USA Biology Olympiad (USABO) Practice Exam

What distinguishes radiata organisms from bilateria?

1. Radiata exhibit radial symmetry.

2. Radiata have complex body systems.

3. Radiata reproduce only sexually.

4. Radiata are exclusively marine organisms.

The correct answer is: Radiata exhibit radial symmetry.

Radiata organisms are primarily distinguished by their radial symmetry, which means that their body plans are arranged around a central axis, allowing for multiple planes of symmetry. This is quite different from bilateria, which exhibit bilateral symmetry, where the body can be divided into mirror-image halves along one plane. In organisms with radial symmetry, like jellyfish or corals, body parts are typically arranged in a circular pattern around the central point, allowing them to interact with their environment from all directions, which is beneficial in their often aquatic habitats. The other distinctions mentioned in the other options do not accurately characterize radiata. For instance, while radiata can possess various levels of body complexity, it's not accurate to say they have complex body systems like many bilaterally symmetrical organisms, which tend to have more specialized tissues and organ systems. Additionally, radiata reproduce both sexually and asexually, so reproductive method is not a defining characteristic of this group. Furthermore, not all radiata are exclusively marine; some live in freshwater environments, such as certain species of jellyfish. Thus, the defining feature that sets radiata apart from bilateria is indeed their radial symmetry.