1. The locomotion of Patella has been studied by histological, photographic and experimental techniques. The foot consists principally of dorso-ventral and transverse muscles and has no longitudinal muscle fibres near to its sole. The pedal haemocoel is limited to a region of small spherical cavities, extending for 0.5 mm. above the sole, two pedal sinuses and several lateral vertical channels.

2. Patella moves forward by means of retrograde alternate ditaxic pedal locomotory waves, but during rarely observed backward movement the waves are direct. During tight turns the limpet uses forward locomotion on one half of the foot and backward locomotion on the other.

3. During the passage of a retrograde locomotory wave the foot is lifted off the substrate by about 0.2 mm. and the pressure beneath the sole falls by about 6 cm. of water.

4. A model is proposed to account for locomotion utilizing the dorso-ventral muscles as the sole propulsive agents in the hydraulic system of the foot. This system consists principally of the dorso-ventral and transverse muscles, the spherical cavities of the pedal haemocoel and the fluid enclosed beneath each locomotory wave. Both fluid systems may be utilized during contraction and relaxation of different groups of dorso-ventral muscles.

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