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Notes Platyhelminthes

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Table of Contents

Section 1 Platyhelminthes

Section 2 Nematoda and Rotifera

Flatworms, Roundworms, and Rotifers

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Taxonomy

  • Kingdom Animalia
  • Phylum Platyhelminthes
    • Class Turbellaria (planaria – flatworms)
    • Class Trematoda (parasitic flukes)
    • Class Cestoda (tapeworms)

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Structure and Function of Flatworms

  • The phylum Platyhelminthes includes organisms called flatworms.
  • They are more complex than sponges but are the simplest animals with bilateral symmetry.
  • Their bodies develop from three germ layers:
    • ectoderm
    • mesoderm
    • endoderm
  • They are acoelomates with dorsoventrally flattened bodies. (completely solid)
  • They exhibit cephalization.

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Characteristics continued…

  • Lack respiratory and circulatory systems – no need for them due to thinness of body so oxygen and carbon dioxide can pass by diffusion
  • Has only one opening, the mouth, which serves as the food entrance and the waste removal
  • Have organs!
  • 3 classes
    • Turbellaria (planaria)
    • Cestoda (tapeworms)
    • Trematoda (flukes)

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Class Turbellaria

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Class Turbellaria

  • The majority of species in the class Turbellaria live in the ocean.
  • The most familiar turbellarians are the freshwater planarians of the genus Dugesia.
  • Planarians have a spade-shaped anterior end and a tapered posterior end.
  • Have earlike auricles which are sense organs

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5 auricle

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How do they eat?

  • Planarians feed on decaying plant or animal matter and smaller organisms.
  • Planarians have a centrally located mouth attached to a muscular pharynx
  • Food enters the mouth, goes to the pharynx, and then enters the gastrovascular cavity or intestines.
  • Nutrients are absorbed in the intestines and waste is expelled through the mouth

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Digestion continued

They need to constantly remove water….

  • Planarians eliminate excess water through a network of excretory tubules.
    • Each tubule is connected to several flame cells, which have cilia to help push the water out.
    • The water is transported through the tubules and excreted from pores on the body surface.

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Class Turbellaria, continued

Neural Control in Planarians

  • The planarian nervous system is more complex than the nerve net of cnidarians.
  • The cerebral ganglia serve as a simple brain.
  • A planarian’s nervous system gives it the ability to learn and complete complex tasks (like mazes)
  • Planarians sense light with eyespots.
  • Other sensory cells respond to touch, water currents, and chemicals in the environment.

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Class Turbellaria, continued

Reproduction in Planarians

  • Planarians are hermaphrodites that can reproduce sexually or asexually.
  • During asexual reproduction, their body undergoes fission and the two halves regenerate missing parts.

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Planarian

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Class Trematoda

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Class Trematoda

  • The class Trematoda consist of parasitic flukes.
  • Leaf shaped
  • Have complex life cycles
  • Some are endoparasites; others are ectoparasites.
    • Endoparasites – live inside their hosts, usually have a protective covering called a tegument
    • Ectoparasites – live outside their hosts

Structure of Flukes

  • A fluke clings to the tissues of its host by an anterior sucker and a ventral sucker.
  • A fluke’s nervous system is similar to a planarian’s, but simpler.

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Tegument

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Classes Trematoda continued

Reproduction and Life Cycle of Flukes

  • Most flukes have highly developed reproductive systems and are hermaphroditic.
  • Fertilized eggs are stored in a fluke’s uterus until they are ready to be released.
  • Flukes have complicated life cycles that involve more than one host species.

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Schistosomiasis

  • For example, the trematode blood flukes of the genus Schistosoma use humans as a primary hosts and snails as intermediate hosts. They can cause schistosomiasis.
    • Commonly in Africa & South America
    • Get it by swimming in infected waters with snails that are carrying the parasite
    • Causes bowel and stomach shut down, fatigue
    • Diagnosed by eggs in stool
    • Drug treatment, studies are in effect for vaccine

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Life Cycle of Schistosoma

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Class Cestoda

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Class Cestoda

  • About 5,000 species of tapeworms make up the class Cestoda.
  • Tapeworms can live in the intestines of almost all vertebrates.

Structure of Tapeworms

  • Tapeworms are surrounded by a tegument.
  • They attach to the host with a scolex.
  • The body is a series of many sections called proglottids.
  • Tapeworms have no light-sensing organs, no mouth, no gastrovascular cavity, and no digestive organs.

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Class Cestoda, continued

Reproduction and Life Cycle of Tapeworms

  • Nearly all tapeworms are hermaphrodites. Each proglottid contains male and female reproductive organs and little else.
  • New proglottids are added to the front of the tapeworm. Older proglottids grow, mature, and begin producing eggs.
  • Eggs in one proglottid are usually fertilized by sperm from a different proglottid, possibly a different individual.
  • An example is the beef tapeworm, Taenia saginatus. Its primary host is a human and its intermediate host is a cow. Its larvae form cysts in the muscle tissue of the cow.

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Four Videos

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Life Cycle of Beef Tapeworm

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