What is the advantage of radial symmetry for sessile animals such as hydras and bilateral symmetry for mobile animals such as planaria? What major evolutionary trends accompany bilateral symmetry?

- Radial symmetry is advantageous because sessile animals can "sit down", take food, or sense harmful environmental conditions from different directions.  

- Bilateral symmetry allows motile animals to move straight forward.

- The major evolutionary advantages of bilateral symmetry include cephalization, the formation of a head and tail area and a more directional motion.

Explanation:

Radial symmetry is advantageous for sessile organisms since it enables the uniform distribution of the sensory receptors around the body. In consequence, sessile organisms can react to environmental stimuli from every direction. On the other hand, bilateral symmetry allows motile organisms the arrangement of a specialized nervous system from the anterior end of the organism (i.e., the 'head'). Moreover, another important advantage of bilateral symmetry is the ability to equalize environmental pressures on both sides of the body, thereby enabling a rectilinear motion.