Phylum Platyhelminthes

Phylum Platyhelminthes

1. Characteristics

   • triploblastic - more complex than cnidarians!
   • acoelomate
   • bilateral symmetry
   • unsegmented worms with dorsoventrally flattened body
   • cephalisation - have distinct head
   • free living, parasitic
   • organ system level of organization
     • digestive system
     • reproductive
     • nervous
     • excretory
   • NO circulatory or respiratory system

2. Class Tubellaria

   • General Features
     • bilateral symmetry
     • triploblastic
     • acoelomate
     • mostly free living
     • Temnocephalans live on crayfish shells
     • locomotion via cilia on ventral surface only and/or rhythmic muscular contractions
   • Dugesia sp.
     • Digestive System
       • consists of mouth, pharynx, and 3-branched intestine
       • no anus
       • pharynx used to suck in small prey
       • some secrete digestive enzymes on prey then suck up partially digested food
     • Nervous System
       • ganglia located in head region serves as brain - connected to two long parallel nerve cords running down body
       • ocelli - shadow response
     • Excretory system
       • waste product is ammonia
       • waste collected in flame cells then disposed of through nephridiopores on ventral surface of worm
       • excretory pores - waste leaves planarian through pores on ventral surface
     • Reproductive system
       • sexual
         • hermaphrodites pair up and fertilise each others eggs
       • asexual
         • transverse longitudinal fission (split in half)

3. Class Trematoda

   • General Features
     • flatworms - dorsoventrally flattened
     • always parasitic
     • heavy infestation can cause debilitating symptoms
     • complex lifecycle - often have mollusc and vertebrate hosts
     • adulthood reached in final host - humans
     • one or two suckers sometimes supplemented with hooks - for attachment
     • non-segmented body
   • Clonorchis sp. (flukes)
     • Features
       • Have 2 suckers to attach to host
       • Siimple digestive system
       • most hermaphrodites, many capable of self fertilization
     • Complex Life Cycle
       • 1) Snail - snail eats egg containing miracidium, which hatch and develop into sporocyst. Sporocysts develop into redia, which develop into cecaria. Cecaria break out of snail and enter stream
         • 2) Fish - cecaria attach to fish and develop into metacercarial cyst with juvenile flukes inside, lodge in fish muscle
           • Water - miracidia encapsulated within eggs are released in feces and enter stream
             • 3) Human - human eats infected raw fish, flukes travel to bile ducts of liver, attach with suckers, and produce eggs. Eggs carried to intestine then excreted.
   • Schistosoma sp. (blood fluke)
     • complex life cycle
     • Enter host by penetrating skin
     • Cause schistosomiasis - 250 million people infected
       • Symptoms - fever, chills, liver and spleen enlargement, skin rash, abdominal pain, malnutrituion...
     • Live in blood vessels - cause considerable damage
     • Break through blood vessels to get to intestine or bladder - leave the body

4. Class Cercomeromorpha

   • Adaptations to a parasitic lifestyle
     • Attachment organs (scolex with suckers and hooks)
     • Body is very thin - absorption across body
     • Protective cuticle - protection against digestive enzymes
   • All parasitic
   • Hermaphroditic
   • Taenia sp. Lifecycle
     • Eggs passed out into environment
       • Eggs ingested by pigs or other vertebrate
         • Eggs hatch and move from intestine to muscle
           • Humans eat pig meat
             • Tapeworm attaches to intestine in human
   • Differ from other platyhelminthes
     • body made of many segments (proglottids) - each proglottid is a reproductive unit
     • mature proglottids fill with eggs, detach and carried out of host via feces

5. Echinococcus granulosus

   • General Features
     • impacts wildlife and domestic animals
     • transmitted to dogs by feeding them hydatid infected offal of kangaroos
     • wild animals defecate on pastures - transmitted to livestock
     • transported to australia during european colonization
     • forms cysts in the internal organs
     • Hydatid disease causes losses in livestock with the downgrading of edible meat by-products becase of the presence of hydatid cysts
   • Lifecycle
     • 1) dogs infected when they eat cysts in the liver, heart, and lungs from infected stock
       • 2) taperworms develop inside dogs and shed many small eggs onto pasture
         • 3a) Humans can pick up worm eggs from infected dogs
           • 3a) Hydatidic cysts can develop in human internal organs causing serious health problems
             • 3a) Livestock can become infected by eating tapeworm eggs on pasture from feces of infected dogs
               • 4) Cysts develop in the liver, lungs, and heart

6. Gecko Tapeworm

   • Hosts - frogs, birds, snakes, geckoes
   • Dog or cat eat hosts - become infected
   • Often large numbers in hosts
   • Symptoms - vomiting and weight loss
   • Spirometra erinacei also infects frogs and is a major cause of frog disease and death
   • Live in intestine of hosts, can grow up to 1.5m