What animal group includes octopuses, squid, and cuttlefish?

The animal group that includes octopuses, squid, and cuttlefish is the cephalopods.

Cephalopoda is a class within the mollusks, the broader group that also includes snails, clams, and slugs. Cephalopods are marine animals with prominent heads, bilateral symmetry, and arms or tentacles modified from the ancestral molluscan foot. Most can propel themselves with a siphon and many can release ink when threatened.

Octopuses, squid, and cuttlefish belong to the subclass Coleoidea. The nautilus is also a cephalopod, but it belongs to the separate subclass Nautiloidea and retains an external chambered shell. That is why “mollusks” is too broad, while “crustaceans” is incorrect: crustaceans include crabs, lobsters, and shrimp, which are arthropods.

Cephalopods have a long evolutionary history. Their earliest known representatives appeared more than 400 million years ago, and living species occupy habitats from near the surface to the deep ocean. Their sophisticated eyes, nervous systems, camouflage, and flexible arms make them especially important in studies of animal behavior and intelligence.