USA Biology Olympiad (USABO) Practice Exam

Which organism group is known for having members with bilateral symmetry?

1. Flatworms and annelids.

2. Cnidarians and sponges.

3. Fungi and bacteria.

4. Algae and prokaryotes.

The correct answer is: Flatworms and annelids.

The correct answer identifies flatworms and annelids as organisms that exhibit bilateral symmetry, which is a characteristic where the body can be divided into two equal halves along a single plane, typically through the midline. This symmetry is significant in the evolution of animal forms, as it allows for a more streamlined body structure that can enhance movement and coordination. Both flatworms, which belong to the phylum Platyhelminthes, and annelids, which belong to the phylum Annelida, display this bilateral arrangement. This symmetry often aligns with the presence of a distinct head (cephalization) and tail end, facilitating more complex behaviors, such as movement and predation. For instance, cephalization is an adaptive advantage in many bilateral organisms, allowing them to process sensory information more effectively. In contrast, the other organism groups listed do not primarily exhibit bilateral symmetry. Cnidarians and sponges are primarily radially symmetrical, which suits their nature as predominantly sessile organisms. Fungi and bacteria have more varied and often asymmetric forms, while algae and prokaryotes also do not consistently demonstrate bilateral symmetry. This distinction is essential in understanding how body plans have evolved in different multicellular and unicellular organisms.