The animal body plan

• Symmetry


  The arrangement of the parts of an organism or object around an axis or line so that the parts are balanced
• Parazoa


  • Sponges, the only animal group which lack defined tissues and organs and have no symmetry in their body plan
• Eumetazoa


  • All other animals, have a definite shape and symmetry
• Radial symmetry


  The parts of the body are arranged around a central axis in such a way that any plane passing through the central axis divides the organism into two equal halves
• Animals with radial symmetry


  • Cnidaria (jellyfish, sea anemones, corals)
• Radial symmetry


  • Primitive type of symmetry, occurs in sessile animals or animals which move slowly, allows the animal to receive sensory information, food and oxygen from all directions, sense predators coming from any direction
• Bilateral symmetry


  There is only a single plane which divides the body into two equal halves; a right and a left half
• Bilateral symmetry


  • Permits different parts of the body to evolve in different ways, allows more efficient movement from place to place compared to radial symmetry, enables a defined head and tail region
• Cephalisation


  The nervous system in bilaterally symmetrical animals is in the form of longitudinal nerve cords, and early in evolution nerve cells became grouped in the anterior end of the body, leading to the evolution of a definite head and brain
• Germ layers


  The three primary cell layers that form during embryonic development: ectoderm, mesoderm, endoderm
• Triploblastic organisation


  • All bilaterally symmetrical animals have ectoderm, mesoderm and endoderm layers
• Diploblastic organisation


  • Radially symmetrical animals have only ectoderm and endoderm layers

  
• Acoelomates


  • Animals with no body cavity
• Pseudocoelomates


  • Animals with a fluid-filled body cavity called a pseudocoel located between the mesoderm and the endoderm
• Coelomates


  • Animals with a fluid-filled body cavity called a coelom located entirely within the mesoderm, allowing for more extensive organ growth, efficient circulatory system, hydrostatic skeleton, and independent movement of digestive and body wall muscles
• Segmentation


  The subdivision of the animal body into segments, also known as metamerism or metameric segmentation
• Segmentation


  • Repetition of body segments allows for functional redundancy, flexibility in locomotion, and increase in body size using minimum genetic information
• Tagmatisation


  The fusion of body segments into structural and functional body units, e.g. head/thorax/abdomen in insects
• Tagmatisation


  • Allows specialisation of body segments so each tagma has a different function, and appendages can become specialised for different functions
• Jointed appendages


  External body parts or natural prolongations that protrude from an organism's body and have joints permitting greater range of movement
• Flexibility in locomotion


  • More complex movements possible e.g. permits burrowing activity
• Increase in body size

  • Using a minimum of genetic information due to replication of most segments
• Tagmatisation


  Fusion of body segments into structural and functional body units e.g. head/ thorax/ abdomen in insects
• Advantages of tagmatisation


  • Specialisation of body segments so that each tagma has a different function
  • Appendages on the different tagmata become specialised for different functions
• Appendage


  An external body part, or natural prolongation, that protrudes from an organism's body
• Jointed/ articulated appendages


  Appendages have joints permitting a much better flexibility of movement
• Jointed appendages first appeared in the arthropods
• Types of appendages in arthropods

  • Antennae
  • Mouthparts (including mandibles, maxillae and maxillipeds)
  • Wings
  • Gills
  • Walking limbs (pereiopods)
  • Swimming limbs (pleopods)
  • Sexual organs (gonopods)
  • Parts of the tail (uropods)
• Typically, each body segment carries one pair of appendages in arthropods
• Arthropods having different mouth parts are adapted to feeding in different ways
• Chordates also have jointed limbs making locomotion highly efficient
• Tetrapods


  Vertebrate animals having four legs or leg-like appendages
• All tetrapods have a basic pentadactyl (five-digit) limb structure
• Homology


  An organ or structure possessed by members of different taxonomic groups that originally derived from the same structure in a common ancestor
• Modifications of the pentadactyl limb in different tetrapod groups

  • In the monkey, the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees
  • In the whale, the forelimbs become flippers for steering and maintaining equilibrium during swimming
  • In the bat, the forelimbs have turned into wings for flying by great elongation of four digits, while the hook-like first digit remains free for hanging from trees

  
• Transition from water to land

  • Involves the invasion of a habitat that is more hazardous to life in many respects
  • Requires modification of almost every system in the vertebrate body
• Problems animals must overcome on land

  • Water loss
  • Density
  • Temperature regulation
  • Oxygen content
• Despite its hazards the terrestrial environment offers a great variety of new habitats and safe shelter for the protection of vulnerable eggs and young
• Cleidoic egg


  The egg laid by reptiles and birds that enables reproduction to be entirely adapted to life on land
• Features of the cleidoic egg


  • Contains food and protective membranes for supporting embryonic development
  • Shell provides protection against mechanical damage and pathogenic invasion, and is porous allowing gas exchange
  • Food is provided by yolk from the yolk sac the embryo develops within the amnion, cushioned by amniotic fluid