General Zoology (Biol 111H)

The Triploblastic, Acoelomate Body Plan
Chapter 19
Flatworms

The Flatworms
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  The Triploblastic, Acoelomate Animals 
   Several minor phyla (we will introduce but not study in detail).
     For Example:
       Phylum: Mesozoa (middle-animals)
            General Characteristics:
               - marine, endoparasites (mollusk), no organs, ciliated, sexes separate, approximately 100 species .

    Phylum: Placozoa (flattened-animals)

        General Characteristics:
marineflatworm.jpg (21534 bytes)  marineflatworm2.jpg (14219 bytes)
           - all marine.
           - single species (Tricoplax adherans).
          - Consist of two epithelial layers with fiber cells (used in locomotion).
          - rediscovered in the Red sea in 1960.

Flatworms that we will study in detail are Triploblastic, having an Acoelomate Body and include the:
     Phylum: Platyhelminthes
       Class: Turbellaria (free-living)
       Class: Monogenea
       Class: Trematoda (flukes)
       Class: Cestoidea (tapeworms)

     Phylum: Nemertea

Welcome to the Flatworms
  Phylum: Platyhelminthes
   Class: Turbellaria
     General Characteristics:

      - Approximately 300 species known world wide.

      - Represent the free living "flatworms."

      - Aquatic (both freshwater and marine); someterrestrial .

      - Some with color (pigments).

      -Triploblastic; dorso-ventrally compressed with anterior / post ends.

      - Acoelomate; no segmentation.

      - No  larvaestages.

      - Incomplete digestive system.

      - Cephalized anterior ganglia.

      - Excretory / Osmoregulatory System is based on Protonephrida.

      - Hermaphroditic (monoecious).

      - Animals with primitive organ systems.

      - Locomotion by cilia and undulations

      - Digestion System and Nutrition:

          1. Blind -  pouch System, branched to increase surface area. 

          2.  Pharynx highly developed.

          3. Mostly carnivorous or scavengers, some herbivorous.

          4. Some use "chemosensors" - food detection.

          5. Digestion is extracellular.

      - Muscle System:

          1. circular, and

          2. longitudinal.

      - Nervous System, generally well developed with posterior fibers and anterior ganglia.


      - Sensory, some with eyes and statocysts.

      - No circulatory or skeletal Systems.

      - Highly developed reproductive System.

      - No  respiratory System - by diffusion only

  A Generalized Cross-section of a Turbellarian

    For example:
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  A Side -view of Turbellarian Digestive System
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     some marine forms lack a digestive cavity.
      blind cavity, highly branched to simple pouch.
      muscular pharynx that functions as an ingestive organ, being ciliated and typically "housed" in pharyngeal sheath.
              Extracellular  Distribution.
              Pharyngeal gland proteolytic enzymes (catalyze protein digestion).
              Mostly carnivorous, some scavengers and others herbivorous.
              Food detected by chemosensory cells.
insideplanarian.gif (43565 bytes)

Turbellarian Nervous System    
     Nervous System

     - Three types giving rise to 3 separate orders of the Class Turbellaria.

          1. Order: Acoela - most primitive

              a subepidermal nerve plexus. 

              similar to nerve-net of cnidarians.

              anterior statocyst detects position.


          2. Order: Poycladida

              similar to Acoela but with a more centralized  nerve-net and central ganglia.

          3. Order: Tricladida

              also a subepidermal nerve - net but pair of lateral nerve cords

              nerve cords connected by commissures forming a ladder - like system.

              nerve cords connect anteriorly to ganglia.
     - Generalized Nervous System of Three Orders of Turbellaria 
                    Order:              Acoela                                    Polycladida                                  Tricladida

     - Central anterior ganglia function as a primitive brain.

     - Neurons organized into 

          1. Sensory - nerve fibers leading to the "brain"


          2. Motor - nerve fibers leading away from the "brain"


          3. Association - nerve fibers that connect or integrate with other neurons


      - These attributes in neuron inter-relationship (integration) and central control by the primitive brain 
             demonstrate  major advances in the Central Nervous System (CNS).

      - Sensory

          1. Tactal - located in auricles of sensory lobes, detectind pressure (mechanoveceptor)
                          of water currents.

          2. Chemosensory - dense clusters of sensory cells in auricles, aid in locating food and each other.
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          3. Ocelli - eye spots of dense cell masses located anteriorly and lateral to central ganglia ;
                          serve as photoreceptors allowing response to light (most worms are negatively phototropic)
                          have black photosensitive pigment;  Ocelli cell receptors have neuron connections to 
                          "brain".

