Animal Phylogeny and Development Chapter 10 and 11

Introduction to Germ Layers
  • Two Cell Layers
  • Basic structure of organisms like jellyfish (diploblasts).
  • All cells experience a similar environment (e.g., aqueous medium).
  • Waste disposal: Cells dump waste outside directly.
Three Germ Layers (Triploblastic)

  • Significance of the Third Germ Layer

  • Allows more complex body structures and functioning.

  • Advances to organ systems and specialized functions.

  • Bilateral Symmetry

  • Organisms with three germ layers typically exhibit bilateral symmetry.

  • Inclusion of the third germ layer (mesoderm) enhances movement and organ differentiation.

Coelom Formation
  • Definition: Coelom
  • Body cavity surrounded by mesoderm, facilitating more complex body structures and organ systems.
  • Evolved for improved locomotion, notably seen in various animal phyla.
Flatworms (Phylum Platyhelminthes)

  • Characteristics

  • Mainly parasitic, capable of complex life cycles. Some are free-living.

  • Lack a coelom and, therefore, exhibit limited mobility compared to more advanced animals.

  • Mind Control: Some flatworms can manipulate the behavior of host organisms (e.g., ants) for their advantage.

  • Examples:

  • Trematodes (e.g., liver flukes): Complex life cycles involving multiple hosts (e.g., snails and ants) to facilitate reproduction.

Evolution of Coelomate Animals
  • Coelomates
  • Evolved for advantages in movement and organ function.
  • Mollusks as a Representative Group
  • Exhibit high diversity, including bivalves, gastropods, and cephalopods (e.g., squids, octopi).
  • Basic structure includes a foot for movement, a visceral mass, and a mantle that secretes shells (where applicable).
Arthropods: Molting Process

  • Diversity

  • Includes insects, arachnids, crustaceans, etc.

  • Characterized by jointed legs and exoskeleton of chitin.

  • Molting (

  • Definition: Shedding of the exoskeleton to allow for growth.

  • Magic of Growth: Grows in distinct steps rather than gradually.

  • Uses enzymes to digest the old exoskeleton and facilitates growth of a new one beneath it.

Chordates
  • Derived Traits of Chordates
  • Notochord: Stiff rod of cells signaling structural development.
  • Dorsal Hollow Nerve Cord: Forms the central nervous system.
  • Pharyngeal Slits: Allow for respiratory and filter feeding functions (e.g., gills in fish).
  • Post-anal Tail: Extension of the body beyond the anus, present at some developmental stage.
Evolutionary Examples
  • Cephalochordates: Early chordates without defined heads.
  • Vertebrates: Characteristics include a backbone and enhanced complexity in body plan.
Summary of Taxonomic Groups
  • Flatworms: Triploblastic without coelom, mainly parasitic.
  • Mollusks: Diverse group with a coelom and complex body structures.
  • Arthropods: Significant diversity; undergo molting as a growth process.
  • Chordates: Defined by specific derived traits; include both invertebrates and vertebrates.
  • Hemichordates: Transitional group leading towards chordates, with shared characteristics but distinct features.