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