Final answer:
Bilateral symmetry and cephalization are the features that make an earthworm a protostome bilateral animal.
Step-by-step explanation:
Bilateral symmetry and cephalization are the features that make an earthworm a protostome bilateral animal.
Bilateral symmetry means that the earthworm can be divided into right and left sides that are mirror images of each other. This symmetry allows for streamlined and directional motion.
Cephalization refers to the concentration of nervous tissues and sensory organs in the head of the organism. The earthworm's head is where it first encounters its environment.