External shells are absent on the Dibranchia . The oldest known Dibranchia are the fossil Belomoids . The internal shell here was much thickened at the apical and lateral walls and in some the anterior dorsal wall projected over the viscera. In the Spirula, a common worldwide deep-water squid, the shell is internal. It is a loosely coiled tube divided by septa into small chambers.
All cephalopods swim by rapidly expelling water from their mantle cavity.
The squids are highly specialized at swimming, and bear a pair of posterior, lateral fins that act as stabilizers. The mantle contains both longitudinal and circular muscle fibers. On inhalation, the circular muscles relax and the longitudinal muscles contract enlarging the mantle cavity. This creates a suction and pulls in water through the space located between the anterior edge of the mantle and the head. When the mantle cavity is full the increasing pressure created by the water causes the circular muscles to contract that in turn closes the opening through which the water entered.