. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 456 J. W. SANGER AND J. M. SANGER. FIGURE 9. Cross-section of the smooth adductor with surface coupling between vesicles of the sarcoplasmic reticulum (arrows) and the cell surface. Scale = ^m. vesicles and tubes of the sarcoplasmic reticulum. The area occupied by both elements of the sarcoplasmic reticulum in smooth muscle cells was about '/2% of the non-nuclear cross-sectional area of the smooth muscle adductor cell (average of 15 measurements). DISCUSSION In scallops, both cross-striated and smooth adductor muscles po
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 456 J. W. SANGER AND J. M. SANGER. FIGURE 9. Cross-section of the smooth adductor with surface coupling between vesicles of the sarcoplasmic reticulum (arrows) and the cell surface. Scale = ^m. vesicles and tubes of the sarcoplasmic reticulum. The area occupied by both elements of the sarcoplasmic reticulum in smooth muscle cells was about '/2% of the non-nuclear cross-sectional area of the smooth muscle adductor cell (average of 15 measurements). DISCUSSION In scallops, both cross-striated and smooth adductor muscles possess a lacy network of sarcoplasmic reticulum localized just under the cell surface (cf. Figs. 4 and 8). Both systems have flattened vesicles coupled to the surface cell membrane via short structures, , surface couplings (Franzini-Armstrong, 1972). In the two muscles the vesicles are interconnected via tubules. The uniform diameter of the cross-striated sarcoplasmic reticulum tubules is in contrast to the non-uniform bore of the smooth muscle sarcoplasmic reticulum. The alignment of the tubes is along the long axis of the myofilaments and cell fiber. A comparison of the two muscles in cross-sections (cf. Figs. 1 and 6) and in tangential sections (cf. Figs. 4, 10 and 8, 11) reveals at first glance that there is much more sarcoplasmic reticulum in the cross-striated muscle than in the smooth muscle. Our measurements of the sarcoplasmic reticulum demonstrated that 4% of the cross-sectional area of the cross-striated muscle is occupied by sarcoplasmic reticulum while in smooth muscle it is only The average cell size of the scallop striated muscle cell has a width of 10 urn and a thickness of yum (area 15 Mm2) while the average diameter of a smooth muscle cell was 6 yum (area : 28 Mm2). Thus the striated cells have about half the cross-sectional area of a smooth muscle cell, but eight times as much sarcoplasmic reticulum, resulting in a sixteen-fold advantage for the
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