Bilateria

Bilateria refers to animals that exhibit bilateral symmetry, meaning their bodies can be divided into two mirror-image halves along a central axis. This body plan is established during embryonic development and is characterized by three tissue layers (triploblastic) and the organization of tissues into organs.12

Unlike radially symmetrical animals like jellyfish, Bilateria typically move actively. This directed movement is facilitated by sensory organs concentrated at the anterior (head) end and locomotion organs arranged along the sides. This bilateral symmetry is fundamental to their organization, with many internal organs and external features like limbs being paired.2

Flatworms are considered early Bilateria and lack a fluid-filled body cavity (coelom). This absence limits muscle contraction and necessitates adaptations like extensive gut systems or circulatory systems to supply oxygen and nutrients. The development of body plans in Bilateria is controlled by regulatory genes, notably Hox genes, which play a crucial role in establishing the organism's structure from the earliest embryonic stages.23