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Animal Characteristics and Development
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.
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