A light and electron microscopic examination of muscles in the walking legs of the horseshoe crab, Limulus polyphemus (L.)

Abstract

An examination was made of the gross morphology and ultrastructure of skeletal muscles in the legs of the horseshoe crab, Limulus polyphemus (L.). The origins, insertions, actions, and innervation of each head of the seven distal leg muscles are described. A detailed description of the transverse tubular system and the sarcoplasmic reticulum is also given. Of special interest is the possibility that the muscle fibers may be split into "units" of different diameters.The sarcomere length of fibers in the seven muscles was determined to see if both long and short sarcomere length fibers are present. The distribution of fiber sarcomere lengths in each muscle is unimodal, and mean values for each of the different muscles range from 7.6 to 8.5 μ. The ultrastructural features of fibers in each muscle also are similar. Therefore, it appears that horseshoe crab leg skeletal muscles are composed structurally of a fairly uniform population of fibers and that fast and slow categories of fibers, at least as defined for other arthropods, are not present in the horseshoe crab muscles studied here.