a1 The Laboratory, Marine Biological Association, Citadel Hill, Plymouth, PL1 2PB
a2 Department of Zoology, Williamson Building, Manchester University, Manchester, M13 9PL
Jet propulsion in the small siphonophores Chelophyes and Abylopsis was investigated by measuring chamber pressures and thrust exerted in tethered nectophores, and by cinephotography of free swimming colonies. Although of similar design, the two siphon-ophores swim at different speeds, and with very different chamber pressures in the locomotor nectophores. Estimates of drag incurred during swimming, and of the cost of the process in the two forms are discussed.