Figure 6. A schematic diagram of the two types of isocrinid crawling. In the top panel (6.1. finger-tip pull), movement of the arm tip is generated by aboral flexure through an angle ₁ occurring on a distal articulation, producing a forward displacement, X₁. In the bottom panel (6.2. elbow-crawl), movement of the arm tip is generated by aboral flexure through an angle ₂ occurring on a proximal articulation, producing a forward displacement, X₂. Assuming a constant rate of flexure, elbow-crawl results in a higher horizontal velocity.