USA Biology Olympiad (USABO) Practice Exam

What is a characteristic of bilateria organisms?

1. They exhibit bilateral symmetry.

2. They have radial body symmetry.

3. They lack tissues and organs.

4. They reproduce only asexually.

The correct answer is: They exhibit bilateral symmetry.

Bilateria, a broad group within the animal kingdom, are distinguished primarily by their bilateral symmetry. This means that these organisms have a body plan that is symmetrical on either side of a central axis, allowing for the development of distinct head and tail ends (anterior and posterior) as well as a top and bottom (dorsal and ventral). This symmetry is crucial for the organization of their nervous systems and facilitates more efficient movement and predation strategies, as it allows for a directional approach to the environment. Other body symmetry forms, such as radial symmetry, are found in different groups, like cnidarians, but bilateria specifically possess bilateral symmetry. The presence of tissues and organs is also a feature of bilaterally symmetrical organisms, contrasting with simpler organisms that do not have these structures. Lastly, while many bilateria can reproduce asexually, they are not limited to this method, as sexual reproduction is common within this group. In summary, the defining characteristic of bilateria is their bilateral symmetry, which greatly influences their anatomy and behavior.