Shells were the mollusks’ response to what looks like an abrupt change in the lives of animals: the invention of predation. There are various ways of dealing with the fact that you are suddenly surrounded by creatures who can see and would like to eat you, but one way, a molluscan specialty, is to grow a hard shell and live within or beneath it.

P. Godfrey-Smith, Other minds: the octopus and the evolution of intelligent life (2017), loc. 550