Lecture 7 - Platyhelminthes

• Most animals have bilateral symmetry.
• The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development.
• Animals that have no space between their gut and body wall are said to be acoelomate.
• These animals are also triploblastic – they have three embryonic germ layers.
• Although flatworms undergo triploblasticdevelopment, they are acoelomates.
• Although flatworms undergo triploblastic development, they are acoelomates.
• These acoelomate phyla are protostomes and have spiral cleavage.
• Acoelomate phyla belong to the superphylum Lophotrochozoa
• Members of phylum Platyhelminthes live in marine,

  freshwater, and damp terrestrial habitats.
• Some contains syncytial tegument (neodermis)
• Flatworms are flattened dorsoventrally and have a

  gastrovascular cavity.
• The osmoregulatory system consists of protonephridia (excretory or osmoregulatory organs closed at the inner end) with flame cells.
• Many can reproduce asexually as well as sexually.

  • Asexual reproduction via fission.
• Reproduction

  • Most are monoecious (hermaphroditic)
  • During breeding season each individual develops both male and female organs, which usually opens through a common genital pore.
• Flatworms (phylum Platyhelminthes) are divided into four classes:

  1. Class Turbellaria - planaria
  2. Class Trematoda - parasitic flukes
  3. Class Monogenea - parasitic monogenetic flukes
  4. Class Cestoda - tapeworms
• Turbellarians are nearly all free-living and mostly marine.

  • Distinguished on the basis of the gut
• Order Tricladida - have a three branched intestine
• Class Turbellaria

  The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.
• Trematodes live as parasites in or on other animals.
• Class Trematoda

  18,000 to 24,000 known species, two subclasses:

  • Apidogastrea are obligate parasites of molluscs and some vertebrates including cartilaginous fish.
  • Digenea are obligate parasite of both molluscs and vertebrates.
• Subclass Digenea - digenetic flukes, have a complex life cycle with a mollusc (snail) as the first host and a vertebrate as the final, or definitive, host.
• Class Monogenea

  • All monogeneans are parasites.
  • Often found in the gills or external surfaces of fishes.
• Class Cestoda - Tapeworms are also parasitic and lack a digestive system.
• The scolex is equipped with suckers and hooks for attachment to the host.
• Each proglottid contains a set of reproductive organs.
• Class Cestoda

  • have sensory endings in the tegument that are modified cilia
  • Contains microtriches greatly enlarges the surface area