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Shortening and stiffness were measured simultaneously in the aboral ligament of arms of sea lilies. Arm pieces were used from which oral tissues (including muscles) were removed, leaving only collagenous ligaments connecting arm ossicles. Chemical stimulation by means of artificial seawater with an elevated concentration of potassium caused both a bending movement and stiffness changes (either softening or stiffening). The movement lasted for 1.5-10 min, and bent posture was maintained. The observation that contraction was not necessarily associated with softening provided evidence against the hypothesis that the shortening of the aboral ligaments was driven by the elastic components that had been charged by the oral muscles and released their strain energy at the softening of the aboral ligaments. The speed of ligamental shortening was slower by at least one order of magnitude than that of muscles. Acetylcholine (10(-5)-10(-3) M) caused both contraction and softening. We conclude that the aboral ligament shows two mechanical activities based on different mechanisms: one is active contraction and the other is connective tissue catch in which passive mechanical properties show mutability. We suggest that there is neural coordination between the two mechanisms.

Contraction and Stiffness Changes in Collagenous Arm Ligaments of the Stalked Crinoid Metacrinus rotundus (Echinodermata)