A spasmoneme spring.
 

            

(A) The spasmoneme in Vorticella, shown in its fully extended (left), fully contracted (middle), and partially extended (right) states (52). The body measures 50 µm across.

(B) The extended state (left) consists of aligned filaments held apart by negative charges (blue).
Calcium (red) neutralizes the charge, and the filaments condense (right)  into a rubberlike material.
 

 L. Mahadevan  and P. Matsudaira,
Motility Powered by Supramolecular Springs and Ratchets
Science, Volume 288, Number 5463, Issue of 7 Apr 2000,  pp. 95-99.