Pseudocoelomate Animals

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Pseudocoelomate Animals

• There are 9 pseudocoelomate phyla.
• These are a diverse lot most of which are small,
  and some microscopic, although a few are
  relatively large.
• Most are free living although some are
  exclusively (Acanthocephala) parasitic or include
  many parasitic species (Nematoda).

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Pseudocoelomate Animals

• All have a pseudocoelom, a cavity surrounding the
  gut.
• Unlike a true coelom, a pseudocoelom is not a
  cavity surrounded by mesoderm. Instead it is a
  persistent blastocoel and lacks a peritoneum.

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Pseudocoelomate Animals

• All pseudocoelomates have a body wall of muscles
  and epidermis that surrounds the pseudocoel.
• All pseudocoelomates except for the
  Acanthocephala have a complete gut.

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Pseudocoelomate Phyla

• The pseudoceolomate phyla are listed below. In
  the interests of time we will discuss only the
  first three
• Rotifera wheel animals
• Acanthocephala spiny-headed worms
• Nematoda roundworms
• Gastrotricha
• Entoprocta
• Nematomropha
• Kinorhyncha
• Priapulida
• Loricifera

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Phylum Rotifera

• Rotifers are named for their characteristic
  ciliated crown or corona, which when it beats
  looks like a rotating wheel.
• Rotifers are tiny animals (most are 100-300µm
  long and the largest only 3mm long) the majority
  of which live in freshwater and are benthic
  inhabitants (live on the bottom).
• About 2000 species have been described.

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Phylum Rotifera

• The beating of the cilia in the corona draws in
  plankton-containing water for food.
• The mouth opens to a modified muscular pharynx
  known as a mastax, which is a structure unique to
  rotifers.
• The mastax has a set of complex jaws, which are
  used to grasp and chew food.

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Phylum Rotifera

• One group of rotifers, the Bdelloid rotifers, are
  very unusual in that there are no males,
  hermaphrodites, or evidence of meiosis.
• Molecular evidence suggests that there has been
  only asexual reproduction in this group for
  several million years.

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Phylum Rotifera

• Because of the problem of accumulation of
  deleterious mutations in lineages of exclusively
  asexually reproducing animals (a process known as
  Mullers ratchet) it is unclear how the bdelloids
  have been able to dispense with sexual
  reproduction entirely.
• Other rotifers reproduce using a combination fo
  sexual and asexual reproduction.

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Phylum Acanthocephala

• Acanthocephalans are commonly known as
  spiny-headed worms because of the spiny eversible
  proboscis they use to attach to the gut of their
  host.
• All 1100 species of Acanthocephalan are
  endoparasitic and most parasitize fish, birds and
  mammals.

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Phylum Acanthocephala

• The body wall is covered with numerous minute
  depressions which enormously increase the surface
  are of the tegument and facilitates (as in
  cestodes) the absorption of food from the hosts
  gut.
• As is true in cestodes, Acanthocephalans lack a
  gut.

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Phylum Acanthocephala

• Acanthocpehalans have a lifecycle in which a
  vertebrate is the definitive host and an
  invertebrate the intermediate host.
• Acanthocephalans, as other parasites do, modify
  the behavior of the intermediate host to enhance
  the chances of its being eaten.

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Phylum Acanthocephala

• For example, acanthocephalans that parasitize
  Gammarus, a small freshwater crustacean, cause
  the Gammarus to alter its behavior in the
  presence of ducks, a common predator.
• Instead of diving to the bottom when a duck
  appears, the Gammarus swims into the light and
  grasps tightly onto a piece of vegetation,
  greatly increasing its chances of being eaten.

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Phylum Acanthocephala

• The change in behavior appears to be caused by
  the Acanthocephalan pumping a serotonin-boosting
  molecule into the Gammarus brain.
• This causes the Gammarus to think its having sex
  and cling as it would if mating. Interestingly,
  the parasites manipulation also causes female
  Gammarus to mimic the males mating behavior.

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Phylum Acanthocephala

• Another Acanthocephalan that parasitizes pill
  bugs causes them to reverse their normal behavior
  and avoid humid, dark areas.
• Instead they wander in the open where they are
  much more vulnerable to birds, the
  acanthocephalans definitive host.
• The parasites manipulations are very effective.
  Although fewer than 1 of pill bugs are typically
  infected with acanthocephalan parasites 30 of
  pill bugs delivered to nestlings are infected.

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Phylum Nematoda

• The nematodes are by far the most important group
  of pseudoceolomates both in terms of numbers
  (about 10,000 species) and their impact on
  humans.
• Most nematodes are under 5cm and many are
  microscopic. However, some parasitic forms can
  be over a meter in length.

