Ans.
Symmetry in animals can be defined as the balanced distribution of identical body shapes or parts within an organism's body. All animals, except sponges have either radical or asymmetry.
Sponges belong to phylum Porifera, which exhibit asymmetrical body plans. Their body shapes are adapted for maximum efficiency of water flow through their central cavity.
Thus, the correct word to be filled in the blank is 'sponges.'
