1. The burrowing activity of naticid gastropods, described by use of ciné film and electronic recordings, shows four stages, together termed the digging cycle, which are repeated cyclically until burial is complete.
2. In each cycle the retraction and extension of the propodium is integrated with the alternate swelling of two regions of the foot by the movement of blood within the pedal sinus. The shell is pulled into the sand with the propodium dilated by the fluid pressure generated by the columellar and intrinsic pedal muscles. The role of water in the expansion of the foot of Polinices is discussed.
3. In a comparison of the burrowing process of naticids with that of other molluscs a general similarity is observed throughout the phylum and in other soft-bodied invertebrates. The major differences between the burrowing of gastropods and that of bivalves are discussed and indicate that in former burrowing is an adaption of normal surface locomotion, whereas the bivalves are more primitively adapted to an infaunal mode of life.