The animals with bilateral symmetry in young stage, and radial pentamerous symmetry in the adult stage, belong to the phylum:

Step-by-step solution

In Echinoderms, larval forms possess bilateral symmetry while adults have radial symmetry. Mollusc possess bilateral symmetry. Cnidarian have bilaterally and radial symmetry. Annelid have bilateral symmetry. Caution Class Gastropods (Mollusc) are characterised by their asymmetric body plan, which develops through process called torsion (rotation of visceral organs in anticlockwise direction through an angle of 180^ on the rest of the body during larval development).