1. Recordings have been made of the pressures in the mantle cavity of some coastal cephalopods, both at rest and while swimming, under conditions as near normal as possible. Pressures of up to 180 cm. of water were developed by Sepia officinalis (250 g. weight), 300 cm. by Loligo vulgaris (350 g.) 170 cm. by Octopus vulgaris (370 g.) and 400 cm. by Eledone moschata (600 g.).
2. The momentum produced by the efflux of the jet of water from the mantle cavity was recorded on an isometric myograph, attached to the head of the animal by a thread, as a tension. The swimming tensions, derived from maximum jet pressures, were in general equivalent to the body weight in Loligo, Sepia and Eledone but in Octopus never exceeded half body weight.
3. In Octopus, however, the arms are powerfully developed and, using five arms for attachment to the side of the tank, they can exert holding tensions of up to 100 times their body weight. In an Octopus of 1 g. body weight this is equivalent to a tension of 2 kg./cm.2 in the longitudinal muscle at the base of the arms.
4. Comparisons of the tensions and pressures obtained in simultaneous recordings during jet swimming showed, that, with the exception of Octopus, the tension developed is generally equal to twice the cross sectional area of the jet multiplied by the pressure.
5. The theoretical maximal velocity for a single jet cycle in Loligo and Eledone was in accord with observed velocities and the much lower theoretical velocity of Octopus is discussed.