1. The stiffness (flexural rigidity) of some echinoderm sperm flagella was measured, using a flexible glass microneedle.

2. Values of 0.3-1.5 × 10−21 N m2 were obtained for the stiffness of live flagella which were immobilized with CO2-saturated sea water.

3. The immobilized live flagellum was uniform in stiffness along its entire length, except in a particular plane of imposed bending in which flexible regions were observed.

4. Demembranated flagella (Hemicentrotus pulcherrimus) in an ATP-free solution were about ten times stiffer (1.1 × 10−20 N m2) than immobilized live ones (0.5-0.9 × 10−21 N m2). The stiffness was decreased by addition of ATP to the solution and became equivalent to that of live ones when the solution contained 10 mM ATP.

5. In the demembranated flagella, the effects of ADP and ATP on the stiffness were similar. Other nucleotide phosphates and inorganic phosphate did not reduce the stiffness.

6. Young's modulus of microtubules is estimated to be 2.5 × 109 Nm2 on the basis that the microtubules have no tight connexion with one another in immobilized live flagella.

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