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Title: Platyhelminthes (Flatworms)


1
Platyhelminthes (Flatworms) and Nematoda
(Roundworms)
2
Section 26.3 Summary pages 706-710
Platyhelminthes (Flatworms)
  • Flatworms are the least complex worms
  • acoelomates with thin, solid bodies.

Planarian
3
Section 26.3 Summary pages 706-710
Platyhelminthes (Flatworms)
  • approximately 14,500 marine and freshwater
    species of flatworms

They are found in bodies of water and moist
habitats
Planarian
4
Section 26.3 Summary pages 706-710
Platyhelminthes (Flatworms)
  • The most well-known members are the parasitic
    tapeworms and flukes, which cause diseases in
    other animals
  • The most commonly studied flatworms are the
    free-living planarians

Free living not parasitic
5
Section 26.3 Summary pages 706-710
Planarians
  • Planarians have many characteristics common to
    all species of flatworms.
  • The bodies are flat, with a dorsal and a ventral
    surface. All flatworms have bilateral symmetry.

Dorsal Top Ventral Bottom
6
Section 26.3 Summary pages 706-710
Planarians
  • Most of the nervous system is located in the
    heada characteristic common to other bilaterally
    symmetrical animals.

Nerve cell mass
7
Section 26.3 Summary pages 706-710
Planarians
  • Some flatworms have a nerve net, and others have
    the beginnings of a central nervous system.

Nerve cell mass
Nerve cord
  • Planarians have two nerve cords that run the
    length of the body.

8
Section 26.3 Summary pages 706-710
Planarians
  • Nervous system includes eyespots that can detect
    the presence or absence of light and sensory
    cells that can detect chemicals and movement in
    water.

Eyespots
9
Section 26.3 Summary pages 706-710
Planarians
  • At the anterior end of the nerve cord is a
    ganglion (plural, ganglia). The ganglion receives
    messages from the eyespots and sensory pits, then
    communicates with the rest of the body along the
    nerve cords.

Ganglia
  • Messages from the nerve cords trigger responses
    in a planarians muscle cells.

10
Section 26.3 Summary pages 706-710
Reproduction in Planarians
  • Most flatworms including planarians, are
    hermaphrodites.
  • During sexual reproduction, individual planarians
    exchange sperm, which travel along special tubes
    to reach the eggs.
  • Fertilization occurs internally. Zygotes are
    released in capsules into the water, where they
    hatch into tiny planarians.

11
Section 26.3 Summary pages 706-710
Reproduction in Planarians
  • Planarians also can reproduce asexually.
  • When a planarian is damaged, it has the ability
    to regenerate.
  • If a planarian is cut horizontally, the section
    containing the head regenerate a new tail, and
    the tail section will regenerate a new head.

12
Section 26.3 Summary pages 706-710
Feeding and Digestion
  • Planarians feed on dead or slow-moving organisms.
  • It extends a tube-like, muscular organ, called
    the pharynx (FAHR inx), out of its mouth. Enzymes
    released by the pharynx begin digesting food
    outside the animals body.

Extended pharynx
13
Section 26.3 Summary pages 706-710
Feeding and Digestion
  • Food particles are sucked into the digestive
    tract, where they are broken up.
  • Cells lining the digestive tract obtain food by
    endocytosis.
  • Food is thus digested in individual cells.

14
Section 26.3 Summary pages 706-710
Parasitic Flatworms
  • Parasite an organism that lives on or in
    another organism and depends upon that host
    organism for its food.

15
Section 26.3 Summary pages 706-710
Parasitic Flatworms
  • Parasitic flatworms have mouthparts with hooks
    that keep the flatworm firmly attached inside its
    host.
  • They do not need to move to seek out or find food.
  • Parasitic flatworms do not have complex nervous
    or muscular tissue.

Parasitic flatworms make up two classes,
Tapeworms and Parasitic Flukes
16
Section 26.3 Summary pages 706-710
Tapeworms
Hooks
  • knob-shaped head called a scolex and detachable,
    individual sections called proglottids.

