MORPHOLOGICAL BASIS AND MECHANICS OF ARM MOVEMENT IN THE STALKED CRINOID METACRINUS-ROTUNDUS (ECHINODERMATA, CRINOIDA)

We obtained live specimens of the stalked crinoid Metacrinus rotundus from Suruga Bay, Japan, in 1992 and managed to keep them for several m onths in aquaria. Video observations of crawling individuals showed th at both the aboral bending of the arm and the oral bending can be powe r strokes. When climbing up a lattice, the crinoids bend their arms ab orally to pull the body upward. The oral side of the arm contains musc les, but the aboral side has ligaments without muscles. We found that ligaments on the aboral side are divided into two parts, the aboral li gament and the newly found fossa ligament. Ultrastructurally, the abor al ligament resembles typical echinoderm catch connective tissue. Its microfibrils exhibit a variable banding pattern after special staining and might be an elastic material. The fossa ligament contains only co llagen fibrils which are often closely connected. Biomechanically the frozen and rethawed aboral ligament behaves like a spring. It shows be tween 2.4 and 29.1% stress relaxation, and it is so stiff that the ora l muscles would not be able to stretch it. We suggest that the connect ive tissue of the ligament is a catch connective tissue and softens wh en the arm moves. During long-term filtration posture the ligament wou ld stiffen so as to maintain its posture for a long time without muscl e contraction and thus without using much energy.