Which organism is not an arthropod?

Which organism is not an arthropod?

• A. Sea Star
• B. Spider
• C. Crab
• D. Ant

Answer: (A)

Arthropods are animals with segmented bodies, an external exoskeleton made of chitin, and jointed appendages. Sea stars do not have these features; they are echinoderms, a different group characterized by radial symmetry and a water vascular system used for movement and feeding, with an internal skeleton rather than a hard external one. Spiders, crabs, and ants all display the arthropod pattern—segmented bodies, external exoskeletons, and jointed legs or appendages—so the sea star is the non-arthropod in this set.

Arthropods are animals with segmented bodies, an external exoskeleton made of chitin, and jointed appendages. Sea stars do not have these features; they are echinoderms, a different group characterized by radial symmetry and a water vascular system used for movement and feeding, with an internal skeleton rather than a hard external one. Spiders, crabs, and ants all display the arthropod pattern—segmented bodies, external exoskeletons, and jointed legs or appendages—so the sea star is the non-arthropod in this set.