Transcript ch18Nematoda

CHAPTER 18
The Ecdysozoas
Phylum Nematoda
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Nematode:
Trichinella
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Diversity
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Many protostomes possess a cuticle
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Non-living outer layer secreted by epidermis
Cuticle restricts growth and must be molted
via ecdysis (shedding)
Members of Ecdysozoa molt cuticle as they
grow
Regulation of molting achieved by the
hormone ecdysone
Nematodes are known as Small Ecdysozoans
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Diversity
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Nematoda (Roundworms) are
Pseudocoelomate
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Flatworms = acoelomates
Diversity
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About 25,000 species are described
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Found in virtually all habitats in all biomes
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As many as half a million may exist
Topsoil may contain billions per acre
Nematode parasites exist in nearly all animal
and plant species
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Phylum Nematoda: Roundworms
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Free-living nematodes feed on bacteria,
yeasts, fungal hyphae, and algae
Predatory nematodes eat rotifers,
tardigrades, small annelids, and other
nematodes
Nematodes are food for mites, insects,
larvae, and fungi
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Phylum Nematoda: Roundworms
Form and Function
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Distinguishing Characteristics
Cylindrical shape
 Nonliving cuticle
 Cuticle shed during juvenile growth stages
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superphylum Ecdysozoa
Lack motile cilia or flagella
 Longitudinal muscles in body wall
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Female Ascaris (Roundworm)
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Phylum Nematoda: Roundworms
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Digestion
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Alimentary canal consists of mouth, pharynx, nonmuscular intestine, short rectum and anus
Pharynx sucks food in
Intestinal wall is one cell thick
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Food moves back as new food enters and the body
moves
Mouth and Anus = Complete Gut
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Phylum Nematoda: Roundworms
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Ring of nerve tissue and ganglia around the pharynx
lead to dorsal and ventral nerve cords
Sensory organs at head and tail
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Amphids = chemoreceptors near anterior end
Parasitic nematodes have a bilateral pair of
phasmids near posterior end that resemble amphids
Most are dioecious with males smaller than females
Fertilization is internal
Eggs are stored in uterus until deposited
Cuticle is shed between each of four juvenile stages
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Amphid (chemoreceptors in Anterior region)
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Phylum Nematoda: Roundworms
Representative Nematode Parasites
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Some are parasites of humans
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Most are tropical
Ascaris lumbricoides
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Occurs in up to 25% of people in some areas of
the southeastern U.S.
More than 1.27 billion affected worldwide
Some species of Ascaris is found in the intestine
of horse and pigs.
A female Ascaris may lay 200,000 eggs a day,
which pass out in host’s feces
Embryos develop into juveniles in 2 weeks
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Male and Female Ascaris Roundworm
Male is smaller (top), Female is larger
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Phylum Nematoda: Roundworms
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Viable eggs remain after signs of fecal matter have
disappeared
Survive for long periods in soil
Host swallows embryonated eggs, juveniles hatch,
and burrow through intestinal wall
Carried through the heart to the lungs, they break
into alveoli and are carried up to tracheae
Coughed up and swallowed, they mature in the
intestine after two months
They feed on intestinal contents and may block or
perforate the intestines
Infection rates tend to be higher in children and
males
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Life Cycle of Ascaris Roundworm
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Phylum Nematoda: Roundworms
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Hookworms
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Anterior end of these small (9–11 mm) worms has a
hook-like curve
Necator americanus, most common hookworm.
Sexes are separate
Large plates in mouth cut into intestinal wall and
suck host’s blood
Pump through more blood than they digest
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Heavy infections cause anemia
Eggs pass out in feces and juveniles hatch in soil
If human skin comes in contact with soil, infective
juveniles burrow through skin to blood
Travel in blood to the lungs, are coughed up to be
swallowed, and mature in the intestine
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Hookworm:
A. Mouth showing cutting plates
B. Hookworm attached to dog intestine, cutting plates pinch of intestine wall
Hookworm secretes an anticoagulant to prevent blood from clotting
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Life cycle of Hookworm
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Phylum Nematoda: Roundworms
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Trichina Worm
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Trichinella spiralis causes a potentially lethal
trichinosis
Adult worms burrow into intestinal wall and
females directly produce juvenile worms
Juveniles penetrate blood vessels and circulate
throughout the body to all tissues and spaces
Penetrate skeletal muscle cells
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Cells lose striations and becomes a nurse cells
(continues to nourish) to the parasite
When poorly cooked meat containing encysted
juveniles is eaten, worms are liberated and mature
in the intestine
Heavy infection in mammals causes death
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Muscle infected with
Trichina worm
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Phylum Nematoda: Roundworms
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Pinworms
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Most common worm parasite in the U.S. but
causes little disease
Adults live in large intestine
Females, about 12 mm long, migrate to anal region
at night and lay eggs, causing itching
Scratching the anal region contaminates hands
and bedclothes
Eggs develop rapidly and become infective within
six hours at body temperature
When swallowed, hatch in duodenum and mature
in large intestine
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A.Female pinworm from intestine
B.Group of Pinworm eggs released from anus
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Phylum Nematoda: Roundworms
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Filarial Worms
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Eight species of filarial nematodes infect humans
 Some cause serious diseases
Wucheria bancrofti and Brugia malayi live in
lymphatic system
 Cause inflammation and blockage of the
lymphatics vessels
 Females release live young, tiny microfilariae,
into blood and lymph
 Mosquitoes ingest microfilariae when they feed
 Worms develop to infective stage and move into
the mosquito bite wound when it feeds
 Elephantiasis is caused by repeated exposure
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Swelling and growth of connective tissue causes
enormous swelling of body parts
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Phylum Nematoda: Roundworms
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River blindness or onchocerciasis is carried by
black flies and infects 37 million people in tropics
Dog heartworm, Dirofilaria immitis, is carried by
mosquitoes and is the most common U.S. filarial
worm
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Life Cycle of Filarial Worm
Wuchereria bancrofti
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