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