Animal Diversity and Evolution

Animal Unit: What to Expect

  • Overview of topics covered in the study of animals.
    • Animal diversity and evolution
    • Body plans, physiology, gas exchange
    • Overview of organ systems:
    • Movement
    • Reproduction
    • Immunity
    • Behavior
    • Interesting Animal Behaviors: Sleeping, exercising.

Inspirational Quote

  • Charles Darwin: "…from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
    • Source: On the Origin of Species

Unit 2: Animals

  • Course Information:
    • Date: February 16, 2026
    • Instructor: Dr. Ana Longo
    • Course Code: BSC2011-Spring 2026

Outline of Animal Diversity and Evolution 1

  • Chapter 22: Animal Origins and Diversity
    • 22.1 Distinct Body Plans Evolved among the Animals
    • 22.2 Large Multicellular Animal Groups Arose in Parallel
    • 22.3 Protostomes Have an Anterior Brain and a Ventral Nervous System
    • 22.4 Arthropods Are Diverse and Abundant

Learning Objectives

  • Explain the functional relationships between:
    • Body symmetry
    • Body cavity structure
    • Segmentation
    • Appendages
    • Nervous systems
  • Apply knowledge of phylogenetic relationships among major groups of animals to describe:
    • Evolution and loss of major structural features
  • Describe morphological innovations of arthropods and their effects on arthropod diversity.

Defining Features of Animals

  • What makes an animal an animal?
    • Multicellularity
    • Heterotrophic metabolism
    • Internal digestion
    • Movement and nervous systems
  • All animals share a common ancestor (monophyletic).

Animal Phylogeny

  • Early development and cellular organization:
    • Choanocytes and spicules are fundamental to sponges.
    • Presence of unique cell junctions, collagen, and proteoglycans in the extracellular matrix.
    • Genomic support as a factor for evolutionary relationships.
    • Distinct organ systems evolved, leading to various animal lineages:
    • Demosponges, Calcareous sponges
    • Nerve nets in Cnidarians and Ctenophores
    • Classification of animals based on:
    • Body symmetry, embryonic development, and specialized structures (notochord in Chordates and nerve nets in simpler organisms).

Early Development Across Triploblastic Animals

  • Key Developmental Patterns:
    • Deuterostomes:
    • The blastopore develops into the anus; the mouth forms later.
    • Protostomes:
    • The blastopore develops into the mouth; the anus forms later.

Body Plan Features

  • Key features of body plans:
    • Symmetry
    • Body cavity structure
    • Segmentation
    • External appendages
    • Nervous system

1. Symmetry

  • Types of symmetry:
    • Asymmetrical: No plane of symmetry (e.g., sponges and placozoans).
    • Radial symmetry: Body parts arranged around a central axis (e.g., cnidarians).
    • Bilateral symmetry: Can be divided into mirror-image halves along one plane.

2. Body Cavity Structure (Coelom)

  • Structure:

    • Tube-within-a-tube model:
    • Inner tube: Digestive tract.
    • Outer tube: Body wall (skin and muscles).
    • Coelom: Fluid-filled space between tubes.

  • Importance of Coelom:

    • Provides a space for organ development and movement.

  • Types of Body Cavity:

    • Acoelomates:
    • Example: Flatworms.
    • No enclosed body cavity.
    • Pseudocoelomates:
    • Example: Roundworms.
    • Pseudocoel is fluid-filled and lined with mesoderm.
    • Coelomates:
    • Example: Earthworms.
    • Coelom is fully surrounded by mesoderm.

3. Segmentation

  • Function:
    • Facilitates specialization of body regions.
    • Allows precision in movement and body shape alteration.
  • Example: Radiation of arthropods based on segmented body plans.

4. Appendages

  • Types of appendages:
    • Enhance movement capabilities.
    • Include: antennae, claws, mouthparts, and reproductive organs.

5. Nervous Systems

  • Types of Nervous Systems:
    • Nerve nets: Diffuse systems found in organisms like ctenophores and cnidarians.
    • Well-coordinated central nervous systems: Present in Bilaterians, allowing for enhanced coordination of muscle actions.

Early Animal Branches

  • Four major groups that do not belong to Bilaterians:
    • Sponges
    • Ctenophores
    • Placozoans
    • Cnidarians
  • Evolutionary Insights:
    • Sponges and ctenophores represent early splits in animal evolution. Disagreement exists on which group diverged first.
    • Large multicellular animals evolved multiple times across different lineages.

Evolutionary Pathway from Protists to Animals

  • Choanoflagellate protists:
    • Not animals but are closely related.
  • Specialized cells in colonies of protists evolved to perform distinct functions (movement, nutrition), leading to larger and more complex forms.

Notable Examples of Sponges

  • Examples:
    • Xestospongia sp.
    • Euplectella aspergillum
    • Sycon sp.
  • Carnivorous Sponges:
    • Feature spicules resembling hooks for capturing prey.

Ctenophores

  • Characteristics:
    • Radial symmetry and diploblastic body plan.
    • Move using cilia arranged on eight ctenes (comb-like plates).
  • Example: Mnemiopsis leidyi.

Placozoans

  • Description:
    • Known as "the blobs"; asymmetric and diploblastic.
    • Composed of only four cell types; lack a mouth, gut, or nervous system.

Cnidarians

  • Groups:
    • Include jellyfishes, sea anemones, corals, and hydrozoans.
  • Functions:
    • The gastrovascular cavity allows for digestion, circulation, gas exchange, and acts as a hydrostatic skeleton.
  • Special Structure:
    • Nematocysts: Harpoon-like structures with toxins for capturing prey.

Conclusion and Next Steps

  • Review previous chapters for upcoming discussions.
  • Next Class: Chapter 22 - Continuing with Deuterostomes.
    • Focus on echinoderms, hemichordates, and chordates.
    • Explore the impact life on land had on vertebrate diversification.