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Nematodes
Round Worms
Remember:
pg 2 Nematode Evolution
pg 3 Nematode Notes
There are many, many nematodes counting species as well as individuals
 20,000 nematode species
have been described but
probably this is only a small
fraction of the actual diversity
 The majority of nematodes
are small predators or
saprophytes (they eat
decomposing organic matter)
Nematode anatomy
 Long worms with often
tapered ends
 Unsegmented
 Liquid filled body cavity
(not lined by mesoderm:
pseudocoel)
 Surrounded by rigid
cuticle
 Simple organ and neuroanatomy
 Hypodermis organizes
worm into quadrants
cuticula
 Epicuticle: thin layer of
lipids and proteins
 Cortex: made from
cross-linked cuticilin
proteins and collagen
fibers
 Basal layers are
formed by collagen
fibers
 Basal layer is fibrous
and closest to
hypodermis
cuticula
 Nematodes molt 4 times
 The old cuticle separates from the epidermis and the innermost layer is
partially hydrolyzed
 A new cuticula is secreted by the hypodermis starting with the
epicuticle
 The old cuticle is shed
The hydrostatic skeleton
and motility
 Nematodes have stiff motility
 Nematodes depend entirely on
longitudinal muscles
Nervous system
 The brain consists of a
single ring around the
pharynx
 A dorsal and a ventral
nerve extend back
through the length of the
worm
 Labial and amphid nerves
extend forward to
enervate sensory organs
(amphids and various
papilla)
 Amphids are simple
organs of mechano- and
chemoreception. The
sensory organelles are
modified cilia
Nematode digestive system,
pumping against the pressure
 Nematodes have a
complete (but simple)
digestive system with a
mouth at the very tip of the
animal.
 The intestine is
nonmuscular, and
consists of a single layer
tall columnar cells which
carry microvilli and an
underlying basal lamina
 Food has to be pumped
against the pseudocoel
pressure by the muscular
esophagus
 Food is passed rapidly
(minutes)
Reproduction
 Most nematodes are
dioecious (meaning they
have two sexes, male
and female), but
hermaphrodites occur
as well
 Males are smaller than
females, and often have
additional external
features at the posterior
end of the worm
Development
 The development proceeds
through 4 larval stages
before reaching adulthood
 Molting is required at each
step
 Many nematodes can
develop a specialized
resting or Dauer version of
the L3 larval stage
 In parasitic nematodes this
L3 larvae is often the
infectious stage for the final
host
Nematodes impact human
life
 Nematodes cause
numerous human
diseases
 Abundant pathogens in
livestock and pets
 Pests of many crops
 Provide powerful
genetic models to study
the basis of
development, aging and
many diseases
including cancer
Wuchereria bancrofti
Necator americanus
Enterobius vermincularis
Ascaris
Trichinella spiralis
Trichuris trichuria
Nematoda Evolution
Phylum Nematoda
Class Secernentea
Class Adenophorea
Class Secernentea
Ventrally coiled or derived therefrom
Three esophageal glands,
some with phasmids(nerves near anus);
both free-living and parasitic forms.
Wuchereria bancrofti
 Aka: Elephantiasis pg
392
 Ingested by mesquitos
and develop into their
infective stage where
they are transmitted into
a new host
 Caused by adult filaria
worms of Wuchereria
bancrofti, which live in
lymph passages and
block the flow of lymph.
Wuchereria bancrofti
Necator americanus
 Hookworms pg 389
 Named because their anterior
end curves dorsally,
suggesting a hook
 Large plates in their mouths
cut into the intestinal mucosa
of the host where they suck
blood and pump it through
their intestine, partially
digesting it and absorbing the
nutrients.
Enterubius vermicularis
Pinworms pg 391
Cause little disease
Most common nematode parasite in the
United States (30% of children and 16% of
adults)
Live in intestine and cecum
Causes itching
Ascaris
 Intestinal worm pg 388
 Usually selected to study in Zoology
Thus we know more about their structure, physiology
and biochemistry than any other nematode
 A female may lay 200,000 eggs a day carried by the
host feces.
 Eggs have an amazing tolerance to adverse conditions
Ascaris
Infection typically occurs when eggs are
ingested with uncooked vegetables or when
children put unsanitary things in their mouths.
Ascauis megalocephala – found in the intestines
of horses
Ascaris lumbricoides – most common found in
humans
Class Adenophorea
Amphids (nerves near mouth) generally
well-developed, pocketlike
5 or more esophageal glands
Phasmids (similar to Amphids but located
on the posterior end) absent
Excretory system lacking lateral canals
Mostly free-living but includes some
parasites
Trichinella spiralis
 Trichinosis pg 390
 Infection of all hosts occurs
through larvae encysted in
muscle tissue
 Females produce living
young that enter the blood
stream and are eventually
found in almost any tissue of
body space
Trichinella
Trichinella
 Muscle invasion stage begins in the
second week after infection
 Fever and perorbital edema are followed
by myalgia (muscle pain) and weakness
 Characteristic splinter hemorrhages can
be found under finger nails
 Fever and chills can persist for weeks
 Headache is common and dizziness may
develop
 Muscle swelling, aching and tenderness
occurs often
 Deaths are rare and due to myocarditis
(inflammation of the heart muscle),
encephalitis and pneumonia (larvae in
the diaphragm)
Trichuris trichuria
 Whip Worm pg
 The third most common round
worm of humans. Worldwide,
with infections more frequent in
areas with tropical weather and
poor sanitation practices, and
among children (800 million
infected).
 Moderate to heavy infections
cause abdominal pain, diarrhea,
weight loss
 Adults can live for years so
worm burden can build up
Trichuris