Scolex
Sucker
Proglottid
  • Proglottids contain muscles, nerves, flame cells,
    and male and female reproductive organs.

Mature proglottid with fertilized eggs
  • Some adult tapeworms that live in animal
    intestines can be more than 10 m in length and
    consist of 2000 proglottids.

17
Section 26.3 Summary pages 706-710
Flukes
  • A fluke is a parasitic flatworm that spends part
    of its life in the internal organs of a
    vertebrate, such as a human or sheep.
  • It obtains its nutrition by feeding on cells,
    blood, and other fluids of the host organism.

18
Section 26.3 Summary pages 706-710
Flukes
  • Blood flukes of the genus Schistosoma cause a
    disease in humans known as schistosomiasis.
  • Schistosomiasis is common in countries where rice
    is grown.
  • Blood flukes are common where the secondary host,
    snails, also are found.

19
Section 26.3 Summary pages 706-710
The life cycle of a fluke
Adult flukes
Larva
Embryos released
Human host
Snail host
Larva
20
Section 26.4 Summary pages 711-713
Nematoda (Roundworms)
  • widely distributed, living in soil, animals, and
    both freshwater and marine environments.
  • Most are free-living, but many are parasitic.

Roundworm
21
Section 26.4 Summary pages 711-713
Nematoda (Roundworms)
  • tapered at both ends.
  • thick outer covering protects them in harsh
    environments. Covering is shed 4 times as they
    grow.

Tapered ends
Round body shape
22
Section 26.4 Summary pages 711-713
Nematoda (Roundworms)
  • No circular muscles, but have lengthwise muscles.
  • One muscle contracts, another muscle relaxes.
    This alternation causes roundworms to move in a
    thrashing fashion.

23
Section 26.4 Summary pages 711-713
Nematoda (Roundworms)
  • Have a pseudocoelom and are the simplest animals
    with a tubelike digestive system.
  • Roundworms have two body openingsa mouth and an
    anus! This is a first for us!

Intestine
Anus
Mouth
Round body shape
24
Section 26.4 Summary pages 711-713
Nematoda (Roundworms)
  • Eyespots are reduced in parasitic roundworms.
  • Approximately half of roundworm species are
    parasites, and about 50 species infect humans.

Examples Ascaris, Pinworms, Trichenella, and
Hookworms
25
Section 26.4 Summary pages 711-713
Ascaris
  • the most common roundworm infection in humans
  • Children infected more often than adults
  • Eggs are found in soil and enter a human through
    the mouth.
  • Eggs hatch in the intestines, move into the
    bloodstream, and eventually to the lungs, where
    they are coughed up, swallowed, and begin the
    cycle again.

26
Section 26.4 Summary pages 711-713
Pinworms
Pinworms are the most common human roundworm
parasites in the United States.
  • highly contagious because eggs can survive for up
    to two weeks on surfaces.
  • live eggs are ingested and mature in the hosts
    intestinal tract.
  • female pinworms exit the hosts anus and lay eggs
    on nearby skin.
  • eggs fall onto bedding or other surfaces.

27
Section 26.4 Summary pages 711-713
Trichinella
  • Trichinella causes a disease called trichinosis
    (tri keh NOH sis).
  • Found in raw or undercooked pork, pork products,
    or wild game.

Trichinella
28
Section 26.4 Summary pages 711-713
Hookworms
  • Hookworm infections are common in humans in warm
    climates where they walk on contaminated soil in
    bare feet.
  • Hookworms cause people to feel weak and tired due
    to blood loss.

29
Section 26.4 Summary pages 711-713
Roundworm Parasites
  • Nematodes can infect and kill pine trees, cereal
    crops, and food plants such as potatoes.
  • They are particularly attracted to plant roots
    and cause a slow decline of the plant.
  • They also can infect fungi and can form symbiotic
    associations with bacteria.
  • Nematodes also can be used to control pests.
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