Platyhelminthes (Flatworms)

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Platyhelminthes (Flatworms) and Nematoda
(Roundworms)
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Section 26.3 Summary pages 706-710
Platyhelminthes (Flatworms)

• Flatworms are the least complex worms

• acoelomates with thin, solid bodies.

Planarian
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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
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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
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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
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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
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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.

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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
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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.

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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.

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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.

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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
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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.

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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.

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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
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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.

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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.

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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.

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Section 26.3 Summary pages 706-710
The life cycle of a fluke
Adult flukes
Larva
Embryos released
Human host
Snail host
Larva
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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
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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
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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.

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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
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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
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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.

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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.

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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
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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.

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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.