Analysis of ciné records indicates that the locomotion of Actinosphaerium eichhorni and Actinophrys sp. includes a definite rolling motion, in addition to evident horizontal and vertical displacements. Such movements could be correlated with significant changes in the lengths of supportive axopods, but not with axopodial rowing or sliding movements. The data also do not support a model of locomotion based simply on those systematic shifts in the cell's centre of gravity that would be caused by sequential collapse of supportive axopods. Although active bending of attached axopods cannot be discounted, locomotion would seem to result from forces generated between the cytosome and substratum by attached axopods undergoing changes in length. The observations suggest, moreover, that axopodial retraction is more important than elongation in the generation of motive force.

It is proposed that the relative magnitude of each locomotory component is determined by the dimensional parameters of the particular species. As a consequence, changes in axopodial length can account for both the ‘rolling’ and ‘gliding’ behaviour reported in the literature.

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