Bilateria and Flatworms Notes

Bilateria

• All animals with bilateral symmetry and are triploblastic (having three germ layers).

• Divided into two major groups based on blastopore fate:

  • Protostomes: Blastopore forms the mouth.

    • Spiralia: exhibit spiral cleavage, are mostly aquatic, and move via cilia and muscle contractions.

      • Lophotrochozoa: Possess either a trochophore larva or a lophophore feeding structure.

        • Trochophore: Free-living larva with a spherical body and a band of cilia; found in annelids and mollusks.

        • Lophophore: Feeding structure with ciliated tentacles surrounding the mouth; found in brachiopods and bryozoans.

    • Platyzoans: Flatworms (Platyhelminthes).

  • Deuterostomes: Blastopore forms the anus.

  • Ecdysozoa: Animals that molt and undergo ecdysis (shedding of an outer layer).

    • Hard exoskeleton splits for growth.

    • Includes arthropods and nematodes.

Platyhelminthes (Flatworms)

• Major group within Platyzoans.

• Key characteristics:

  1. Dorsoventrally flattened.

  2. Ciliated and soft-bodied.

  3. Acoelomate (lacking a body cavity).

• Habitats: marine, freshwater, and terrestrial environments.

• Feeding strategies: carnivores, detritivores, or parasites.

Platyhelminthes Anatomy

• Acoelomate with three tissue layers.

• Single opening into a gastrovascular cavity.

  • Functions as both a digestive and circulatory system.

  • Extracellular digestion occurs within the gastrovascular cavity.

• Developed nervous and excretory systems.

Platyhelminthes Digestive System

• Gastrovascular cavity extends throughout the body.

• Single opening (mouth) for both nutrient intake and waste removal via a pharynx.

• Muscular pharynx rips food into small pieces.

• Swallowed into gastrovascular cavity where extracellular digestion occurs.

Platyhelminthes Excretion and Osmoregulation

• Network of excretion tubules for osmoregulation.

• Bulb-like flame cells: Flagellated cells that move water through excretory pores.

• Metabolic wastes diffuse directly into the gastrovascular cavity for removal.

Platyhelminthes Nervous System

• Some cephalization (concentration of sensory organs at the anterior end).

• Cerebral ganglion located anteriorly, with two nerve cords running throughout the body.

• Some have rudimentary eyespots: light-sensitive cells; most move away from light.

Platyhelminthes Reproductive System

• Hermaphroditic: Each individual has both male and female reproductive organs.

• Most require copulation with another individual, exchanging sperm.

Platyhelminthes Asexual Reproduction

• Capable of regeneration: can form new individuals from parts of the body.

• 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 with an outer layer called the neodermis.

    • Resistant to digestive and immune systems of hosts.

    • Mostly endoparasites, but some are ectoparasites; lack eyespots.

    • Trematoda (Flukes):

      • Attach within the body of a host 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:

            1. 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 tadpole-like larva that leaves the snail (cercaria).

            2. 2nd host: Fish

               • Cercaria stage is free-living and aquatic.

               • Seeks out fish, burrows into muscle layer, and forms a metacercaria cyst.

            3. 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 up to 30 years in humans and can lead to liver cirrhosis.

    • Cercomeromorpha (Tapeworms):

      • Attaches 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:

        • 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 the 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 the mouth and aid in movement

  • Toes: Used to attach to objects in the environment

  • Complete digestive system with a mouth and an anus