USA Biology Olympiad (USABO) Practice Exam

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The subkingdom Eumetazoa branches into which two taxonomic clades?

Radiata and bilateria.
Invertebrates and vertebrates.
Chordates and non-chordates.
Prokaryotes and eukaryotes.

The correct answer is: Radiata and bilateria.

The subkingdom Eumetazoa is known for its distinct tissue organization and complexity compared to other animal groupings. It diverges into two major taxonomic clades: Radiata and Bilateria. Radiata includes organisms that exhibit radial symmetry, such as cnidarians (jellyfish, corals, and sea anemones). These animals are characterized by their body plan, which can be divided into similar halves around a central axis. In contrast, Bilateria encompasses organisms with bilateral symmetry, where the body can be divided into mirror-image halves along a single plane. This group includes a vast array of animals, ranging from flatworms to mammals, and is recognized for more complex body structures and organ systems. Understanding the distinction between these two clades is fundamental in studying the evolutionary relationships among animals, as it reflects key developmental and anatomical differences that arose early in animal evolution.