. Electro-physiology. Electrophysiology. 20 ELECTRO-PHYSIOLOGY CHAP. catory muscles of many molluscs are yellow, pink, and even deep red. Knoll discovered a remarkable instance of " plasmic " muscle- cells among Invertebrates, in the thin muscle-bands of the mantle of Salpa (S. maxima, africana, pclesii). The cross-striped, cylin- drical fibres are very long, with conical ends. They are easily split up longitudinally into finer bundles and fibrils, owing to the excessive abundance of sarcoplasm, which intersects the plexus of fibrils, and as we see in transverse section, not only col
. Electro-physiology. Electrophysiology. 20 ELECTRO-PHYSIOLOGY CHAP. catory muscles of many molluscs are yellow, pink, and even deep red. Knoll discovered a remarkable instance of " plasmic " muscle- cells among Invertebrates, in the thin muscle-bands of the mantle of Salpa (S. maxima, africana, pclesii). The cross-striped, cylin- drical fibres are very long, with conical ends. They are easily split up longitudinally into finer bundles and fibrils, owing to the excessive abundance of sarcoplasm, which intersects the plexus of fibrils, and as we see in transverse section, not only collects in the axis of each fibre, but also runs out in wide, radial tracts towards the periphery, thus dividing the contractile and FIG. section of two muscie-i distinctly fibrillated cortical stratum Knoll.) .. cells from >' 'h-nii. (Knoll. into separate laminae (-tig. - 1). The same plan of structure is here to some extent repeated, on a larger scale, that prevails in the far more delicate radial striation of the cortical zone of muscle-cells in many worms and molluscs. But there is one important difference ; the contractile substance is no longer (as in all previous cases) exclusively at the periphery of the formative cell, but appears in more or less con- spicuous bundles (muscle-columns) within the central sarcoplasm also. Hence, as Knoll has pointed out, the Salpa muscles in a measure represent the transition to certain arthropod and vertebrate muscles, in which the same structural arrangement is present. A transverse section through the cardiac muscles of Crustacea often exhibits an unmistakable similarity Fl0t ^ _Tra]ls. to Salpa in disposition of sarcoplasm and con- verse section of J A . cardiac muscle- tractile substance, except that the sarcoplasm is, Ceii of Lobster. where possible, even more richly developed, and all the "muscle-columns" lie within the formative cell (Fig. 12). The unusual quantity of protoplasm is explained in both case
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