In cephalopods, what is a pen?

The gladius (plural: gladii), or pen, is a hard internal body part found in many cephalopods of the superorder 'Decapodiformes' (particularly squids) and in a single extant member of the 'Octopodiformes', the vampire squid ('Vampyroteuthis infernalis').

It is so named for its superficial resemblance to the Roman short sword of the same name and is a vestige of the ancestral mollusc shell, which was external. The gladius is located dorsally within the mantle and usually extends for its entire length. Composed primarily of chitin, it lies within the shell sac, which is responsible for its secretion.