Which phylum includes insects, arachnids, and crustaceans?

Which phylum includes insects, arachnids, and crustaceans?

• A. Annelida
• B. Arthropoda
• C. Chordata
• D. Echinodermata

Answer: (B)

All three groups are arthropods, a phylum defined by jointed appendages, a segmented body, and an external skeleton made of chitin. This combination is the hallmark that ties insects, arachnids, and crustaceans together, even though they differ in specifics (insects with three body parts and six legs; arachnids with two main body regions and eight legs; crustaceans with varied forms and many paired appendages). Because they share these distinctive traits, they are classified in the same phylum. The other options describe different phyla: Annelida are segmented worms without an exoskeleton or jointed limbs; Chordata includes animals with a notochord and dorsal nerve cord; Echinodermata comprise marine animals with radial symmetry and a calcareous endoskeleton, not a chitinous exoskeleton and jointed limbs.

All three groups are arthropods, a phylum defined by jointed appendages, a segmented body, and an external skeleton made of chitin. This combination is the hallmark that ties insects, arachnids, and crustaceans together, even though they differ in specifics (insects with three body parts and six legs; arachnids with two main body regions and eight legs; crustaceans with varied forms and many paired appendages). Because they share these distinctive traits, they are classified in the same phylum. The other options describe different phyla: Annelida are segmented worms without an exoskeleton or jointed limbs; Chordata includes animals with a notochord and dorsal nerve cord; Echinodermata comprise marine animals with radial symmetry and a calcareous endoskeleton, not a chitinous exoskeleton and jointed limbs.