ABSTRACT
In the ophiuroid arm the vertebral ossicles of adjacent segments are interconnected by four muscle blocks. During arm autotomy in Ophiocomina nigra (Abildgaard) and most other species so far examined, the aboral (upper) intervertebral muscles in the autotomising segment always part cleanly from the ossicle at their distal insertion, and the oral (lower) muscles always part from the ossicle at their proximal insertion. The muscles are linked to the vertebrae by connective tissue fibres which are strongly PAS-positive and show hyaluronidase-labile gamma metachromasia with toluidine blue, indicating a rich content of glycoproteins and glycosaminoglycans respectively. In the electron microscope they appear finely granular and contain aligned microfilaments 2 - 3 nm in diameter.
