1. The action of the mantle of Sepia and Loligo has been monitored under nearly natural conditions. Respiratory movements are confined to the anterior mantle whilst during jet cycles the circular muscles contract powerfully throughout the mantle.
2. Contraction of circular muscle results in thickening of the mantle and expulsion of water from the mantle cavity. Activity of radial muscles causes the mantle to become thinner and to expand in surface area so as to inhale water.
3. During such movements these two groups of muscles antagonize each other directly without the participation of a discrete skeleton.