BIO L3

• Flatworms are divided into four classes:

  Turbellaria, Monogenia, Trematoda, and Cestoidea
• Turbularians are nearly all free-living (nonparasitic) and most are marine.
• Planarians, members of the genus Dugesia, are carnivores or scavengers in unpolluted ponds and streams. Planarians and other flatworms lack organs specialised for gas exchange and circulation. Their flat shape places all cells close to the surrounding water and fine branching of the digestive system distributes food throughout the animal. Nitrogenous wastes are removed by diffusion and simple ciliated flame cells help maintain osmotic balance.
• How do planarians move?

  using cilia on the ventral epidermis, gliding along a film of mucus they secrete. Some turbellarians use muscles for undulatory swimming
• A planarian has a head with a pair of eyespots to detect light and lateral flaps that function mainly for smell. The planarian nervous system is more complex and centralised than the nerve net of cnidarians. Planarians can learn to modify their responses to stimuli.
• Planarians can reproduce asexually through regeneration. The parent constricts in the middle, and each half regenerates the missing end. Planarians can also reproduce sexually. These hermaphrodites cross-fertilize.
• Three parasitic Platyhelminthes:


  Trematoda, Cestoidea and Monogenea
• General features of parasites:

  Structural and behavioral adaptions, specialised yet simplified, Complex life cycles , Intermediate hosts and larval reproduction
• define a definitive host:

  animal harboring the adult or sexually mature stage of the parasite.
• define an intermediate host:


  animal in which development occurs but in which adulthood is not reached
• Parasites that live on their hosts are termed

  ectoparasites
• Parisites that live inside their host are termed

  endoparasites
• The monogeneans (class Monogenea) and the trematodes (class Trematoda) live as parasites in or on other animals.
• Trematodes 


  .... parasitise a wide range of hosts, and most species have complex life cycles with alternation of sexual and asexual stages.
• Many tetratodes require an intermediate host in which the larvae develop before infecting the final hosts (usually a vertebrate) where the adult worm lives.
• the blood fluke ..

  Schistosoma infects 200 million people, leading to body pains, anemia, and dysentery.
• The transmission of a blood fluke occurs in fresh water where the cercaria (larval stage of Schistoma) come in contact with and penetrates human skin. Adult worms live within blood vessels of colon and urinary bladder, releasing eggs into stool and urine which can end up in fresh water, hatch and metamorphose through an intermediate snail host to free-swimming cercaria which can penetrate human skin. The resultant larvae (a schistosomule) enters venous system passes through lungs and matures in the portal system to the adult stage.
• lung flukes 


  Paragonomiasis
• lung flukes are prevalent in the Far East; also areas in Central America and Africa. Transmission is related to the consumption of raw fresh water crabs and crayfish which contain the larvae (metacercaria) of Paragonimus
• what shape are tetramodes usually?

  leaf shaped
• what are platyhelminths covered in?


  Covered by a cytoplasmic tegument, rather than a cuticle
• Platyhelminthes have a body cavity 


  flase
• their digestive system is ....

  absent or rudimentry
• how do Platyhelminthes reproduce?

  Most species are hermaphroditic; Adults reproduce sexually, larvae can reproduce asexually
• Excretory system containing ciliated cells extending into excretory tubules; in living state, motion of cilia reminiscent of flames and when seen under microscope indicates viability
• Most species are flattened dorsoventrally (hence flat worms).