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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. 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.
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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.
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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.
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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
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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)
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1. What is a flatworm? 2. What are the three groups of flatworms?
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Textbook Introduction
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Roundworms Have a pseudocoelom, or "false coelom"; not a true fluid- filled, tissue-lined coelom Body plan: "tube within a tube"
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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.
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Rely solely on diffusion, no internal transport,
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Simple nervous system with several ganglia Sexual: most species have separate sexes. Internal Fertilization: Male deposits sperm inside female.
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Textbook Introduction
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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)
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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
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Closed circulatory system: blood is contained within a network of blood vessels.
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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
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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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