ABSTRACT
Electron microscopic studies on the crinoid Antedon bifida show that smooth muscle cells which have been observed previously in the oral connective tissue of the arms belong to an extended three-dimensional network of fine muscle strands. These strands can be regarded as protrusions of the somatocoelic mesothelia and are composed of contractile cells and sparse neuronal cells. The neurons and their processes contain dense core vesicles of about 75 nm diameter. In part the neurons belong to the plexus inside the coelomic mesothelia and inside the muscle strands, in part they are situated outside in the connective tissue. The functional significance of this muscular system remains unknown; possibly it may stabilize the form of the coelomic channels even during arm movements.