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Phylum Nematoda

• Nematodes use their pseudocoelom as a hydrostatic
  skeleton.
• The body has a thick cuticle (made primarily of
  collagen) secreted by the underlying epidermis,
  which resists the high hydrostatic pressure
  exerted by the fluid in the pseudocoelom.

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Phylum Nematoda

• Beneath the epidermis is a layer of longitudinal
  muscles.
• Muscles in nematodes are not arranged in
  antagonistic pairs, the antagonistic role is
  played by the cuticle.
• Contraction of a longitudinal muscle on one side
  is transmitted through the hydrostatic skeleton
  and stretches the cuticle on the opposite side of
  the body.
• When the muscle relaxes, the cuticle contracts
  and the body returns to its resting position.

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Phylum Nematoda

• Nematodes have a complete gut with a mouth,
  muscular pharynx, intestine, rectum, and anus.
• Most nematodes are dioecious and males are
  smaller than females.
• Fertilization is internal and juveniles go
  through several developmental stages, each time
  molting or shedding their cuticle.

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Free-living nematodes

• Free-living nematodes live in the sea, in fresh
  water, and in the soil. They occur worldwide in
  all environments and most live in the
  interstitial spaces of sediments and soils.
• Vast numbers of nematodes occur. One square
  meter of sea bottom mud has been estimated to
  hold 4.4 million nematodes and 90,000 were
  counted on a single decomposing apple.

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Free-living nematodes

• The slender, tapered body of nematodes equips
  them to live in interstitial spaces.
• Most free-living nematodes are less than 2.5mm in
  length and often are microscopic. The largest
  soil dwelling nematodes may be 7mm long and the
  largest marine forms a whopping 5cm.

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Free-living nematodes

• Most free-living nematodes are carnivorous.
• However, some feed on algae and fungi and some
  are detritivores. Others feed on plants,
  especially the roots.

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Free-living nematodes

• Many root feeding nematodes are major
  agricultural pests. These species pierce root
  cells and suck out their contents.
• Nematodes are estimated to destroy 12 of the
  worlds cash crops annually.

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Parasitic nematodes

• There are a great many species of parasitic
  nematodes and they attack virtually all groups
  of animals and plants.
• Parasitic forms include ascarids, hookworms,
  Guinea worms, trichina worms, pinworms, and
  filarial worms.

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Ascaris lumbricoides large roundworm of humans

• Its estimated that worldwide as many as 1.4
  billion people are infected with Ascaris
  lumbricoides which lives in the small intestine.
• Females may be a foot long and produce 200,000
  eggs a day.
• Infection occurs when parasite eggs are eaten
  with uncooked food or when soiled fingers are put
  into the mouth.

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Ascaris lumbricoides large roundworm of humans

• The larvae penetrate the intestinal wall and
  travel through the blood stream to the lungs
  where they break out of the alveoli (often
  causing pneumonia).
• Then they make their way up the trachea where
  they are swallowed and eventually settle in the
  small intestine.

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Ascaris lumbricoides large roundworm of humans

• In the intestines the worms cause abdominal
  symptoms and allergic reactions and may produce
  an intestinal blockage.

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9.8
Male (top) and female Ascaris lumbricoides
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Hookworms

• Hookworms are named for the dorsal curve in their
  anterior end.
• Hookworms are quite small, the commonest species
  Necator americanus is only 11mm long. However,
  because they feed on blood a heavy infection can
  produce severe anemia.

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Hookworms

• Large plates in the hookworms mouth are used to
  cut the intestinal lining of the host.
• The parasite then pumps blood through its gut,
  partially digesting it before excreting it.
• Because hookworms suck more blood than they use,
  they can cause debilitating anemia. In children a
  hookworm infection can stunt growth and cause a
  general lack of energy.

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9.9
Section through hookworm attached to dog
intestine
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Hookworms

• Hookworms do not permanently attach in one spot,
  but move around the gut and reattach when they
  are ready to feed.
• Hookworms have evolved sophisticated
  anti-clotting factors that keep platelets from
  clumping and forming a clot while the hookworm is
  feeding.

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Hookworms

• When the hookworm releases, a clot forms and the
  tissue can recover.
• By using this approach instead of producing a
  crude blood thinner to ensure blood flow,
  hookworms prevent hemophilia developing in their
  hosts, which would be fatal for the hookworm.

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Hookworms

• The life cycle of hookworms is very similar to
  that of ascarids and infection occurs in the same
  way by contact with eggs in soil or food.

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Guinea worms

• Guinea worm infections (also referred to as
  Dracunculiasis) are now confined to sub-Saharan
  Africa. Adults are threadlike nematode worms
  that can grow to 1 meter in length.
• The adult lives in humans and the intermediate
  host is tiny crustaceans.
• Humans become infected when they drink water
  containing the crustaceans.

