Animal Characteristics and Development

What Does It Mean To Be An Animal?

  • Animals are eukaryotic organisms.
  • Animal cells have no cell wall.
  • Animals are heterotrophic.
  • Animals are multicellular.
  • Animals are aerobic (use oxygen).
  • Animals develop from an embryo.
  • Animals reproduce mostly sexually, with the diploid stage being the dominant stage (99.9% reproduce sexually).
  • Animals are motile at some stage in their life cycle.
  • Most animals possess nervous tissue and muscles.
  • Animals possess a Hox gene.

Embryonic Development

  • When a sperm fertilizes an egg, a zygote is formed.
  • The zygote undergoes mitosis to produce two cells, then four, eight, sixteen, etc.
  • The zygote develops into a ball of cells called a blastula.
  • The blastula undergoes gastrulation, where it folds inward, forming three tissue layers:
    • Endoderm (innermost layer)
    • Mesoderm (middle layer)
    • Ectoderm (outer layer)
  • Endoderm gives rise to parts of the gut and some organs.
  • Ectoderm forms skin and part of the nervous system.

Blastula and Gastrulation

  • Blastula: a hollow ball of cells with an empty space inside called the blastocoel.
  • Gastrulation: the process where the blastula folds inward, giving rise to the three germ layers (endoderm, mesoderm, ectoderm).

Animal Body Plans

  • Animal body plans can be divided into three categories:
    • Radial symmetry: round shape (e.g., sea star, sand dollar, sea urchin)
    • Bilateral symmetry: elongated shape with a left and right side (e.g., humans, dogs, cats, horses)
    • Asymmetrical symmetry: irregular shape with no definite form (e.g., sponges)

Bilateral Symmetry and Body Regions

  • Bilateral animals have distinct body regions:

    • Anterior: head region or front region
    • Posterior: tail region or region away from the head
    • Dorsal: back region
    • Ventral: stomach or front side

  • In non-upright animals (e.g., fish, horse):

    • Dorsal side faces upward.
    • Ventral side faces downward.

  • In humans:

    • Dorsal side is the back.
    • Ventral side is the stomach or front.
    • Caudal means tail.

Germ Layers

  • Germ layers are tissues in animals.
  • Animals can have one, two, or three germ layers.
  • Triploblastic animals have three germ layers: endoderm, mesoderm, and ectoderm.
  • Diploblastic animals have two germ layers: endoderm and ectoderm (cnidarians like sea anemones, corals, jellyfish).
  • Sponges, considered the oldest evolved animals, do not have germ layers.

Body Cavity (Coelom)

  • Body cavity refers to the space inside the body.
  • Animals with a true body cavity are called coelomates and have a true coelom.
  • A true coelom is a body cavity lined with mesoderm tissue.

Pseudo coelom

  • Animals with a pseudo coelom have a body cavity that is not completely lined with mesoderm tissue.
  • The body cavity in pseudo coelomates comes from the blastocoel.

Acoelomate

  • Acoelomate animals have no body cavity.
  • Their bodies are completely solid, except for the digestive tract (e.g., flatworms).

Embryo Development

  • Embryo development can be divided into two categories:
    • Protostome development
    • Deuterostome development

Protostome Development

  • In protostome development, cell division is spiral and determinate.
    • Spiral: Cells divide in a spiral manner.
    • Determinate: The fate of each cell is already determined.

Deuterostome Development

  • In deuterostome development (e.g., humans), cell division is radial and indeterminate.
    • Radial: Cells divide upward.
    • Indeterminate: The fate of the cells has not been determined yet.
    • Tay-Sachs Example
      • Doctors can remove one cell from the blastula (day 3-5) and analyze its DNA to see if it contains the gene for Tay-Sachs.
      • If the cell does not contain the gene, the embryo can be used for implantation.
      • This is possible because the cells are indeterminate, so the remaining cells can compensate for the removed cell.

Protostome vs. Deuterostome

  • In protostome development
    • The first pore to form becomes the mouth
    • the body cavity is formed through schizocoelous
      • during gastrulation, masses of mesoderm tissue develop and form the coelom.
  • In deuterostome development
    • the first pore becomes the anus
    • the body cavity is formed through enterocoelous.
    • mesoderm tissue is attached to the archenteron, and it splits off to form the coelom.

Evolutionary Origins

  • All animals are believed to have evolved from a common ancestor, an algae known as choanoflagellate.
  • Choanocytes are cells found in sponges.
  • Sponges are considered the basal group of all animals.

Animal Phylogeny

  • There are about 35 phyla of animals.
  • The lecture will cover sponges, cnidarians, flatworms, annelids, mollusks, roundworms, arthropods, and mammals.