THE HISTOLOGY OF THE RETRACTOR MUSCLES OF THYONE BRIAREUS LESUEUR

¹ From the Marine Biological Laboratory, Woods Hole and the Department of Zoölogy, University of Minnesota

The retractor muscles of Thyone consist of long spindle-shaped smooth muscle fibers. The fibers appear to be uninucleate. The nuclei usually occupy a lateral position and are attached to the fiber by a delicate layer of cytoplasm. Occasional nuclei are found completely within the fibers.

In the center of the muscle the fibers occur in bundles of two to fifteen. No such organization is exhibited in the periphery of the muscle.

The fibers do not remain associated with a single bundle throughout their length but pass freely from one bundle to the other.

Measurements of fiber length were not obtained, but it is evident from study of teased preparations that they. do not extend the full length of the muscle. Fiber diameter varies from 5-10 micra in contraction to 2-4 micra in extension.

The connective tissue forms a lattice-like network which is concentrated about each bundle and in the periphery of the muscle. The heaviest fibers of the reticulum tend to run perpendicular to the axes of the muscle fibers.