Phylum Nematoda
AKA Unsegmented roundworms.
Nematodes
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Advancements over flatworms:
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Has a two-hole digestive tract.
Not as advanced as “higher” worms,
such as earthworms because:
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They lack body segmentation.
Nematodes, cont’d
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Unsegmented roundworms are classified into
about 16,000 species, but the actual number
of nematode species could be as high as
500,000.
Nematode facts:
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Colorless
Range in length from microscopic to several
meters long.
May be free-living or parasitic.
Nematode Habitat
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Nematodes live in:
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Parasitic nematodes live within a host
Free-living nematodes live in marine,
freshwater, or damp soil environments.
The Nematode lifestyles
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Nematodes generally live one of two
types of lifestyles: free-living or
parastic.
Free-living Lifestyle
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Eating habits:
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Herbivores – eat plants
Carnivores – eat animals
Omnivores – eat both plants & animals
Saprophagous -Eat dead organic matter (from
animals only), Yummy
Free-living; and have the eating habits
mentioned on the previous slide.
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The free-living nematodes are important because
they add organic matter to the soil and putting
holes in the soil to better allow water movement
through the soil.
Parasitic Lifestyle
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Parasitic – feed off of a host.
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These worms feed of the blood or
tissue fluids of their hosts.
We will learn about four types of
parastic nematodes: filarial worms,
hookworms, trichina worms, and
ascaris worms.
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Interesting tidbits:
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The filarial worms cause the disease
elephantitis, pictured to the left.
Trichina worms cause trichinosis – the
horrid disease contracted from eating
undercooked pig products.
Nematode Body Plan
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Nematodes have three cell layers:
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Ectoderm – outer tissue layer (epidermis)
Mesoderm – middle tissue layer (muscle)
Endoderm – innermost tissue layer (body cavity)
They are known as psuedocoelomates
because they have an internal cavity that is
not lined with peritoneum – therefore it is not
a “true” coelom.
They are bilaterally symmetrical – as is every
organism we study from here on out.
Nematode Epidermis
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The outside of the nematode is made up of a
tough, flexible, noncellular layer known as the
cuticle.
The cuticle is secreted by epidermal cells.
It functions to:
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Resist penetration – in free-living.
Resist enzymatic digestion – in parasitic.
Maintains internal hydrostatic pressure.
The cuticle usually molts 4 times during
maturation.
Nematode Digestive System
One way digestive system; remember
this means that food goes in one way
and out another. Just like us.
 The digestive tract is a linear
progression, as shown below:
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mouth  pharynx  intestines  rectum  anus
 Food
is pushed through this system
by hydrostatic pressure.
Nematode Musculature
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The nematode body wall has only longitudinal
muscles.
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Remember longitudinal means lengthwise, so they
only run from the anterior to the posterior end of
the worm.
These muscles are used for movement.
When these muscles contract it causes the
thrashing movements from head to tail.
They lack circular muscles so they cannot crawl as
we saw the leech do on dry surfaces.
Excretory System
Aquatic species have ventral glands
(called renettes) posterior to the
pharynx that absorb waste from the
pseudocoelom and empties the waste
through the excretory pore.
 Parasitic nematodes have a more
advanced excretory system.
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Parasitic Excretory System
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Their system is known as a tubular system
that develops from the renette system of the
free-living worms.
The renettes unite to form two large canals,
known as the excretory canals that open to
the outside by an excretory pore which is
located by the head.
A little odd to excrete wastes near your head,
eh?
Excretory System Diagram
Reproductive System
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Most nematodes are:
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Dioecious: two different types of reproductive
cells; ex. sperm & egg.
Dimorphic: two different sexes; ex. male and
female.
As you might guess, they reproduce sexually.
The males are slightly smaller than the
females. Why?
Female Reproductive System
Consists of a pair of ovaries attached to
an oviduct that has a swollen proximal
end that forms a seminal receptacle.
 Each oviduct becomes a tubular uterus,
and the two uteri come together to
form a vagina that opens to the outside
through a genital pore.
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Male Reproductive System
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Most male nematodes have only a single testis
attached to the vas deferens which expands into a
seminal vesicle which connects to the cloaca.
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What are all these things?
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Vas deferens – aka sperm duct, releases sperm
Seminal vesicle – stores sperm cells
Cloaca – hole that sperm is ejected from
They also have a flap of tissue called the bursa that aids
in the transfer of sperm to the female genital pore.
Reproductive System Diagram
Brief Parasitic Nematode Info.
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Pinworms
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Hookworms
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Most common roundworm in the U.S.
Adults reside in the large intestine.
Enter humans by being eaten.
Found in the southern U.S.
Adults live in the small intestine of humans.
Enter host through the skin, usually between the toes. That makes you
want to walk around barefoot, doesn’t it?
Trichina
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Live in humans & other omnivores (like piggys)
Adults live in the small intestine of it’s host; larvae encyst in the
stomach and skeletal tissue (ouch!)
Enter host by being eaten.
Nematode Nervous System
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Nematodes have two nerve cords in their
bodies.
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Ventral nerve cord – runs along the “belly.”
Dorsal nerve cord – runs along the “back.”
They have a central nervous system
consisting of a circular brain.
The nervous system allows the nematode to
detect its environment and react to it.
Aquatic nematodes have a pair of ocelli
(eyes).