Phylum Platyhelminthes

• Platyhelminthes


  • Bilateral symmetry
  • Acoelomate - Lack a body cavity(coelom)
  • 3 Layers: endoderm, ectoderm, mesoderm
  • Gastrovascular cavity
  • Mouth - one opening
  • Cephalization-nerves and sense organs at anterior end
• Flatworms have three germ layers (triploblastic); they are the first animal we have discussed in this class with true organs and tissues

  
• Flatworms are acoelomate, which means the spaces between the organs are filled with mesoderm tissue called parenchyma

  
• Nervous system in flatworms

  • Anterior organs called ganglia that form a simple brain
• Excretory system in flatworms

  • Two lateral canals connected to structures called flame cells
• Flatworms have no circulatory or respiratory organs; respiration occurs by simple diffusion

  
• Types of flatworms


  • Turbellarians
  • Trematoda
  • Monogenea
  • Cestoda

  
• Turbellarians


  Free-living, soft, flat bodies; bodies are covered in cilia; mouth located on ventral surface near the center of the body; mostly hermaphroditic; some can reproduce asexually by fission

  
• Turbellarians


  • Dugesia (planaria)
  • Microstomum
  • Planocera

  
• Trematoda, Subclass Digenea


  Parasitic lifestyle with two hosts; first intermediate host is a mollusk; second definitive host is a vertebrate; sexual reproduction occurs in the definitive host; occasionally a third host (intermediate) plays a role in the life cycle; parasitism can occur in almost every system of the body including the digestive, respiratory, circulatory, excretory, and reproductive systems
• Trematoda, Subclass Digenea


  • Fasciola
  • Clonorchis
  • Schistosoma
• Trematoda, Subclass Digenea life cycle

  1. Egg

  2. Miracidium

  3. Sporocyst

  4. Redia

  5. Cercaria

  6. Metacercaria
• Egg is released from the host's excrement and must reach a source of water to be released
• In the water the egg releases a free-swimming, ciliated larva called the miracidium
• The miracidium penetrates and enters the tissues of a mollusk, and transforms into a sporocyst
• Sporocysts then reproduce asexually to produce redia
• Redia reproduce asexually to produce cercaria
• Cercaria emerge from the mollusk and penetrate a second intermediate host or they can develop as a metacercaria on vegetation or in the animal host
• Metacercaria are eventually ingested by a definitive host and develop into adult flukes
• Monogenea


  Ectoparasites that attach to the gills of fish; hook-like mouth called an opisthaptor; occasionally they are found in the urinary bladders of frogs and turtles, but mostly on the skin or gills of fish; a certain kind even lives in the eye of a hippopotamus; common but cause little damage to their host; they can become a serious threat when fish become crowded, like in the case of fish farming; life cycle simple with only a single host
• Examples of Monogenea

  • Dactylogyrus
  • Polystoma
  • Gyrodactylus
• Cestoda (Tapeworms)


  Long flat bodies with a hook-like mouth called a scolex used for attaching to the intestinal wall of its host; the rest of the body is followed by regular repeating units of reproducing sections called proglottids; the main body of a tapeworm is a chain of proglottids called a strobila; no digestive tract, nutrients are absorbed across the skin by diffusion; all tapeworms require at least two hosts, adults are parasitic in the digestive tract of vertebrates

  
• Almost ALL vertebrate species can become infected by tapeworms, but adult tapeworms do very little harm to their hosts

  
• Humans often get infected by consuming raw or rare beef, pork, or fish (larva form cysts in the muscle tissue)
• Children often become infected by putting their fingers in their mouth after touching infected animals
• Tapeworm life cycle


  1. Larva ingested by intermediate host

  2. Larva migrate to skeletal muscle and become larval cysts (cysticerci)

  3. Larval cysts (bladder worms) can develop in the brain leading to severe brain damage and death (cerebral cysticercosis)

  
• Ribbon Worms and Jaw Worms are similar to animals in the Phylum Platyhelminthes in several ways, but have some important differences, placing them in their own phyla

  
• Ribbon Worms


  • Have flame cells and ganglia; some can regenerate like planarians; have a complete digestive tract with both a mouth and an anus; have a circulatory system

  
• Jaw Worms


  • Somewhat resemble rotifers, newly discovered species yet to be properly classified
• Flatworms and Roundworms belong to the Kingdom Animalia and the Phyla Platyhelminthes and Nematoda respectively

  
• Characteristics of Flatworms and Roundworms

  • Three germ layers (endoderm, ectoderm, mesoderm)
  • Bilateral symmetry
  • Cephalization (has a head)

  
• Flatworms do not have a coelom (fluid filled body cavity that usually contains organs)

  
• Functions of Flatworms


  • Feeding - most are parasites
  • Senses - brain (ganglia) and some have eyespots (vision)
  • Reproduction - many are hermaphrodites, can also reproduce asexually by fission

  
• Free-living flatworms


  • Planarian (Dugesia)
• Planarian (Dugesia)


  Lives in freshwater, mostly a scavenger but can also feed on protists
• Tapeworms


  Parasites that live in the intestines and absorb nutrients from the host; have a scolex (structure with suckers and/or hooks) and proglottids (body segments)

  
• Human diseases caused by parasitic flatworms

  • Trichinosis - eating undercooked pork
  • Schistosomiasis - blood fluke, snails
  • Hookworm - walking barefoot
  • Tapeworm - contaminated food
  • Roundworms - food or pets
• Atworms


  Live in fresh or marine water, ex. Planarian
• Dugesia


  Lives in freshwater, mostly a scavenger but can also feed on protists
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  4/10/2022