The pen of the squid stabilizes the mollusk while it is swimming. The pen is a stiff structure running the length of the animal's body.
Squid are mollusks and are related to animals with shells such as snails. In the squid, the shell has been modified into the pen, a strip of cartilage buried inside the flesh of the animal. Other close relatives of the squid include the cuttlefish, the nautilus and the octopus, which all have different stabilizing, or supporting, structures . The nautilus retains an external shell, the cuttlefish has a pen made out of a hard bone-like material and the octopus has lost all traces of a shell.