Bilateral symmetry allowed for the evolution of heads and brains by concentrating sensory organs and the brain to the front of the body, and also enabled effective motion through the symmetrical positioning of legs. Both parasitic organisms and sessile organisms commonly rely on external reproduction to spread their offspring since they cannot move or find mates on their own, making external reproduction an advantageous adaptation for their lifestyle.

1. A) Explain how bilateral symmetry contributed to motion and cephalization (the evolution of a
head and brain)? B) Explain the common adaptations for reproduction seen in both: organisms
with a parasitic life-style and sessile organisms (those that do not move). C) Explain why those
adaptations are beneficial for each life-style.
Solution
When the animal has right and left sides morror images of each other such a symettry is called
bilateral symmetry.
Bilateral symmetry has contributed to both cephalization and locomotion in animals possessing
them. Animals wuth bilateral symmetry has locomotive organs legs on opposite sides of the body
in symmetrical position. This symmetrical arrangement of locomotive organs enables these
animalsto move thotougjly in steady direction.
Bilateral symmetry resulted in the concentration of brain ,nerves, sense organs ,eyes to the
anterior side of the body forming head.