Exercise 1 - Basis of Classification in the Animal Kingdom

• The six-kingdom classification is used. These are Archaea, Bacteria, Fungi, Plantae and Animalia or Metazoa
• Today, there are more than 30 animal phyla with the most recent phylum, Xenoturbellida, named in 2006.
• Symmetry describes the pattern of arrangement of body parts around an axis.
• An asymmetrical body plan shows an irregular arrangement and is uncommon in animals.
• Bilateral symmetry describes a body plan wherein an animal can be cut along its length to yield left and right halves that are mirror-images of each other.
• Radially symmetrical animals have body parts that are equally arranged around its axis.
• A radially symmetrical body form can be specialized into biradial or pentaradial symmetry depending on the presence of certain body parts and their arrangement.
• Terms for locating body regions of radially symmetrical animals:

  • Oral - at the mouth end
  • Aboral - opposite the mouth end
• Terms for locating body regions of bilaterally symmetrical animals:

  • Anterior - head end
  • Posterior - rear or tail
  • Dorsal - back side
  • Ventral - abdominal or belly side
• Terms for locating body regions of bilaterally symmetrical animals:

  • Medial - midline of the body
  • Lateral - to the sides
  • Distal - far from the midline
• Terms for locating body regions of bilaterally symmetrical animals:

  • Proximal - near midline
  • Pectoral - chest region
  • Pelvic - hip region
• Planes of division in a bilateral animal:

  Sagittal – plane passing through the midline which divides the animals into right and left halves. 

  Frontal – plane passing through the midline which divides the animal into dorsal and ventral halves.

  Transverse/cross-section – plane which cuts through the animal perpendicular to both the sagittal and frontal planes.
• Planes of division in a bilateral animal:

  • Sagittal – plane passing through the midline which divides the animals into right and left halves. 
  • Frontal – plane passing through the midline which divides the animal into dorsal and ventral halves.
  • Transverse/cross-section – plane which cuts through the animal perpendicular to both the sagittal and frontal planes.
• In early development of animals, the single- celled zygote undergoes a series of mitotic divisions called cleavage.
• After a few hours, a small ball of cell not larger than the parent cell is formed and each cell that comprises this early embryo is called a blastomere.
• A blastula can be described as a spherical structure consisting of one or two layers of cells surrounding a fluid-filled cavity called the blastocoel.
• Gastrulation, on the other hand, is initiated with the inward pushing of cells in a process called invagination.
• The site of the invagination is the blastopore. 
• This movement forms a new cavity called the archenteron, the precursor of the gut.
• The outer layer of the gastrula becomes the ectoderm and the inner lining of the archenteron becomes the endoderm.
• All bilateral animals are triploblastic (possess all three germ layers including the mesoderm), most of whom feature a body cavity or coelom.
• Those that do not have a coelom are called acoelomates.
• Those that do have a body cavity are either pseudocoelomate or eucoelomate.