The fine structure of the abdominal musculature of the copepod Macrocyclops albidus was investigated by electron microscopy. Tubules penetrate into the muscle fibers from the sarcolemma, continuity between the wall of the tubules and the sarcolemma being clear. A dense network of tubules envelops the myofibrils, its interstices being occupied by cisternal elements. At the Z lines the tubules traverse the interior of myofibrils, giving off branches which course longitudinally within the substance of the myofibrils. These branches are also accompanied by elongate, non-intercommunicating cisternae. Comparison of this fast acting copepod muscle with other vertebrate and invertebrate muscles indicates that the complexity of the tubular system is a function of the myofibrillar geometry, whereas the degree of development of the cisternal system is related to the contraction speed of the muscle.

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