Flatworms
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 parisitic flatworms.
  • Describe the symmetry of this group.
    Platyhelminthes  is a phylum composed of three classes of flatworms.
    The class Turbellaria  (planarians) is free-living, while the remaining classes 
       
    including the Trematoda  (flukes) and Cestoda  (tapeworms) are parisitic.
    Members of the phylum Platyhelminthes are dorsoventrally flattened
    with a triploblastic body composed of three different tissue layers: 
       
    ectoderm, endoderm, and the mesoderm. These animals are bilaterally
       
    symmetrical, meaning that a slice directly through the middle can produce
       
    two mirror images. 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.
    The best known member of the class Turbellaria is the free-living
    planarian,  Dugesia. 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.
    Water balance or osmoregulation is maintained by protonephridia
    which terminate in specialized flame cells. 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.
    Members of the class Trematoda consist of the parisitic 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 hermaphrodidic (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.
    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.
    Review Questions
    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?
    (Answers to review questions)
       
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