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This paper describes the locomotion of salps by jet propulsion, from a combination of measurements of chamber pressures, static thrust, and electromyographic activity, with kinematic records of free‐swimming and tethered salps. From such measurements, estimates are made of the thrust exerted, the drag incurred, and the work performed by single salps, and by chains of linked individuals. It is concluded that salp jet propulsion is a more economical process than is jet propulsion in other animals.

The digging activity of Ensis arcuatus shows six stages, together termed the 'digging cycle' which are repeated cyclically and are similar to those of other burrowing bivalves. A digging cycle involves the integration of pedal protraction and retraction with the opening and closing of the valves, much of the musculature of the body playing a part in each cycle. Extension and probing of the foot involves only the intrinsic pedal musculature which generates low pressures (10 cm) in the pedal haemocoele. The hinged shell acts as the basis of a fluid/muscle system which allows the strength of adduction to be used in digging and consists of two separate fluid filled chambers, the haemocoele and the mantle cavity. Adduction of the valves generates high pressures (100 cm) in each equally and simultaneously. In the haemocoele this pressure gives rise to the characteristic bulbous form of the foot which ensures a secure anchorage so that at pedal retraction the shell is drawn down. In the mantle cavity the pressure produces powerful jets of water which assist movement of the shell by loosening the adjacent sand. Estimates of the muscle tensions indicate that the pressures recorded may be obtained at tensions of not more than 2 Kg/cm 2 .

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When bivalves burrow into soft substrates the foot is first extended and then dilated to obtain a firm anchorage before retraction pulls the shell downward. Pedal dilation is principally caused by adduction of the valves. The hinged shell futnctions as a hydraulic machine in which the strength of the adductor muscles is transferred to the distal part of the foot by means of the body fluids.