Which phylum includes snails, clams, and octopuses?

• A. Nematode
• B. Arthropod
• C. Mollusk
• D. Annelid

Answer: (C)

Snails, clams, and octopuses all share a common body-plan that defines their bigger group: a soft, usually invertebrate animal with a mantle, a muscular foot used for movement, and a visceral mass that contains most of the internal organs. Many mollusks produce a calcium carbonate shell, though some, like octopuses, have lost the external shell over evolution. This combination of features places them in the same phylum, Mollusca, which includes gastropods (snails), bivalves (clams), and cephalopods (octopuses).

Nematodes are roundworms with a simple, unsegmented body; arthropods have segmented bodies, an exoskeleton, and jointed legs; annelids are segmented worms. None of these groups encompasses all three creatures together, which is why mollusks is the correct classification.