Bilateria and Flatworms

Bilateria

  • Bilateria includes all animals featuring bilateral symmetry and a triploblastic structure.
  • Within Bilateria are two groups:
    • Protostomes:
      • Blastopore forms the mouth.
      • Spiralia:
        • Exhibit spiral cleavage.
        • Mostly aquatic, moving via cilia and muscle contractions.
        • Lophotrochozoa:
          • Two forms: trochophore and lophophore.
          • Trochophore: Free-living larva with a spherical body and a band of cilia.
          • Annelids and mollusks
          • Lophophore: Feeding structure with ciliated tentacles surrounding the mouth.
          • Brachiopods and bryozoans
      • Ecdysozoa:
        • Animals that molt and undergo ecdysis.
        • Hard exoskeleton splits for growth.
        • Arthropods and nematodes.
    • Deuterostomes:
      • Blastopore forms the anus.

Platyzoans (Flatworms)

  • Major group: Platyhelminthes.
  • Key Characteristics:
    1. Dorsoventrally flattened.
    2. Ciliated and soft-bodied.
    3. Acoelomate.
  • Habitats: marine, freshwater, and terrestrial.
  • Feeding: carnivores, detritivores, or parasites.

Platyhelminthes Anatomy

  • Acoelomate with three tissue layers.
  • Single opening into the gastrovascular cavity serves as both a digestive and circulatory system, enabling extracellular digestion.
  • Developed nervous and excretory systems.

Platyhelminthes Digestive System

  • Gastrovascular cavity extends throughout the body.
  • Single opening (mouth) facilitates both nutrient intake and waste expulsion via a pharynx.
  • Food is broken into smaller pieces by the muscular pharynx and then swallowed into the gastrovascular cavity, where extracellular digestion takes place.

Platyhelminthes Excretion and Osmoregulation

  • Network of excretion tubules for osmoregulation, featuring bulb-like flame cells.
    • Flame cells: Flagellated cells that move water through an excretory pore.
  • Metabolic wastes directly diffuse into the gastrovascular cavity for removal.

Platyhelminthes Nervous System

  • Some cephalization.
    • Cerebral ganglion located anteriorly, with two nerve cords running through the body.
  • Some possess rudimentary eyespots.
    • Light-sensitive cells, causing most platyhelminthes to move away from light.

Platyhelminthes Reproductive System

  • Hermaphroditic.
    • Each individual has both male and female reproductive organs.
  • Most require copulation with another individual, where both exchange sperm.

Platyhelminthes Asexual Reproduction

  • Capable of regeneration.
    • If a platyhelminthes is cut in half, each half can grow into a new individual.

Platyhelminthes Groups

  • Two major groups:
    • Turbellaria:
      • Free-living flatworms.
      • Named after turbulence from their mode of locomotion (beating cilia moves water).
      • Mostly marine and freshwater; many have eyespots.
    • Neodermata:
      • Parasitic flatworms, possessing an outer layer called the neodermis.
      • Resistant to digestive and immune systems.
      • Mostly endoparasites, but some are ectoparasites; they lack eyespots.
      • Trematoda (Flukes):
        • Attach within the host's body via suckers, anchors, or hooks.
        • Feed on host cells and fluids.
        • Often have complex life cycles involving multiple hosts.
          • Liver Fluke Lifecycle:
            • Three different hosts:
              • 1st host: Snails
                • Fertilized egg ingested by snails, grows into a ciliated miracidium.
                • Forms into a bag-like sporocyst and develops into an elongated, non-ciliated redia.
                • Matures into a tadpole-like larva that leaves the snail - Cercaria.
              • 2nd host: Fish
                • Cercaria stage is free-living and aquatic.
                • Seeks out fish, burrows into muscle layer, and forms a metacercaria cyst.
              • 3rd host: Terrestrial vertebrate
                • Fish with a metacercaria cyst is consumed raw.
                • Cyst is digested by enzymes, releasing the liver fluke, which moves towards the bile duct.
                • Capable of living 30 years in humans, and can lead to liver cirrhosis.
      • Cercomeromorpha (Tapeworms):
        • Attach to the inner wall of intestines and directly absorbs nutrients from the host's digestive tract.
        • Tapeworms were a diet fad in the early 1900s.
          • Tapeworm Anatomy:
            • Three distinct regions:
              • Scolex: Attachment via hooks and suckers.
              • Neck: Unsegmented region with high regenerative properties.
              • Proglottid: Series of segmented regions with reproductive organs.
                • Hermaphroditic!
                • Forms continuously from the neck and matures.
                • Terminal proglottids are filled with eggs, which either break loose or rupture.
                • Carried out of host through feces.
                • Embryos are scattered into the environment to be picked up by another host.

Rotifera

  • Wheel animals.
  • Tiny, smaller than some protists, but multicellular.
  • Major characteristics:
    1. Unsegmented pseudocoelomate
    2. Contains both a mouth and an anus
    3. Cephalization
  • Rotifera Anatomy
    • Corona: Cilia ring to sweep food into mouth and movement.
    • Toes: Used to attach to objects in environment.
  • Complete digestive system with a mouth and an anus.