What term describes animals that display the dorsal hollow nerve cord, notochord, pharyngeal gill slits, and a post-anal tail at some developmental stage?

• A. Chordates
• B. Echinoderms
• C. Arthropods
• D. Mollusks

Answer: (A)

Chordates are animals defined by having four hallmark features at some point in their development: a dorsal hollow nerve cord, a flexible notochord, pharyngeal gill slits, and a post-anal tail. The dorsal hollow nerve cord runs along the back and forms from ectoderm; the notochord provides axial support and is often replaced by a vertebral column in many lineages; pharyngeal gill slits function in filter feeding or gas exchange in aquatic relatives; and the post-anal tail extends beyond the anus and aids movement. Even when some features are reduced or only present during embryonic stages—as in humans, who retain only tiny tailbud remnants—the presence of these traits at development marks chordates. Other major animal groups, like echinoderms, arthropods, and mollusks, do not exhibit this combination of features in development, so they are not chordates.