Trends in Animal Evolution

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.

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

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

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.

Body Symmetry

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

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