1. Continuous recordings of the hydrostatic pressure in the coelom of Arenicola marina show a resting pressure of about 2 cm. of water in a non-burrowing worm. During burrowing a series of pressure peaks is produced and these gradually increase in amplitude up to 110 cm. as burrowing progresses.
2. The pressure peaks are of 2 sec. duration, occur at intervals of 5-7 sec., and for each there is a major contraction of the circular muscles followed by the shortening of the longitudinal muscles. The main power stroke in producing the high pressure is the contraction of the longitudinal muscles of most of the trunk segments. The sequence of muscular contractions and the phases of burrowing are considered.
3. The pressure is utilized at the anterior end of the worm both to aid passage through the sand and to anchor the head while the posterior segments are pulled into the burrow.
4. At maximum pressures the tension developed in the circular muscle of the body wall is estimated to be 3 kg./cm.2, while the resting pressure corresponds to less than 7% of this.