animal classification

• Prokaryotic and eukaryotic cells


  Types of cells
• Endosymbiotic origin of plastids and mitochondria

  How plastids and mitochondria evolved
• Radial and bilateral symmetry


  Types of body symmetry in animals
• Diploblastic and triploblastic organisation


  Number of embryonic germ layers in animals
• Acoelomate and coelomate body plans


  Types of body cavities in animals
• Metameric segmentation


  Body segmentation in some animals
• Jointed appendages


  Limbs with joints in some animals
• Pentadactyl tetrapod limb


  Limb structure with 5 digits in some animals
• Transition of gills to lungs


  Evolution of respiratory structures in some animals
• Cleidoic egg


  Type of egg with a shell in some animals
• Animals are so diverse that few criteria fit them all
• Some features e.g. being consumers, apply to all animals
• Barnacles are sessile (cannot move from place to place)
• Others, e.g. being mobile, have exceptions
• There is enough evidence to show that animals have descended from a common ancestor
• Heterotrophy


  All animals obtain energy and organic molecules by ingesting other organisms
• Multicellularity


  All animals are multicellular
• No cell walls


  Animal cells are usually quite flexible, held together by extracellular frames of structural proteins
• Internal digestion


  Animals use internal processes to break down materials from their environment into the organic molecules they need
• Movement and Nervous system


  Muscle tissue and nervous systems are unique to animals, many animal body plans are specialised for movement and detection
• Sessile


  Cannot move from place to place
• Sedentary


  Move rarely or slowly, although they have muscles
• Embryonic development


  Zygote undergoes cleavage, forms blastula, then gastrula, then larva in most animals
• The genes that govern the development of body symmetry, body cavities, segmentation, and appendages are widely shared among the different animal groups
• Animals share body plans, although they vary greatly
• Key features of animal body plans


  • Symmetry
  • Body cavity structure
  • Segmentation
  • Appendages
  • Nervous system development
• Symmetry


  The overall shape of an animal described by its symmetry
• Symmetrical animals


  • Asymmetric (sponges)
  • Bilateral
  • Radial
• Radial symmetry


  Can be divided into similar halves by any plane that contains the main axis
• Radially symmetrical animals


  • Sessile (remain fixed in one place)
  • Drift with water currents
  • Move slowly but equally well in any direction
• Bilateral symmetry


  Body can be divided into similar halves on only one plane, from anterior to posterior
• Cephalisation


  Concentration of sensory organs and nerve tissues at the anterior end or head
• Advantages of bilateral symmetry


  • Efficient in seeking food, shelter and mates
  • Efficient in avoiding predators
• Echinoderms show secondary radial symmetry as adults, but their larvae are bilaterally symmetrical
• Diploblastic


  Develop from two embryonic tissue layers - ectoderm and endoderm
• Triploblastic


  Develop from three embryonic tissue layers - ectoderm, mesoderm and endoderm
• Acoelomate


  Have no body cavity
• Pseudocoelomate


  Have a body cavity called pseudocoel located between the mesoderm and endoderm
• Coelomate


  Have a fluid-filled body cavity that develops entirely within the mesoderm
• Functions of the coelom


  • Acts as a hydrostatic skeleton
  • Enables independent operation of body wall and gut
  • Permits larger body size
  • Transports materials
  • Stores waste
  • Provides space for organ enlargement
  • Plays a role in osmoregulation