Chapter 12
SMALLER ECDYSOZOANS
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Diversity
• Many protostomes possess a cuticle
– Non-living outer layer secreted by epidermis
• Cuticle restricts growth and must be molted via
ecdysis
• Members of Ecdysozoa molt cuticle as they grow
• Regulation of molting achieved by the hormone
ecdysone
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Phylum Nematoda: Roundworms
• 12,000 species have been named
– As many as half a million may exist
• Found in virtually all habitats in all biomes
– Topsoil may contain billions per acre
• Nematode parasites exist in nearly all animal and
plant species
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Phylum Nematoda: Roundworms
Form and Function
Outer body covering is a thick, noncellular cuticle, secreted by
the underlying epidermis- hypodermis
Cuticle serves to contain the hydrostatic pressure exerted by
fluid in the pseudocoelom
• Collagen is the primary protein in layers of the cuticle
• Muscles
– Longitudinal muscles lie beneath the cuticle
• No circular muscles
– Run in four bands, marked off by epidermal cords that
project inward to pseudocoelom
– Unlike other animals, the muscle extends to the nerve
cords
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Phylum Nematoda: Roundworms
• Hydrostatic skeleton-muscle contractions press on
fluid
• No circular muscles to compliment longitudinal
muscle in movement (antagonist) so the cuticle
assists.
• Fluid is force to opposite side when longitudinal
muscles constrict forcing the cuticle to expand
• Compression and stretching of the cuticle returns the
body to resting position when muscles relax
• Produces the thrashing movement of nematodes
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Phylum Nematoda: Roundworms
• Digestion
– Gut tube consists of mouth, pharynx, non-muscular
intestine, short rectum and anus
– Muscular pharynx sucks food in
– Intestinal wall is one cell thick-no muscles
• Food moves back as new food enters and the body moves
– Defecation occurs from opening the anus and allowing
pseudocoelomic pressure to expel waste
• Some parasitic adults have an anaerobic metabolism;
aerobic metabolism are absent
• Free-living nematodes and free-living stages of
parasitic nematodes have both anaerobic and aerobic
metabolism
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Phylum Nematoda: Roundworms
• Ring of nerve tissue and ganglia around the pharynx give rise
to small nerves to the anterior end and to two nerve cords,
one dorsal and one ventral
• Sensory organs at head and tail
• Most are dioecious with males smaller than females
• Male has copulatory spicules
• Fertilization is internal
• Nematode sperm has no flagella, in female reproductive tract
sperm is ameboid and moves by pseudopods
• Eggs are stored in uterus until deposited
• Cuticle is shed between each of four juvenile stages
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Figure 12_03
Representative Nematode Parasites
Representative Nematode Parasites
• Some are parasites of humans
– Most are tropical
• Ascaris lumbricoides
– Occurs in up to 25% of people in some areas of the
southeastern U.S.
– More than 1.27 billion affected worldwide
– A. suum is found in pig intestines
– A female Ascaris may lay 200,000 eggs a day, which pass
out in host’s feces
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Figure 12_04a
Representative Nematode Parasites
– Survive for long periods in soil
– When humans eat undercooked vegetables contaminated
with shelled juveniles, or when children put soiled fingers or
toys in their mouths, consumed juveniles hatch, and burrow
through intestinal wall
– Carried through the heart to the lungs, they enter into
alveoli causing pneumonia and are carried up to tracheae
– Coughed up and swallowed, they mature in the intestine
– They feed on intestinal contents and may block the
intestines
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Figure 12_04b
Representative Nematode Parasites
• Hookworms
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–
–
–
Anterior end of these small worms has a hook-like curve
Necator americanus, most common hookworm.
Sexes are separate
Large plates in mouth cut into intestinal mucosa and suck
host’s blood
– Pump through more blood than they digest
• 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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Figure 12_05
Representative Nematode Parasites
• Trichina Worm
– Trichinella spiralis causes a potentially lethal trichinosis
– Adult worms burrow into intestinal mucosa of the host’s
small intestines and females directly produce juvenile
worms
– Juveniles penetrate blood vessels and circulate throughout
the body to all tissues and spaces
– Penetrate skeletal muscle cells, redirecting gene expression
of the musculature
• Cells lose striations and becomes a nurse cells to the parasite
– When poorly cooked meat (pork or bear) containing
encysted juveniles is swallowed, worms are liberated and
mature in the intestine
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Representative Nematode Parasites
– Infect humans, pigs, rats, cats and dogs
• Pigs can become infected eating uncooked scraps of infected meat
or rats
– Four other sibling species with variable distribution,
freezing resistance, etc.
