Morphology and histology of the disc autotomy plane in Ophiophragmus filograneus (Echinodermata,Ophiurida)

Summary Normal and autotomized specimens of Ophiophragmus filograneus were studied by gross dissection, light microscopy, transmission electron microscopy, and scanning electron microscopy to determine how autotomy occurs at the major region of disc attachment. Each of the 10 genital bars is attached to an arm by a broad ligament consisting of a thick outer layer of collagenous connective tissue and an inner layer of neurosecretory cells innervated by a lateral ectoneural branch of the radial nerve. Neurosecretory cell processes extend into the collagenous layer. During autotomy the collagenous fibers separate, weakening the ligament near its insertion along the genital bar. Collagen fiber separation is probably caused by neurosecretions. There is no evidence for ossicle dissolution. The results indicate that the histological organization of the genital bar ligaments is identical to that of mutable collagenous tissues in other echinoderms and that the autotomy process is similar to that described for other echinoderms.