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Bilaterians and Lophotrochozoans: A Deep Dive into Body Plans, Development, and Evolutionary Significance

Bilaterians and Lophotrochozoans

Features of Bilaterians

  • Proloplastic Development: Ancestor of bilaterians had a through gut.
  • Worm Body: Basic body plan.
    • Through gut: Mouth, gastrointestinal tract, anus.
  • Bilateral Symmetry: Division into left and right halves.
    • Anterior (head) and posterior (butt) sides.
    • Dorsal and ventral sides are distinct.
  • Cephalization: Concentration of nervous system in a head region.
    • Central processing organ (brain) in the head.
  • Expansion of Regulatory Genes: Possession of at least seven Hox genes on one chromosome.
    • Hox genes control developmental biology processes; how cells know their position and how organs develop.

Embryonic Development

  • Blastopore: The first indentation in the embryo during development.
  • Protostomes: Animals where the blastopore becomes the mouth.
    • Mouth-first development.
  • Deuterostomes: Animals where the blastopore becomes the anus (e.g., humans).
    • Anus-first development.
  • Protostomes vs. Deuterostomes: Almost like mirror images.
    • In deuterostomes, the nerve cord is on the back, while in protostomes, it's on the front.
  • Protostome Groups: Two major groups: Lophotrochozoans and Ectisozoans.

Arrow Worms (Chaetognatha)

  • Minor Group: Only about 120 species.

  • Predators: Fearsome carnivores in plankton environments.

  • Morphology: Torpedo-like shape with reversible claws on their face to grab food.

  • Transparency: Transparent bodies allow observation of internal organs.

  • Lack Circulatory System: Gas exchange happens directly across their bodies.

    • They are very tiny and thin.

  • Mixture of protostome and deuterostome traits

  • protostome development - blastopore develops into the mouth

  • deuterostome development - blastopore develops into the anus

  • Arrow worms show deuterostome development but have other protostome features.

    • e.g. ventral nerve chord

  • Have evolved protostome features, but retain the ancestral feature of deuterostome development.

  • Evolutionary Significance: Anus-first development is now considered the ancestral condition of bilaterians, with protostomes having changed to mouth-first development.

  • Phylogenetic Placement: Potentially a sister clade of the lophotrochozoans.

Lophotrochozoans

  • Etymology: "Animals with lophophores and trochophores."
    *Features:
    *Lophophore: A band of tentacles around the mouth.
    *Analogous to cnidarian tentacles, but evolved independently - not homologous to cnidarian tentacles.
    *Mostly seen in sessile (planted) adults.
    *Used for grabbing prey and gas exchange.
    *Surface area elaborations facilitate gas exchange.
    *Independently evolved multiple times within the group
    *Trochophore: A larval form with a band of cilia.
    *Small, vaguely spherical larvae.
    *Band of cilia around the middle.
    *Apical tuft: Little band of cilia that can pull the animal through the water.
    * Larval stage thought to be synapomorphy of the clade.
    *Present in common ancestor but has been secondarily lost in some groups.

  • Diversity: Body plans have a lot of variety.

    • Most are worms; mollusks are the exception with appendages.
    • Mollusks and annelids are the two largest clades.
      *Two major clades:
      *Small worms
      *Big worms
      *Bryozoans.

Bryozoans

  • Etymology: "Moss animals" (bryo = moss, zoa = animal).
  • Colonial Animals: Live in colonies of genetically identical individuals.
    • Asexual reproduction: Individuals bud off the original animal.
    • Physically attached: Connected via a string of tissue called a stolen.
    • Nutrient Sharing: Nutrients are shared among colony members.
  • Differentiation of Roles: Specialization within the colony.
    • Feeding: Use lophophores (ring of tentacles).
    • Defense: Modified tentacles as claw-like weapons.
    • Reproduction: Produce offspring and release larvae.
  • Size: Very small; individual animals are about 1-2 millimeters.
  • Habitat: Mostly marine, but also found in ponds.

Flatworms (Platyhelminthes)

  • Morphology: Dorsal-ventrally flattened.

    • All internal body cells are close to the surface.
    • Lack a circulatory system.
    • Many have bands of cilia for swimming.
      *Free Living Flatworms
      *Beautiful with striking coloration
      *Serve as warning of bad taste.
      *Simultaneous hermaphrodites - possess both egg and sperm producing reproductive organs

  • Free-living Flatworms: Possess both egg and sperm producing reproductive organs.

  • Aquatic Ecosystems: Important role.

  • Parasitism: Parasitic flatworms are more common than free-living ones. Give up everything for reproduction.

    • Chinese liver fluke is a common example.
    • Simplified internal organs; feed from the host.

  • Schistosomiasis: A parasitic flatworm that affects humans.

    • Causes stunting of growth, anemia, and learning disabilities.
    • Enters through bare feet in contaminated water.
    • Life Cycle: Worms penetrate skin, mature in the body, and shed eggs through waste products.
    • Control: Often focuses on eliminating the intermediate host (freshwater snails).

Rotifers

  • Wheel Animals: Have spinning discs of cilia on their heads that brings food to the mought.
  • Morphology: Foot with "toes" for movement and inching around.
  • Unique Reproduction: Some groups (Deloid rotifers) consist only of females that reproduce asexually.
    • Horizontal Gene Transfer: Incorporate DNA from the environment to increase diversity.
  • Survival: Can survive in dried-up ponds by turning into "rotifer tumbleweeds."
  • Parasitic Rotifers: Have simplified internal organs and use a spiny proboscis to burrow into the host's gut wall.

Gastrotrichs

  • Morphology: Dorsal-ventrally flattened like a smooshed rotifer, with cilia all over their body (hairy back).
  • Ecological Importance: Live in high population densities in aquatic ecosystems.
  • Role: Connect the microbial world to the macro animal world by feeding on bacteria and microbial eukaryotes and then being eaten by larger animals.

Ribbon Worms (Nemertea)

  • Proboscis: Use a proboscis (appendix on their head) to catch prey. *The appendage is inside of a body cavity; can squirt out their proboscis in the environment.
    • The proboscis is shot out, and is as long as the animal itself
    • The proboscis has a barb on the end that injects a stun.
  • Size: Most are small, but some can be very long (up to 177 feet).
  • Proboscis Origin: Proboscis is believed to be a highly modified lophophore.

Brachiopods

  • Morphology: Have a two-part shell like a clam.
  • Feeding: Have a lophophore inside the shell.
  • Attachment: Attached to a stalk, with part of the body buried in the sand.

Phoronids

  • Morphology: Have feeding tentacles.
  • Gut: Mouth is near the tenacles, and the anus is located close to the mouth.
  • U-shaped Gut: Develop a U-shaped gut as adults.
  • Chitin Tube: Enclosed in a tube of chitin.

Annelids

*Terrestrial Environment - live on land.

  • Segmentation: Bodies are made of repeating units which results in gene duplication events
    Most annelids are worms with no appendages

  • Synapomorphy: segmentation

  • Result of gene duplication