Pen, the internal skeleton of squid made of chitin-like material(noun)
Wiktionary.org : Text is available under the Creative Commons Attribution-ShareAlike License
Use "gladius" in a sentence
"Called the gladius, it was the classic weapon of the gladiator."
"It would be a Roman short sword called a gladius, about 27 inches long, 2 and a half inches wide, weighing three pounds, with a tapered point for thrusting and two sharp edges for cutting."
"Now, if it was a squid-like creature, there is a significant chance of finding the fossil remains of the squid pen, which as a fossil remain would be called the gladius."