Platyhelminthes

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Platyhelminthes

• After completing this tutorial you should be able
  to
• Explain the basic body plan of members of the
  phylum Platyhelminthes.
• Identify representatives of the classes
  Turbellaria, Trematoda, and Cestoda.
• Compare and contrast the anatomy and morphology
  of free-living and parasitic flatworms.
• Describe the symmetry of this group.

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Platyhelminthes is a phylum of flatworms.

• Class Turbellaria (planarians) is free-living.
• Trematoda (flukes)
• Cestoda (tapeworms) are parisitic.
• Members of the phylum Platyhelminthes are
  dorsoventrally flattened with body composed of
  three different tissue layers ectoderm,
  endoderm, and the mesoderm.
• These animals are bilaterally symmetrical.
• This phylum demonstrates an organ-system level of
  organization.
• The front or anterior portion of the body bears
  most of the sense organs as compared to the rear
  or posterior of the body.
• Most flatworms have a single opening to the
  digestive tract and they have no respiratory or
  circulatory systems but are simple and flat
  enough so diffusion is sufficient for these
  processes.
• Protonephredia and flame cells regulate water
  balance.

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Turbellaria the Planarians
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Planaria

• They are usually found in slow-moving
  streams near stones, leaves or debris. The head
  of these animals has lateral tactile projections
  termed auricles. There are also photoreceptor
  sensory organs termed ocelli on the anterior
  dorsal surface. Their nervous system is based on
  two longitudinal ventral nerve cords connected by
  the anterior ganglia, or primitive brain. This
  concentration of sense organs in the anterior end
  is termed cephalization. Planarians secrete a
  slime track over which they glide. Gliding is
  accomplished by beating the epidermal cilia in
  the slime track. These flatworms ingest food by
  means of a tube-like pharynx located on the
  midventral line. Digestion begins extracellularly
  with enzymes secreted onto the food before it is
  sucked into the intestine by the pharynx. The
  intestine may have lateral branches, adding
  surface area for increased absorption of
  nutrients. The mouth, pharynx, and intestine make
  up the entire digestive tract. A cross section of
  Dugesia will show the ectoderm which produces the
  epidermis. The layer surrounding the digestive
  tract is derived from endoderm. The space between
  the two layers is filled with tissues from the
  mesoderm.

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Three layers

• A cross section of Dugesia will show the ectoderm
  which produces the epidermis. The layer
  surrounding the digestive tract is derived from
  endoderm. The space between the two layers is
  filled with tissues from the mesoderm.

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Trematoda the Flukes

• Members of the class Trematoda consist of the
  parasitic flukes. The flukes live within one or
  more host animals during their life cycle. They
  have a well-developed digestive system with the
  mouth at the anterior end. They are characterized
  by a thick cuticle as well as one or more suckers
  surrounding the mouth. These suckers are used for
  attachment to the host's internal body surface.
  These organisms are typically hermaphroditic
  (have male and female sex organs). With the aid
  of a drawing of Clonorchis sinensis (a human
  liver fluke) locate the oral sucker, pharynx,
  esophagus, excretory bladder, ovary, uterus,
  seminal receptacle, testes, seminal vesicles and
  vas deferens

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Flukes
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Liver fluke
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Cestoda the tapeworms

• Members of the class Cestoda are
  endoparisitic tapeworms which completely lack a
  digestive tract. At the anterior end a scolex
  possessing hooks and suckers is found which
  attaches to the host's digestive system.
  Posterior to the scolex is the neck region which
  leads to sections termed proglottids. Each
  proglottid possesses both male and female
  reproductive organs. Farther down, the gravid
  proglottids contain thousands of fertilized eggs.
  In the drawing and pictures of Taenia pisiformis
  (tapeworm) observe the scolex with hooks and
  suckers, proglottids, ovary, testes, uterus, and
  excretory canal.

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Tapeworms
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Tapeworm proglottid
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Review

1. What is the function of the numerous branches of
   the digestive tract of Dugesia?
2. What is meant by the term cephalization?
3. The scolex is common to the class _____________?
4. This flatworm class lacks a digestive tract.
5. What does the term hermaphrodidic mean?

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Answers to Review Questions (Flatworms)

• They add surface area for increased absorption of
  nutrients.
• The concentration of sense organs in the anterior
  end.
• Cestoda.
• Cestoda.
• Having functional male and female sex organs.

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Scientists identify gene required for flatworms
to maintain their stem cells

• Freshwater flatworms, called planaria,
  possess extraordinary regenerative capabilities
  by virtue of a population of stem cells they
  maintain throughout their lives. Researchers
  recently identified a key gene that maintains
  planarian stem cells.