Pearls are sometimes produced when an irritation enters the mantle cavity and covered by the pearly layer secreted by the mantle. The oldest part of the shell is the area surrounding the hinge, called the umbo . Squids, octopuses, cuttlefish, and Nautiluses are examples of this class. All members of this class are marine. Squid and octopuses are among some of the most intelligent of the invertebrates .
All cephalopods are carnivorous.
They contain tentacles and a central mouth containing a sharp beak. Many of these organisms contain a large ink sac that is used when trying to escape from potential predators. They have well developed eyes and brains. They contain special pigment cells called chromatophores which enable them to change color. They have gills in the mantle cavity.