Stalk contraction and zooid contraction of living Vorticella convallaria were studied by high-speed video cinematography. Contraction was monitored at a speed of 9000 frames per second to study the contractile process in detail. Complete stalk contraction required approximately 9 ms. The maximal contraction velocity, 8.8 cm/s, was observed 2 ms after the start of contraction. We found that a twist appeared in the zooid during contraction. As this twist unwound, the zooid began to rotate like a right-handed screw. The subsequent stalk contraction steps, the behavior of which was similar to that of a damped harmonic oscillator, were analyzed by means of the equation of motion. From the beginning of stalk contraction, the Hookean force constant increased, and reached an upper limit of 2.23 x 10(-4) N/m 2-3 ms after the start of contraction. Thus, within 2 ms, the contraction signal spread to the entire stalk, allowing the stalk to generate the full force of contraction. The tension of an extended stalk was estimated to be 5.58 x 10(-8) N from the Hookean force constant of a stalk. This value coincides with that of the isometric tension of a glycerol-treated V. convallaria, confirming that the contractile system of V. convallaria is well preserved despite glycerol treatment.

Using glycerinated spasmoneme of giant Zoothamnium sp., the physical properties of spasmoneme before and after Ca2+-induced contraction (pCa 4.5) were investigated. The volume change of spasmoneme contraction was measured under zero tension. The length and diameter decreased by about 50% of their initial value as a result of contraction, which means that contraction is nearly isotropic. Thus the volume of spasmoneme decreased drastically by 86% of its original value. The swollen ratio of extended and contracted spasmoneme were 0.07 and 0.37, respectively. Tension-extension relationships of extended and contracted spasmonemes were measured. By applying the theory of rubber elasticity, the number of segments of a chain in originally extended spasmoneme was only 3.3, i.e., the chain was almost a straight one. On the other hand, the number of segments of a chain in contracted spasmoneme was more than 100, i.e., the chain was essentially a random one. Furthermore, the total number of chains in single spasmoneme was the same in extended and contracted spasmoneme. This means that the interchain cross-links of chains were not influenced by addition or removal of Ca2+. Moreover, the molecular weight of a chain is estimated to be at most about 50 kd. By considering all these results, it is concluded that the contractile mechanism of spasmoneme originates in the intramolecular folding and unfolding induced by Ca2+ binding and detaching.

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The amino acid composition and hydrophobicity scale (hydropathy) of calcium‐binding proteins contained in the contractile spasmoneme of Carchesium polypinum was compared with other calcium‐binding proteins from eukaryotes. Spasmins which may hind at most 4 calcium ions simultaneously and initiate spasmoneme contraction cooperatively belong to a super family of proteins including; centrin/caltractin and calmodulin. Based on chemical modification of tryptophan and methionine, these residues are involved in contraction but the spasmin proteins contain little or none of these amino acids. Based on this evidence, it is suggested that another, non‐calcium binding protein(s) is involved in spasmoneme contraction.