The cytological organization of three insect visceral muscles has been examined in the electron microscope. In each instance, the fibres were found to be striated, and the striation pattern has been shown to reflect the distribution along the sarcomere of two sets of myofilaments. In transverse sections of the fibre at the level of the A band, these muscles have been found to exhibit an unusual myofilament array in which each thick (myosin) filament is surrounded by twelve thin (actin) filaments rather than six, as in insect flight muscle and vertebrate skeletal muscle. The distribution of T-system tubules and cisternae of the sarcoplasmic reticulum in these visceral fibres is described, and compared with the corresponding membrane systems in other striated muscles.

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