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Platyhelminthes, Nemtoda, and Annelida
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There are three major groups of worms:
o 1) Flatworms (Phylum Platyhelminthes)
o 2) Roundworms (Phylum Nematoda)
o 3) Segmented Worms (Phylum Annelida)
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Textbook Introduction
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Flatworms : have tissues and
internal organ systems.
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Flatworms are acoelomates,
meaning without coelom.
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coelom: fluid-filled body cavity
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They have right and left sides, and
most have enough cephalization to
have what is called a head.
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Most cells are in contact with
external environment, so they
rely on diffusion.
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No gills, heart, blood or blood
vessels.
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Some have flame cells:
specialized cells that remove
excess water and wastes.
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Head encloses several ganglia: groups of nerve cells
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Not complex enough to be called a brain
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Many have eyespots: groups of cells that can detect light.
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Most are hermaphrodites: both male and female
reproductive organs
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Sexual reproduction: two worms join and deliver sperm to
each other; eggs are then laid in clusters
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Asexual reproduction: fission - organism splits in two and
each half grows to form a complete organism
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Turbellarians: free-living flatworms, most live in marine or
fresh water. (planarians)
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Trematoda: parasitic flatworms, most flukes infect the
internal organs of their host. (flukes)
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Cestoda : long, flat, parasitic worms that are adapted to life
inside the intestine of the host. (tapeworms)
1.
What is a flatworm?
2.
What are the three groups of flatworms?
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Textbook Introduction
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Roundworms
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Have a pseudocoelom, or "false coelom"; not a true fluidfilled, tissue-lined coelom
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Body plan: "tube within a tube"
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Digestive tract has two openings – mouth and anus
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Many free-living forms are predators with grasping mouth
parts to catch and eat small animals.
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Other eat decaying matter, and several types are parasitic
and cause disease in humans.
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Rely solely on diffusion, no internal transport,
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Simple nervous system with several ganglia
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Sexual: most species have separate sexes.
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Internal Fertilization: Male deposits sperm inside female.
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Textbook Introduction
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Segmented Worms
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Ring-like appearance, or "segmented" body parts
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Many have bristles, (setae), attached to each segment
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Have a true coelom that is lined with tissue came from
mesoderm (middle layer)
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Many use a pharynx; some have sharp jaws to attack prey.
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Earthworms: pharynx pumps food and soil into a tube
called the esophagus.
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Food enters the crop where it is stored, and then through
the gizzard, where it is ground into small pieces
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Closed circulatory system: blood is contained within a
network of blood vessels.
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Aquatic annelids have gills
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Gill: organ specialized for the exchange of gases
underwater.
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Earthworms take in oxygen and release carbon dioxide
through their moist skin
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Most reproduce sexually.
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Two worms attach to each other, exchange sperm, then
store it.
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When eggs are ready, the clitellum secretes a mucous ring
into which eggs and sperm are released and fertilization
takes place.
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The ring slips off the worm and forms a protective cocoon
from which young worms hatch weeks later.
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