– Heavy infections cause death
– 25 cases of trichinosis are reported per year in the US
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Figure 12_06
Representative Nematode Parasites
• Pinworms
– Most common worm parasite in the U.S.
– Adults live in large intestine and cecum
– Females, about 12 mm in length, 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
– Members of this order have haploid males from unfertilized
eggs
– Females are diploid and come from fertilized eggs
(haplodiploidy)
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Representative Nematode Parasites
• Filarial Worms
– Eight species of filarial nematodes infect humans
• Some cause serious diseases
– Wucheria bancrofti and Brugia malayi live in lymphatic
system
• Cause inflammation and obstruction 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 bites a human
• Elephantiasis is caused by repeated exposure
– Swelling and growth of connective tissue causes enormous
swelling of body parts-scrotum, legs, arms
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Representative Nematode Parasites
– 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
– Heartworm pills
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Figure 12_08
Figure 12_09
Representative Nematode Parasites
– Dracunculus medinensis, the guinea worm
- Larvae live within planktonic copepods (water fleas)
- Upon ingestion of contaminated water, water fleas are
digested by human host, but larvae survive and penetrate
the stomach/intestinal wall
- Worms live and mate within body cavity
- Gravid females migrate through tissues to the lower
extremities and produce an open ulcer
- Causing burning pain, the human host immerses leg into
water and female releases thousands of eggs into water
- Eggs are eaten by copepods and the cycle continues
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Representative Nematode Parasites
• Only known treatment is to carefully remove
ulcerated females carefully with a stick;
treated this way for thousands of years
Phylum Nematomorpha
Diversity
• “Horsehair worms” resemble coarse hairs
• Adult structures resemble those seen in nematodes:
cuticle, epidermal cords, only longitudinal muscles,
and a similar nervous system pattern
– Currently placed as the sister taxon to nematodes
• About 320 species are known
– Occur worldwide
• Adults are free-living in moist habitats
• Juveniles are parasites of arthropods
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Phylum Nematomorpha
– Larvae encyst within host and do not emerge from an
aethropod host unless water is nearby
– Juveniles of freshwater forms use terrestrial insects as
hosts
– Marine nematomorphs infect certain crabs
• Digestive system is vestigial
– Larvae absorb food from arthropod hosts
Adults can absorb organic molecules through vestigial gut
and body wall
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Figure 12_10
Phylum Tardigrada
• Known as “water bears”
• Very small, less than 1 mm long
• About 900 live in a water film around mosses and
lichens
• Some live in freshwater and some are marine
• Most are terrestrial that occupy a film of water
surrounding mosses and lichens
• Trunk bears eight short unjointed legs, each with
claws
• A pair of stylets and sucking pharynx protrude to
pierce nematodes or plant cells
• Body covered by non-chitinous cuticle that is molted
four or more times during lifetime
• Most of the body cavity a hemocoel
• No circulatory or respiratory systems
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• Cryptobiosis
– Terrestrial tardigrades can suspend metabolism to
survive harsh conditions
– Tardigrades can dehydrate from 85% water to only
3% water
• In this state they can resist extreme temperatures,
ionizing radiation, oxygen deficiency, etc. for years
– When water is available, they become
metabolically active again
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• Reproduction
– Sexes are separate
– In parthenogenetic freshwater and moss-dwelling species,
males are unknown
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Figure 12_15
Phylogeny and Adaptive Diversification
Phylogeny
• Evolutionary relationships among ecdysozoans are
not well-understood
• Members of this clade do not share a common
cleavage pattern
– Nematodes and nematomorphs
• Cleavage is unique, not spiral or radial
– Cleavage in tardigrades
• Has yet to be studied
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Phylogeny and Adaptive Diversification
• Recent phylogenies place Nematoda and
Nematomorpha as sister taxa since they share a
collagenous cuticle
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Phylogeny and Adaptive Diversification
• Tardigrades have some similarities to rotifers,
particularly in their reproduction and cryptobiotic
tendencies
• Tardigrades and arthropods also share arthropodtype setae and muscles inserted on the cuticle
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Phylogeny and Adaptive Diversification
Adaptive Diversification
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Nematodes show the most impressive adaptation
Found in almost every habitat available to animals
Body structure is plastic enough to allow adaptation
Life cycle ranges from simple to complex
Have been known to survive in suboptimal
conditions
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