Blood pressure was measured at both ends of the heart of Pyura praeputialis (Heller) after removing the tunic.

For posterior anterior heart waves average upstream pressures were 23–25 mm H2O (positive): corresponding downstream pressures averaged 8 mm H2O (negative). For anterior posterior waves average upstream pressures were 17–18 mm H2O (+) and downstream values were 7–8 mm H2O (-). Maximum pulse amplitudes recorded were about 30 mm H2O (upstream).

Speed of the peristaltic wave was 25-31 mm/s. In one experiment the speed was demonstrably different over the two halves of the heart (48 mm/s over the rear half and 29 mm/s over the front half).

Number of peristaltic waves per series (i.e. between successive reversals) varied from 20 to 178. Duration of each series varied from 120 s to 690 s. Wave frequency ranged from 8 to 21 per min. Reversal frequency ranged from 5 to 30 reversals per hour.

Most preparations showed periods of reduced heart activity (“rest periods”) during the the 2 4 h of the experiment. All showed spasmodic contractions of the mantle muscles which caused pressure “surges” in the vascular system.

It is shown that, in both directions of beat, most or all of the pressure wave is contributed by the front half of the heart (half towards which peristaltic wave is travelling). This can be related to the “reversed spiral” structure of the heart: each “half” of the heart (i.e. each spiral) serves primarily as the pump for one direction.