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In the early years of this century Vera t t i (15) and a few other uncommonly observant cytologists (11) demonst ra ted in the sarcoplasm of striated muscle a delicate plexus of strands sur rounding all of the myofibrils. The transverse elements of this network were located in a constant relat ion to the crossbanded pat tern of the myofibrils. The majori ty of cytologists of tha t period seem not to have accepted the reality of this network, possibly because of their mistrust of the capricious metallic impregnat ion methods required for its demonstration. Therefore, it received little fur ther study unti l some 50 years later when Bennet t and Porter (3) and Andersson (1) directed a t tent ion to a vacuolar or tubu la r interfibri l lar componen t of the sarcoplasm visible in electron micrographs of striated muscle. Extending these observations, Porter and Palade (9) later demonst ra ted tha t these membranous profles are par t of an elaborate reticular system of anastomosing tubules tha t surrounds all the myofibrils. The majori ty of the tubu la r elements of this sarcoplasmic reticulum run longitudinally, bu t there is, for each muscle, a characterist ic repeat ing pa t te rn of transverse channels occurr ing in register with par t icular bands in the cross-striations of the myofibrils. These transverse components , called the " t r iads" of the ret iculum, consist of pairs of parallel tubes separated by an intermediate row of small vesicles. The continuous na ture of the ret iculum, its in t imate relat ion to the myofibrils and to the cell surface have led to the suggestion tha t the membranes of this system may be involved in impulse conduct ion from the sarcolemma to the contractile elements in the interior of the muscle fiber (4, 7, 9). Compara t ive studies by Peachey and Porter (7) correlat ing cell size, speed of contraction, and degree of development of the reticulum have provided evidence tending to support this hypothesis. The investigation reported here extends these comparat ive observations on the sarcoplasmic re t iculum to a par t icular ly fastacting fish muscle. A n u m b e r of teleost fish including the sea robins (Prionotus), the toadfish (Opsanus), the croakers (Micropogon) and drums (Sciaenops) are capable of making clearly audible sounds by a variety of mechanisms. The deep resonant sounds of the toadfish are made by rapid contract ion of the intrinsic striated muscle in the taut wall of its gas-filled swim-bladder (14). In recent studies of the neuromuscular mechanism of sound production in this species, Skoglund (12) has shown tha t the muscle involved reaches its contract ion peak and returns to complete relaxat ion very much more rapidly than do most ver tebra te muscles. The present paper describes an elaborate development of the sarcoplasmic re t iculum and other fine structural features of this muscle which may be related to its unusual physiological properties.

THE SARCOPLASMIC RETICULUM OF A FAST-ACTING FISH MUSCLE