 The Turbellarian Reproductive System

      Reproductive System

      - Arises from mesodermal tissue.

      - Mostly hermaphroditic (monoecious)

      - Sexual

          1. Well developed male / female with gonads, ducts , bursa that receives 
                    sperm for storage, and copulatory structure (penis).

         2. Usually involves cross-fertilization when sperm-mass is exchanged.

      - Asexual

         1. Transverse fission to produce two zooids.(Gk.living animal) 

         2. Development is direct.

         3. Produce cocoon (summer & fall) or have "Muller's" larva (ciliated with arms for feeding/swimming);
                 may bePaedomorphic (Gk. pais =  child) being sexually mature as a larva (neotony).

      - Cross-sections of Turbellarian Reproductive System
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                         mostly hermaphroditic (monoecious)
                         Ectolecithal = yolk outside ova
                                     Endolecithal = yolk contained w/i contained cytoplasm


    - Copulation in Turbellarian Worms

    - Hypodermic Impregnation in Turbellarian Worms

Class:  Monogenea
lifecycle.gif (4949 bytes)
       General Characteristics:

        - mostly ectoparasites on fish, or sometimes turtles, frogs, crayfish.

        - fertilized egg (zygote) has "thread" that sticks to the gill of a fish host.
        - matures to a ciliated larva known as an oncomiracidium that swims to another fish host
               to mature into an adult worm.

 Class: Trematoda (flukes)
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    (Gk. trematodes = pierced)

    Many of considerable economic importance.

    General Characteristics: 

    - No cilia in adult stage.

    - dorsoventrically compressed.

    - Muscle System - both circular and longitudinal.

    - Excretory System - flame - cell Protonephridial system.

    - No circulatory System.

    - Respiration via diffusion.

    - Nervous System with a pair of anterior ganglia and lateral fibers.

    - Digestive System - dead-ended pouch.
        has anterior and ventral suckers used as attachment organs (used also in classification of flukes

    - Reproductive system - well developed, monoecious and hermaphroditic.

    - No skeletal system (not found in any Platyhelminthes)

 Subclass: Aspidogastrea

   General Characteristics: 
    - all parasitic flukes to mollusk.

    - all with large "hold - fast " organs, known as the opisthaptor.

    - no oral sucker.

    - complex life cycle.

    - in some, a vertebrate may serve as thefinal  host becoming infected when eating a mollusk
             that carry immature aspidogastreans.

 Phylum: Platyhelminthes
  Class: Trematoda
    Subclass: Digenea
                (Gk. di = two + genea = generations )
    General Characteristics:
    - Adults  endoparasitic to vertebrates.

    - at least 2 different life cycle forms in 2 or more hosts.

    - parasites have an oral sucker surrounding  a mouth and 
           a ventral sucker known as the acetabulum.

    - complex life cycle involving a molluscan (snail) intermediate host and typically 2 or more final
                 hosts (definitive) of various warm blooded(endothermic) animals, including man.
digcycl.gif (7568 bytes)

  Class: Cestoidea
parasite.jpg (230140 bytes)
      General Characteristics:
      - Tapeworms, over 3500 species known.

      - Parasitic, from few mm to 15m in lengths.

      - No digestive tract or system.

      - Have great reproductive  potential.

      - Are of considerable economic importance.

      - Two subclasses:

       1. Subclass: Cestodaria

            Body not subdivided into individual proglottids.
            Larva found in crustaceans, and adult fish.

       2. Subclass: Eucestoda

            True tapeworms with scolex, neck, strobila with strobila of many proglottids.
            Adult tapeworms found in the G.I. tract of vertebrates.

 Phylum: Nemertea


   General Characteristics:

    - usually marine

    - small phylum of 600 to 1000 species

    - dorso-ventrally  compressed


    - similar to Turbellarian but show some advancements in that they -

         have rudimentary circulatory systems with hemoglobin to transport O2.

    - reach lengths of 30 meters.


    - Have external cilia for locomotion.

    - Some with "eyes" and show bilateral symmetry

    - Muscle system highly developed with circular and longitudinal muscles.

    - complete Digestive system

    - Sexes separate, fertilization external.

    - Respiration through body wall.

    - Excretory System is well developed, protonephridia

    - Nervous System with pair of well developed ganglia, dorsal to mouth 

      connecting with nerve fibers forming nerve trunk.


     - Circulatory system consists of:

       1. Three longitudinal vessels.

       2. Contains blood with hemoglobin.

       3. Forming a "closed" system, no pump.


     - Reproductive System



     - Acoelomate and Triploblastic

     - No Skeletal System.
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