Bilateria and Flatworms Notes

Bilateria

All animals with bilateral symmetry and are triploblastic.

  • Protostomes: Blastopore forms the mouth.

  • Deuterostomes: Blastopore forms the anus.

Protosomes

Spiralia

Spiral cleavage, mostly aquatic, move via cilia and muscle contractions.

  • Lophotrochozoa: Two forms, trochophore and lophophore

    • Trochophore: Free-living larva with a spherical body and band of cilia.

    • Lophophore: Feeding structure with ciliated tentacles surrounding the mouth.

Ecdysozoa

Animals that molt and undergo ecdysis with a hard exoskeleton that splits for growth. Examples include arthropods and nematodes.

Platyzoans (Flatworms)

  • Major group: Platyhelminthes

  • Characteristics:

    1. Dorsoventrally flattened

    2. Ciliated and soft-bodied

    3. Acoelomate

  • Habitats: Marine, freshwater, and terrestrial.

  • Nutrition: Carnivores, detritivores, or parasites.

Platyhelminthes Anatomy

  • Acoelomate with three tissue layers.

  • Single opening into the gastrovascular cavity for extracellular digestion; functions as both a digestive and circulatory system.

  • Developed nervous and excretory systems.

Platyhelminthes Digestive System

Gastrovascular cavity extends through the body with a single opening (mouth via pharynx) for nutrient entrance and waste exit. Food is ripped into small pieces via the muscular pharynx and swallowed into the gastrovascular cavity where extracellular digestion occurs.

Platyhelminthes Excretion and Osmoregulation

Network of excretion tubules for osmoregulation with bulb-like flame cells (flagellated cells that move water through the excretory pore). Metabolic wastes directly diffuse into the gastrovascular cavity for removal.

Platyhelminthes Nervous System

Some cephalization with a cerebral ganglion located anteriorly and two nerve cords running through the body. Some have rudimentary eyespots (light-sensitive cells).

Platyhelminthes Reproductive System

Hermaphroditic; individuals carry both male and female reproductive organs and most require copulation with another individual to exchange sperm. Capable of asexual reproduction via regeneration; if cut in half, each half can grow into a new individual.

Platyhelminthes Groups

Turbellaria

Free-living flatworms; locomotion via beating cilia. Mostly marine and freshwater, many have eyespots.

Neodermata

Parasitic flatworms with an outer layer called the neodermis (resistant to digestive and immune systems). Mostly endoparasites, some ectoparasites; lack eyespots.

  • Trematoda (Flukes): Attach via suckers, anchors, or hooks; feed on host cells and fluids; complex life cycles involving multiple hosts.

    • Liver Fluke Lifecycle:

      • 1st host: Snails

      • 2nd host: Fish (Cercaria stage seeks out fish and burrows to form metacercaria cyst)

      • 3rd host: Terrestrial vertebrate

  • Cercomeromorpha (Tapeworms): Attach to inner wall of intestines to absorb nutrients.

    • Tapeworm Anatomy:

      • Scolex: Attachment via hooks and suckers.

      • Neck: Unsegmented region with high regenerative properties.

      • Proglottid: Segmented regions with reproductive organs (hermaphroditic).

Rotifera

Wheel animals; tiny, multicellular

  • Characteristics:

    1. Unsegmented pseudocoelomate

    2. Complete digestive system (mouth and anus)

    3. Cephalization

  • Anatomy:

    • Corona: Cilia ring to sweep food into mouth and movement.

    • Toes: Used to attach to objects.