The organization and fine structure of the muscles of the scolex of the cysticercoid of Hymenolepis microstoma

The organization and fine structure of the muscles of the scolex of the cysticercoid of Hymenolepis microstoma are described. The contractile apparatus consists of thick (175–325 Å diameter × 1.4 μm) and thin (60–80 Å diameter × 1 μm) filaments. The thick filaments are occasionally attached to the thin filaments by cross bridges. The thin filaments are attached to the dense bodies or to a dense zone at the sarcolemma at muscle insertions. In contracted muscle the thick filaments appear as quasi‐hexagonal arrays or in lines. Each thick filament is surrounded by an orbit of up to 12 thin filaments, which in turn may be shared by adjacent thick filaments. Thin filaments may be present in quasi‐rectangular or hexagonal groupings indicating some low order degree of actin lattice. The fusiform dense bodies (1,500 Å × 900 Å), consisting of up to 25 discrete substructures, are distributed uniformly throughout the myofiber and/or attached to the sarcolemma at attachment plaques. The sarcoplasmic reticulum, consisting of a presumed anastomosing network of tubules is structurally connected to the sarcolemma by periodic deposits of electron opaque material. Sarcoplasmic extensions of the myofiber(s) contain the nucleus, Golgi complexes, rough endoplasmic reticulum, ribosomes, β‐glycogen, mitochondria and membrane bound electron dense structures. Upon activation of the metacestode, groups of α‐glycogen and enlargement of the rough endoplasmic reticulum were observed. Microtubules which were conspicuously absent from the sarcoplasm of the unactivated worms appeared adjacent to the myofibers in activated worms.