1. Trends in Animal Evolution

2. Complexity in animals
Complex animals tend to have high levels of cell specialization and internal body organization, bilateral body symmetry, a front end or head with sense organs and a body cavity.

3. Early development
Animals that reproduce sexually begin life as a zygote (fertilized egg)
The zygote undergoes a series of divisions to form a blastula (hollow ball of cells)
A blastopore forms, creating a single opening tube (later becomes the digestive tract)
Protostome- an animal whose mouth is formed from the blastopore
Ex. Most invertebrates
Deuterostome- an animal whose anus is formed from the blastopore
Ex. Echinoderms and all vertebrates

4. Early development
Cells of most animal embryos differentiate into 3 layers
Endoderm- innermost germ layer that develops into the lining of the digestive tract and much of the respiratory system
Mesoderm- middle layer that gives rise to the muscles, much of the circulatory system, reproductive system, and excretory organ system.
Ectoderm- outermost layer, gives rise to sense organs, nerves and outer layers of the skin

5. Body Symmetry
Every animal (except sponges) exhibits some type of body symmetry in its anatomy or body structure
Radial symmetry- body parts that repeat around the center of the body
Ex. Sea anemone
Bilateral symmetry- a single imaginary plane divides the body into two equal halves
Ex. Crayfish, humans, etc.

6. Body Symmetry

7. Cephalization
Cephalization- the concentration of sense organs and nerve cells at the front end of the body
Responds more quickly to the environment and in more complex ways than simpler animals can
As sense organs evolve (ex. Eyes), they have a tendency to gather at the anterior end (front) as it comes in contact with the environment first
In general, the more complex the animal, the more pronounce their cephalization

8. Body Cavity formation
Body cavity- a fluid-filled space that lies between the digestive tract and the body wall
Provides a space for internal organs
In some animals, it contains fluids that are involved in circulation, feeding, and excretion