The muscle cells of the ventricle, the branchial heart and the branchial heart appendages ofRossia macrosoma(Delle Chiaje) are studied. The ventricle myocardium has three muscle layers, while the other two organs exhibit a loose arrangement of muscle cells. The muscle cells of the ventricle, the branchial heart and the branchial heart appendages are similar in structure. The nuclei are surrounded by myofibrils. In the myofibrils A-, I- and discontinuous Z-bands are seen. The diameters of the thick filaments are 300–400Å, their length varies from 1.7 to 3.9 μ. Thin filaments have a diameter of approximately 85Å. The ratio between thick and thin filaments is roughly 1 t
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