The
gladius (plural:
gladii), or
pen, is a hard internal bodypart 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.