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Pseudocoelomate Animals


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. – PowerPoint PPT presentation

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Title: Pseudocoelomate Animals

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).

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

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.

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

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.

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
  • The mastax has a set of complex jaws, which are
    used to grasp and chew food.

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.

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.

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
  • All 1100 species of Acanthocephalan are
    endoparasitic and most parasitize fish, birds and

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
  • As is true in cestodes, Acanthocephalans lack a

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.

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.

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.

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.

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
  • Most nematodes are under 5cm and many are
    microscopic. However, some parasitic forms can
    be over a meter in length.

Phylum Nematoda
  • Nematodes use their pseudocoelom as a hydrostatic
  • 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.

Phylum Nematoda
  • Beneath the epidermis is a layer of longitudinal
  • 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.

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.

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.

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.

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.

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.

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.

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.

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.

Ascaris lumbricoides large roundworm of humans
  • In the intestines the worms cause abdominal
    symptoms and allergic reactions and may produce
    an intestinal blockage.

Male (top) and female Ascaris lumbricoides
  • 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.

  • 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.

Section through hookworm attached to dog
  • 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

  • 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.

  • 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.

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.

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.

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.

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.

(No Transcript)
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.
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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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

Diriofilaria immitis Dog heartworm
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

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.

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.