Answer: The correct answer for the blank is-
sponges.
Living organisms can be divided into three types based on their body plan that is asymmetrical ( no symmetry in the body as it can not be divided into two halves by any plane), bilaterally symmetrical ( body can be divisible into two equal halves by single plane when it passes the organism, and radially symmetrical organisms ( body divided into two equal halves when any plane passes through central axis).
Sponges ( belonging to the phylum porifera under invertebrates) are the asymmetrical organisms as there is no central axis and it can not be divided into two halves.
Except for sponges, all animals possess either radial or bilateral symmetry.