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Guinea worms

• The immature worm penetrates the gut wall and
  wanders through the body, maturing and growing.
• After about a year the female makes her way to
  the surface of the skin (usually in the legs)
  causing very painful blistering.

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Guinea worms

• To ease the pain, sufferers immerse their feet in
  water. This bursts the blisters and the female
  worm then protrudes from the sore and lays her
  eggs, thus continuing the life cycle.

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Guinea worms

• There is no cure for Guinea worms and the only
  way to remove one is to slowly over the course of
  weeks wind the worm out on a stick.
• If the worm breaks,
• a serious bacterial
• infection results.

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Interestingly, the traditional symbols for
medicine and healing the staff of Asclepias
(showing a snake entwined around a staff) and
the caduceus (which shows two snakes entwined
about a winged staff) very likely are derived
from the Guinea worm removal technique.
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Guinea worms

• Guinea worm infection is avoidable with
  relatively simple precautions such as preventing
  people walking in drinking water sources and
  boiling or filtering water before drinking it.

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Guinea worms

• Since the mid 1980s a campaign to eradicate
  Guinea worms coordinated by the U.N. and the
  Carter Center has had tremendous success.
• In 1986, an estimated 3.5 million people were
  infected, but by 2000 the number of cases had
  been reduced to about 75,000.

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Guinea worms

• Guinea worms have been eliminated from Pakistan,
  India, and Iran and infections greatly reduced
  over much of sub-Saharan Africa.
• The major barrier to elimination at this point is
  the ongoing conflict in southern Sudan where the
  majority of cases now occur.

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Trichina worms

• Trichinella spiralis is a tiny nematode that
  causes the potentially fatal disease trichinosis.
  
• Humans typically become infected by eating
  undercooked pork. Trichinella lives in cysts
  formed in individual muscle cells of the host.

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Trichina worms

• Trichinella when it hatches from an ingested
  cysts in its hosts gut drills through the wall
  of the gut where females produce living young.
• These juveniles travels in the circulatory system
  to a muscle.
• The juvenile penetrates an individual muscle cell
  and breaks the cell down so it can be remade.

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Trichina worms

• Trichinella, just as a virus does, manipulates
  the host cells DNA. It causes the cell to
  recruit a blood supply to supply food to the cell
  and also produce collagen to form a cyst around
  the cell.
• The Trichinella juvenile awaits ingestion by
  another host. When ingested it emerges from its
  cysts enters the mucosal lining of gut, develops
  into an adult and continues the life cycle.

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Trichina worms

• Adults usually do not persist long in the gut
  before being expelled by the hosts immune system.

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Trichina worms

• Humans are an inadvertent host of Trichinella.
• Normal life cycles involve pigs and rats.
• In humans, infection with a few Trichinella
  parasites may cause no symptoms, but heavy
  infections can cause intense muscle pain and in
  some cases death.

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Filarial worms

• At least 8 species of filarial worms infect
  humans especially in tropical regions.
• Approximately 250 million people infected with
  Wuchereia bancrofti or Brugia malayi
• These worms live in the lymphatic system and
  females can be 10cm long.

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Filarial worms

• Females release live young microfilariae into
  the blood and lymphatic system.
• The microfilariae are picked up by mosquitoes
  where they develop, become infective and can
  infect another person.

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Filarial worms

• In some people exposed to persistent infections
  with these parasites, elephantiasis may develop.
• This is caused by blockage of lymphatic ducts and
  inflammation. There may be excessive growth of
  connective tissue and enormous swelling of
  infected parts including legs, arms and scrotum.

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Elephantiasis of leg caused by filarial worms
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Filarial worms

• The most common filarial worm in the U.S. is the
  cause of heartworm.
• The microfilariae are transmitted by mosquitoes.

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Diriofilaria immitis Dog heartworm
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River blindness

• River blindness is also caused by filarial worms.
  
• In this case the insect that transmits the
  disease is a blackfly.
• 18- 30 million people are infected worldwide
  (mainly central Africa and parts of South
  America) and more than 300,000 have been made
  blind.

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River blindness

• When a black fly becomes infected, the worm
  larvae spread to its salivary glands. When it
  bites someone the larvae pass into the skin.
• The larvae develop into adults and form nodules
  under the skin. The adults breed and produce
  thousands of larvae, which spread all over the
  body - including the eyes.

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River blindness

• The worst problem is caused when problem is when
  the parasites die. The immune system produces a
  severe inflammation, which if it occurs in the
  eye it can cause blindness.
• People infected at birth with river blindness
  commonly become blind by their 40s.