Catch in the primary spines of the sea urchin Eucidaris tribuloides: a brief review and a new interpretation.

Previous models of reversible catch in echinoid spines, as a property of muscle or of collagen, are briefly reviewed and discussed. This brief review offers a new interpretation of catch in primary spines of Eucidaris tribuloides, viewing the collagen and small muscles of the catch ligament working together as a variable-length tendon. In the model presented, changes in ligament length when out of catch are accommodated by sliding of discontinuous, interdigitating and cross-link-stabilized columns of collagen fibrils, the muscle layer external to the ligament effecting spine movement. Catch is viewed as a consequence of contraction of small muscles inserted on the collagen columns within the ligament. Ligament shortening tightens the profuse (ca. 30,000/mm2) and highly ordered collagen insertion loops within the stereoms of the spine base and test, and catch results from the multiplicative effect of these friction sites in series. New data are presented on novel structural cross-links between collagen fibrils. The cross-links stabilize the ligament columns. The central ligament in Eucidaris plays a purely passive mechanical role in maintaining the alignment of the spine-test articulation. It contains no muscle and neither contracts nor undergoes catch; its insertions are simple, unlike the complex stereom insertions of the main ligament.