Which of the following is NOT a cephalopod?

Which of the following is NOT a cephalopod?

• A. Octopus
• B. Squid
• C. Cuttlefish
• D. Jellyfish

Answer: (D)

Cephalopods are mollusks defined by a distinct head region, a muscular mantle, and arms or tentacles around the mouth, often with jet propulsion via a siphon, plus a highly developed nervous system. Octopus, squid, and cuttlefish are classic examples that fit this pattern, with complex brains and specialized body plans.

Jellyfish, however, come from a different branch of life, the cnidarians. Their bodies are based on a simple, radial symmetry with two tissue layers (diploblastic) and a gastrovascular cavity, lacking a true head, a centralized brain, or a mantle. They have tentacles with stinging cells but no arms, no molluskan shell or mantle, and no closed circulatory system like cephalopods have.

Because of these fundamental differences in body plan and classification, jellyfish are not cephalopods.