Cephalopods originated from a form close to the primitive gastropod by the forward growth of the so-called molluscan foot. The structure of the archimollusc was quite simple. It had a cap-like shell secreted by a layer of tissue known as the mantle. The archimollusc was an animal that moved over the seabed by rhythmic contractions of its ventral muscles.

Figure 3: The class Cephalopoda is named from the close union of the head with the foot, which evolved and subdivided to produce two new types of organs.

The emergence of the cephalopods represents a more important adaptive step towards being a free-swimmer. The direct success of class Cephalopoda emerges from the hydrostatic system which ensures the organism's buoyancy, and the acquisition of a chambered shell and siphon are the features that ensures this. The mantle cavity is a constant feature of all mollusks and has been put to various uses. The outlet of the mantle has undergone a differential modification, most markedly in the cephalopods where water, taken into the mantle cavity and squirted out as a jet through a funnel, is the primary means of propulsion.