A model of contractile tubules showing how they contract in the heliozoan Echinosphaerium.

In the large heliozoan Echinosphaerium, contractile tubules (formerly called X-bodies) are located between the axopodial membrane and the axonemal microtubules. When axopodial contraction occurs, the tubules have been thought to be transformed from a tubular to a granular form, as seen in ultra-thin sections. Our detailed morphological observations of the contractile tubules, however, have revealed that this so-called granulation of the contractile tubules is mediated by self-twisting and supercoiling during contraction. We also examined the localization of calcium during axopodial contraction using a potassium pyroantimonate assay. Ca-Sb deposits were detected on contractile tubules only during the twisting and coiling of this organelle. Our results indicate that axopodial contraction is enforced by the twisting and coiling of contractile tubules, which action probably is mediated by Ca2+ ions.