Chapter 32

• Animals


  Multicellular, heterotrophic eukaryotes
• Characteristics that define the animal kingdom

  • Cells supported by structural proteins: collagen
  • Nervous tissue and muscle tissue
  • Most animals reproduce sexually, with the diploid stage usually dominant
  • Most animals have at least one larval stage
• Larva


  Sexually immature and morphologically distinct from the adult; it eventually undergoes metamorphosis to become a juvenile
• Juvenile


  Resembles an adult, but is not yet sexually mature
• Biologists have identified 1.3 million living animal species to date; far more are estimated to exist
• The common ancestor of all living animals likely lived

  770 million years ago
• Choanoflagellates


  A group of protists that are the closest living relatives to animals
• Early members of the animal fossil record include the Ediacaran biota


  560 million years ago
• Evidence of predation has also been found in fossils of the Ediacaran period


  635–541 million years ago
• The Cambrian explosion marks the earliest fossil appearance of many major groups of living animals


  535–525 million years ago
• Bilaterians


  • Bilaterally symmetric form
  • Complete digestive tract
  • One-way digestive system
• Hypotheses regarding the cause of the Cambrian explosion and decline of Ediacaran biota

  • New predator-prey relationships
  • A rise in atmospheric oxygen
  • The evolution of the specific gene complexes and RNAs involved in gene regulation
• Animals began to make an impact on land

  450 million years ago
• Vertebrates made the transition to land

  365 million years ago
• During the Mesozoic era, dinosaurs were the dominant terrestrial vertebrates
• The first mammals emerged during the Mesozoic era
• The beginning of the Cenozoic era followed mass extinctions of both terrestrial and marine animals
• These extinctions included the large, nonflying dinosaurs and the marine reptiles
• Mammals increased in size and exploited vacated ecological niches during the Cenozoic era
• The global climate cooled during the Cenozoic era
• Body plan


  A set of morphological and developmental traits used to categorize animals
• Radial symmetry


  • A type of symmetry found in a flowerpot, with a top and a bottom, but no front and back, or left and right
  • Radially symmetrical animals are often sessile or planktonic (drifting or weakly swimming)
• Bilateral symmetry


  • The two-sided symmetry of a shovel, with a dorsal (top) side and a ventral (bottom) side, a right and left side, and anterior (front) and posterior (back) ends
  • Bilateral animals typically move actively and have a central nervous system
• Sponges and a few other groups lack true tissues
• Diploblastic animals

  Have only ectoderm and endoderm
• Triploblastic animals


  Also have a mesoderm
• All bilaterally symmetrical animals are triploblastic
• Coelom


  A true body cavity derived from mesoderm
• Coelomates


  Animals that possess a true coelom
• Pseudocoelom


  A body cavity derived from the mesoderm and endoderm
• Pseudocoelomates


  Triploblastic animals that possess a pseudocoelom
• Acoelomates


  Triploblastic animals that lack a body cavity
• Functions of a body cavity

  • Fluid cushions the suspended organs
  • Fluid acts like a skeleton against which muscles can work
  • Enables internal organs to grow and move independently of the outer body wall
• Cleavage


  Rapid cell division of the zygote leading to formation of a multicellular, hollow blastula
• Gastrulation


  The process that forms a gastrula with different layers of embryonic tissues
• Protostome development


  Cleavage is spiral and determinate, with the blastopore becoming the mouth
• Deuterostome development


  Cleavage is radial and indeterminate, with the blastopore becoming the anus
• Indeterminate cleavage makes possible identical twins and embryonic stem cells
• In protostome development, the splitting of solid masses of mesoderm forms the coelom
• In deuterostome development, the mesoderm buds from the wall of the archenteron to form the coelom