Fine structure and innervation of an annelid muscle with the longest recorded sarcomere

The fine structure of myoepithelial cells of the proventriculus of a marine annelid ( Syllis spongiphila ) is described. The contractile system of these muscle fibers includes a single medial Z band. The thick filaments possess a ~140 A paramyosin-like periodicity, but the filament disposition in these cells corresponds to that of other striated muscles, and actin orbitals in the A band number up to 20. The distance between the centers of the two H bands in the largest cells is ca . 40 µ Dyads involving T-system invaginations and isolated vesicles of the sarcoplasmic reticulum are situated at all sarcomere levels. Insertions of the contractile material onto invaginations of the inner and outer cell surfaces are described. Presumed polyaxonal neuromuscular junctions are established across wide synaptic clefts and include terminals with spherical and non-spherical synaptic vesicles. Inclusions occupying the core of each muscle cell appear to be rich in magnesium.