Chapter 8  Acoelomate Animals: Flatworms

NEW  FEATURES:

       

A. PHYLUM PLATYHELMINTHES – "Flatworms"

   Characteristics:

• Bilateral symmetry

• Dorsoventrally flattened

• Epidermis cellular or syncytial

• Muscular system arising from mesoderm

• Incomplete digestive system (gastrovascular cavity)

• Nervous system - Anterior ganglia and lateral nerve cords

• Excretory system - lateral canals with flame cells (protonephridia)

• Free living or parasitic

• Monecious or diecious, - complex reproductive cycles

• Respiration by diffusion

1. Class Turbellaria  free- living flatworms – freshwater, marine and terrestrial

Highly variable in size and shape; most are aquatic and marine

• Morpholgy

 

 

•  Feeding and Digestion  -

 

 

• Sensory Structures -

 

 

• Nervous System -

 

• Excretion and Water Balance -

 

 

• Respiration –

 

• Reproduction - mostly momoecious/hermaphroditic

Asexual Reproduction  - fission

    Sexual Reproduction - egg ( in cocoon) > juvenile > adult

 

• Regeneration well studied in developmental biology

 

 

• Mimicry and Camouflage

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Parasitism as a Life Strategy

 

Benefits to the Parasite?

 

 Challenges

 

Adaptations Possessed by Parasites

 

 

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  2. Class Tremotoda – Flukes – common endoparasites of vertebrates

 

           

      Characteristics:

           All parasitic; (usually endoparasites of vertebrates)

           Variety of suckers and hooks for attachment.

           Glands to produce encystment material for immature stages.

           Complex life cycle usually involving two or more hosts

                        “digenetic” means:

     Fluke Anatomy

Digestive

 

Reproductive

 

Excretory System

 

General Stages of a Fluke Life Cycle  Draw a circle in the space below and place the stages (letters A, B, C, etc) in proper sequence

        A  Adult

      B   Egg

      C    Miracidium

      D    Sporocyst

      E    Redia

      F   Cercaria

      G   Metacercaria

 

definitive (final) host:

 

intermediate host:

 

Specific Example: Chinese Liver Fluke of Humans

Adult lives in bile passageways of humans, dogs, etc

 

Chinese Liver Fluke Life Cycle see text Fig. 8.9 on p. 145  Clonorchis sinensis

 

Variation in Fluke Life Histories

• Sheep Liver Fluke Life Cycle    Fasciola hepatica
• no second intermediate stage

• Schistosomes - Blood Flukes   Schisotosoma sp.
  • "Schistosomiasis" a major infective disease
• Life cycle abbreviatd and modified - redia stage omitted
• Live in blood vessels, not digestive organs
• Cercarias burrow in final host
• Possible control measures:

• “Swimmer’s Itch”   a local phenomenon  - 

 

 

 

 

            

            primary host:

            intermediate host:

           effect on humans:

 

•  Fluke Diseases of Fish

            Yellow grub

 

            Black spot

  3. Class Cestoda – Tapeworms

       Characteristics:

          All parasitic - mostly intestinal parasites of vertebrates

          Complex life cycle usually involving two hosts.

          Morphology: 

                  No mouth or digestive system 

                

                  Scolex with hooks for attachment.

                  Proglottids

                  Integument compensates for lack of digestive structures - microvilli

Reproduction

monoecious - each proglottid male & female

Proglottids not self-fertilizing

Examples of Tapeworms:

             Taenia saginata (Beef Tapeworm); also Tainiarhynchus saginata

                           Cattle and Humans

            Taenia solium (Pork Tapeworm)

                           Hogs and Humans

    Diphyllobothrium latum (Fish Tapeworm)

                Crustaceans, Fish and Humans

             

Example of a tapeworm life cycle: beef tapeworm - see Fig. 8.16, p. 150 for life cycle

 

 

 

 

 

 

 

 4. Class Monogenea - Monogenetic Flukes – “one-host” flukes

       Characteristics: 

Generally ectoparasites on vertebrates

Haptor - on each end, multiple suckers

Single host - Single generation; no intermediate host; one egg - one adult

            Sexual reproduction - eggs hatch into ciliated larvae; settle on new host; gradual development

 

B.  PHYLUM NEMERTEA - Nemertean Worms ("Ribbon Worms" and "Proboscis Woems")

     Characteristics:

            Cylindrical proboscis and flattened dorsoventrally

            Free living, marine

 Swim via cilia and well-developed musculature

     Structurally similar to Turbellaria except:

         First group to have complete digestive system (mouth and anus);

         Has a  true circulatory system - (no heart, but has blood vessels and Hb)

  Predaceous - Extensible proboscis.

  Dioecious, but reproduce both sexually and asexually (fragmentation)

 

C.  PHYLUM GNATHOSTOMULIDA - Jaw Worms

      Characteristics:

       Tiny, worm-like

        

       Gut generally incomplete, although a few possess rudimentary anus

       No special excretory, circulatory, or gas exchange structures

       Hermaphroditic