Here’s a pop quiz for you: How many tentacles does an octopus have? If you said “eight,” sorry, but you fail. An octopus does have eight limbs. But technically, they’re known as arms, not tentacles.

An octopus is a cephalopod—a group that includes squid, cuttlefish, and nautilus. Each of them has a whole bunch of limbs—from eight for the octopus, to more than 90 for the nautilus. The animals use those limbs to look for and catch prey, to move along the sea floor, and even to build houses.

An arm is a limb that’s covered with suction cups from beginning to end. A tentacle has suckers only at its end, which is usually wide and heavy.

Each arm of an octopus contains its own brain. That means the arm can operate on its own, or it can be controlled by the central brain. An octopus uses its arms to build its own housing and to climb in and out of shells and other objects. It uses two arms to crawl along the bottom, so those arms are sometimes called legs.

Squid and cuttlefish each have eight arms and two longer tentacles. Cuttlefish fire out their tentacles at high speed to catch prey. And the tentacles of some squid have sharp hooks to help them snag prey in open water.

The nautilus, which lives in its own shell, is different from its cephalopod cousins. It can have more than 90 limbs. They don’t have suckers, but they’re still called arms. Sharp features on the arms extend to catch prey— “arming” the nautilus for survival on the ocean floor.