Lecture Notes: Importance and Diversity of Annelids and Arthropods

Introduction

• Annelids and arthropods are some of the most successful organisms on Earth.
  • Annelids process significant amounts of soil, making plant life possible.
  • Arthropods vastly outnumber humans, with 80% of known animal species being arthropods.

Importance of Segmentation

• Segmentation: Key trait in animal complexity.
  • Repetition of anatomically identical units.
  • Allows adaptation and specialization.
• Three major segmented phyla: Annelida, Arthropoda, and Chordata.
  • Common ancestor 600 million years ago.
• In humans: Vertebrae, ribs, brain folds.

Phylum Annelida

• Synapomorphies: Traits setting groups apart from ancestors.
  • Example: Segmentation in annelids ("little rings").
  • Chitae: Bristles for movement.
• Plesiomorphies: Basic shared traits.
• Classes within Annelida:
  • Oligochaeta: Earthworms.
    • Few chitae.
    • Important for soil health (aeration, nutrient cycling).
  • Hirudinea: Leeches.
    • Parasitic with powerful suckers.
  • Polychaeta: Marine bristle worms.
    • Diverse, known for burrow-casting on beaches.

Phylum Arthropoda

• Diversity: Most numerous animals.
  • Segmentation led to diverse forms (scorpions, butterflies, lobsters).
• Synapomorphies:
  • Segmented body: Head, thorax, abdomen.
  • Exoskeleton made of chitin.
  • Paired, jointed appendages.
• Subphyla:
  • Cheliceriformes: Spiders, scorpions, horseshoe crabs.
    • Arm lips, simple eyes, no antennae.
    • Arachnids: Eight legs, cephalothorax.
  • Myriapoda: Millipedes and centipedes.
    • Terrestrial with many legs and antennae.
  • Hexapoda: Insects.
    • Three-part body, six legs, compound eyes.
    • Extremely diverse with many species.
  • Crustacea: Crabs, lobsters, shrimp.
    • Marine, three body segments.

Evolutionary History and Adaptations

• Insects evolved 410 million years ago from crustaceans.
• Adaptations:
  • High oxygen levels allowed large sizes during Carboniferous Period.
  • Relationship with flowering plants led to pollination strategies.
  • Unique flying ability through metamorphosis.

Metamorphosis in Insects

• Two types:
  • Partial Metamorphosis: Nymphs resemble adults.
  • Complete Metamorphosis: Larvae transform into adults.

Crustaceans

• Mainly marine, with adaptations like bioluminescence and specialized appendages.
• Lobsters and crayfish have multiple types of appendages.

Conclusion

• Annelids and arthropods demonstrate incredible diversity and evolutionary success.
• Understanding segmentation helps illuminate evolutionary paths and relationships.