Abstract
The retractor muscles of Phascolosoma consist of densely packed spindle-shaped fibers which have an average length of 1 mm. and an average diameter of 6 or 7 micra. The fibers are uninucleate, the nuclei usually occupying an eccentric position midway between the fiber terminals. Fine fibrils are densely and uniformly distributed throughout the fibers. They are essentially straight parallel structures. Vertical alignment of the contraction nodes of adjacent fibers in isometrically contracted muscle produces cross-striations simulating the Q and J discs of skeletal muscle.