshwater, in the soil, and as parasites on a wide array of
animal and plant species. The body
cavity is a Pseudocoelom
and their size varies from microscopic up to two meters in length. Characteristics of
The Phylum Nematoda are uniform
morphologically and there are many variations in the small size range. More detailed morphology is found on the
head and tail ends, and thus the teeth and copulatory bursae are extensively
used in identification. Although
there are a few hermaphroditic forms, most species have separate sexes. Some species alternate as free-living and
parasitic, some are facultative parasites and some obligate parasites. Most parasitic species do not have
intermediate hosts. When there are
intermediate hosts these are almost always arthropods. Type Animal = Ascaris lumbricoides Habitat.-- This species
inhabits the intestine of humans or hogs, but it is not cross-infective
between the two hosts. Body
Plan.-- There is a mouth and an anus. The female being larger and the male
smaller with a curled tail express sexual dimorphism. Lateral lines are distinguished on the
sides. The female's genital pore is
inconspicuous on the ventral surface about one-third the way back from the
anterior end. The male genital pore
is located at the posterior end and is the same opening as the anus. Body
Surface.-- A heavy cuticle is
present. The epidermis,
which is often called a hypodermis because it is located under a cuticle, is
a syncytium,
which is not as prevalent as in the Acanthocephala.. There is a longitudinal muscle
on the inside of the epidermis. It is
shaped like a dugout
canoe with the wood portion
representing the contractile part and the hollow portion the cytoplasm. The cytoplasmic part projects into the
body cavity, while the contractile part faces the epidermis.
There are no circulatory
muscles. A pseudocoelom
occurs inside the muscle layer, and suspended within are the digestive
organs.
Food
& Digestion.-- The food is the
same food as that of the host, which is partially predigested. The mouth leads into a heavy muscular
pharynx. The intestine is very flat
and runs the entire length of the animal without muscular walls. There is no food absorbed through the body
wall. Circulation.--
No circulatory system is present. Respiration.--
There are no special respiratory structures, and respiration occurs by
diffusion. The roundworms are largely
anaerobic. Excretion.--
One pair of excretory canals lie on the lateral lines. The epidermis is thickened at the site of
the lateral lines and bears a canal.
Each canal consists of one single cell with a hollow cavity. A cross-connection runs between the
lateral canals.
An excretory pore lies
mid-ventrally just back of the mouth.
Rossett cells (one per each canal)
are associated with each tube in other species of nematodes. There is no sign of flame cells. Cilia
or Flagella.-- There are neither
cilia nor flagella in any part of the body.
Even the sperm lack flagellated tails and are amoeboid. Support
& Protection.-- The cuticle, which
is constantly replaced as in the Cestoda, furnishes support and protection. Locomotion
.-- Movement is back and forth in a figure 8. The lack of circular muscles restricts movements greatly. Ascaris
is not attached to the wall of the intestine, so it must remain constantly in
motion forward in the gut. Sensitivity.--
A few sense organs occur in parasitic nematodes and almost none in Ascaris. The fleshy lips are sense organs, and tactile and olfactory
papillae occur at both ends.
Free-living forms may possess eyes. Nervous System.--
There is a continuous ring around the pharynx, which is analogous to the
cerebral ganglion in flatworms. Six
anterior and six posterior nerve cords run from the ring, but only two, the
mid-dorsal and mid ventral, are of any consequence. Reproduction.--
In the male the testes is a very long coiled tube. The vas deferens is the thicker portion of the tube, while the
seminal vesicle is an even thicker part.
The seminal vesicle opens into the gut at a cloaca. The anus serves for both the digestive and
reproductive systems. Peneal
spicules
occur at the posterior
end and serve as claspers. Some
species have a copulatory bursa like the Acanthocephala. The female's ovaries are two coiled
tubes. Each oviduct enlarges abruptly
to form a uterus. The uterus runs
fairly straight to the oviduct. Two
uteri units are located just before the genital pore. During fertilization and
development, the sperm are introduced into the genital pore of the
female. The amoeboid sperm unite with
eggs as they come down the oviduct.
The fertilized egg forms a heavy proteinaceous shell and is warty or
bumpy and extremely resistant. The
eggs develop very little in the body of the female roundworm. They pass out of the body through the
genital pore. There can be millions
of eggs from one individual roundworm. Life
Cycle.-- Infection is direct
and there is no intermediate host.
The host swallows an egg and the juvenile worm hatches from the
egg. The worms burrow to the vascular
system, enter the blood and become stuck in capillaries of a hog's
lungs. They burrow through the
capillaries and into the air spaces of the lungs. Here they are coughed up and swallowed, entering the intestine
for the second time where permanent residence is assumed. The process takes about ten days Medical
Importance.-- In the host there
is an occlusion of the gut and the bile ducts become blocked. Secondary respiratory infections occur in
the damaged lungs of pigs. However, Ascaris
is generally not too important economically. Several genera of Nematoda are of
importance as disease incitants. Trichinella spinale attacks
hogs and humans and is intertransferrable.
The larvae burrow into the blood stream where they then encyst in the
muscles. The disease is called Trichinosis. The hookworm genera Necator and
Ancylostomum hang
on the intestine of the host where they suck blood and body fluids. Active larvae penetrate through unbroken
skin. The genus Wuchereria
is transmitted by
mosquitoes and causes the diseases called Filariasis. In extreme cases "Elephantiasis"
can result. The adult parasites cause
a stoppage of lymph flow while the larvae live in the host's blood. The genus Dracunculus
is the Guiana
worm, which is known as the
Fiery
Serpent in the Bible. The worm is transmitted by the genus Cyclops. The adult female lives subcutaneously in the host where it may
assume several feet in length. Body
fluids of the worm are extremely caustic.
Extraction is accomplished by winding the worm on a stick. Plant Pests.-- Many
soil-inhabiting nematode species are important in agriculture because they
attack the roots of crop plants, causing death or deformation. Soil steam treatment or chemical
fumigation has been widely used for control. ------------------------------------ Please see following plates for Example Structures of the
Nematoda: Plate 36 = Phylum: Nematoda: Ascaris lumbricoides female Plate
37 = Phylum: Nematoda: Ascaris
lumbricoides Plate
38 = Phylum: Nematoda: Dioctophyme venale, Dracunculus medinense,
Enterobius vermicularis, Ascaris sp. ============== |
