Does any major group of animals include members as strikingly different as molluscs? Think of a cockle and an octopus. The former spends its life within a shell buried in sand, where it filters microscopic food, surfacing only to cast its eggs or sperm into the plankton. Nothing it does requires a brain, or even a head – and indeed it has neither. An octopus, on the other hand, is an agile, inquisitive, dextrous, colour-changing, shape-shifting genius.
Most molluscs belong to one of three classes. Bivalves (cockles, clams, mussels, oysters and the like) are characterised by a two-piece hinged shell, and are mostly sedentary filter-feeders. Gastropods (snails, slugs, conches etc) usually possess a single, coiled shell and are mobile grazers or predators. Then there are the cephalopods, an exclusively marine group comprising octopuses, cuttlefish, squids and nautiluses, as well as the extinct ammonites and belemnites. Like other molluscs, cephalopods are rubbery, non-segmented animals usually equipped with some sort of shell. But they have taken the basic molluscan body plan and transformed it almost beyond recognition.
Let’s start with the shell. Among living species, only the ammonite-like nautiluses have a conventional external shell. In squid, it has been reduced to a long, flattened internal structure called a ‘pen’, which stiffens the body. In cuttlefish, it is a larger, lightweight, chalky internal structure involved in maintaining buoyancy. Most octopuses, meanwhile, have no shell at all.
A mollusc’s shell is secreted by an organ called the mantle, visible as fleshy lips around the shell’
