What do pens in squids evolve from?

Pens in squids are believed to have evolved from the internal shells of their ancestors, which were similar to those of modern-day cuttlefish and other cephalopods. These structures provided buoyancy and support, gradually becoming reduced and more streamlined as squids adapted to a fully aquatic lifestyle. The pen serves as a rigid support structure for muscle attachment and helps maintain the squid's body shape, facilitating efficient movement in the water.