What do we mean by sessile how Radial Symmerty is advantageous 4nthem

Sessile means which is not able to move , remain attached to one position with substratum.

Radial symmetry- Cnidarians are radially symmetrical means when any plane passing through the central axis of the body divides the organism into two identical halves. It is beneficial for them as they can extend their body parts in all directions from the central point.