Cephalopods had a gland that produced the shell, and the evolutionary trend for cephalopods during the Devonian was a reduction in shell size.

One of three reductions occurred; the shell either:

The shell became compressed to increase the hydrodynamic efficiency of the shell, which in turn would have made the cephalopod more suited for rapid axial movement, even though this would have compromised its strength. Cuttlefish have a remnant of their shell inside their body, while octopi and squids no longer have a shell.

Figure 14: A Belemnite.
Their shell was compromised into an orthoconic form.
Figure 15: Octopus rubescens. Octopi no longer have a shell and they secrete a poison to evade off predators. Figure 16: A cuttlefish. They still have a remnant of their shell inside their body.

© Sarah Vandervlugt.1998