Evolution: Protostomes and Deuterostomes

Once multicellular organisms evolved bilateral symmetry, they soon split into two major evolutionary lines: Protostomes and Dueterostomes. Examples of protostomes include insects, earthworms, and clams; examples of dueterostomes include humans, snakes, and sea urchins.

What is the scientific/evolutionary basis of dividing most multicellular animals into these two groups and list two (2) specific differences theses two groups exhibit.